On Laboratory Arts
by Richard Threlfall · English · Gutenberg #22784 · 539,148 chars · public domain (US)
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Title: On Laboratory Arts
Author: Richard Threlfall
Release date: September 27, 2007 [eBook #22784]
Language: English
Other information and formats: www.gutenberg.org/ebooks/22784
Credits: Produced by Jon Richfield
*** START OF THE PROJECT GUTENBERG EBOOK ON LABORATORY ARTS ***
Produced by Jon Richfield
ON LABORATORY ARTS
BY
RICHARD THRELFALL, M.A.
PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;
MEMBER OF THE INSTITUTE OF ELECTRICAL ENGINEERS;
ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL ENGINEERS;
MEMBER OF THE PHYSICAL SOCIETY
London
MACMILLAN AND CO, LIMITED
NEW YORK: THE MACMILLAN COMPANY
1898
All rights reserved
PREFACE 5
CHAPTER I 8
HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
PURPOSES 8
§ 4. Soft Soda Glass.
§ 6. Flint Glass.
§ 9. Hard or Bohemian, Glass.
§ 10. On the Choice of Sizes of Glass Tube.
§ 11. Testing Glass.
§ 13. Cleaning Glass Tubes.
§ 14. The Blow-pipe.
§ 18. The Table.
§ 19. Special Operations.
§ 20. Closing and blowing out the End of a Tube.
§ 21. To make a Weld.
§ 22. To weld two Tubes of different Sizes.
§ 24. To weld Tubes of very small Bore.
§ 30. To cut very thick Tubes.
§ 31. To blow a Bulb at the End of a Tube.
§ 32. To blow a bulb in the middle of a tube.
§ 33. To make a side Weld.
§ 34. Inserted Joints.
§ 35. Bending Tubes.
§ 36. Spiral Tubes.
§ 37. On Auxiliary Operations on Glass.
§ 38. Boring small Holes.
§ 39. For boring large holes through thick glass sheets.
§ 41. Operations depending on Grinding: Ground-in Joints.
§ 42. Use of the Lathe in Glass-working.
§ 46. Making Ground Glass.
§ 47. Glass-cutting.
§ 48. Cementing.
§ 49. Fusing Electrodes into Glass.
§ 51. The Art of making Air-light Joints.
APPENDIX TO CHAPTER I.
ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
ROENTGEN'S RADIATION.
CHAPTER II.
GLASS-GRINDING AND OPTICIANS' WORK.
§ 61. Details of the Process of Fine Grinding.
§ 62. Polishing.
§ 63. Centering.
§ 65. Preparing Small Mirrors for Galvanometers.
§ 66. Preparation of Large Mirrors or Lenses for Telescopes.
§ 69. The Preparation of Flat Surfaces of Rock Salt.
§ 70. Casting Specula for Mirrors.
§ 71. Grinding and polishing Specula.
§ 72. Preparation of Flat Surfaces.
§ 73. Polishing Flat Surfaces on Glass or on Speculum Metal.
CHAPTER III.
MISCELLANEOUS PROCESSES.
§ 74. Coating Glass with Aluminium and Soldering Aluminium.
§ 75. The Use of the Diamond-cutting Wheel.
§ 76. Arming a Wheel.
§ 77. Cutting a Section.
§ 78. Grinding Rock Sections, or Thin Slips of any Hard Material.
§ 79. Cutting Sections of Soft Substances.
§ 80. On the Production of Quartz Threads.
§ 84. Drawing Quartz Threads.
§ 86. Drawing Threads by the Catapult.
§ 87. Drawing Threads by the Flame alone.
§ 88. Properties of Threads.
§ 90. On the Attachment of Quartz Fibres.
§ 91. Other Modes of soldering Quartz.
§ 92. Soldering.
§ 94. Preparing a Soldering Bit.
§ 95. Soft Soldering.
§ 97. Soldering Zinc.
§ 98. Soldering other Metals.
§ 99. Brazing.
§ 100. Silver Soldering.
§ 101. On the Construction of Electrical Apparatus--Insulators.
§ 102. Sulphur.
§ 103. Fused Quartz.
§ 104. Glass.
§ 105. Ebonite or Hard Rubber.
§ 106. Mica.
§ 107. Use of Mica in Condensers.
§ 108. Micanite.
§ 109. Celluloid.
§ 110. Paper.
§ 111. Paraffined Paper.
§ 112. Paraffin.
§ 113. Vaseline, Vaseline Oil, and Kerosene Oil.
§ 114. Imperfect Conductors.
§ 116. Conductors.
§ 117. Platinoid.
§ 119. Platinum Silver.
§ 120. Platinum Iridium.
§ 121. Manganin.
§ 122. Other Alloys.
§ 123. Nickelin.
§ 124. Patent Nickel.
§ 125. Constantin.
126. Nickel Manganese Copper.
CHAPTER IV.
ELECTROPLATING AND ALLIED ARTS.
§ 127. Electroplating.
§ 128. The Dipping Bath.
§ 130. Scratch-brushing.
§ 131. Burnishing.
§ 132. Silver-plating.
§ 133. Cold Silvering.
§ 134. Gilding.
§ 135. Preparing Surfaces for Gilding.
§ 136. Gilding Solutions.
§ 137. Plating with Copper.
§ 138. Coppering Aluminium.
§ 140. Alkaline Coppering Solution.
§ 141. Nickel-plating.
142. Miscellaneous Notes on Electroplating.
§ 143. Blacking Brass Surfaces.
§ 144. Sieves.
§ 145. Pottery making in the Laboratory.
APPENDIX.
PLATINISING GLASS.
PREFACE
EXPERIMENTAL work in physical science rests ultimately upon the
mechanical arts. It is true that in a well-appointed laboratory,
where apparatus is collected together in greater or less profusion,
the appeal is often very indirect, and to a student carrying out a set
experiment with apparatus provided to his hand, the temptation to
ignore the mechanical basis of his work is often irresistible.
It often happens that young physicists are to be found whose
mathematical attainments are adequate, whose observational powers are
perfectly trained, and whose general capacity is unquestioned, but who
are quite unable to design or construct the simplest apparatus with
due regard to the facility with which it ought to be constructed.
That ultimate knowledge of materials and of processes which by long
experience becomes intuitive in the mind of a great inventor of course
cannot be acquired from books or from any set course of instruction.
There are, however, many steps between absolute ignorance and
consummate knowledge of the mechanical arts, and it is the object of
the following pages to assist the young physicist in making his first
steps towards acquiring a working knowledge of "laboratory arts."
However humble the ambition may be, no one can be more keenly alive
than the writer to the inadequacy of his attempt; and it is only from
a profound sense of the necessity which exists for some beginning to
be made, that he has had the courage to air his views on matters about
which there are probably hundreds or thousands of people whose
knowledge is superior to his own.
Moreover, nothing has been further from the writer's mind than any
idea of "instructing" any one; his desire is--if happily it may so
befall--to be of assistance, especially to young physicists or
inventors who wish to attain definite mechanical ends with the minimum
expenditure of time. Most people will agree that one condition
essential to success in such an undertaking is brevity, and it is for
this reason that alternative methods as a rule have not been given,
which, of course, deprives the book of any pretence to being a
"treatise." The writer, therefore, is responsible for exercising a
certain amount of discretion in the selection he has made, and it is
hardly to be hoped that he has in all--or even in the majority of
cases--succeeded in recommending absolutely the best method of
procedure.
This brings another point into view. Before all things the means
indicated must be definite and reliable. It is for this reason that
the writer has practically confined himself to matters lying within
his own immediate experience, and has never recommended any process
(with one or two minor exceptions, which he has noted) which he has
not actually and personally carried through to a successful issue.
This, although it is a matter which he considers of the highest
importance, and which is his only title to a hearing, has
unfortunately led to a very personal tone in the book.
With regard to the arts treated of in the following pages, matters
about which information is easily acquired--such as carpentering,
blacksmithing, turning, and the arts of the watchmaker--have been
left on one side. With regard to the last, which is of immense use in
the laboratory, there happen to be at least two excellent and handy
books, viz. Saunier's Watchmakers' Handbook, Tripplin, London, 1892;
and Britton's Watchmakers' Dictionary and Guide.
With regard to carpentering, turning, and blacksmithing, almost any
one who so desires can obtain a little practical experience in any
village. A short chapter has been devoted to glass-blowing, in spite
of there being an excellent and handy book by Mr. Shenstone (The
Methods of Glass-blowing, Rivington) on the subject already in
existence. The reason for this exception lies in the fact that the
writer's methods differ considerably from those advocated by Mr.
Shenstone.
The chapter on opticians' work has had to be compressed to an extent
which is undesirable in dealing with so complex and delicate an art,
but it is hoped that it will prove a sufficient introduction for
laboratory purposes. In this matter the writer is under great
obligations to his friend and assistant, Mr. James Cook, F.R.A.S, who
gave him his first lessons in lens-making some twenty years ago. To
Mr. John A. Brashear of Allegheny, Pa, thanks are due for much
miscellaneous information on optical work, which is included verbatim
in the text, some of it contained originally in printed papers, and
some most kindly communicated to the writer for the purpose of this
book. In particular, the writer would thank Mr. Brashear for his
generously accorded information as to the production of those "flat"
surfaces for which he is so justly famous.
The writer is also indebted to Mr. A. E. Kennelly for some
information as to American practice in the use of insulating material
for electrical work, and to his friends Mr. J. A. Pollock and Dr. C.
J. Martin for many valuable suggestions. For the illustrations
thanks are due to Mrs. Threlfall and Mr. James Cook. With regard to
matters which have come to the writer's knowledge by his being
specifically instructed in them from time to time, due acknowledgment
is, it is hoped, made in the text.
With regard to the question as to what matters might be included and
what omitted, the general rule has been to include information which
the author has obtained with difficulty, and to leave on one side that
which he has more easily attained. All the "unities" have been
consistently outraged by a deliberate use of the English and metric
systems side by side. So long as all the materials for mechanical
processes have to be purchased to specifications in inches and feet,
it is impossible to use the centimetre consistently without
introducing inconvenience. However, everybody ought to, and probably
does, use either system with equal facility.
No attempt has been made at showing how work can be done without
tools. Though, no doubt, a great deal can be done with inferior
appliances where great economy of money and none of time is an object,
the writer has long felt very strongly that English physical
laboratory practice has gone too far in the direction of starving the
workshop, and he does not wish, even indirectly, 'to give any
countenance to such a mistaken policy. Physical research is too
difficult in itself, and students' time is too valuable, for it to be
remunerative to work with insufficient appliances.
In conclusion, the writer would ask his readers to regard the book to
some extent as tentative, and as a means to the procuring and
organising of information bearing upon laboratory arts. Any
information which can be given will be always thankfully received, and
the author hereby requests any reader who may happen to learn
something of value from the book to communicate any special
information he may possess, so that it may be of use to others should
another edition ever be called for.
CHAPTER I
HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
PURPOSES
§ 1. THE art of glass-blowing has the conspicuous advantage, from the
point of view of literary presentation, of being to a great extent
incommunicable. As in the case of other delightful arts--such as
those treated of in the Badminton Library, for instance--the most
that can be done by writing is to indicate suitable methods and to
point out precautions which experience has shown to be necessary, and
which are not always obvious when the art is first approached. It is
not the object of this work to deal with the art of glass-blowing or
any other art after the manner befitting a complete treatise, in which
every form of practice is rightly included. On the contrary, it is my
wish to avoid the presentation of alternative methods.
I consider that the presentation of alternative methods would, for my
present purpose, be a positive disadvantage, for it would swell this
book to an outrageous size; and to beginners--I speak from
experience--too lavish a treatment acts rather by way of obscuring
the points to be aimed at than as a means of enlightenment. The
student often does not know which particular bit of advice to follow,
and obtains the erroneous idea that great art has to be brought to
bear to enable him to accomplish what is, after all, most likely a
perfectly simple and straightforward operation.
This being understood, it might perhaps be expected that I should
describe nothing but the very best methods for obtaining any proposed
result. Such, of course, has been my aim, but it is not likely that I
have succeeded in every case, or even in the majority of cases, for I
have confined myself to giving such directions as I know from my own
personal experience will, if properly carried out, lead to the result
claimed. In the few cases in which I have to refer to methods of
which I have no personal experience, I have endeavoured to give
references (usually taking the form of an acknowledgment), so that an
idea of their value may be formed. All methods not particularised may
be assumed by the reader to have come within my personal experience.
§ 2. Returning to glass-blowing, we may note that two forms of
glass-blowing are known in the arts, "Pot" blowing and "Table"
blowing. In the former case large quantities of fluid "metal"
(technical term for melted glass) are assumed to be available, and as
this is seldom the case in the laboratory, and as I have not yet felt
the want of such a supply, I shall deal only with "table" blowing.
Fortunately there is a convenient book on this subject, by Dr.
Shenstone (Rivingtons), so that what I have to say will be as brief as
possible, consistent with sufficiency for everyday work. As a matter
of fact there is not very much to say, for if ever there was an art in
which manual dexterity is of the first and last importance, that art
is glass-working.
I do not think that a man can become an accomplished glass-blower from
book instructions merely--at all events, not without much unnecessary
labour,--but he can learn to do a number of simple things which will
make an enormous difference to him both as regards the progress of his
work and the state of his pocket.
§ 3. The first thing is to select the glass. In general, it will
suffice to purchase tubes and rods; in the case where large pieces
(such as the bulbs of Geissler pumps) have to be specially prepared by
pot-blowing, the student will have to observe precautions to be
mentioned later on. There are three kinds of glass most generally
employed in laboratories.
§ 4. Soft Soda Glass, obtained for the most part from factories in
Thuringia, and generally used in assembling chemical apparatus.--This
glass is cheap, and easily obtainable from any large firm of apparatus
dealers or chemists. It should on no account be purchased from small
druggists, for the following reasons:-
(a) It is usually absurdly dear when obtained in this way.
(b) It is generally made up of selections of different age and
different composition, and pieces of different composition, even if
the difference is slight, will not fuse together and remain together
unless joined in a special manner.
(c) It is generally old, and this kind of glass often devitrifies with
age, and is then useless for blowpipe work, though it may be bent
sufficiently for assembling chemical apparatus. Devitrified glass
looks frosty, or, in the earlier stages, appears to be covered by
cobwebs, and is easily picked out and rejected.
§ 5. It might be imagined that the devitrification would disappear
when the glass is heated to the fusing point; and so it does to a
great extent, but for many operations one only requires to soften the
glass, and the devitrification often persists up to this temperature.
My experience is that denitrified glass is also more likely to crack
in the flame than good new glass, though the difference in this
respect is not very strongly marked with narrow tubes.
§ 6. Flint Glass.
Magnificent flint glass is made both in England and France. The
English experimenter will probably prefer to use English glass, and,
if he is wise, will buy a good deal at a time, since it does not
appear to devitrify with age, and uniformity is thereby more likely to
be secured. I have obtained uniformly good results with glass made by
Messrs. Powell of Whitefriars, but I daresay equally good glass may
be obtained elsewhere.
For general purposes flint glass is vastly superior to the soft soda
mentioned above. In the first place, it is very much stronger, and
also less liable to crack when heated--not alone when it is new, but
also, and especially, after it has been partly worked. Apparatus made
of flint glass is less liable to crack and break at places of unequal
thickness than if made of soda glass. This is not of much importance
where small pieces of apparatus only are concerned, because these can
generally be fairly annealed; and if the work is well done, the
thickness will not be uneven. It is a different matter where large
pieces of apparatus, such as connections to Geissler pumps, are
concerned, for the glass has often to be worked partly in situ, and
can only be imperfectly annealed.
Joints made between specimens of different composition are much more
likely to stand than when fashioned in soda glass. Indeed, if it is
necessary to join two bits of soda glass of different kinds, it is
better to separate them by a short length of flint glass; they are
more likely to remain joined to it than to each other. A particular
variety of flint glass, known as white enamel, is particularly
suitable for this purpose, and, indeed, may be used practically as a
cement.
§ 7, It is, however, when the necessity of altering or repairing
apparatus complicated by joints arises that the advantage of flint
glass is most apparent. A crack anywhere near to a side, or inserted
joint, can scarcely ever be repaired in the case of soda glass
apparatus, even when the glass is quite thin and the dimensions small.
It should also be mentioned that flint glass has a much more brilliant
appearance than soda glass. Of course, there is a considerable
difference between different kinds of flint glass as to the melting
point, and this may account for the divergency of the statements
usually met with as to its fusibility compared with that of soda
glass. The kind of flint glass made by Messrs. Powell becomes
distinctly soft soon after it is hot enough to be appreciably luminous
in a darkened room, and at a white heat is very fluid. This fluidity,
though of advantage to the practised worker, is likely to give a
beginner some trouble.
§ 8. As against the advantages enumerated, there are some drawbacks.
The one which will first strike the student is the tendency of the
glass to become reduced in the flame of the blow-pipe. This can be
got over by proper adjustment of the flame, as will be explained later
on. A more serious drawback in exact work is the following. In
making a joint with lead glass it is quite possible to neglect to fuse
the glass completely together at every point; in fact, the joint will
stand perfectly well even if it be left with a hole at one side, a
thing which is quite impossible with soft soda glass, or is at least
exceedingly unusual. An accident of this kind is particularly likely
to happen if the glass be at all reduced. Hence, if a joint does not
crack when cold, the presumption is, in the case of soda glass, that
the joint is perfectly made, and will not allow of any leak; but this
is not the case with flint glass, for which reason all joints between
flint glass tubes require the most minute examination before they are
passed. If there are any air bubbles in the glass, especial care must
be exercised.
§ 9. Hard or Bohemian, Glass.
This is, of course, used where high temperatures are to be employed,
and also in certain cases where its comparative insolubility in water
is of importance. It is very unusual for the investigator to have to
make complicated apparatus from this glass. Fused joints may be made
between hard glass and flint glass without using enamel, and though
they often break in the course of time, still there is no reason
against their employment, provided the work be done properly, and they
are not required to last too long.
§ 10. On the Choice of Sizes of Glass Tube.
It will be found that for general purposes tubes about one-quarter
inch in inside diameter, and from one-twentieth to one-fortieth of an
inch thick, are most in demand. Some very thin soda glass of these
dimensions (so-called "cylinder" tubes) will be found very handy for
many purposes. For physico-chemical work a good supply of tubing,
from one-half to three-quarters of an inch inside diameter, and from
one-twentieth to one-eighth inch thick, is very necessary. A few
tubes up to three inches diameter, and of various thicknesses, will
also be required for special purposes.
Thermometer and "barometer" tubing is occasionally required, the
latter, by the way, making particularly bad barometers. The
thermometer tubing should be of all sizes of bore, from the finest
obtainable up to that which has a bore of about one-sixteenth of an
inch. Glass rods varying from about one-twentieth of an inch in
diameter up to, say, half an inch will be required, also two or three
sticks of white enamel glass for making joints.
To facilitate choice, there is appended a diagram of sizes from the
catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and
the experimenter will be able to see at a glance what sizes of glass
to order. It is a good plan to stock the largest and smallest size of
each material as well as the most useful working sizes.
Fig. 1.
§ 11. Testing Glass.
"Reject glass which has lumps or knots, is obviously conical, or has
long drawn-out bubbles running through the substance." If a scratch be
made on the surface of a glass tube, and one end of the scratch be
touched by a very fine point of fused glass, say not more than
one-sixteenth inch in diameter, the tube, however large it is (within
reason), ought to crack in the direction of the scratch. If a big
crack forms and does not run straight, but tends to turn
longitudinally, it is a sign that the glass is ill annealed, and
nothing can be done with it. If such glass be hit upon in the course
of blow-pipe work, it is inadvisable to waste time upon it; the best
plan is to reject it at once, and save it for some experiment where it
will not have to be heated.
The shortest way of selecting glass is to go to a good firm, and let
it be understood that if the glass proves to be badly annealed it will
be returned. Though it was stated above that the glass should not be
distinctly conical, of course allowance must be made for the length of
the pieces, and, on the other hand, a few highly conical tubes will be
of immense service in special cases, and a small supply of such should
be included.
The glass, as it is obtained, should be placed in a rack, and covered
by a cloth to reduce the quantity of dust finding its way into the
tubes. It has been stated by Professor Ostwald that tubes when reared
up on end tend to bend permanently. I have not noticed this with lead
glass well supported. Each different supply should be kept by itself
and carefully described on a label pasted on to the rack, and tubes
from different lots should not be used for critical welds. This
remark is more important in the case of soda than of lead glass.
In the case of very fine thermometer tubes it will be advisable to
cover the ends with a little melted shellac, or, in special cases, to
obtain the tubes sealed from the works. Soda glass can generally be
got in rather longer lengths than lead glass; the longer the lengths
are the better, for the waste is less.
It is useful to be able to distinguish the different kinds of glass by
the colour. This is best observed by looking towards a bright surface
along the whole length of the tube and through the glass. Lead glass
is yellow, soda glass is green, and hard glass purple in the samples
in my laboratory, and I expect this is practically true of most
samples. [Footnote: Some new lead glass I have is also almost purple
in hue. If any doubt exists as to the kind of glass, it may be tested
at once in the blow-pipe flame, or by a mixture of oils of different
refractive indices, as will be explained later.]
§ 12. The question of the solubility of glass in reagents is one of
great importance in accurate work, though it does not always meet with
the attention it deserves. It is impossible here to go into the
matter with sufficient detail, and the reader is therefore referred to
the Abstracts of the Chemical Society, particularly for the years 1889
and 1892. The memoir by F. Kohlrausch, Wied. Ann. xliv, should be
consulted in the original. The following points may be noted. A
method of testing the quality of glass is given by Mylius (C. S. J.
Abstracts, 1889, p. 549), and it is stated that the resistance of
glass to the action of water can generally be much increased by
leaving it in contact with cold water for several days, and then
heating it to 300° to 400° C. This improvement seems to be due to the
formation of a layer of moist silica on the surface, and its
subsequent condensation into a resisting layer by the heating. Mylius
(C. S. J. Abstracts, 1892, p. 411), and Weber, and Sauer (C. S.
J. Abstracts, 1892, p. 410) have also shown that the best glass for
general chemical purposes consists of:
Silica, 7 to 8 parts
Lime, 1 part
Alkali, 1.5 to 1.1 parts.
This is practically "Bohemian" tube glass.
The exact results are given in the Berichte of the German Chemical
Society, vol. xxv. An excellent account of the properties of glass
will be found in Grove's edition of Miller's Elements of Chemistry.
§ 13. Cleaning Glass Tubes.
This is one of the most important arts in chemistry. If the tubes are
new, they are generally only soiled by dust, and can be cleaned fairly
easily--first by pushing a bit of cotton waste through with a cane,
or pulling a rag through with string--and then washing with sand and
commercial hydrochloric acid. I have heard of glass becoming
scratched by this process, and breaking in consequence when heated,
but have never myself experienced this inconvenience. In German
laboratories little bits of bibulous paper are sometimes used instead
of sand; they soon break into a pulp, and this pulp has a slightly
scouring action.
As soon as the visible impurities are removed and the tube when washed
looks bright and clean, it may be wiped on the outside and held
perpendicularly so as to allow the water film to drain down. If the
tube be greasy (and perhaps in other cases) it will be observed that
as the film gets thinner the water begins to break away and leave dry
spots. For accurate work this grease, or whatever it is, must be
removed; and after trying many plans for many years, I have come back
to the method I first employed, viz. boiling out with aqua regia.
For this purpose, close one end of the tube by a cork (better than a
rubber bung, because cheaper), and half fill the tube with aqua regia;
then, having noted the greasy places, proceed to boil the liquid in
contact with the glass at these points, and in the case of very
obstinate dirt--such as lingers round a fused joint which has been
made between undusted tubes--leave the whole affair for twelve hours.
If the greasiness is only slight, then simply shaking with hot aqua
regia will often remove it, and the aqua regia is conveniently heated
in this case by the addition of a little strong sulphuric acid.
The spent aqua regia may be put into a bottle. It is generally quite
good enough for the purpose of washing glass vessels with sand, as
above explained.
However carefully a tube is cleaned before being subjected to blowpipe
operations, it will be fouled wherever there is an opening during the
process of heating, unless the extreme tip only of an oxidising flame
be employed. Even this should not be trusted too implicitly unless
an oxygas or hydrogen flame is employed.
When a tube or piece of apparatus has been cleaned by acid, so that on
clamping it vertically, dry spaces do not appear, it may be rinsed
with platinum distilled water and left to drain, the dust being, of
course, kept out by placing a bit of paper round the top. For
accurate work water thus prepared is to be preferred to anything else.
When the glass is very clean interference colours will be noticed as
the water dries away.
Carefully-purified alcohol may in some cases be employed where it is
desired to dry the tube or apparatus quickly. In this case an alcohol
wash bottle should be used, and a little alcohol squirted into the top
of the tube all round the circumference. The water film drags the
alcohol after it, and by waiting a few minutes and then adding a few
more drops of alcohol, the water may be practically entirely removed,
especially if a bit of filter paper be held against the lower end of
the tube. It is customary in some laboratories to use ether for a
final rinse, but unless the ether is freshly distilled and very pure,
it leaves a distinct organic residue.
When no more liquid can be caused to drain away, the tube may be dried
by heating it along its length, beginning at the top (to get the
advantage of the reduction of surface tension), and so on all down.
It will then be possible to mop up a little more of the rinsing
liquid. When the tube is nearly dry a loose plug of cotton wool may
be inserted at the bottom. The wool must be put in so that the fibres
lie on an even surface inside the tube, and the wool must be blown
free from dust. Ordinary cotton wool is useless, from being dusty and
the fibres short, and the same remark applies to wadding. Use nothing
but what is known as "medicated" cotton wool with a good long fibre.
The tube will usually soon dry of itself when the cover is lifted an
inch or so. If water has been used, the air-current may be assisted
by means of the water-pump, the air being sucked from the top, so that
the wool has an opportunity of acting as a dust filter; a very slow
stream of air only must be employed. For connecting the tube to the
pump, a bit of India-rubber tube about an inch in diameter, with a
bore of about one-eighth of an inch, may be employed. The end of the
rubber tube is merely pressed against the edge of the glass.
These remarks apply, with suitable modification, to all kinds of
finished apparatus having two openings. For flasks and so on, it is
convenient to employ a blowing apparatus, dust being avoided by
inserting a permanent plug of cotton wool in one of the leading tubes.
The efficiency of this method is greatly increased by using about one
foot of thin copper tube, bent into a helix, and heated by means of a
Bunsen burner; the hot air (previously filtered) is passed directly
into the flask, bottle, or whatever the apparatus may be. This has
proved so convenient that a copper coil is now permanently fastened to
the wall in one of the rooms of my laboratory.
The above instructions indicate greater refinement than is in general
necessary or proper for tubes that have to be afterwards worked by the
blow-pipe. In the majority of cases all that is necessary is to
remove the dust, and this is preferably done by a wad of cotton waste
(which does not leave shreds like cotton wool), followed by a bit of
bibulous filter paper. I would especially warn a beginner against
neglecting this precaution, for in the process of blowing, the dust
undergoes some change at the heated parts of the apparatus, and forms
a particularly obstinate kind of dirt.
In special cases the methods I have advocated for removing dirt and
drying without covering the damp surfaces with dust are inadequate,
but an experimenter who has got to that stage will have nothing to
learn from such a work as this.
§ 14. The Blow-pipe.
I suppose a small book might easily be written on this subject but
what I have to say--in accordance with the limitation imposed--will
be brief. For working lead glass I never use anything but an oxygas
blow-pipe, except for very large work, and should never dream of using
anything else. Of course, to a student who requires practice in order
to attain dexterity this plan would be a good deal too dear. My
advice to such a one is--procure good soda glass, and work it by
means of a modification of a gas blow-pipe, to be described directly.
The Fletcher's blow-pipes on long stems are generally very
inconvenient. The flame should not be more than 5 or 6 inches from
the working table at most, especially for a beginner, who needs to
rest his arms on the edge of the table to secure steadiness.
The kind of oxygas blow-pipe I find most convenient is indicated in
the sketch. (Fig. 2) I like to have two nozzles, which will slip on
and off, one with a jet of about 0.035 inch in diameter, the other of
about double this dimension. The oxygen is led into the main tube of
the blow-pipe by another tube of much smaller diameter, concentric
with the main tube (Fig. 3, at A). The oxygen is mixed with the gas
during its escape from the inner tube, which is pierced by a number of
fine holes for the purpose, the extreme end being closed up. The
inner tube may run up to within half an inch of the point where the
cap carrying the nozzle joins the larger tube.
Fig. 2.
Fig. 3.
If it is desired to use the blow-pipe for working glass which is
already fixed in position to a support, it will be found very
advantageous to use a hooked nozzle. The nozzle shown in the sketch
is not hooked enough for this work, which requires that the flame be
directed 'backwards towards the worker. With a little practice such a
flame may be used perfectly well for blowing operations on the table,
as well as for getting at the back of fixed tubes.
To warm up the glass, the gas supply is turned full on, and enough
oxygen is allowed to pass in to clear the flame. The work is held in
front of, but not touching, the flame, until it is sufficiently hot to
bear moving into the flame itself. The, work is exposed to this flame
until, in the case of lead glass, traces of reduction begin to appear.
When this point is reached the oxygen tap is thrown wide open. I
generally regulate the pressure on the bags, so that under these
circumstances the flame is rather overfed with oxygen. This condition
is easily recognised, as follows. The flame shrinks down to a very
small compass, and the inner blue cone almost disappears; also
flashes of yellow light begin to show themselves--a thing which does
not occur when the proportions of the gases are adjusted for maximum
heating effect.
For many purposes the small dimensions of the flame render it very
convenient, and the high temperature which can be attained at exact
spots enables glass to be fused together after a certain amount of
mixing, which is an enormous advantage in fusing lead glass on to hard
glass. The lead glass should not be heated hot enough to burn, but,
short of this, the more fluid it is the better for joints between
dissimilar samples.
It will be noticed that the blow-pipe can be rotated about a vertical
axis so as to throw the flame in various directions. This is often
indispensable.
§ 15. In general the oxygen flame does not require to be delivered
under so high a pressure as for the production of a lime light. In
England, I presume, most experimenters will obtain their oxygen ready
prepared in bottles, and will not have to undergo the annoyance of
filling a bag. If, however, a bag is used, and it has some advantages
(the valves of bottles being generally stiff), I find that a pressure
produced by placing about two hundredweight (conveniently divided into
four fifty-six pound weights) on bags measuring 3' x 2'6" x 2' (at the
thicker end) does very well. To fill such a bag with oxygen, about
700 grms of potassium chlorate is required.
If the experimenter desires to keep his bag in good order, he must
purify his oxygen by washing it with a solution of caustic soda, and
then passing it through a "tower" of potash or soda in sticks, and,
finally, through a calcium chloride tower. This purifying apparatus
should be permanently set up on a board, so that it may be carried
about by the attendant to wherever it is required. Oxygen thus
purified does not seem to injure a good bag--at least during the
first six or seven years:
In order to reduce the annoyance of preparing oxygen, the use of the
usual thin copper conical bottle should be avoided. The makers of
steel gas bottles provide retorts of wrought iron or steel for
oxygen-making, and these do very well. They have the incidental
advantage of being strong enough to resist the attacks of a servant
when a spent charge is being removed.
The form of retort referred to is merely a large tube, closed at one
end, and with a screw coupling at the other; the dimensions may be
conveniently about 5 inches by 10. The screw threads should be filled
with fireclay (as recommended by Faraday) before the joint is screwed
up. Before purchasing a bottle the experimenter will do well to
remember that unless it is of sufficiently small diameter to go into
his largest vice, he will be inconvenienced in screwing the top on and
off. Why these affairs are not made with union joints, as they should
be, is a question which will perhaps be answered when we learn why
cork borers are still generally made of brass, though steel tube has
long been available.
Fig. 4.
These little matters may appear very trivial--and so they are--but
the purchaser of apparatus will generally find that unless he looks
after details himself, they will not be attended to for him. Whether a
union joint is provided or not, let it be seen that the end of the
delivery tube is either small enough to fit a large rubber tube
connection going to the wash-bottle, or large enough to allow of a
cork carrying a bit of glass tube for the same purpose to be inserted.
This tube should not be less than half an inch in inside diameter.
Never use a new bottle before it has been heated sufficiently to get
rid of grease and carbonaceous dirt. A convenient oxygen-making
apparatus is shown in Fig. 4, which is drawn from "life."
§ 16. For large blow-pipe work with lead glass I recommend a system
of four simple blow-pipes, in accordance with the sketch annexed. I
first saw this system in operation in the lamp factory of the
Westinghouse Electric Company at Pittsburg in 1889, and since then I
have seen it used by an exceedingly clever "trick" glass-worker at a
show. After trying both this arrangement and the "brush flame"
recommended by Mr. Shenstone, I consider the former the more
convenient; however, I daresay that either can be made to work in
competent hands, but I shall here describe only my own choice.
[Footnote: A brush flame is one which issues from the blow-pipe nozzle
shaped like a brush, i.e. it expands on leaving the jet. It is
produced by using a cylindrical air jet or a conical jet with a large
aperture, say one-eighth of an inch. See Fig. 25.]
As will be seen, the blow-pipe really consists of four simple brass
tube blow-pipes about three-eighths of an inch internal diameter and 3
inches long, each with its gas and air tap and appropriate nozzle.
Each blowpipe can turn about its support (the gas-entry pipe) to some
extent, and this possibility of adjustment is of importance, The air
jets are merely bits of very even three-sixteenths inch glass tubing,
drawn down to conical points, the jets themselves being about 0.035
inch diameter.
Fig. 5.
The flames produced are the long narrow blow-pipe flames used in
blow-pipe analysis, and arranged so as to consist mostly of oxidising
flame. The air-supply does not require to be large, nor the pressure
high--5 to 10 inches of water will do--but it must be very regular.
The "trick" glass-blower I referred to employed a foot bellows in
connection with a small weighted gasometer, the Westinghouse Company
used their ordinary air-blast, and I have generally used a large
gas-holder with which I am provided, which is supplied by a Roots
blower worked by an engine.
I have also used a "velocity pump" blower, which may be purchased
amongst others from Gerhardt of Bonn. The arrangement acts both as a
sucking and blowing apparatus, and is furnished with two manometers
and proper taps, etc. As I have reason to know that arrangements of
this kind work very ill unless really well made, I venture to add that
the Gerhardt arrangement to which I refer is No. 239 in his
catalogue, and costs about three pounds. It hardly gives enough air,
however, to work four blow-pipes, and the blast requires to be
steadied by passing the air through a vessel covered with a rubber
sheet.
In default of any of these means being available, one of Fletcher's
foot-blowers may be employed, but it must be worked very regularly. A
table mounted with one blow-pipe made on this plan, and worked by a
double-acting bellows, is recommended for students' use. For working
flint glass, the air jet may be one-eighth of an inch in diameter and
the pressure higher--this will give a brush flame. See Fig. 25.
It will be seen, on looking at the sketch of the blowpipe system, that
the pair of blow-pipes farther from the observer can be caused to
approach or recede at will by means of a handle working a block on a
slide. It often happens that after using all four blow-pipes at once
it is necessary to have recourse to one blow-pipe only, and to do this
conveniently and quickly is rather an object. Now, in my arrangement
I have to turn off both the gas and air from the farther system, and
then put in a bit of asbestos board to prevent the nozzles being
damaged by the flame or flames kept alight. As I said before, when
some experience is gained, glassblowing, becomes a very simple art,
and work can be done under circumstances so disadvantageous that they
would entirely frustrate the efforts of a beginner. This is not any
excuse, however, for recommending inferior arrangements.
Consequently, I say that the pipes leading in gas and air should be
all branches of one gas and one air pipe, in so far as the two remote
and one proximate blow-pipe are concerned, and these pipes should come
up to the table to the right hand of the operator, and should have
main taps at that point, each with a handle at least 2 inches long.
By this arrangement the operator can instantly turn down all the
blow-pipes but one, while, if the inverse operation is required, all
the three pipes can be started at once. [Footnote: I find, since
writing the above, that I have been anticipated in this recommendation
by Mr. G. S. Ram, The Incandescent Lamp and its Manufacture, p.
114.]
The separate air and gas taps must be left for permanent regulation,
and must not be used to turn the supply on or cut it off. In some
respects this blow-pipe will be found more easy to manage than an
oxygas blow-pipe, for the glass is not so readily brought to the very
fluid state, and this will often enable a beginner who proceeds
cautiously to do more than he could with the more powerful instrument.
Though I have mentioned glass nozzles for the air supply, there is no
difficulty in making nozzles of brass. For this purpose let the end
of a brass tube of about one-eighth of an inch diameter be closed by a
bit of brass wire previously turned to a section as shown (Fig. 6),
and then bored by a drill of the required diameter, say -.035 inch. It
is most convenient to use too small a drill, and to gradually open the
hole by means of that beautiful tool, the watchmaker's "broach." The
edges of the jet should be freed from burr by means of a watchmaker's
chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin,
1882, p. 232, § 342), or by the alternate use of a slip of Kansas
stone and the broach.
Fig. 6.
The construction of this blow-pipe is so simple, that in case any one
wishes to use a brush flame, he can easily produce one simply by
changing his air jets to bits of the same size (say one-eighth to
one-sixteenth of an inch) tubing, cut off clean. To insure success,
the ends of the tubes must be absolutely plane and regular; the
slightest inequality makes all the difference in the action of the
instrument. If a jet is found to be defective, cut it down a little
and try again; a clean-cut end is better than one which has been
ground flat on a stone. The end of a tube may, however, be turned in
a manner hereafter to be described so as to make an efficient jet.
Several trials by cutting will probably have to be made before success
is attained. For this kind of jet the air-pressure must be greatly
increased, and a large Fletcher's foot-blower or, better still, a
small double-action bellows worked with vigour will be found very
suitable. A fitting for this auxiliary blow-pipe is shown in Fig. 5
at B.
Professor Roentgen's discovery has recently made it necessary to give
more particular attention to the working of soft soda glass, and I
have been obliged to supplement the arrangements described by a table
especially intended for work with glass of this character. The
arrangement has proved so convenient for general work that I give the
following particulars. The table measures 5 feet long, 2 feet 11
inches wide, and is 2 feet 9 inches high.
Fig. 7.
It is provided with a single gas socket, into which either a large or
small gas tube may be screwed. The larger tube is 5.5 inches long and
0.75 of an inch in diameter. The smaller tube is the same length, and
half an inch in diameter. The axis of the larger tube is 3.5 inches
above the table at the point of support, and is inclined to the
horizontal at an angle of 12°. The axis of the smaller tube is 2.5
inches above the surface of the table, and is inclined to the
horizontal at the same angle as the larger one.
The air jets are simply pieces of glass tube held in position by
corks. The gas supply is regulated by a well-bored tap. The air
supply is regulated by treading the bellows--no tap is requisite.
The bellows employed are ordinary smiths' bellows, measuring 22 inches
long by 13 inches wide in the widest part. They are weighted by lead
weights, weighing 26 lbs. The treadle is connected to the bellows by
a small steel chain, for the length requires to be invariable. As the
treadle only acts in forcing air from the lower into the upper chamber
of the bellows, a weight of 13 lbs. is hung on to the lower cover, so
as to open the bellows automatically.
The air jets which have hitherto been found convenient are:
for the small gas tube
(1) a tube 0.12 inch diameter drawn down to a jet of 0.032 inch
diameter for small work;
(2) plain tubes not drawn down of 0.14 inch, 0.127 inch, and -0.245
inch diameter, and for the large gas tube, plain tubes up to 0.3 inch
in diameter.
The table is placed in such a position that the operator sits with his
back to a window and has the black calico screen in front of him and
to his right. The object of the screen is to protect the workman
against draughts. The table is purposely left unscreened to the left
of the workman, so that long tubes may be treated.
§ 17. Other appliances which will be required for glass-blowing are
of the simplest character.
(1) Small corks for closing the ends of tubes.
(2) Soft wax--a mixture of bees' wax and resin softened by linseed
oil to the proper consistency, easily found by trial, also used for
temporarily closing tubes.
(3) A bottle of vaseline for lubricating.
(4) An old biscuit tin filled with asbestos in shreds, and an asbestos
towel or cloth for annealing glass after removal from the flame. As
asbestos absorbs moisture, which would defeat its use as an annealing
material, it must be dried if necessary.
(5) A Glass-Cutter's Knife. This is best made out of a fine
three-cornered file, with the file teeth almost ground out, but not
quite; it should be about 2 inches long. After the surface has been
ground several times, it may be necessary to reharden the steel. This
is best done by heating to a full red and quenching in mercury. The
grindstone employed for sharpening the knife should be "quick," so as
to leave a rough edge. I have tried many so-called glass knives "made
in Germany," but, with one exception, they were nothing like so good
as a small French or Sheffield file. In this matter I have the
support of Mr. Shenstone's experience.
(6) A wire nail, about 2 inches long, mounted very accurately in a
thin cylindrical wooden handle about 5 inches long by one-quarter of
an inch diameter, or, better still, a bit of pinion wire 6 inches
long, of which 1.5 inches are turned down as far as the cylindrical
core, An old dentists' chisel or filling tool is also a very good
form of instrument.
(7) A bit of charcoal about 3.5 inches long and 2 wide, and of any
thickness, will be found very useful in helping to heat a very large
tube. The charcoal block is provided with a stout wire handle, bent
in such a manner that the block can be held close above a large glass
tube on which the flames impinge. In some cases it is conveniently
held by a clip stand. By the use of such a slab of charcoal the
temperature obtainable over a large surface can be considerably
increased.
I have seen a wine-glass (Venetian sherry-glass) worked on a table
with four blow-pipes, such as is here described, with the help of a
block of hard wood held over the heated glass, and helping the
attainment of a high temperature by its own combustion.
(8) Several retort stands with screw clips.
(9) Some blocks of wood about 5" X 2" X 2" with V-shaped notches cut
in from the top.
(10) A strong pair of pliers.
(11) An apparatus for cleaning and drying the breath, when blowing
directly by the mouth is not allowable. The apparatus consists of a
solid and heavy block of wood supporting a calcium-chloride tube
permanently connected with a tube of phosphorus pentoxide divided into
compartments by plugs of glass wool. Care should be taken to arrange
these tubes so as to occupy the smallest space, and to have the stand
particularly stable. The exit tube from the phosphorus pentoxide
should be drawn down to form a nozzle, from, say, half an inch to
one-eighth of an inch in diameter, so as to easily fit almost any bit
of rubber tube. The entry to the calcium chloride should be
permanently fitted to about a yard of fine soft rubber tubing, as
light as possible. The ends of this tube should terminate in a glass
mouthpiece, which should not be too delicate.
As an additional precaution against dust, I sometimes add a tube
containing a long plug of glass wool, between the phosphorus pentoxide
and the delivery tube, and also a tube containing stick potash on the
entry side of the calcium chloride tube, but it may safely be left to
individual judgment to determine when these additions require to be
made. In practice I always keep the affair set up with these
additions. The communication between all the parts should be
perfectly free, and the tubes should be nearly filled with reagents,
so as to avoid having a large volume of air to compress before a
pressure can be got up.
The arrangement will be clear by a reference to Fig. 8, which
illustrates the apparatus in use for joining two long tubes. I have
tried blowing-bags, etc, but, on the whole, prefer the above
arrangement, for, after a time, the skill one acquires in regulating
the pressure by blowing by the mouth and lips is such an advantage
that it is not to be lightly foregone.
Fig. 8.
§ 18. The Table.
The system of four blow-pipes is, of course, a fixture. In this case
the table may be about a yard square, and may be covered with asbestos
mill-board neatly laid down, but this is not essential. The table
should have a rim running round it about a quarter of an inch high.
The tools should be laid to the right of the worker, and for this
purpose the blow-pipes are conveniently fixed rather to the left of
the centre of the table, but not so far as to make the leg of the
table come so close to the operator as to make him uncomfortable, for
a cheerful and contented spirit ought to be part of the glass-worker's
outfit.
The most convenient height for a blow-pipe table--with the blow-pipes
about 2 inches above the table top--is 3 feet 2 inches. Nothing is
so convenient to sit upon as a rough music-stool with a good range of
adjustment. The advantage of an adjustable seat lies in the fact that
for some operations one wants to be well over the work, while in
others the advantage of resting the arms against the table is more
important.
§ 19. Special Operations.
The preliminary to most operations before the blow-pipe, is to draw
down a tube and pull it out to a fine point. This is also the
operation on which a beginner should exercise himself in the first
instance. I will suppose that it is desired to draw out a tube about
one-quarter of an inch in diameter, with the object of closing it,
either permanently or temporarily, and leaving a handle for future
operations in the shape of the point, thin enough to cool quickly and
so not delay further work.
For this simple operation most of the glass-blower's skill is
required. The tube must be grasped between the first finger and thumb
of both hands, and held so that the part to be operated on lies evenly
between the two hands. The distance between the operator's thumbs may
conveniently vary from 2.5 to 4 inches. Releasing the grip of the
left hand, let the operator assure himself of his ability to easily
rotate the tube about its axis--by the right thumb and finger--he
will incidentally observe by the "feel" whether the tube is straight
or not.
A good deal of progress can be made from this point before the tube is
heated at all. The operator can acquire a habit of instinctively
rotating the tube by both hands, however the tube itself be moved
about in space, or however it be pushed or pulled. The habit of
constant and instinctive rotation is literally about one-third of the
whole art of glassblowing. It is unlikely, however, that the beginner
will discover that he has not got this habit, until a few failures
draw his attention to it.
The glass tube being held in position lightly yet firmly, and the
operator being sure that he feels comfortable and at his ease, and
that the blow-pipe flame (a single flame in this instance) is well
under control, the preliminary heating may be commenced. With a tube
of the dimensions given this is a very simple affair. Turn the air
partly off, or blow gently, to get a partly luminous gas flame; hold
the tube about an inch from the end of this flame, and turn it round
and round till it commences to soften.
In the case of soda glass it is usual to employ the gas flame only,
but I find that it is better in most cases to use the hot air of a
gently-blown flame, rather than have the disadvantage of the soot
deposited in the alternative operation. When the glass begins to
soften, or even before, it may be moved right into the blow-pipe
flame, and the latter may be properly urged.
It is not possible to give quite explicit and definite instructions,
applicable to every case, as to when the time is ripe for passing the
work into the flame, but the following notes will indicate the general
rules to be observed:-
(1) A thick tube must be warmed more slowly and raised to a higher
temperature than a thin tube.
(2) The same remark applies to a tube of large diameter, as compared
with one of small diameter, whatever the thickness.
(3) In the case of very large or thick tubes the hot air is
advantageously employed at first, and to complete the preliminary
heating, the luminous flame alone may be used. The object of this is
to enable the operator to judge, by the presence of soot, its
inability to deposit--or its burning off if deposited--of the
temperature of the glass, and of the equality of this temperature all
over the surface, for a large and thick tube might be heated quite
enough to enable it to be safely exposed to the full heat before it is
appreciably yielding to the fingers. In general, when the soot burns
off freely, or lead glass begins to show the faintest sign of
reduction, or soda glass begins to colour the flame, it is more than
safe to proceed.
In order to turn on the full flame the operator will form a habit of
holding the work in the left hand only, and he will also take care not
to let anything his right hand may be doing cause him to stop rotating
the tube with his left thumb and finger.
The preliminary adjustment of air or oxygen supply will enable the
change to a flame of maximum power to be made very quickly. The tube
having been introduced with constant rotation, it will soon soften
sufficiently to be worked. The beginner will find it best to decide
the convenient degree of softness by trial.
With soda glass it does not much matter how soft the glass becomes,
for it remains viscous, but with lead glass the viscosity persists for
a longer time and then suddenly gives place to a much greater degree
of fluidity. [Footnote: This is only drawn from my impressions
acquired in glass-working. I have never explicitly tested the matter
experimentally.]
It is just at this point that a beginner will probably meet with his
first difficulty. As soon as the glass gets soft he will find that he
no longer rotates the glass at the same speed by the right and left
hand, and, moreover, he will probably unconsciously bend the tube, and
even deform it, by pushing or pulling.
The second third of the art of the glass-blower consists in being able
to move both hands about, rotating a tube with each thumb and finger,
and keeping the distance between the hands, and also the speed of
rotation, constant. Nothing but long practice can give this facility,
but it is essential that it be acquired to some extent, or no progress
can be made. Some people acquire a moderate proficiency very quickly,
others, of whom the writer is one, only become reasonably proficient
by months, or even years, of practice.
Supposing that the tube is now ready to be drawn down, the operator
will remove it from the flame, and will gently pull the ends apart,
interrupting his turning as little as possible. If the tube be pulled
too hard, or if the area heated be too small (about three-eighths of
an inch in length in the case given would be proper), it will be found
that the ends of the two portions of the tube will be nearly closed at
a very sharp angle (nearly a right angle to the length of the tube),
that the ends will be thin, and that a long length of very fine tube
will be produced. To heat a short length of tube and pull hard and
suddenly is the proper way to make a very fine capillary tube, but, in
general, this is what we want to avoid.
If the operation be successfully performed, the drawn-down tube will
have the appearance exhibited, which is suitable either for
subsequently closing or handling by means of the drawn-down portion.
The straightness of the point can be obtained by a little practice in
"feeling" the glass when the tube is rotated as it cools just before
it loses its viscous condition.
When the operation is carried out properly the shoulder of the "draw"
should be perfectly symmetrical and of even thickness, and its axis
regarded as that of a cone should lie in the axis of the tube
produced. The operation should be repeated till the student finds
that he can produce this result with certainty, and he should not be
discouraged if this takes several days, or even weeks. Of course, it
is probable that within the first hour he will succeed in making a
tolerable job, but it is his business to learn never to make anything
else.
Fig. 9.
Fig. 10. Diagram of a folded end.
§ 20. Closing and blowing out the End of a Tube.
When it is desired to close the end of a particular bit of tube, this
is easily done by heating the end, and at the same time heating the
end of a waste bit of tube or rod; the ends, when placed in contact,
stick together, and a point can be drawn down as before. [Footnote:
"Point" is here used in the technical sense, i.e. it is a thin tail
of glass produced by drawing down a tube.] Having got a point, it
will be found that the thin glass cools enough to allow of the point
being handled after a few moments.
The most convenient way of reducing the point to a suitable length
(say 1.5 inch) is to fuse it off in the flame, but this must be done
neatly; if a tail is left it may cause inconvenience by catching, or
even piercing the finger and breaking off. The blow-pipe flame being
turned down to a suitable size, and the shoulder of the "draw" having
been kept warm meanwhile, let the tip of the flame impinge on a point
where the diameter is about half that of the undrawn tube, and let the
temperature be very high (Fig. 11). The tube is to be inclined to
the flame so that the latter strikes the shoulder normally, or
nearly so. Then, according to circumstances, little or much of the
glass can be removed at will by drawing off the tail (Fig. 12), till,
finally, a small drop of melted glass only, adheres to the end of the
now closed tube (Fig. 13).
Fig. 11.
Fig. 12.
Fig. 13.
Fig. 14.
When this is satisfactorily accomplished, heat the extreme end of the
tube most carefully and equally, holding it in such a position that
the glass will tend to flow from the bead back on to the tube, i.e.
hold the closed end up to the flame, the tube being, say, at 45
degrees to the horizontal. Then when the temperature is such as to
indicate complete softness lift the tube to the mouth, still holding
the tube pointing with its closed end a little above the horizontal,
and blow gently. A beginner almost always blows too hard.
What is wanted, of course, is a continued pressure, to give the
viscous glass time to yield gradually, if it is uniform; or else
intermittent puffs to enable the thinner parts, if there are any, to
cool more, and hence become more resisting than the thicker ones. In
any case a little practice will enable the operator to blow out a
round and even end--neither thicker nor thinner than the rest of the
tube.
§ 21. To make a Weld.
To begin with, try on two bits of glass of the same size, i.e. cut a
seven-inch length of glass in half by scratching it with the knife,
and pulling the ends apart with a slight inclination away from the
scratch. In other words, combine a small bending moment with a
considerable tensional stress. It is important to learn to do this
properly. If the proportions are not well observed, the tube will
break with difficulty, and the section will not be perpendicular to
the main length. If the knife is in good order it will make a fine
deep scratch--the feel of the glass under the knife will enable the
operator to decide when the scratch is made. The operation of cutting
large tubes will be treated further on. The two halves of the tube
being held one in each hand, and one tube closed at one end, the
extremities to be united will be warmed, and then put in the flame as
before.
Fig. 16.
There are many ways of proceeding--perhaps the easiest is as follows.
As soon as the glass shows signs of melting at the ends--and care
should be taken that much more is not heated--take both bits out of
the flame. Stop rotating for a moment, and resting the arms carefully
on the edge of the table, raise the tubes above the flame and bring
the ends swiftly and accurately together. This is a case of "sudden
death no second attempt at making the ends meet can be allowed; if
the tubes join in any other than a perfectly exact manner a kink more
or less objectionable will result. In practice the operator will
learn to bring the ends together, commencing at one point; i.e. the
axes of the tubes will be inclined at first, so as to cause adherence
at one spot only. If this is not quite "fair", then less damage is
done in moving one tube slightly up or down to get the contact exact.
The tubes will then be closed upon one another as if they were hinged
at the joint. This must be done lightly, yet sufficiently, to ensure
that the glass is actually in contact all round.
Having gone so far, replace the tubes--now one--in the flame, and
carefully rotating the glass, raise the temperature higher than in the
operation just described, in fact the higher the temperature, short of
burning the glass, the better. Take the tube out of the flame and
blow into the open end, turning constantly as before. One puff is
enough. Then turn and pull the glass apart till it is of the same
diameter and thickness throughout, and feel that it is straight as
before.
Though it is in general of high importance that the joint should be
well heated, the beginner will probably find that he "ties up" his
glass as soon as it gets really soft.
If his object is to make one joint--at any cost--then let him be
careful to use two bits of exactly the same kind of glass, and only
get the temperature up to the viscous stage. If the joint be then
pulled out till it is comparatively thin, it will probably stand (if
of soda glass); certainly, if of lead glass, though in this case it
may not be sound. In any case the joint should be annealed in the
asbestos box if practicable, otherwise (unless between narrow tubes)
with the asbestos rag. Care must be taken that the asbestos is dry.
§ 22. To weld two Tubes of different Sizes.
To do this, the diameter of the larger tube must be reduced to that of
the smaller. The general procedure described in drawing down must be
followed, with the following modification. In general, a greater
length of the tube must be heated, and it must be made hotter. The
tube is to be gradually drawn in the flame with constant turning till
the proper diameter and thickness of glass are attained.
Fig. 16.
For this operation time must be allowed if the operator's hands are
steady enough to permit of it; the shoulder should form partly by the
glass sinking in and partly by the process of drawing the hot glass
out. A shoulder properly prepared is shown in the sketch. Beginners
generally make the neck too thin on large tubes, and too thick on
smaller ones. There ought to be no great difference in thickness of
glass between the neck on the larger tube, and the smaller tube. The
diameters should be as nearly as possible alike.
Having drawn down the larger tube to a neck, take it out of the flame,
and as it cools pull and turn till the neck is of the right thickness
and is perfectly straight, i.e. make the final adjustment outside the
flame, and to that end have the neck rather too thick (as to glass)
before it is taken out. It is not necessary to wait till the neck
gets cold before the end can be cut off. Make a scratch as before--this
will probably slightly damage the temper of the file knife, but
that must be put up with. Hold the tube against the edge of the
table, so that the scratch is just above the level of the rim, and
strike the upper part a smart blow with the handle of the glass knife
rather in the direction of its length. [Footnote: A bit of hoop iron
nailed against the side of the table is a very convenient arrangement,
and it need not project appreciably above the general level of the
rim.]
Of course this applies to a tube where economy has been exercised and
the end is short. If the tail is long enough to form a handle, the
tube may be pulled apart as before. As a rule a temporary joint
between a tube and a rod is not strong enough to enable the shoulder
to be broken at the scratch by mere pulling. The ends to be welded
must be broken off very clean and true. Subsequent operations are to
be carried out as already described.
§ 23. The above operations will be easily performed on tubes up to
half an inch in diameter, if they are not too long. It is the length
of tube, and consequent difficulty in giving identity of motion with
the two hands, which make the jointing of long tubes difficult. There
are also difficulties if the tubes are very thin, have a very fine
bore or a very large diameter.
All these difficulties merely amuse a good glass-blower, but to an
experimenter who wants to get on to other things before sufficient
skill is acquired (in the movement of the hands and arms) the
following method is recommended. First, use flint glass. Then,
assuming that any drawing down has to be done, do it as well as
possible, for on this the success of the method to be described
especially depends. Be sure that the tubes to be welded are cut off
clean and are as nearly as may be of the same size at the point of
junction.
To fix the description, suppose it is desired to join two tubes (see
Fig. 8), each about one inch in diameter and a yard long. Get four
clip stands and place them on a level table. Be sure that the stands
are firm and have not warped so as to rock. In each pair of clips
place a tube, so that the two tubes are at the same height from the
table, and, in fact, exactly abut, with axes in the same straight
line. Close one tube by a cork and then fix the blowing apparatus as
shown to the other.
In such an operation as this the drying apparatus may be dispensed
with, and a rubber tube simply connected to one end of the system and
brought to the mouth. Take the oxygen blow-pipe and turn the nozzle
till the flame issues towards you, and see that the flame is in order.
Then turn down the oxygen till it only suffices to clear the smoky
flame, and commence to heat the proposed joint by a current of hot
air, moving the flame round the joint. Finally, bring to bear the
most powerful flame you can get out of the blow-pipe, and carry it
round the joint so quickly that you have the latter all hot at once.
Put down the blow-pipe, and, using both hands, press the tubes
together (which wooden clips will readily allow), and after seeing
that the glass has touched everywhere, pull the tubes a trifle apart.
Apply the blow-pipe again, passing lightly over the thin parts, if
any, and heating thicker ones; having the end of the rubber tube in
his mouth, the operator will be able to blow out thick places. When
all is hot, blow out slightly, and having taken the flame away, pull
the tubes a little apart, and see that they are straight.
Throw an asbestos rag over the joint, loosen one pair of the clamps
slightly, and leave the joint to anneal. It is important that the
least possible amount of glass should be heated, hence the necessity
of having the ends well prepared, and it is also important that the
work should be done quickly; otherwise glass will flow from the upper
side downwards and no strong joint will be obtained.
Fig. 17. Tube being opened at one end.
§ 24. To weld Tubes of very small Bore.
If the bore is not so small as to prevent the entrance of the point of
the iron nail, get the ends of the tubes hot, and open the bore by
inserting the end of the nail previously smeared over with a trace of
vaseline. Work the nail round by holding the handle between the thumb
and first finger of the right hand, the tube being similarly placed in
the left. The tube and nail should be inclined as shown in the
sketch.
Never try to force the operation; the nail soon cools the glass, so
that only a very short time is available after each heat; during this
the tube should be rotated against the nail rather than the nail
against the tube. Be careful not to heat a greater length of tube
than is necessary, or the nail will, by its component of pressure
along the tube, cause the latter to "jump up" or thicken and bulge.
Both ends being prepared, and if possible, kept hot, the weld may be
made as before, and the heating continued till the glass falls in to
about its previous thickness, leaving a bore only slightly greater
than before.
It is in operations such as this that the asbestos box will be found
of great use. As soon as one end of the weld is ready cool it in the
flame till soot deposits, and then plunge it into the asbestos. This
will cause it to cool very slowly, and renders it less likely to crack
when again brought into the flame. Turned-out ends, if the glass is
at all thick, are very liable to crack off on reheating, so that they
must be reintroduced (into the flame) with especial care. This
liability to breakage is reduced, but not eliminated, by the asbestos
annealing.
Figs. 18 and 19.
§ 25. When the bore is very fine, it is best to seal off the tubes,
and blow an incipient bulb near one end of each tube. These bulbs may
be cooled in asbestos, and cut across when cold by means of a scratch
touched at one end (Figs. 18 and 19) by a fine point of highly
incandescent glass. For details of this method see p. 46, Fig. 21.
Time is occasionally saved by blowing off the ends of the bulbs. The
details of this process will be described when the operation of making
thistle-headed tubes is dealt with.
§ 26. When the tubes are both of large diameter, long, and very thin
(cylinder tubes), a considerable amount of difficulty will be
experienced. On the whole, it is best to heat each end separately
till the glass thickens a little, anneal in the flame and in asbestos,
and then proceed as in § 22. If the ends are not quite true, it will
be found that quite a thickness of glass may be "jumped" together at
one side of the tubes, while the edges are still apart at the other.
When this looks likely to happen, incline the tubes as if the joint
were a hinge, and bend back quickly; do not simply continue to push
the tubes together in a straight line, or an unmanageable lump of
glass will be formed on one side.
If in spite of these precautions such a lump does form, proceed as
follows. Take a rod of glass, at least one-eighth of an inch thick,
and warm it in the flame at one end. Heat the imperfect joint till it
softens all round, and then bring the flame right up to the thick
part, and heat that as rapidly and locally as possible. The oxygas
flame does this magnificently. Press the heated end of the glass rod
against the thick part, and pull off as much of the lump as it is
desired to remove, afterwards blowing the dint out by a judicious
puff. Finish off as before.
§ 27. Occasionally, when it is seen that in order to produce a joint
closed all round, one side of the tube would be too much thickened, it
is better to patch the open side. For this purpose take a glass rod
about one-sixteenth inch in diameter, and turn the flame to give its
greatest effect, still keeping rather an excess of air or oxygen. See
that the side of the joint already made is kept fairly hot--it need
not be soft; interrupt any other work often enough to ensure this.
Then, directing the flame chiefly on the thin rod, begin to melt and
pull the glass over the edges of the gap. When the gap is closed get
the lump very hot, so that all the glass is well melted together, and
then, if necessary, pull the excess of glass off, as before described.
It must be remembered that this and the method of the previous section
are emergency methods, and never give such nice joints as a
manipulation which avoids them, i.e. when the ends of the tubes are
perfectly straight and true to begin with. Also note that, as the
tubes cannot be kept in rotation while being patched, it is as well to
work at as low a temperature as possible, consistently with the other
conditions, or the glass will tend to run down and form a drop,
leaving a correspondingly thin place behind.
Fig. 20.
§ 28. A very common fault in cutting a tube of about an inch in
diameter is to leave it with a projecting point, as shown. This can
be slowly chipped off by the pliers, using the jaws to crush and grind
away the edge of the projection; it is fatal to attempt to break off
large pieces of glass all at once.
§ 29. It will be convenient here to mention some methods of cutting
large tubes. With tubes up to an inch and a half in diameter, and
even over this--provided that the glass is not very thick--we may
proceed as follows: Make a good scratch about half an inch long, and
pretty deep, i.e. pass the knife backwards and forwards two or three
times. Press a point of melted glass exactly on one end of the
scratch; the glass point even when pressed out of shape should not be
as large as a button one-twelfth of an inch in diameter. If this
fails at first, repeat the operation two or three times.
Fig. 21.
If a crack does not form, touch the hot place with the cold end of the
nail. If no success is obtained, try the other end of the scratch.
If failure still pursues the operator, let him make another cut on the
opposite side of the tube and try again. In general, the tube will
yield the first or second time the hot drop of glass is applied.
Never apply the drop at the centre of the scratch, or a ragged crack,
which may run in any direction, will result. Very often, with a large
tube, the crack formed by a successful operation will only extend a
short distance. In this case it is desirable to entice the crack
round the tube, and not trust to its running straight when the tube is
pulled apart.
On the whole, the best method in this case is to employ a flame
pencil, which should be kept ready for use. This merely consists of a
bit of glass tube of about the same dimensions as an ordinary lead
pencil, drawn down to a very fine jet at one end. The jet must not be
very long or thin, or the glass will soon fuse up. A few trials will
enable the operator to get the proper proportions, which are such that
the tube has the general appearance of a pencil normally sharpened
(say with a cone of 60'). This tube is best made of hard glass.
Connect it to a gas supply by light flexible tubing, and turn down the
gas till the flame from the end of the jet is not more than one-tenth
of an inch long. Then apply the jet, beginning from the end of the
crack, and gradually draw it (the crack) round the tube. The
operation will be assisted if a rubber ring is slipped on the tube to
begin with, so that the eye has some guide as to whether the flame is
being drawn round properly or not. The ring must, of course, be far
enough away to escape the effect of the flame. The crack will be
found to follow the flame in the most docile manner, unless the tube
is thick or badly annealed. Some operators recommend a pencil of
glowing charcoal, but the flame is undoubtedly better.
§ 30. To cut very thick Tubes.
A large number of methods have been proposed, and nearly everybody has
his favourite. The following has always succeeded with me. First
mark on the tube, by means of a little dead black spirit paint,
exactly where the cut is to be. Then sharpen the glass knife and
scratch a quite deep cut all round: there is no difficulty in making
the cut one-twentieth of an inch deep. It will be proper to lubricate
the knife with kerosene after the first mark is made. [Footnote: The
edge of the knife may be advantageously saved by using an old file
moistened with kerosene for the purpose. I find kerosene is not
worse, but, if anything, better than the solution of camphor in
turpentine recommended by Mr. Shenstone.]
If the glass is about one-eighth of an inch thick, the scratch maybe
conveniently about one-twentieth of an inch deep, but if the glass is
anything like one-quarter of an inch thick, the scratch must be much
deeper, in fact, the glass may be half cut through. To make a very
deep scratch, a wheel armed with diamond dust, which will be described
later on, may be used. However, it is not essential to use a diamond
wheel, though it saves time.
When the cut is made to a sufficient depth proceed thus: Obtain two
strips of bibulous paper or bits of tape and twist them round the tube
on each side of the scratch, allowing not more than one-eighth of an
inch between them. Then add a few drops of water to each, till it is
thoroughly soaked, but not allowing water to run away. Dry out the
scratch by a shred of blotting paper.
Turn down the oxygas flame to the smallest dimensions, and then boldly
apply it with its hottest part playing right into the nick and at a
single point. Probably in about two seconds, or less, the tube will
break. If it does not, rotate the tube, but still so that the flame
plays in the nick. After making the tube very hot all round--if it
has not broken--apply the flame again steadily at one point for a few
seconds and then apply a bit of cold iron. If the tube does not break
at once during these processes, let it cool, and cut the groove
deeper; then try again. [Footnote: This method is continually being
reinvented and published in the various journals. It is of unknown
antiquity.]
Fig. 22.
If the tube breaks after great heating and long efforts, it will
probably leave incipient cracks running away from the break, or may
even break irregularly. A good break is nearly always one that was
easily made. If a number of rings have to be cut, or a number of cuts
made on glass tubes of about the same size, it will be found
economical in the end to mount a glazier's diamond for the purpose. A
simple but suitable apparatus is figured (Fig. 23).
Fig. 23.
The only difficulty is to regulate the position of the diamond so that
it cuts. In order to do this, carefully note its cutting angle by
preliminary trials on sheet glass, and then adjust the diamond by
clamps, or by wriggling it in a fork, as shown. Weight the board very
slightly, so as to give the small necessary pressure, and produce the
cut by rotating the tube by hand. When a cut is nearly completed take
great care that the two ends join, or irregularity will result. This
is not always easy to do unless the tube happens to be straight.
Having got a cut, start a crack by means of a fine light watchmaker's
hammer, or even a bit of fused glass, and entice the crack round the
cut by tapping with the hammer or by means of the flame pencil.
If the cut is a true "cut" the tube will break at once. As a supply
of electrical current for lighting will, in the near future, be as
much a matter of course for laboratory purposes as a gas supply, I add
the following note. To heat a tube round a scratch, nothing--not
even the oxygas blow-pipe--is so good as a bit of platinum or iron
wire electrically heated. If the crack does not start by considerable
heating of the glass, stop the current, unwind the wire, and touch the
glass on the crack either with a bit of cold copper wire or a wet
match stem. I prefer the copper wire, for in my experience the water
will occasionally produce an explosion of cracks. On the other hand,
the cold wire frequently fails to start a crack.
Judging from the appearance of thick tubes as supplied by the dealers,
the factory method of cutting off appears to be to grind a nick almost
through the tube, and right round; and for really thick glass this is
the safest but slowest way; a thin emery wheel kept wet will do this
perfectly. Suitable wheels may be purchased from the "Norton" Emery
Wheel Co. of Bedford, Mass, U.S.A.--in England through Messrs.
Churchill and Co. of London, importers.
§ 31. To blow a Bulb at the End of a Tube.
I must admit at once that this is a difficult operation--at all
events, if a large bulb is required. However, all there is to be said
can be said in few words. In general, when a bulb is required at the
end of a tube it will be necessary to thicken up the glass. A
professional glass-worker will generally accomplish this by "jumping
up" the tube, i.e. by heating it where the bulb is required, and
compressing it little by little until a sufficient amount of glass is
collected. The amateur will probably find that he gets a very
irregular mass in this way, and will be tempted to begin by welding on
a short bit of wide and thick tubing preparatory to blowing out the
bulb.
However, supposing that enough glass is assembled by-either of these
methods, and that it is quite uniform in thickness, let the thickened
part be heated along a circle till it becomes moderately soft, and let
it then be expanded about one-fifth, say by gently blowing. It is
perhaps more important to keep turning the glass during bulb-blowing
than in any other operation, and this both when the glass is in the
flame and while the bulb is being blown. It is also very important to
avoid draughts. In general, a bulb is best blown with the tube in a
nearly horizontal position, but sloping slightly upwards from the
mouth. If it be noticed that a bulb tends to blow out more at one
side than another, let the side of greatest protuberance be turned
down, so that it is at the lowest point, reduce the pressure for an
instant, and then blow again. It will be observed that the bulb will
now expand at the top.
The reason of this is chiefly that the under side cools most rapidly
(according to Faraday, Chemical Manipulation, § 1194), and
consequently can expand no further; but also it is not unlikely that
the glass tends to flow somewhat from the upper side, which remains
hot, and consequently the bulb, when the next puff reaches it, will
tend to yield at this point. By heating several zones the tube will
become gradually expanded.
Fig. 24.
Fig. 25.
Fig. 26.
When the length of the thickened part of the tube only slightly
exceeds its diameter (Fig. 25), let the whole be brought to a
temperature which, with flint glass, should be just short of that of
perfect fluidity; and then, holding the tube horizontally and
constantly turning it, let the bulb be blown out to its full size,
noting the appearances and correcting too great protuberance on any
side by the means above mentioned. If the bulb appears pear-shaped
turn the tube so that the melted mass is directed upwards; if the
bulb have the contrary fault, correct in the corresponding manner.
The bulb when finished may be lightly tapped on the table, when, if
there is any weak place owing to inequality of thickness, the bulb
will break, and the operation may be started afresh. "A good bulb is
round, set truly on the tube, and free from lumps of thick glass or
places of excessive thinness." When the amateur has succeeded in
blowing a bulb two inches in diameter on the end of a strong bit of
thermometer tube--say for an air thermometer--he may well seek the
congratulations of his friends.
In case the bulb is not satisfactory on a first attempt, it may be
melted down again, if the following precautions are taken. Directly
creases begin to appear in the bulb let it be withdrawn from the
flame, and gently blown till the creases come out. By alternate
heating and blowing the glass can be got back to its original form, or
nearly so, but unless the operator shows great skill and judgment, the
probability is that the glass will be uneven. By heating and keeping
the thicker parts in the higher position, and blowing a little now and
again, the glass may be got even, and a new attempt may be made. It
must not be supposed that this process can be carried on indefinitely,
for the glass tends to lose its viscous properties after a time, or,
at all events, it "perishes" in some way, especially if it has been
allowed to get very thin; consequently too frequent attempts on the
same glass are unprofitable. Two or three trials are as many as it
generally pays to make. As a rule the largest possible flame may be
used with advantage in this operation.
§ 32. To blow a bulb in the middle of a tube, the procedure is much
like that already treated, but the manipulation is, if anything,
more difficult, for the further end of the tube must be carried and
turned as well as the end which is held to the lips.
§ 33. To make a side Weld.
This is by no means difficult, but is easier with lead glass than with
soda glass. The tube to which it is desired to make a side connection
having been selected, it is closed at one end by rubber tube stops, or
in any other suitable manner. The zone of the proposed connection is
noted, and the tube is brought to near softness round that circle (if
the tube is made actually soft, inconvenience will arise from the
bending, which is sure to occur). Two courses are then open to the
operator, one suitable to a thick tube, the other to a tube of
moderate thickness.
Taking the former first. Provide a piece of glass rod and warm its
end. Direct a small flame against the spot on the thick tube where
the proposed joint is to be. When the glass becomes almost
incandescent at this spot, put the end of the rod against it and draw
out a thread of glass till sufficient "metal" has been removed. Then
fuse off the thread close to the tube.
Fig. 27.
The subsequent procedure is the same as for thin tubes. In this case
heat the spot by the smallest flame available, and get the spot very
hot. Blow it out gently into a bubble, perhaps extending to a height
equal to its diameter. Then heat the top of the bubble till it is
incandescent and blow violently. This will produce an opening fringed
by glass so thin as to exhibit interference colours. Remove the filmy
part, and heat the frayed edges till they cohere and form an incipient
tube. If the flame has been of a correct size, the tube will now be
of the same diameter as the tube to be welded on, and will project
perhaps one-sixteenth of an inch from the surface of the main tube
(Fig. 28).
Fig. 28.
Fig. 29.
When this stage is reached, again heat the tube all round till it
nearly softens, and by means of the other hand heat the end of the
other tube which it is proposed to weld. Just before the main tube
actually softens, turn it so as to heat the edges of the aperture, and
at the same time get the end of the side tube very hot. Take both out
of the flame for an instant, and press the parts together, instantly
slightly withdrawing the side tube. If the operation is well
performed, it will be found that the point of maximum thickness of
glass is now clear of the main tube. The joint is then to be heated
all round and blown out--a rather awkward operation, and one
requiring some practice, but it can be done.
Fig. 30.
If great strength is wanted, heat the main tube all round the joint
bit by bit, and blow each section slightly outwards. If the operator
is confident in his skill, he should then heat the whole joint to the
softening point, blow it out slightly, and then adjust by pulling and
pushing. Cool first in the gas flame, and then plunge the joint into
the asbestos and cover it up--or if too large, throw the asbestos
cloth round it.
In the case of soda glass this final "general heat" is almost
essential, but it is not so with flint glass, and as the general heat
is the most difficult part of the job, it will be found easier to use
lead glass and omit the general heating. With soda glass a very small
irregularity will cause the joint to break when cold, but flint glass
is much more long-suffering. It is easy to perform the above
operation on small tubes. For large ones it will be found best to
employ flint glass and use the clip stands as in the case of direct
welds, treated above, but, of course, with suitable modifications.
Never let the main tube cool after the hole is made until the work is
done.
§ 34. Inserted Joints.
In many instances the performance of apparatus is much improved by
joints of this kind, even when their use is not absolutely essential.
There are two ways in which inserted joints may be made. The first
method is the easier, and works well with flint glass; but when one
comes to apply it to soda glass there is a danger of the glass
becoming too thick near the joint, and this often leads to a cracking
of the joint as the glass cools.
Fig. 31.
Suppose it is desired to insert the tube B into the tube A (Fig. 31).
Begin by reducing the size of the end of tube A till B will just slip
in quite easily. With B about one-quarter inch in diameter, a
clearance of about one-twentieth of an inch, or less, in all (i.e.
one-fortieth of an inch on each side) will be proper.
Heat B by itself at the proposed zone of junction, and blow out a very
narrow ring; then compress this slightly so that it forms an almost
closed ring of glass. The figure refers to the close of this
operation (Fig. 31, B). It does not matter much whether the ring
remains a mere flattened bulb, or whether it is a solid ring, but it
must be one or the other. Some judgment must be exercised in
preparing the ring. In general, the beginner will collect too much
glass in the ring, and consequently the joint, when made, will either
be thick and liable to crack easily, or it will be blown out into an
erratic shape in endeavours to reduce this thickness. Accordingly,
the operator will, if necessary, thin the tube B by drawing slightly,
if he considers it desirable, before the little enlargement is blown
out. In general, two heats must be used for this operation.
Fig. 32.
Get the approximating parts of both A and B up to a temperature just
below that at which they will adhere, and having closed the other end
of A, place B carefully within it up to the ring, and if it can be
arranged, have a mica wad in A, with a central hole through which the
end of B can project. This will very much facilitate the operation,
especially if B is long, but may be dispensed with by the exercise of
care and skill.
The operation is now simple. Fuse the junction and press the tubes
lightly together, being careful not to collect more glass than can be
helped; finally, blow out the joint and reduce the thickness by mild
drawing (Fig. 33). In order to make a really good joint, two points
must be particularly attended to--the rim must be thin and its plane
perfectly perpendicular to the axis of tube B; the end of tube A must
be cut off quite clean and perpendicular to its axis before B is
inserted. So important are these conditions--especially the latter
that the writer has even occasionally used the grindstone to get the
end of A into a proper condition, an admission which will probably
earn the contempt of the expert glass-worker.
Fig. 33.
Now for the second method, which is often practised in Germany, where
soda glass is chiefly used. With this glass the chief point is to get
a very even and not too thick ring at the junction, and consequently
the extra thickening produced by making a rim on B is rather a
drawback. The method consists in cutting off from B the length which
it is desired to insert, slipping this into A (which may be an
otherwise closed bulb, for instance), and then gradually melting up
the open end of A till the piece of B inside will no longer fall out.
By holding the joint downwards so that the inserted portion of B rests
on the edges of the opening, a joint may be made with the minimum
thickening.
The external part of B, previously heated, is then applied, and the
joint subjected to a "general" heat and blown out. Very nice joints
may be made by this method, and it is perhaps the better one where the
external part of B is to be less in diameter than the inserted part.
It was in this manner that the writer was taught to make glass
velocity pumps, one of which, of a good design, is figured as an
example.
In all cases good annealing should follow this operation. If the
inserted part of the inner tube (B) is anything like an inch in
diameter, and especially if it is of any length, as in some forms of
ozone apparatus, or in a large Bunsen's ice calorimeter, the
arrangements for supporting the inner part must be very good. A
convenient way of proceeding when the inner tube is well supported is
to make the mouth of A only very little larger than the diameter of B,
so that B will only just slip in. Then the mouth of A and the zone of
B may be heated together, and B blown out upon A. This, of course,
must be arranged for, if necessary, by temporarily stopping the inner
end of B.
The inner support of B should be removed as soon as practicable after
the joint is made, or, at all events, should not be perfectly rigid; a
tightly-fitting cork, for instance, is too rigid. The reason is, of
course, that in cooling there may be a tendency to set B a little to
one side or the other, and if it is not free to take such a set, the
joint most probably will give way. Good annealing both with flame and
asbestos is a sine qua non in all inserted work.
Fig. 34.
§ 35. Bending Tubes.
I have hitherto said nothing about bending tubes, for to bend a tube
of a quarter of an inch in diameter, and of ordinary thickness, is
about the first thing one learns in any laboratory, while to bend
large tubes nicely is as difficult an operation as the practice of
glass-blowing affords. However, even in bending a narrow tube it is
possible to proceed in the wrong way. The wrong way is to heat a
short length of the tube and then bend it rapidly, holding the plane
of the bend horizontal. The right way, per contra, is to use a
batswing burner to heat, say, two inches of the tube with constant
turning till it is very soft, and then, holding the glass so that the
bend will be in a vertical plane passing through one eye (the other
being shut), to make the bend rather slowly.
If an exact angle is required, it is as well to have it drawn out on a
sheet of asbestos board. In this case bend the glass as described
till it is approximately right, and finish by laying it on the
asbestos board and bringing it up to the marks. A suitable bit of
wood may be substituted for the asbestos on occasion.
N.B: The laboratory table is not a suitable piece of wood. A
right-angled bend is often wanted. In this case the corner of a table
will serve as a good guide to the eye, the glass being finished by
being held just above it. If great accuracy is wanted, make a wooden
template and suspend it by a screw from the side of the table, so that
the vertex of the gauge for the interior angle projects downwards,
then finish by bending the tube round it. The wood may be about half
an inch thick.
If a sharp bend is required, heat the tube in the blow-pipe, and bend
it rapidly, blowing out the glass meanwhile. The reason why a long
bend should be held in a vertical plane is that the hot part tends to
droop out of the plane of the bend if the latter be made in a
horizontal position. To bend a tube above half an inch in diameter is
a more or less difficult operation, and one which increases in
difficulty as the diameter of the tube increases.
A U-tube, for instance, may be made as follows: Use the four
blow-pipe arrangement so as to heat a fair length of tube, and get,
say, two inches of tube very hot--almost fluid, in fact--by means of
the carbon block supported from a stand. Remove the tube rapidly from
the flame and draw the hot part out to, say, three inches. Then,
holding the tube so as to make the bend in a vertical plane, bend it
and blow it out together to its proper size.
This operation seems to present no difficulties to experienced
glass-workers, even with tubes of about one inch in diameter, but to
the amateur it is very difficult. I always look on a large U-tube
with feelings of envy and admiration, which the complex trick work of
an elaborate vacuum tube does not excite in the least. It will be
noted that this method may, and often does, involve a preliminary
thickening of the glass.
With tubes over an inch in diameter I have no idea as to what is the
best mode of procedure--whether, for instance, a quantity of sand or
gas coke might not be used to stuff out the tube during bending, but
in this case there would be the difficulty of removing the fragments,
which would be sure to stick to the glass.
Of course, if the bend need not be short, the tube could be softened
in a tube furnace and bent in a kind of way. I must admit that with
tubes of even less than one inch in diameter I have generally managed
best by proceeding little by little. I heat as much of the glass as I
can by means of a gigantic blow-pipe, having a nozzle of about an inch
in diameter, and driven by a machine-blower.
When I find that, in spite of blowing, the tube begins to collapse, I
suspend operations, reheat the tube a little farther on, and so
proceed. If by any chance any reader knows a good laboratory method
of performing this operation, I hope he will communicate it to me.
After all, the difficulty chiefly arises from laboratory heating
appliances being as a rule too limited in scope for such work.
The bending of very thin tubes also is a difficulty. I have only
succeeded here by making very wide bends, but of course the blowing
method is quite applicable to this case, and the effect may be
obtained by welding in a rather thicker bit of tube, and drawing and
blowing it till it is of the necessary thinness. This is, however, a
mere evasion of the difficulty.
§ 36. Spiral Tubes.
These are easily made where good heating apparatus is available. As,
however, one constantly requires to bend tubes of about one-eighth
inch in diameter into spirals in order to make spring connections for
continuous glass apparatus, I will describe a method by which this is
easily done. Provide a bit of iron pipe about an inch and a quarter
in outside diameter. Cover this with a thick sheath of asbestos
cloth, and sew the edges with iron wire. Hammer the wire down so that
a good cylindrical surface is obtained. Make two wooden plugs for the
ends of the iron pipe. Bore one to fit a nail, which may be held in a
small retort clip, and fasten a stout wire crank handle into the other
one. Support the neck of the handle by means of a second clip. In
this way we easily get a sort of windlass quite strong enough for our
purpose.
Fig. 35.
Provide a large blow-pipe, such as the blow-pipe of a Fletcher
crucible furnace, Select a length of tubing and clean it. Lash one
end to the cylinder by means of a bit of wire, and hold the other end
out nearly horizontally. Then start the blow-pipe to play on the tube
just where it runs on to the asbestos cylinder, and at first right up
to the lashing. Get an attendant to assist in turning the handle of
the windlass, always keeping his eye on the tube, and never turning so
fast as to tilt the tube upwards. By means of the blow-pipe, which
may be moved round the tubing, heat the latter continuously as it is
drawn through the flame, and lay it on the cylinder in even spirals.
If the tubing is thin, a good deal of care will have to be exercised
in order to prevent a collapse. A better arrangement, which, however,
I have not yet tried, would, I think, be to replace the blow-pipe by
two bats-wing burners, permanently fastened to a stand, one of them
playing its flame downwards on to the top of the flame of the other.
The angle between the directions of the jets might be, say, 130°, or
whatever is found convenient. In this way the glass would not be so
likely to get overheated in spots, and better work would doubtless
result. However, I have made numbers of perfectly satisfactory
spirals as described. Three or four turns only make a sufficiently
springy connection for nearly all purposes.
§ 37. On Auxiliary Operations on Glass:-
Boring Holes through Glass:- This is much more easily done than is
generally supposed. The best mode of procedure depends on the
circumstances. The following three cases will be considered:-
1. Boring holes up to one-quarter inch diameter through thick glass
(say over one-eighth inch), or rather larger holes through thin glass.
2. Boring holes of any size through thick glass.
3. Boring round holes through ordinary window glass.
§ 38. Boring small Holes.
Take a three-cornered file of appropriate dimensions, and snip the
point off by means of a hammer; grind out most of the file marks to
get sharp corners. Dip the file in kerosene, and have plenty of
kerosene at hand in a small pot. Place the broken end of the file
against the glass, and with considerable pressure begin to rotate it
(the file) backwards and forwards with the fingers, very much as one
would operate a bradawl against a hard piece of wood. The surface of
the glass will shortly be ground away, and then the file bradawl will
make much quicker progress than might be expected. Two or three
minutes should suffice to bore a bit of sheet window-glass.
The following points require attention:
(1) Use any quantity of oil.
(2) After getting through the skin reduce the pressure on the file.
(3) Be sure to turn the file backwards and forwards through a complete
revolution at least.
(4) When the hole is nearly through reduce the pressure.
(5) When the hole is through the glass be exceedingly careful not to
force the file through too rapidly, otherwise it will simply act as a
wedge and cause a complete fracture.
(6) In many cases it is better to harden the file in mercury before
commencing operations; both files and glass differ so much in hardness
that this point can only be decided by a trial. If it is found
necessary to harden the file, use either a large blow-pipe and a coke
or charcoal bed, or else a small forge. A small blowpipe, such as is
generally found in laboratories, does more harm than good, either by
burning the end of the file or raising it to an insufficient
temperature.
(7) To sharpen the file, which is often necessary after passing
through the "skin" of the glass, put it in a vice so that the point
just protrudes clear of the jaws. Then, using a bit of waste iron as
an intermediary anvil or punch, knock off the least bit from the
point, so as to expose a fresh natural surface. The same result may
be brought about by the use of a pair of pliers.
If several holes have to be bored, it is convenient to mount the file
in the lathe and use a bit of flat hard wood to press up the glass by
means of the back rest. A drilling machine, if not too heavy, does
very well, and has the advantage of allowing the glass to remain
horizontal so that plenty of oil can be kept in the hole.
Use a very slow speed in either case--much slower than would be used
for drilling wrought iron. It is essential that the lubricant should
flow on to the end of the file very freely, either from a pipette or
from the regular oil-feed. If a little chipping where the file
pierces the back surface is inadmissible, it is better, on the whole,
to finish the bore by hand, using a very taper file. It is not
necessary to use a special file for the lathe, for a well-handled file
can be chucked very conveniently in a three-jaw chuck by means of the
handle.
Mr. Shenstone recommends a lubricant composed of camphor dissolved in
turpentine for general purposes. With the object of obtaining some
decisive information as to the use of this lubricant, and to settle
other points, I made the following experiments. Using an old
three-cornered French file, I chipped off the point and adjusted the
handle carefully. I also ground out the file marks near the point,
without hardening the file in mercury. Using kerosene and turpentine
and camphor, I began to bore holes in a hard bit of 3/32 inch window
glass.
Each hole was bored to about one-eighth inch in diameter in four
minutes with either lubricant. After hardening the file in mercury
and using kerosene, I also required four minutes per hole. After
mounting the file in a lathe which had been speeded to turn up brass
rods of about one-half-inch diameter, and therefore ran too fast, I
required one and a half minutes per hole, and bored them right
through, using kerosene. On the whole, I think kerosene does as well
as anything, and for filing is, I think, better than the camphor
solution. However, I ought to say that the camphor-turpentine
compound has probably a good deal to recommend it, for it has survived
from long ago. My assistant tells me he has seen his grandfather use
it when filing glass.
I beg to acknowledge my indebtedness to Mr. Pye, of the Cambridge
Scientific Instrument Company, for showing me in 1886 (by the courtesy
of the Company) the file method of glass-boring; it is also described
by Faraday in Chemical Manipulation, 1228.
It is not necessary, however, to use a file at all, for the twist
drills made by the Morse Drill Company are quite hard enough in their
natural state to bore glass. The circumferential speed of the drill
should not much exceed 10 feet per minute. In this way the author has
bored holes through glass an inch thick without any trouble except
that of keeping the lubricant sufficiently supplied. For boring very
small holes watchmaker's drills may be used perfectly well, especially
those tempered for boring hardened steel. The only difficulty is in
obtaining a sufficient supply of the lubricant, and to secure this the
drill must be frequently withdrawn.
My reason for describing the file method at such length is to be found
in the fact that a Morse drill requires to be sharpened after drilling
glass before it can be used in the ordinary way, and this is often a
difficulty.
I ought to say that I have never succeeded in boring the barrel of a
glass tap by either of these methods. [Footnote: I have been lately
informed that it is usual to employ a splinter of diamond set in a
steel wire holder both for tap boring and for drilling earthenware for
riveting. The diamond must, of course, be set so as to give
sufficient clearance for the wire holder.
For methods of using and setting diamond tools see § 55. It will
suffice to say here that a steel wire is softened and filed at one end
so as to form a fork; into this the diamond is set by squeezing with
pliers. The diamond is arranged so as to present a point in the axis
of the wire, and must not project on one side of the wire more than on
the other. It is not always easy to get a fragment satisfying these
conditions, and at the same time suitable for mounting. A drop of
solder occasionally assists the process of setting the diamond.
In drilling, the diamond must be held against the work by a constant
force, applied either by means of weight or a spring. I made many
trials by this method, using a watchmaker's lathe and pressing up the
work by a weight and string, which passed over a pulley. I used about
40 ounces, and drilled a hole 3/32 in diameter in flint glass at a
speed of 900 revolutions per minute to a depth of one-eighth of an
inch in eight minutes. I used soap and water as a lubricant, and the
work was satisfactory.
Since this was set up, I have been informed by Mr. Hicks of Hatton
Garden that it is necessary to anneal glass rod by heating it up to
the softening point and allowing it to cool very slowly under red-hot
sand or asbestos before boring. If this be done, no trouble will be
experienced. The annealing must be perfect.]
§ 39. For boring large holes through thick glass sheets, or, indeed,
through anything where it is necessary to make sure that
no accident can happen, or where great precision of position and form
of hole is required, I find a boring tube mounted as shown in the
picture (Fig. 36) is of great service. Brass or iron tube borers do
perfectly well, and the end of the spindle may be provided once for
all with a small tube chuck, or the tubes may be separately mounted as
shown. A fairly high speed is desirable, and may be obtained either
by foot, or, if power is available, is readily got by connecting to
the speed cone of a lathe, which is presumably permanently belted to
the motor.
Fig. 36.
After trying tubes armed with diamond dust, as will be presently
explained, I find that emery and thin oil or turpentine, if liberally
supplied below the glass, will do very nearly as well. The tube
should be allowed to rise from the work every few seconds, so as to
allow of fresh emery and oil being carried into the circular grooves.
This is done by lifting the hinged upper bearing, the drill being
lifted by a spiral spring between the pulley and the lower bearing
shown at B. The glass may be conveniently supported on a few sheets
of paper if flat, or held firm in position by wooden clamps if of any
other shape. In any case it should be firmly held down and should be
well supported. Any desired pressure upon the drill is obtained by
weighting the hinged board A.
§ 40. The following method was shown to me by Mr. Wimshurst, but I
have not had occasion to employ it myself. It is suitable for boring
large holes through such glass as the plates of Mr. Wimshurst's
Influence machines are usually made of. A diamond is mounted as the
"pencil" of a compass, and with this a circle is drawn on the glass in
the desired position. The other leg of the compass of course rests on
a suitable washer.
To the best of my recollection the further procedure was as follows.
A piece of steel rod about one-eighth inch in diameter was ground off
flat and mounted in a vice vertically, so as to cause its plane end to
form a small horizontal anvil. The centre (approximately) of the
diamond-cut circle of the glass was laid on this anvil so as to rest
evenly upon it, and the upper surface (i.e. that containing the cut)
was then struck smartly with a hammer, completely pulverising the
glass above the anvil. The hole was gradually extended in a similar
manner right up to the diamond cut, from which, of course, the glass
broke away.
A similar method has long been known to glaziers, differing from the
preceding in that a series of diamond cuts are run across the circle
parallel to two mutually perpendicular diameters. A smart tap on the
back of the scored disc will generally cause the fragments to tumble
out. I have never tried this myself, but I have seen it done.
Large discs may easily be cut from sheet glass by drawing a circular
diamond cut, and gradually breaking away the outer parts by the aid of
additional cuts and a pair of pliers or "shanks" (see Fig. 44).
§ 41. Operations depending on Grinding: Ground-in Joints.
The process will be perfectly understood by reference to a simple
case. Suppose it is desired to grind the end of a tube into the neck
of a bottle. If a stoppered bottle is available, the stopper must be
taken out and measured as to its diameter at the top and bottom.
Select a bit of tube as nearly as possible of the same diameter as the
stopper at its thickest part. Draw down the glass in the blow-pipe
flame rather by allowing it to sink than by pulling it out. After a
few trials no difficulty will be experienced in making its taper
nearly equal to that of the stopper, though there will in all
probability be several ridges and inequalities. When this stage is
reached anneal the work carefully and see that the glass is not too
thin. Afterwards use emery and water, and grind the stopper into the
bottle.
There are six special directions to be note
(1 )Turn the stopper through at least one revolution in each
direction.
(2) Lift it out often so as to give the fresh emery a chance of
getting into the joint.
(3) Rotate the bottle as well as the stopper in case there is any
irregularity in the force brought to bear, which might cause one side
of the neck to be more ground than another, or would cause the tube to
set rather to one side or the other.
(4) Use emery passing a 50 sieve, i.e. a sieve with fifty threads to
the inch run (see § 144) to begin with, and when the stopper nearly
fits, wash this thoroughly away, and finish with flour emery,
previously washed to get rid of particles of excessive size; the
process of washing will be fully discussed in the chapter on
glass-grinding, which see.
(5) Any degree of fineness of surface may be obtained by using graded
emery, as will be explained, but, in general, it is unnecessary to
attempt a finer surface than can be got with washed flour emery. A
superficial and imperfect polish may be given by grinding for a short
time with powdered pumice stone.
(6) If the proper taper is not attained by blowing, or if ridges are
left on the tapered part, the process may be both hastened and
improved by giving the taper a preliminary filing with a
three-cornered file and kerosene, just as one would proceed with iron
or brass. A little filing will often save a good deal of grinding and
make a better job.
If a bottle without a tapered neck is to be employed, it is as well to
do the preliminary grinding by means of a cone turned up from a bit of
cast iron. This is put in the lathe and pushed into the mouth of the
bottle, the latter being supported by the hands. Use about the same
surface speed as would be employed for turning cast iron. In this
case the emery is better used with kerosene.
If a cylindrical bit of cast iron about an inch in diameter is turned
down conically nearly to a point, it will save a good deal of trouble
in making separate cones. If it gets ground into rings, and it
becomes necessary to turn it up, use a diamond tool until the skin is
thoroughly removed; the embedded emery merely grinds the edge off any
ordinary steel tool.
For diamond tools see § 55.
§ 42. Use of the Lathe in Glass-working.
If it is necessary to remove a good deal of glass, time may be saved
by actually turning the glass in a lathe. According to the direction
given above for grinding a tube into the neck of a bottle, very little
glass need be removed if the drawing down is well done, so that for
this purpose turning is often unnecessary.
If the taper of the stopper be small and it is permissible to use a
thick tube, or if a solid stopper only has to be provided, or an old
stopper quickly altered to a new form, turning is very useful. The
glass may be "chucked" in any suitable manner, and run at a speed not
exceeding 10 feet per minute. Prepare a three-cornered file by
mercury-hardening and by grinding the end flat so as to form a cutting
angle of about 80°, and use a moderate amount of kerosene lubrication,
i.e. enough to keep the glass damp, but even this is not essential.
Use the file as an ordinary brass turning tool, and press much more
lightly than for metal turning. The glass will be found to scrape off
quite pleasantly.
By chucking glass tubes on wooden mandrells the ends may be nicely
turned in this manner ready for accurate closing by glass plates.
The process of grinding also is made much more rapid--at all events
in the earlier stages--by chucking either the stopper or the bottle
and holding the other member in the fingers, or in a wooden vice held
in the hands. The finishing touches are best given by hand.
I ought to say that I think a good deal of glass-grinding, as
practised in laboratories, might be advantageously replaced by glass
turning or filing and certainly will be by any one who will give these
methods a trial.
If one tube is to be ground into another, as in grinding a retort into
a receiver, the latter must be drawn down from a larger piece, few
beginners being able to widen a tube by the method explained with
sufficient ease and certainty. The other operations are similar to
the operations above described.
§ 43. Funnels often require to be ground to an angle of 60°. For
this purpose it is well to keep a cast-iron cone, tapering from
nothing up to four inches in diameter. This may be mounted on a
lathe, and will be found of great use for grinding out the inside of
funnels. Care must be taken to work the funnel backwards and
forwards, or it will tend to grind so as to form rings, which
interfere with filtering. A rough polish may be given on the lines
explained in the next section.
§ 44. A rough polish may be easily given to a surface which has been
finished by washed flour emery, in the following manner. Turn up a
disc of soft wood on the lathe, and run it at the highest wood-turning
speed. Rub into the periphery a paste of sifted powdered pumice stone
and water.
Any fairly smooth ground glass surface may be more or less polished by
holding it for a moment against the revolving disc. Exact means of
polishing will be described later on. Meanwhile this simple method
will be found both quick and convenient, and is often quite sufficient
where transparency, rather than figure, is required. I daresay a fine
polish may be got on the same lines, using putty powder or washed
rouge (not jewellers' rouge, which is too soft, but glass-polishers'
rouge) to follow the pumice powder, but I have not required to try
this.
§ 45. It is sometimes required to give to ground glass surfaces a
temporary transparency. This is to be done by using a film of oil of
the same refractive index as the glass. Cornu has employed a varnish
consisting of a mixture of turpentine and oil of cloves, but the
yellow-brown colour of the latter is often a disadvantage. It will be
found that a mixture of nut oil and oil of bitter almonds, or of
bromo-napthalene and acetone, can be made of only a faint yellow
colour; and by exact adjustment of the proportions will have the same
refractive index for any ray as crown glass (ordinary window glass).
Procure a sample of the glass and smash it up to small fragments in an
iron mortar. Sift out the fine dust and the larger pieces; bits
about as large as small beads--say one-sixteenth inch every way--do
very well. Boil the sifted glass with strong commercial hydrochloric
acid to remove iron, wash with distilled water and a few drops of
alcohol, dry on blotting paper in the sun or otherwise. Put the dry
glass into a bottle or beaker, and begin by adding almond oil (or
bromo-napthalene), then add nut oil (or acetone) till the glass
practically disappears when examined by sodium light, or light of any
other wave-length, as may be required.
The adjustment of the mixture is a matter of great delicacy, one drop
too much of either constituent, in, say, 50 cubic centimetres, makes
all the difference. The final adjustment is best accomplished by
having two mixtures of the oils, one just too rich in almond, the
other in nut oil; by adding one or other of these, the required
mixture is soon obtained.
It is to be noted
(1) That adjustment is only perfect for light of one wave-length.
(2) That adjustment is only perfect at one temperature.
On examining a bottle of rather larger fragments of glass immersed in
an adjusted mixture by ordinary daylight, a peculiarly beautiful play
of colours is seen.
Of course, if it is only desired to make ground glass fairly
transparent, these precautions are unnecessary, but it seemed better
to dispose of the matter once for all in this connection.
M. Cornu's object was to make a varnish which would prevent reflection
from the back of a photographic plate on to the film. I have had
occasion to require to do the same when using a scale made by cutting
lines through a film of black varnish on a slip of glass. This
succeeded perfectly by making the varnish out of Canada balsam stained
with a black aniline dye.
Mr. Russell, Government Astronomer of New South Wales, finds that the
"halation" of star photographs can be prevented by pouring over the
back of the plate a film of collodion suitably stained.
§ 46. Making Ground Glass.
This is easily done by rubbing the surface of polished glass with a
bit of cast iron and washed "flour of emery." Of course, if the
fineness of grain of the surface is of importance, appropriate sizes
of emery must be employed. The iron may be replaced by a bit of glass
cut with transverse grooves to allow the emery to distribute itself,
or even by a bit of glass without such grooves, provided it does not
measure more than one or two inches each way. If great speed is an
object rather than the fineness of the surface, use a bit of lead and
coarse emery, say any that will pass a sieve with fifty threads to the
inch.
It may perhaps be mentioned here that it is a pity to throw away emery
which has been used between glass and glass. In the chapter dealing
with fine optical work the use of emery of various grades of fineness
will be treated, and the finer grades can only be obtained (to my
knowledge) from emery which has been crushed in the process of glass
or metal grinding, especially the former. A large jam-pot covered
with a cardboard lid does well as a receptacle of washings.
§ 47. Glass-cutting.
This is an art about which more can be learned in five minutes by
watching it well practised than by pages of written description. My
advice to any one about to commence the practice of the art would be
to make friends with a glazier and see it done. What follows is
therefore on the supposition that this advice has been followed.
After some experience of cutters made of especially hardened steel, I
believe better work can generally be got out of a diamond, provided
the cost is not an objection. It is economy to pay a good price for a
good diamond. As is well known, the natural angle of the crystal
makes the best point, and a person buying a diamond should examine the
stone by the help of a lens, so as to see that this condition is
fulfilled. The natural angle is generally, if not always, bounded by
curved edges, which have a totally different appearance from the sharp
edges of a "splinter."
When a purchase is to be made, it is as well for the student to take a
bit of glass and a foot-rule with him, and to test the diamond before
it is taken away. When a good diamond has been procured, begin by
taking cuts on bits of clean window glass until the proper angle at
which to hold the tool is ascertained. Never try to cut over a
scratch, if you value your diamond, and never press hard on the glass;
a good cut is accompanied by an unmistakable ringing sound quite
different from the sound made when the diamond is only scratching.
Perhaps the most important advice that can be given is, Never lend the
diamond to anybody--under any circumstances.
The free use of a diamond is an art which the physicist will do well
to acquire, for quite a variety of apparatus may be made out of glass
strips, and the accuracy with which the glass breaks along a good cut
reduces such an operation as glass-box-making to a question of
accurate drawing.
§ 48. Cementing.
One of the matters which is generally confused by too great a
profusion of treatment is the art of cementing glass to other
substances.
The following methods will be found to work, subject to two
conditions:
(1) The glass must be clean;
(2) it must be hot enough to melt the cement.
For ordinary mending purposes when the glass does not require to be
placed in water (especially if hot) nothing is better than that kind
of glue which is generally called "diamond cement." This may be
easily made by dissolving the best procurable isinglass in a mixture
of 20 per cent water and 80 per cent glacial acetic acid--the exact
proportions are not of consequence.
First, the isinglass is to be tightly packed into a bottle with a wide
neck, then add the water, and let the isinglass soak it up.
Afterwards pour in the acetic acid, and keep the mixture near 100°C.
for an hour or two on the water bath--or rather in it. The total
volume of acetic acid and water should not be more than about half of
the volume of isinglass when the latter is pressed into the bottle as
tightly as possible.
The proper consistency of the cement may be ascertained by lifting a
drop out of the bottle and allowing it to cool on a sheet of glass.
In ten minutes it ought not to be more than slightly sticky, and the
mass in the bottle, after standing a few hours cold, should not be
sticky at all, and should yield, jelly-like, to the pressure of the
finger to only a slight degree. If the glue is too weak, more
isinglass may be added (without any preliminary soaking).
A person making the mixture for the first time almost always gets it
too weak. It is difficult to give exact proportions by weight, as
isinglass and gelatine (which may replace it) differ greatly in
quality. This cement is applied like glue, and will cement nearly
anything as well as glass. Of course, as much cement as possible must
be squeezed out of any joint where it is employed. The addition of
gums, as recommended in some books, is unnecessary.
Ordinary glue will serve perfectly for cementing glass to wood.
"Chipped glass" ware is, I understand, made by painting clean glass
with glue. As the glue dries and breaks by contraction, it chips off
the surface of the glass. I have never seen this done. In nearly all
cases where alcohol is not to be employed very strong joints may be
made by shellac. Orange shellac is stronger than the "bleached"
variety.
A sine qua non is that the glass be hot enough to melt the shellac.
The best way is to heat the glass surfaces and rub on the shellac from
a bit of flake; the glass should not be so hot as to discolour the
shellac appreciably, or its valuable properties will be partly
destroyed. Both glass surfaces being thus prepared, and the shellac
being quite fluid on both, they may be brought together and clamped
tightly together till cool. Shellac that has been overheated, or
dissolved in alcohol, or bleached, is of little use as compared with
the pale orange flaky product. Dark flakes have probably been
overheated during the preliminary refining.
For many purposes a cement is required capable of resisting carbon
bisulphide. This is easily made by adding a little treacle (say 20
per cent) to ordinary glue. Since the mixture of glue and treacle
does not keep, i.e. it cannot be satisfactorily melted up again after
once it has set, no more should be made up than will be wanted at the
time. If the glue be thick, glass boxes for carbon disulphide may be
easily put together, even though the edges of the glass strips are not
quite smooth, for, unlike most cements, this mixture remains tough,
and is fairly strong in itself.
I have found by experiment that most fixed and, to a less degree,
essential oils have little or no solvent action on shellac, and I
suspect that the same remark applies to the treacle-glue mixture, but
I have not tried. Turpenes act on shellac slightly, but mineral oils
apparently not at all. The tests on which these statements are based
were continued for about two years, during which time kerosene and
mineral oils had no observable effect on shellac--fastened
galvanometer mirrors.
§ 49. Fusing Electrodes into Glass.
This art has greatly improved since the introduction of the
incandescent lamp; however, up to the present, platinum seems to
remain the only substance capable of giving a certainly air-tight
result. I have not tried the aluminium-alumina method.
Many years ago it was the fashion to surround the platinum wire with a
drop of white enamel glass in order to cause better adhesion between
it and the ordinary glass. [Footnote: Hittorf and Geissler (Pogg.
Ann. 1864, § 35; English translation, Phys. Soc. London, p. 138)
found that it was impossible to make air-tight joints between platinum
and hard potash glass, but that soft lead glass could be used with
success as a cement.] However, in the case of flint glass, if one may
judge from incandescent lamps, this is not essential--a fact which
entirely coincides with my own experience.
On the other hand, when sealing electrodes into German glass I have
often used a drop of enamel with perfect results, though this is not
always done in Germany. In all cases, however, in which electrodes
have to be sealed in--especially when they are liable to heat--I
recommend flint glass, and in this have the support of Mr. Rain (The
Incandescent Lamp and its Manufacture, p. 131). The exact details
for the preparation of eudiometer tubes are given by Faraday (Chemical
Manipulation, § 1200).
In view of what has preceded, however, I will content myself with the
following notes. Make the hole through which the wire is to protrude
only slightly larger than the wire itself, and be sure that the latter
is clean. Allow the glass to cool sufficiently not to stick to the
wire when the latter is pushed in. Be sure that, on heating, the
glass does not get reduced, and that it flows up to the wire all
round; pull and push the wire a little with a pair of pincers, to
ensure this.
It is not a bad plan to get the glass exceedingly fluid round the
wire--even if the lump has to be blown out a little afterwards--as it
cools. The seal should finally be well annealed in asbestos, but
first by gradually moving it into the hot air in front of the flame.
It was observed by Professor J. J. Thomson and the author some years
ago (Proc. Roy. Soc. 40. 331. 1886) that when very violent
discharges are taken through lightly sealed-in electrodes in
lead-glass tubes--say from a large battery of Leyden jars--gas
appears to be carried into the tube over and above that naturally
given off by the platinum, and this without there being any apparent
want of perfection in the seal. This observation has since been
confirmed by others. Consequently in experiments on violent
discharges in vacuo where certainty is required as to the exclusion of
air, the seals should be protected by a guard tube or cap containing
mercury; this must, of course, be put in hot and clean, on hot and
clean glass, and in special cases should be boiled in situ.
A well-known German physicist (Warburg, I think) recommends putting
the seals under water, but I cannot think that this is a good plan,
for if air can get in, why not water? which has its surface tension
in its favour. The same reasoning prevents my recommending a layer of
sulphuric acid above the mercury-a method used for securing
air-tightness in "mercury joints" by Mr. Gimingham, Proc. R. S.
1874.
Further protection may be attained for many purposes by coating the
platinum wire with a sheath of glass, say half an inch long, fused to
the platinum wire to a depth of one-twentieth of an inch all round.
In some cases the electrodes must be expected to get very hot, for
instance, when it is desired to platinise mirrors by the device of
Professor Wright of Yale. In this and similar cases I have met with
great success by using "barometer" tubes of about one-twelfth of an
inch bore, and with walls, say, one-tenth of an inch thick.
[Footnote: "Barometer" tube is merely very thick-walled glass tubing,
and makes particularly bad barometers, which are sold as weather
glasses.]
This tube is drawn down to a long point--say an inch long by
one-eighth of an inch external diameter, and the wire is fused in for
a length, say, of three-quarters of an inch, but only in the narrow
drawn--down part of the tube. At different times I have tried four
such seals, and though the electrodes were red hot for hours, I have
never had an accident--of course they were well annealed.
Fig. 37.
For directions as to the making of high vacuum tubes, see the section
dealing with that matter.
§ 50. As economy of platinum is often of importance, the following
little art will save money and trouble. Platinum is easily caused to
join most firmly to copper--with which, I presume, it alloys--by the
following method. Hold the platinum wire against the copper wire, end
to end, at the tip of the reducing flame of a typical blowpipe--or
anywhere--preferably in the "reducing" part of the oxygas flame; in
a moment the metals will fuse together at the point of contact, when
they may be withdrawn.
Such a joint is very strong and wholly satisfactory, much better than
a soldered joint. If the work is not carried out successfully so that
a considerable drop of copper-platinum alloy accumulates, cut it off
and start again. The essence of success is speed, so that the copper
does not get "burned." If any considerable quantity of alloy is formed
it dissolves the copper, and weakens it, so that we have first the
platinum wire, then a bead of alloy, and then a copper wire fused into
the bead, but so thin just outside the latter that the joint has no
mechanical strength.
§ 51. The Art of making Air-light Joints.
Lamp-manufacturers and others have long since learned that when glass
is in question not only are fused joints made as easily as others, but
that they afford the only reliable form of joint. An experimenter who
uses flint glass, has a little experience, an oxygas blow-pipe and a
blowing apparatus, will prefer to make his joints in this way, simply
from the ease with which it may be done. When it comes to making a
tight joint between glass and other substances the problem is by no
means so easy. Thus Mr. Griffiths (Phil. Trans. 1893, p. 380)
failed to make air-tight joints by cementing glass into steel tubes,
using hard shellac, and the tubes fitting closely. These joints were
satisfactory at first, but did not last; the length of the joint is
not stated. The difficulty was finally got over by soldering very
narrow platinum tubes into the steel, and fusing the former into the
glass.
Mr. Griffiths has since used an alloy with success as a cement, but I
cannot discover what it is made from. Many years ago Professor Hittorf
prepared good high vacuum tubes by plugging the ends of glass tubes
with sealing wax merely, though in all cases the spaces to be filled
with wax were long and narrow (Hittorf, Pogg. Ann. 1869, § 5,
English translation, Phys. Soc. p. 113). Again, Regnault
habitually used brass ferules, and cemented glass into them by means
of his mastic, which can still be procured at a low rate from his
instrument-makers (Golan, Paris). Lenard also, in his investigations
on Cathode Rays (Wied. Ann, vol. li. p. 224), made use of sealing
wax covered with marine glue.
Surely in face of these facts we must admit that cement joints can be
made with fair success. I do not know the composition of M.
Regnault's mastic, but Faraday (Manipulations, § 1123) gives the
following receipt for a cement for joining ferules to retorts, etc:
Resin 5 parts.
Beeswax 1 part.
Red ochre or Venetian red,
finely powdered and sifted 1 part.
I believe this to be substantially the same as Regnault's mastic,
though I have never analysed the latter.
For chemical work the possibility of evolution of gas from such a
cement must be taken into account, and I should certainly not trust it
for this reason in vacuum tube work, where the purity of the confined
gas could come in question. Otherwise it is an excellent cement, and
does not in my experience tend to crack away from glass to the same
extent as paraffin or pure shellac.
This cracking away from glass, by the way, is probably an effect of
difference in rate of expansion between the glass and cement which
probably always exists, and, if the cement be not sufficiently
viscous, must, beyond certain temperature limits, either produce
cracks or cause separation. Professor Wright of Yale has used a hard
mineral pitch as a cement in vacuum work with success.
My attention has been directed to a fusible metal cement containing
mercury, and made according to the following receipt, given by Mr. S.
G. Rawson, Journal of the Society of Chemical Industry, vol. ix.
(1890), P. 150:-
Bismuth 40 per cent
Lead 25 per cent
Tin 10 per cent
Cadmium 10 per cent
Mercury 15 per cent
This is practically one form of Rose's fusible metal with 15 per cent
mercury added. It takes nearly an hour to set completely, and the
apparatus must be clean and warm before it is applied.
As the result of several trials by myself and friends, I am afraid I
must dissent from the claim of the author that such a cement will make
a really air-tight joint between glass tubes. Indeed, the appearance
of the surface as viewed through the glass is not such as to give any
confidence, no matter what care may have been exercised in performing
all the operations and cleaning the glass; besides which the cement
is rigid when cold, and the expansion difficulty comes in.
On the other hand, if extreme air-tightness is not an object, the
cement is strong and easily applied, and has many uses. I have an
idea that if the joints were covered with a layer of soft wax, the
result would be satisfactory in so far as air-tightness is concerned.
This anticipation has since been verified.
In many cases one can resort to the device already mentioned of
enclosing a rubber or tape-wrapped joint between two tubes in a bath
of mercury, but in this case the glass must be clean and hot and the
mercury also warm, dry, and pure when the joint is put together,
otherwise an appreciable air film is left against the glass, and this
may creep into the joint.
Perhaps the easiest way of making such a joint is to use an outer tube
of thin clean glass, and bore a narrow hole into it from one side to
admit the mercury; if the mercury is to be heated in vacuo, it is
better to seal on a side joint. It is always better, if possible, to
boil the mercury in situ, which involves making the wrapping of
asbestos, but, after all, we come back to the position I began by
taking up, viz. that the easiest and most reliable method is by
fusion of the glass--all the rest are unsuitable for work of real
precision.
I should be ungrateful, however, were I not to devote a few lines to
the great convenience and merit of so-called "centering cement." This
substance has two or three very valuable properties. It is very tough
and strong in itself, and it remains plastic on cooling for some time
before it really sets. If for any reason a small tube has to be
cemented into a larger one, which is a good deal larger, so that an
appreciable mass of cement is necessary, and particularly if the joint
requires to have great mechanical strength, this cement is invaluable.
I have even used a plug of it instead of a cork for making the joint
between a gas delivery tube and a calcium chloride tower. (Why are
these affairs made with such abominable tubulures?)
The joint in question has never allowed the tube to sag though it
projects horizontally to a distance of 6 inches, and has had to
withstand nearly two years of Sydney temperature. The cement consists
of a mixture of shellac and 10 per cent of oil of cassia.
The shellac is first melted in an iron ladle, and the oil of cassia
quickly added and stirred in, to an extent of about 10 per cent, but
the exact proportions are not of importance. Great care must be taken
not to overheat the shellac.
APPENDIX TO CHAPTER I
ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
ROENTGEN'S RADIATION
[Footnote: Written in May 1896.]
WHEN Professor Roentgen's discovery was first announced at the end of
1895 much difficulty was experienced in obtaining radiation of the
requisite intensity for the repetition of his experiments. The
following notes on the production of vacuum tubes of the required
quality may therefore be of use to those who desire to prepare their
own apparatus. It appears that flint glass is much more opaque to
Roentgen's radiation than soda glass, and consequently the vacuum
tubes require to be prepared from the latter material.
Fig. 39.
A form of vacuum tube which has proved very successful in the
author's hands is sketched in Fig. 38. It is most easily constructed
as follows. A bit of tubing about 2 centimetres diameter, 15
centimetres long, and 1.5 millimetre wall thickness, is drawn down to
a point. The larger bulb, about 5 centimetres in diameter, is blown
at one end of this tube. The thinner the bulb the better, provided
that it does not collapse under atmospheric pressure. A very good
idea of a proper thickness may be obtained from the statement that
about 4 centimetres length of the tubing should be blown out to form
the bulb. This would give a bulb of about the thickness of an
ordinary fractionating bulb. Before going any further it is as well
to test the bulb by tapping on the table and by exhausting it by means
of an ordinary water-velocity pump.
The side tube is next prepared out of narrower tubing, and is provided
with a smaller bulb, a blowing-out tube, and a terminal, to be made as
will be described. This side tube is next fused on to the main tube,
special care being taken about the annealing, and the cathode terminal
is then sealed into the main tube. After using clean glass it is in
general only necessary to rinse the tube out with clean alcohol, after
which it may be dried and exhausted.
The success of the operation will depend primarily on the attention
given to the preparation and sealing-in of the electrode facing the
large bulb.
Preparation of Terminals. Some platinum wire of about No. 26
B.W.G--the exact size is unimportant--must be provided, also some
sheet aluminium about 1 millimetre thick, some white enamel cement
glass, and a "cane" of flint-glass tube of a few millimetres bore.
The electrodes are prepared by cutting discs of aluminium of from 1 to
1.5 centimetres diameter. The discs of aluminium are bored in the
centre, so as to admit the "stems" which are made of aluminium wire
of about 1 millimetre diameter. The stems are then riveted into the
discs. The "stems" are about I centimetre long, and are drilled to a
depth of about 3 millimetres, the drill used being about double the
diameter of the platinum wire to be used for making the connections.
The faces of the electrodes--i.e. the free surfaces of the aluminium
discs--are then hammered flat and brought to a burnished surface by
being placed on a bit of highly polished steel and struck by a "set"
provided with a hole to allow of the "stem" escaping damage. The
operation will be obvious after a reference to Figs. 39 and 40; it
is referred to again on page 96.
The platinum wires may be most conveniently attached by melting one
end of the piece of platinum wire in the oxygas blow-pipe till it
forms a bead just large enough to pass into the hole drilled up the
stem of the electrode. The junction between the stein and the
platinum wire is then made permanent by squeezing the aluminium down
upon the platinum wire with the help of a pair of pliers. It is also
possible to fuse the aluminium round the platinum, but as I have had
several breakages of such joints, I prefer the mechanical connection
described.
Fig. 39. Sets for striking aluminium electrodes
Fig. 40.
i. Aluminium electrode.
ii. Aluminium electrode connected to platinum wire.
iii. Aluminium electrode connected to platinum wire and protected by
glass.
iv. Detail of fastening platinum wire.
The stem and platinum wire may now be protected by covering them with
a little flint glass. For this purpose the flint-glass tube is pulled
down till it will just slip over the stem and wire, and is cut off so
as to leave about half a centimetre of platinum wire projecting. The
flint-glass tube is then fused down upon the platinum wire, care being
taken to avoid the presence of air bubbles. At the close of the
operation a single drop of white enamel glass is fused round the
platinum wire at a high temperature, so as to make a good joint with
the protecting flint-glass tube.
The negative electrode being nearly as large as the main tube, it must
be introduced before the latter is drawn down for sealing. After
drawing down the main tube in the usual manner, taking care not to
make it less than a millimetre in wall-thickness, it is cut off so as
to leave a hole not quite big enough for the enamel drop to pass
through. By heating and opening, the aperture is got just large
enough to allow the enamel drop to pass into it, and when this is the
case the joint is sealed, pulled, and blown out until the electrode
occupies the right position--viz. in the centre of the tube and with
its face normal to the axis of the tube.
The glass walls near the negative electrode must not be less than a
millimetre thick, and may be rather more with advantage, the glass
must be even, and the joint between the flint glass and the soda
glass, or between the wire and the soda glass, must be wholly through
the enamel. The "seal" must be well annealed. It will be found that
the sealing-in process is much easier when the stem of the electrode
is short and when the glass coating is not too heavy. Half a
millimetre of glass thickness round the stein is quite sufficient.
The diagram, of the tube shows that the main tube has been expanded
round the edges of the cathode. This is to reduce the heating
consequent on the projection of cathode rays from the edges of the
disc against the glass tube.
The anode is inserted into its bulb in a quite similar manner. If
desired it may be made considerably smaller, and does not need the
careful adjustment requisite in sealing-in the cathode, nor does the
glass near the entry wire require to be so thick.
More intense effects are often got by making the cathode slightly
concave, but in this case the risk of melting the thin glass is
considerably increased. No doubt, Bohemian glass might be used
throughout instead of soft soda glass, and this would not melt so
easily; the difficulties of manipulating the glass are, however, more
pronounced.
It will be shown directly that the best Roentgen effects are got with
a high vacuum, and it is for this reason that the glass near the
cathode seal requires to be strong. The potential right up to the
cathode is strongly positive inside the tube, and this causes the
glass to be exposed to a strong electric stress in the neighbourhood
of the seal.
Although the glass-blowing involved in the making of a so-called focus
tube is rather more difficult than in the case just described, there
is no reason why such a difficulty should not be overcome; I will
therefore explain how a focus tube may be made.
Fig. 41.
A bulb about 3 inches in diameter is blown from a bit of tube of a
little more than 1 inch diameter. Unless the walls of the tube are
about one-eighth of an inch in thickness, this will involve a
preliminary thickening up of the glass. This is not difficult if care
be taken to avoid making the glass too hot. The larger gas jet
described in connection with the soda-glass-blowing table must be
employed. In blowing a bulb of this size it must not be forgotten
that draughts exercise a very injurious influence by causing the glass
to cool unequally; this leads to bulbs of irregular shape.
In the method of construction shown in Fig. 41, the anode is put in
first. This anode simply consists of a square bit of platinum or
platinum-iridium foil, measuring about 0.75 inch by 1 inch, and
riveted on to a bent aluminium wire stem.
As soon as the anode is fused in, and while the glass is still hot,
the side tube is put on. The whole of the anode end is then carefully
annealed. When the annealing is finished the side tube is bent as
shown to serve as a handle when the time comes to mount the cathode.
Before placing the cathode in position, and while the main tube is
still wide open, the anode is adjusted by means of a tool thrust in
through this open end. This is necessary in view of the fact that the
platinum foil is occasionally bent during the operation of forcing the
anode into the bulb.
The cathode is a portion of a spherical surface of polished aluminium,
a mode of preparing which will be given directly. The cathode having
been placed inside the bulb, the wide glass tube is carefully drawn
down and cut off at such a point that when the cathode is in position
its centre of curvature will lie slightly in front of the anode plate.
For instance, if the radius of curvature of the cathode be 1.5 inches,
the centre of curvature may lie something like an eighth of an inch or
less in front of the anode.
The cathode as shown in Fig. 41 is rather smaller than is
advantageous. To make it much larger than is shown, however, the
opening into the bulb would require to be considerably widened, and
though this is not really a difficult operation, still it requires
more practice than my readers are likely to have had. The difficulty
is not so much in widening out the entry as in closing it down again
neatly.
Now as to making the anode. A disc of aluminium is cut from a sheet
which must not be too thick--one twenty-fifth of an inch is quite
thick enough. This disc is bored at the centre to allow of the stem
being riveted in position. The disc is then annealed in the Bunsen
flame and the stem riveted on.
The curvature is best got by striking between steel dies (see Figs.
39 and 40). Two bits of tool steel are softened and turned on the
lathe, one convex and the other concave. The concave die has a small
hole drilled up the centre to admit the stem. The desired radius of
curvature is easily attained by cutting out templates from sheet zinc
and using them to gauge the turning. The two dies are slightly ground
together on the lathe with emery and oil and are then polished, or
rather the convex die is polished--the other one does not matter.
The polishing is most easily done by using graded emery and oil and
polishing with a rag. The method of grading emery will be described
in the chapter on glass-grinding.
The aluminium disc is now struck between the dies by means of a
hammer. If the radius of curvature is anything more than one inch and
the disc not more than one inch in diameter the cathode can be struck
at once from the flat as described. For very deep curves no doubt it
will be better to make an intermediate pair of dies and to re-anneal
the aluminium after the first striking.
When the tube is successfully prepared so far as the glassblowing goes
it may be rinsed with strong pure alcohol both inside and out, and
dried. The straight part of the side tube is then constricted ready
for fusing off and the whole affair is placed on the vacuum pump.
In spite of the great improvements made during recent years in the
construction of so-called Geissler vacuum pumps--i.e. pumps in which
a Torricellian vacuum is continually reproduced--I am of opinion that
Sprengel pumps are, on the whole, more convenient for exhausting
Crooke's tubes. A full discussion of the subject of vacuum pumps will
be found in a work by Mr. G. S. Ram (The Incandescent Lamp and its
Manufacture), published by the Electrician Publishing Company, and it
is not my intention to deal with the matter here; the simplest kind of
Sprengel pump will be found quite adequate for our purpose, provided
that it is well made.
Fig. 42 is intended to represent a modification of a pump based on
the model manufactured by Hicks of Hatton Garden, and arranged to suit
the amateur glass-blower. The only point of importance is the
construction of the head of the fall tube, of which a separate and
enlarged diagram is given. The fall tubes may have an internal
diameter up to 2 mm. (two millimetres) and an effective length of 120
cm.
Free use is made of rubber tube connections in the part of the pump
exposed to the passage of mercury. The rubber employed should be
black and of the highest quality, having the walls strengthened by a
layer of canvas. If such tube cannot be easily obtained, a very good
substitute may be made by placing a bit of ordinary black tube inside
another and rather larger bit and binding the outer tube with tape or
ribbon. In any case the tubing which comes in contact with the
mercury should be boiled in strong caustic potash or soda solution for
at least ten minutes to get rid of free sulphur, which fouls the
mercury directly it comes in contact with it. The tubing is well
washed, rinsed with alcohol, and carefully dried.
Fig. 42.
The diagram represents what is practically a system of three Sprengel
pumps, though they are all fed from the same mercury reservoir and run
down into the same mercury receiver. It is much easier to make three
pumps, each with separate pinch cocks to regulate the mercury supply,
than it is to make three jets, each delivering exactly the proper
stream of mercury to three fall tubes.
Sprengel pumps only work at their highest efficiency when the mercury
supply is carefully regulated to suit the peculiarities of each fall
tube, and this is quite easily done in the model figured. Since on
starting the pump the rubber connections have to stand a considerable
pressure, the ends of the tubes must be somewhat corrugated to enable
the rubber to be firmly wired on to them. The best binding wire is the
purest Swedish iron wire, previously annealed in a Bunsen gas flame.
The wire must never be twisted down on the bare rubber, but must
always be separated from it by a tape binding. By taking this
precaution the wire maybe twisted very much more tightly than is
otherwise possible without cutting the rubber.
The only difficulty in making such a pump as is described lies in the
bending of the heads of the fall tubes. This bending must be done
with perfect regularity and neatness, otherwise the drops of mercury
will not break regularly, or will break just inside the top of the
fall tube, and so obstruct its entrance that at high vacua no air can
get into the tube at all.
The connections at the head of the fall tubes must also be well put on
and the joints blown out so that the mercury in dropping over the head
is not interfered with by the upper surface of the tube. However, a
glance at the enlarged diagram will show what is to be aimed at better
than any amount of description. In preparing the fall tubes it is
generally necessary to join at least two "canes" together. The joint
must be arranged to occur either in the tube leading the mercury to
the head of the fall, or in that part of the fall tube which remains
full of mercury when the highest vacuum is attained. On no account
must the joint be made at the fall itself (at least not by an
amateur), nor in that part of the fall tube where the mercury falls
freely, particularly at its lower end, where the drops fall on the
head of the column of mercury.
When a high vacuum is attained the efficacy of the pump depends
chiefly on the way in which the drops fall on the head of the column.
If the fall is too long the drops are apt to break up and allow the
small bubble of air to escape up the tube, also any irregularity or
dirt in the tube at this point makes it more easy for the bubbles of
air to escape to the surface of the mercury.
Any pump in which the supply of mercury to the fall tube can be
regulated nicely will pump well until the lowest available pressures
are being attained; a good pump will then continue to hold the air
bubbles, while a bad one will allow them to slip back [Footnote: For
special methods of avoiding this difficulty see Mr. Ram's book.] ...
Though three fall tubes are recommended, it must not be supposed that
the pump will produce a Crooke's vacuum three times more rapidly than
one fall tube. Until the mercury commences to hammer in the pump the
three tubes will pump approximately three times faster than one tube,
but as soon as the major portion of the air collected begins to come
from the layer condensed on the glass surface of the tube to be
exhausted and from the electrodes, the rate at which exhaustion will
go on no longer depends entirely on the pump.
In order that bubbles of air may not slip back up the fall tube it is
generally desirable to allow the mercury to fall pretty briskly, and
in this case the capacity of the pump to take air is generally far in
excess of the air supply. One advantage of having more than one fall
tube is that it often happens that a fall tube gets soiled during the
process of exhaustion and no longer works up to its best performance.
Out of three fall tubes, however, one is pretty sure to be working
well, and as soon as the mercury begins to hammer in the tubes the
supply may be shut off from the two falls which are working least
satisfactorily.
Thus we are enabled to pump rapidly till a high degree of exhaustion
is attained, having practically three pumps instead of one, whereas
when the final stages are reached, and three pumps are only a drawback
in that they increase the mercury flow, the apparatus is capable of
instant modification to meet the new conditions.
The thistle funnels at the head of the fall tubes are made simply by
blowing bulbs and then blowing the heads of the bulbs into wider ones,
and finally blowing the heads of the wider bulbs off by vigorous
blowing. The stoppers are ground in on the lathe before the tubes are
attached to the fall tubes. The stoppers require to be at least half
an inch long where they fit the necks, and must be really well ground
in. The stoppers must first be turned up nicely and the necks ground
out by a copper or iron cone and emery. The stoppers are rotated on a
lathe at quite a slow speed, say 30 or 40 feet per minute, and the
necks are held against them, as described in the section dealing with
this art. The stoppers must in this case be finished with "two
seconds" emery, and lastly with pumice dust and water (see chapter on
glass-grinding).
Unless the stoppers fit exceedingly well trouble will arise from the
mercury (which is poured into the thistle heads to form a seal) being
forced downwards into the pump by atmospheric pressure.
The joints between the three fall tubes and the single exhaust main
are easily made when the tubes are finally mounted, the hooked nozzle
of the oxygas blow-pipe being expressly made for such work.
It is, on the whole, advisable to make the pump of flint glass, or at
all events the air-trap tube and the fall tubes. A brush flame from
the larger gas tube of the single blowpipe table is most suitable for
the work of bending the tubes. The jointing of the long, narrow bore
fall tubes is best accomplished by the oxygas flame, for in this way
the minimum of irregularity is produced; the blowing tubes will of
course be required for the job, and the narrow tubes must be well
cleaned to begin with.
The air trap is an important though simple part of the pump. Its
shoulder or fall should stand rather higher than the shoulders of the
fall tubes, so that the mercury may run in a thin stream through a
good Torricellian vacuum before it passes down to the fall tubes.
This is easily attained by regulating the main mercury supply at the
pinch cock situated between the tube from the upper reservoir and the
air-trap tube, the other cocks being almost wide open.
It might be thought that the mercury would tend to pick up air in
passing through the rubber connections to the fall tubes, but I have
not found this to be the case in practice. There is, of course, no
difficulty in eliminating the rubber connections between the fall
tubes and the mercury supply from the air trap, but it impresses a
greater rigidity on the structure and, as I say, is not in general
necessary. It must not be forgotten that the mercury always exercises
considerable pressure on the rubber joints, and so there is little
tendency for gas to come out of the rubber.
The thistle funnels at the head of the fall tubes provide a simple and
excellent means of cleaning the fall tubes. For this purpose some
"pure" sulphuric acid which has been boiled with pure ammonium
sulphate is placed in each thistle funnel, and when the fall tube is
dirty the connection to the mercury supply is cut off at the pinch
cock so as to leave the tube between this entry and the head of the
fall tube quite full of mercury, and the sulphuric acid is allowed to
run down the fall tube by raising the stopper. The fall tube should
be allowed to stand full of acid for an hour or so, after which it
will be found to be fairly clean.
Of course the mercury reservoir thus obtains a layer of acid above the
mercury, and as it is better not to run the risk of any acid getting
into the pump except in the fall tubes, the reservoir is best emptied
from the bottom, by a syphon, if a suitable vessel cannot be procured,
so that clean mercury only is withdrawn.
The phosphorus pentoxide tube is best made as shown simply from a bit
of wide tube, with two side connections fused to the rest of the pump.
It is no more trouble to cut the tube and fuse it up again when the
drying material is renewed than to adjust the drying tube to two fixed
stoppers, which is the alternative. The practice here recommended is
rendered possible only by the oxygas blow-pipe with hooked nozzle.
The connection between the pump and tube to be exhausted is made
simply by a short bit of rubber tube immersed in mercury.
The phosphorus pentoxide should be pure, or rather free from
phosphorus and lower oxides; unless this be the case, the vapour
arising from it is apt to soil the mercury in the pump. The
phosphorus pentoxide is purified by distilling with oxygen over
red-hot platinum black; if this cannot be done, the pentoxide should
at least be strongly heated in a tube, in a current of dry air or
oxygen, before it is placed in the drying tube.
The mercury used for the pump must be scrupulously clean. It does
not, however, require to have been distilled in vacuo. It is
sufficient to purify it by allowing it to fall in a fine spray into a
large or rather tall jar of 25 per cent nitric acid and 75 per cent
water. The mercury is then to be washed and dried by heating to, say,
110° C. in a porcelain dish.
Exhausting a Roentgen Tube.
With a pump such as has been described there is seldom any advantage
in fusing an extra connection to the vacuum tube so as to allow of a
preliminary exhaustion by means of a water pump. About half an hour's
pumping may possibly be saved by making use of a water pump.
The tube to be exhausted is washed and dried by careful heating over a
Bunsen burner and by the passage of a current of air. The exhausting
tube is then drawn down preparatory to sealing off, and the apparatus
placed upon the pump. It is best held in position by a wooden clamp
supported by a long retort stand.
Exhaustion may proceed till the mercury in the fall tubes commences to
hammer. At this point the tube must be carefully heated by a Bunsen
flame, the temperature being brought up to, say, 400° C. The heating
may be continued intermittently till little or no effect due to the
heating is discernible at the pump. When this stage is reached, or
even before, the electrodes may be connected up to the coil and a
discharge sent through the tube.
Care must be taken to stop the discharge as soon as a purple glow
begins to appear, because when this happens, the resistance of the
tube is very low, the electrodes get very hot, and may easily get
damaged by a powerful discharge, and the platinum of the anode (if a
focus tube is in question) begins to be distilled on to the glass.
The heating and sparking are to be continued till the resistance of
the tube sharply increases. This is tested by always having a spark
gap, conveniently formed by the coil terminals, in parallel with the
tube. If the terminals are points, it is convenient to set them at
about one quarter of an inch distance apart.
As soon as sparks begin to pass between the terminals of the spark gap
it becomes necessary to watch the process of exhaustion very
carefully. In the first place, stop the pump, but let the coil run,
and note whether the sparks continue to flow over the terminals. If
the glass and electrodes are getting gas free, the discharge will
continue to pass by the spark gap, but if gas is still being freely
given off, then in perhaps three minutes the discharge will return to
the tube, and pumping must be recommenced. The Roentgen effect only
begins to appear when the tube has got to so high a state of
exhaustion that the resistance increases rapidly.
By pumping and sparking, the resistance of the tube may be gradually
raised till the spark would rather jump over 2 inches of air than go
through the tube. When this state is attained the Roentgen effect as
tested by a screen of calcium tungstate should be very brilliant. No
conclusion as to the equivalent resistance of the tube can be arrived
at so long as the discharge is kept going continually. When the spark
would rather go over an inch of air in the spark gap than through the
tube the pumping and sparking may be interrupted and the tube allowed
to rest for, say, five minutes. It will generally be found that the
equivalent resistance of the tube will be largely increased by this
period of quiescence. It may even be found that the spark will now
prefer to pass an air gap 3 inches long.
In any case the sparking should now be continued, the pump being at
rest, and the variations of tube resistance watched by adjusting the
spark gap. If the resistance falls below an equivalent of 2 inches of
air in the gap the pump must be brought into action again and
continued until the resistance as thus estimated remains fairly
constant for, say, ten minutes. When this occurs the narrow neck of
the exhaust tube may be strongly heated till the blow-pipe flame
begins to show traces of sodium light. The flame must then be
withdrawn and the discharge again tested. This is necessary because
it occasionally happens that gas is given off during the heating of
the neck to the neighbourhood of its fusion temperature.
If all is right the neck may now be fused entirely off and the tube is
finished. Tubes of the focus pattern with large platinum anodes are
in general (in my experience) much more difficult to exhaust than
tubes of the kind first described. This is possibly to be attributed
mainly to the gas given off by the platinum, but is also, no doubt,
due to the tubes being much larger and exposing a larger glass
surface. The type of tube described first generally takes about two
hours to exhaust by a pump made as explained, while a "focus" tube has
taken as long as nine hours, eight of which have been consumed after
the tube was exhausted to the hammering point.
The pressure at which the maximum heating of the anode by the cathode
rays occurs is a good deal higher than that at which the maximum
Roentgen effect is produced. There is little doubt that the Roentgen
radiation changes in nature to some extent as the vacuum improves
either as a primary or secondary effect. It is therefore of some
importance to test the tube for the purpose for which it is to be used
during the actual exhaustion. It has been stated, for instance, that
the relative penetrability of bone and flesh to Roentgen radiation
attains a maximum difference at a certain pressure; this is very
likely the case. Whether this effect is a direct function of the
density of the gas in the tube, or whether it is dependent on the
voltage or time integral of the current during the discharge, are
questions which still await a solution.
The preparation of calcium tungstate for fluorescent screens is very
simple.
Commercial sodium tungstate is fused with dried calcium chloride in
the proportion of three parts of the former to two parts of the
latter, both constituents being in fine powder and well mixed
together. The fusion is conducted in a Fletcher's crucible furnace in
a clay crucible. The temperature is raised as rapidly as possible to
the highest point which the furnace will attain--i.e. a pure white
heat. At this temperature the mixture of salts becomes partly fluid,
or at least pasty, and the temperature may be kept at its highest
point for, say, a quarter of an hour. At the end of this time the
mass is poured and scraped on to a brick, and when cold is broken up
and boiled with a large excess of water to dissolve out all soluble
matter. The insoluble part, which consists of a gray shining powder,
is washed several times with hot water, and is finally dried on
filter paper in a water oven.
In order to prepare a screen the powder is ground slightly with very
dilute shellac varnish, and is then floated over a glass plate so as
to get an even covering. Unless the covering be very even the screen
is useless, and no pains should be spared to secure evenness. It is
not exactly easy to get a regular coat of the fluorescent material,
but it may be done with a little care.
CHAPTER II
GLASS-GRINDING AND OPTICIANS' WORK
§ 52. As no instructions of any practical value in this art have, so
far as I know, appeared in any book in English, though a great deal of
valuable information has been given in the English Mechanic and
elsewhere, I shall deal with the matter sufficiently fully for all
practical purposes. On the other hand, I do not propose to treat of
all the methods which have been proposed, but only those requisite for
the production of the results claimed. The student is requested to
read through the chapter before commencing any particular operation.
§ 53. The simplest way will be to describe the process of manufacture
of some standard optical appliance, from which a general idea of the
nature of the operations will be obtained. After this preliminary
account special methods may be considered in detail. I will begin
with an account of the construction of an achromatic object glass for
a telescope, not because a student in a physical laboratory will often
require to make one, but because it illustrates the usual processes
very well; and requires to be well and accurately made.
A knowledge of the ordinary principles of optics on the part of the
reader is assumed, for there are plenty of books on the theory of
lenses, and, in any case, it is my intention to treat of the art
rather than of the science of the subject. By far the best short
statement of the principles involved which I have seen is Lord
Rayleigh's article on Optics in the Encyclopaedia Britannica, and this
is amply sufficient.
The first question that crops up is, of course, the subject of the
choice of glass. It is obvious that the glass must be uniform in
refractive index throughout, and that it must be free from air bubbles
or bits of opaque matter. [Footnote: The complete testing of glass
for uniformity of refractive index can only be arrived at by grinding
and polishing a sufficient portion of the surfaces to enable an
examination to be made of every part. In the case of a small disc it
is sufficient to polish two or three facets on the edge, and to
examine the glass in a field of uniform illumination through the
windows thus formed. Very slight irregularities will cause a "mirage"
easily recognised.]
The simplest procedure is to obtain glass of the desired quality from
Messrs. Chance of Birmingham, according to the following abbreviated
list of names and refractive indices, which may be relied upon:-
Density. Refractive Index.
C D F G
Hard crown
2.85 1.5146 1.5172 1.5232 1.5280
Soft crown
2.55 1.5119 1.5146 1.5210 1.5263
Light flint
3.21 1.5700 1.5740 1.5839 1.5922
Dense flint
3.66 1 6175 1.6224 1.6348 1.6453
Extra dense flint
3.85 1.6450 1.6504 1.6643 1.6761
Double extra dense flint
4.45 1.7036 1.7103 1.7273 ...
The above glasses may be had in sheets from 0.25 to 1 inch thick, and
6 to 12 inches square, at a cost of, say, 7s. 6d. per pound.
Discs can also be obtained of any reasonable size. Discs 2 inches in
diameter cost about £1 per dozen, discs 3 inches in diameter about
10s. each. The price of discs increases enormously with the size. A
16-inch disc will cost about £100.
For special purposes, where the desired quality of glass does not
appear on the list, an application may be made to the Jena Factory of
Herr Schott. In order to give a definite example, I may mention that
for ordinary telescopic objectives good results may be obtained by
combining the hard crown and dense flint of Chance's list, using the
crown to form a double convex, and the flint to form a double concave
lens. The convex lens is placed in the more outward position in the
telescope, i.e. the light passes first through it.
The conditions to be fulfilled are:
(1) The glass must be achromatic;
(2) it must have a small spherical aberration for rays converging to
the principal focus.
It is impossible to discuss these matters without going into a
complete optical discussion. The radii of curvature of the surfaces,
beginning with the first, i.e. the external face of the convex lens,
are in the ratio of 1, 2, and 3; an allowance of 15 inches focal
length per inch of aperture is reasonable (see Optics in Ency.
Brit.), and the focal length is the same as the greatest radius of
curvature. Thus, for an object glass 2 inches in diameter, the first
surface of the convex lens would have a radius of curvature of 10
inches, the surface common to the convex and concave lens would have a
radius of curvature of 20 inches, and the last surface a radius of
curvature of 30 inches. This would also be about the focal length of
the finished lens. The surfaces in contact have, of course, a common
curvature, and need not be cemented together unless a slight loss of
light is inadmissible.
I will assume that a lens of about 2 inches diameter is to be made by
hand, i.e. without the help of a special grinding or polishing
machine; this can be accomplished perfectly well, so long as the
diameter of the glass is not above about 6 inches, after which the
labour is rather too severe. The two glass discs having been obtained
from the makers, it will be found that they are slightly larger in
diameter than the quoted size, something having been left for the
waste of working.
It is difficult to deal with the processes of lens manufacture without
entering at every stage into rather tedious details, and, what is
worse, without interrupting the main account for the purpose of
describing subsidiary instruments or processes. In order that the
reader may have some guide in threading the maze, it is necessary that
he should commence with a clear idea of the broad principles of
construction which are to be carried out. For this purpose it seems
desirable to begin by roughly indicating the various steps which are
to be taken.
(1) The glass is to be made circular in form and of a given diameter.
(2) Called Rough Grinding. The surfaces of the glass are to be made
roughly convex, plane, or concave, as may be required; the glass is to
be equally thick all round the edge. In this process the glass is
abraded by the use of sand or emery rubbed over it by properly shaped
pieces of iron or lead called "tools."
(3) The glass is ground with emery to the correct spherical figure as
given by a spherometer.
(4) Called Fine Grinding. The state of the surface is gradually
improved by grinding with finer and finer grades of emery.
(5) The glass is polished by rouge.
(6) The glass is "figured." This means that it is gradually altered in
form by a polishing tool till it gives the best results as found by
trial.
In processes 2 to 5 counterpart tool surfaces are required--as a rule
two convex and two concave surfaces for each lens surface. These
subsidiary surfaces are worked (i.e. ground) on discs of cast iron
faced with glass, or on slate discs; and discs thus prepared are
called "tools."
Taking these processes in the order named, the mode of manufacture is
shortly as follows:-
(1) The disc of glass, obtained in a roughly circular form, is mounted
on an ordinary lathe, being conveniently cemented by Regnault's mastic
to a small face plate. The lathe is rotated slowly, and the glass is
gradually turned down to a circular figure by means (1) of a tool with
a diamond point; or (2) an ordinary hand-file moistened with
kerosene, as described in § 42; or (3) a mass of brass or iron served
with a mixture of emery--or sand--and water fed on to the disc, so
that the disc is gradually ground circular.
The operation of making a circular disc of given diameter does not
differ in any important particular from the similar operation in the
case of brass or iron, and is in fact merely a matter of turning at a
slow speed.
(2 and 3) Roughing or bringing the surfaces of the glass roughly to
the proper convex or concave shape. This is accomplished by
grinding, generally with sand in large works, or with emery in the
laboratory, where the time saved is of more importance than the value
of the emery.
Discs of iron or brass are cast and turned so as to have a diameter
slightly less than that of the glass to be ground, and are, say, half
an inch thick. These discs are turned convex or concave on one face
according as they are to be employed in the production of concave or
convex glass surfaces. The proper degree of convexity or concavity
may be approximated to by turning with ordinary turning tools, using a
circular arc cut from zinc or glass (as will be described) as a
"template" or pattern. This also is a mere matter of turning.
The first approximation to the desired convex or concave surface of
the glass is attained (in the case of small lenses, say up to three
inches diameter) by rotating the glass on the lathe as described above
(for the purpose of giving it a circular edge) and holding the tool
against the rotating glass, a plentiful supply of coarse emery and
water, or sand and water, being supplied between the glass and metal
surfaces. The tool is held by hand against the surface of the
revolving glass, and is constantly moved about, both round its own
axis of figure and to and fro across the glass surface. In this way
the glass gradually gets convex or concave.
The curvature is tested from time to time by a spherometer, and the
tool is increased or decreased in curvature by turning it on a lathe
so as to cause it to grind the glass more at the edges or in the
middle according to the indications of the spherometer.
This instrument, by the way--so important for lens makers--consists
essentially of a kind of three-legged stool, with an additional leg
placed at the centre of the circle circumscribing the other three.
This central leg is in reality a fine screw with a very large head
graduated on the edge, so that it is easy to compute the fractions of
a turn given to the screw. The instrument is first placed on a flat
plate, and the central screw turned till its end just touches the
plate, a state of affairs which is very sharply discernible by the
slight rocking which it enables the instrument to undergo when pushed
by the hand. See the sketch.
On a convex or concave surface the screw has to be screwed in or out,
and from the amount of screwing necessary to bring all four points
into equal contact, the curvature may be ascertained.
Let a be the distance between the equidistant feet, and d the distance
through which the screw is protruded or retracted from its zero
position on a flat surface. Then the radius of curvature rho is given
by the formula 2rho = a2/3d +d.
Fig. 43.
The process of roughing is not always carried out exactly as
described, and will be referred to again.
(4) The glass being approximately of the proper radius of curvature on
one side, it is reversed on the chuck and the same process gone
through on the other side. After this the glass is usually dismounted
from the lathe and mounted by cement on a pedestal, which is merely a
wooden stand with a heavy foot, so that the glass may be held
conveniently for the workman. Sometimes a pedestal about four feet
high is fixed in the floor of the room, so that the workman engaged in
grinding the lens may walk round and round it to secure uniformity.
For ordinary purposes, however, a short pedestal may be placed on a
table and rotated from time to time by hand, the operator sitting down
to his work.
Rough iron or brass tools do not succeed for fine grinding--i.e.
grinding with fine emery, because particles of emery become embedded
in the metal so tightly that they cannot be got out by any ordinary
cleaning. If we have been using emery passing say a sieve with 60
threads to the inch, and then go on to some passing say 100 threads to
the inch, a few of the coarser particles will adhere to the "tool",
and go on cutting and scratching all the time grinding by means of the
finer emery is in progress.
To get over this it is usual to use a rather different kind of
grinding tool. A very good kind is made by cementing small squares of
glass (say up to half an inch on the side), on to a disc of slate
slightly smaller than the lens surface to be formed (Fig. 51). The
glass-slate tool is then "roughed" just like the lens surface, but, of
course, if the lens has been roughed "convex" the tool must be roughed
"concave".
The "roughed" tool is then used to gradually improve the fineness of
grinding of the glass. For this purpose grinding by hand is resorted
to, the tool and lens being supplied continually with finer and finer
emery. Fig. 52 gives an idea of the way in which the tool is moved
across the glass surface. Very little pressure is required. The tool
is carried in small circular sweeps round and round the lens, so that
the centre of the tool describes a many-looped curve on the lens
surface. The tool must be allowed to rotate about its own axis; and
the lens and pedestal must also be rotated from time to time.
Every few minutes the circular strokes are interrupted, and simple,
straight, transverse strokes taken. In no case (except to correct a,
defect, as will be explained) should the tool overhang the lens
surface by more than about one quarter the diameter of the latter.
After grinding say for an hour with one size of emery fed in by means
of a clean stick say every five minutes, the emery is washed off, and
everything carefully cleaned. The process is then repeated with finer
emery, and so on.
The different grades of emery are prepared by taking advantage of the
fact that the smaller the particles the longer do they remain
suspended in water. Some emery mud from a "roughing" operation is
stirred up with plenty of water and left a few seconds to settle, the
liquor is then decanted to a second jug and left say for double the
time, say ten seconds; it is decanted again, and so on till four or
five grades of emery have been accumulated, each jug containing finer
emery than its predecessor in the process.
It is not much use using emery which takes more than half an hour to
settle in an ordinary bedroom jug. What remains in the liquid to be
decanted is mostly glass mud and not emery at all. The process of
fine grinding is continually checked by the spherometer, and the art
consists in knowing how to move the grinding tool so as to make the
lens surface more or less curved. In general it may be said that if
the tool is moved in small sweeps, and not allowed to overhang much,
the Centre of the lens will be more abraded, while if bold free
strokes are taken with much overhanging, the edges of the lens will be
more ground away.
By the exercise of patience and perseverance any one will succeed in
gradually fine grinding the lens surface and keeping it to the
spherometer, but the skill comes in doing this rapidly by varying the
shape of the strokes before any appreciable alteration of curvature
has come about.
Polishing.
The most simple way of polishing is to coat the grinding tool with
paper, as will be described, and then to brush some rouge into the
paper. The polisher is moved over the work in much the same way as
the fine grinding tool, until the glass is polished. Many operators
prefer to use a tool made by squeezing a disc of slate, armed with
squares of warm pitch, against the lens surface (finely ground), and
then covering these squares with rouge and water instead of emery and
water as in the fine grinding process.
The final process is called "figuring." It will in general be
unnecessary with a small lens. With large lenses or mirrors the final
touches have to be given after the optical behaviour of the lens or
mirror has been tested with the telescope itself, and this process is
called "figuring." A book might easily be written on the optical
indications of various imperfections in a mirror or lens. Suffice it
to say here that a sufficiently skilled person will be able to decide
from an observation of the behaviour of a telescope whether a lens
will be improved by altering the curvature of one or all of the
surfaces.
A very small alteration will make a large difference in the optical
properties, so that in general "figuring" is done merely by using the
rouge polishing tool as an abrading tool, and causing it to alter the
curves in the manner already suggested for grinding. There are other
methods based on knocking squares out of the pitch-polisher so that
some parts of the glass may be more abraded than others.
The "figuring" and polishing may be done by hand just like the
grinding. There are machines, however, which can be made to execute
the proper motions, and a polisher is set in such a machine, and the
mechanical work done is by no means inconsiderable. In fact for
surfaces above six inches in diameter few people are strong enough to
work a polisher by hand owing to the intense adhesion between it and
the exactly fitting glass surface.
Such is a general outline of the processes required to produce a lens
or mirror. These processes will now be dealt with in much greater
detail, and a certain amount of repetition of the above will
unfortunately be necessary: the reader is asked to pardon this. It
will also be advisable for the reader to begin by reading the whole
account before he commences any particular operation. The reason for
this is that it has been desirable to keep to the main account as far
as possible without inserting special instructions for subsidiary
operations, however important they may be; consequently it may not
always be quite clear how the steps described are to be performed. It
will be found, however, that all necessary information is really
given, though perhaps not always exactly in the place the reader might
at first expect.
§ 54. All the discs that I have seen, come from the makers already
roughly ground on the edges to a circular figure--but occasionally the
figure is very rough indeed--and in some cases, especially if small
lenses have to be made, it is convenient to begin by cutting the glass
discs out of glass sheet, which also may be purchased of suit-able
glass. To do this, the simplest way is to begin by cutting squares
and then cutting off the corners with the diamond, the approximate
circular figure being obtained by grinding the edges on an ordinary
grindstone.
If the pieces are larger, time and material may be saved by using a
diamond compass, i.e. an ordinary drawing compass armed with a
diamond to cut circles on the glass, and breaking the superfluous
glass away by means of a pair of spectacle-maker's shanks (Fig. 44),
or what does equally well, a pair of pliers with soft iron jaws. With
these instruments glass can be chipped gradually up to any line,
whether diamond-cut or not, the jaws of the pincers being worked
against the edge of the glass, so as to gradually crush it away.
Fig. 44.
Assuming that the glass has been bought or made roughly circular, it
must be finished on the lathe. For this purpose it is necessary to
chuck it on an iron or hardwood chuck, as shown in Fig. 46. For a
lens below say an inch in diameter, the centering cement may be used;
but for a lens of a diameter greater than this, sufficient adhesion is
easily obtained with Regnault's mastic, and its low melting point
gives it a decided advantage over the shellac composition.
The glass may be heated gradually by placing it on the water bath, or
actually in the water, and gradually bringing the water up to the
boiling-point. The glass, being taken out, is rapidly wiped, and
rubbed with a bit of waste moistened, not wet, with a little
turpentine: its surface is then rubbed with a stick of mastic
previously warmed so as to melt easily. The surface of the chuck
being also warm, and covered with a layer of melted cement, it is
applied to the glass. The lathe is turned slowly by hand, and the
glass pushed gradually into the most central position; it is then
pressed tight against the chuck by the back rest, a bit of wood being
interposed for obvious reasons.
When all is cold the turning may be proceeded with. The quickest way
is to use the method already described (i.e. actual turning by a file
tool); but if the student prefers (time being no object), he may
accomplish the reduction to a circular form very easily by grinding.
Fig. 45.
Fig. 46.
For this purpose he will require to make the following arrangements
(Fig. 45). If the lathe has a slide rest, a piece of stout iron may
be bent and cut so as to fit the tool rest, and project beneath the
glass. The iron must be fairly rigid, for if it springs appreciably
beneath the pressure of the glass, it will not grind the latter really
round. The lathe may run rather faster than for turning cast iron of
the same size. Coarse emery, passing through a sieve of 80 threads to
the inch (run), may be fed in between the glass and iron, and the
latter screwed up till the disc just grinds slightly as it goes round.
A beginner will generally (in this as in all cases of grinding
processes) tend to feed too fast--no grinding process can be hurried.
If a slide rest is not available, a hinged board, carrying a bit of
iron, may (see Fig. 45) be arranged so as to turn about its hinge at
the back of the lathe; and it may be screwed up readily enough by
passing a long set-screw through the front edge, so that the point of
the screw bears upon the lathe bed. I may add that emery behaves as
if it were greasy, and it is difficult to wet it with clean water.
This is easily got over by adding a little soap or alcohol to the
water, or exercising a little patience.
A good supply of emery and water should be kept between the disc and
the iron; a little putty may be arranged round the point of contact
on the iron to form a temporary trough. In any case the resulting
emery mud should on no account be thrown away, but should be carefully
kept for further use. The process is complete when the glass is
perfectly round and of the required diameter as tested by callipers.
§ 55. The next step is to rough out the lens, and this may easily be
done by rotating it more slowly, i.e. with a surface speed of ten
feet per minute, and turning the glass with a hard file, as explained
in § 42. If it is desired to employ the slide rest, it is quicker and
better to use a diamond tool--an instrument quite readily made, and
of great service for turning emery wheels and the like,--a thing, in
fact, which no workshop should be without. A bit of diamond bort, or
even a clear though off-colour stone, may be employed.
An ordinary lathe tool is prepared by drawing down the tool steel to a
long cone, resembling the ordinary practice in preparing a boring
tool. The apex of the cone must be cut off till it is only slightly
larger than the greatest transverse diameter of the diamond splinter.
The latter may have almost any shape--a triangular point, one side of
a three-sided prism is very convenient. A hole is drilled in the
steel (which must have been well softened), only just large enough to
allow the diamond to enter--if the splinter is thicker in the middle
than at either end, so much the better--the diamond is fastened in
position by squeezing the soft steel walls tightly down upon it.
Personally I prefer to use a tool holder, and in this case generally
mount the diamond in a bit of brass rod of the proper diameter; and
instead of pinching in the sides of the cavity, I tin them, and set
the diamond in position with a drop of soft solder.
Fig. 47.
In purchasing diamond bort, a good plan is to buy fragments that have
been employed in diamond drilling, and have become too small to reset;
in this case some idea as to the hardness of the bits may be obtained.
Full details as to diamond tool-making are given in books on
watch-making, and in Holtzapffell's great work on Mechanical
Manipulation; but the above notes are all that are really
necessary--it is, in fact, a very simple matter. The only advantage of
using a diamond tool for glass turning is that one does not need to be
always taking it out of the rest to sharpen it, which generally happens
with hard steel, especially if the work is turned a little too fast.
I recommend, therefore, that the student should boldly go to work
"free hand" with a hard file; but if he prefer the more formal
method, or distrust his skill (which he should not do), then let him
use a diamond point, even if he has the trouble of making it. When
using a diamond it is not necessary to employ a lubricant, but there
is some advantage in doing so.
The surface of the lens can be roughly shaped by turning to a template
or pattern made by cutting a circular arc (of the same radius as the
required surface) out of a bit of sheet zinc. Another very handy way
of making templates of great accuracy is to use a beam compass
(constructed from a light wooden bar) with a glazier's diamond instead
of a pencil. A bit of thin sheet glass is cut across with this
compass to the proper curvature--which can be done with considerable
accuracy and the two halves of the plate, after breaking along the
cut, are ground together with a view to avoiding slight local
irregularities, by means of a little fine emery and water laid between
the edges. In this process the glass is conveniently supported on a
clean board or slate, and the bits are rubbed backwards and forwards
against each other.
§ 56. It is not very easy for a beginner to turn a bit of
anything--iron, wood, or glass--with great accuracy to fit a template,
and consequently time may be saved by the following procedure, applied as
soon as the figure of the template is roughly obtained. A disc of
lead or iron, of the same diameter as the glass, and of approximately
the proper curvature, is prepared by turning, and is armed with a
handle projecting coaxially from the back of the disc. The glass
revolving with moderate speed on the lathe, the lead tool, supplied
with coarse emery and water, is held against it, care being taken to
rotate the tool by the handle, and also to move it backwards and
forwards across the disc, through a distance, say, up to half an inch;
if it is allowed to overhang too much the edges of the glass disc will
be overground. By the use of such a tool the glass can readily be
brought up to the template.
The only thing that remains, so far as the description of this part of
the process goes, is to give a note or two as to the best way of
making the lead tools, and for this purpose the main narrative of
processes must be interrupted. The easiest way is to make a set of
discs to begin with. For this purpose take the mandrel out of the
lathe, and place it nose downwards in the centre of an iron ring of
proper diameter on a flat and level iron plate.
The discs are made by pouring lead round the screw-nose of the
mandrel. This method, of course, leaves them with a hole in the
centre; but this can be stopped up by placing the hot disc (from
which the mandrel has been unscrewed) on a hot plate, and pouring in a
sufficiency of very hot lead; or, better still, the mandrel can
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