Conversations on Chemistry, V. 1-2
by Mrs. Marcet · English · Gutenberg #26908 · 841,354 chars · public domain (US)
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Title: Conversations on Chemistry, V. 1-2
Author: Mrs. Marcet
Release date: October 13, 2008 [eBook #26908]
Language: English
Other information and formats: www.gutenberg.org/ebooks/26908
Credits: Produced by Louise Hope
*** START OF THE PROJECT GUTENBERG EBOOK CONVERSATIONS ON CHEMISTRY, V. 1-2 ***
Produced by Louise Hope
[Transcriber’s Note:
DO NOT TRY THIS AT HOME.
This e-text comes in three different forms: unicode (UTF-8), Latin-1 and
ascii-7. Use the one that works best on your text reader.
--If “œ” displays as a single character, and apostrophes and quotation
marks are “curly” or angled, you have the utf-8 version (best). If any
part of this paragraph displays as garbage, try changing your text
reader’s “character set” or “file encoding”. If that doesn’t work,
proceed to:
--In the Latin-1 version, “œ” is two letters, but the word “aëriform”
is usually written with dieresis (dots) over the “e”, and “æ” is a
single letter. Apostrophes and quotation marks will be straight
(“typewriter” form). Again, if you see any garbage in this paragraph
and can’t get it to display properly, use:
--The ASCII-7 or rock-bottom version. All necessary text will still be
there; it just won’t be as pretty.
The full caption of each Plate is given after its first mention in the
text--generally a few pages before the Plate’s physical appearance, as
specified in the caption.
Many terms used in this book are different from today’s standard
terminology. Note in particular:
oxy-muriatic acid = the element chlorine
phosphat of lime = calcium diphosphate _or_ the element calcium
glucium = the element beryllium
muriatic acid = hydrochloric acid
muriat of lime = calcium chloride
oxymuriate of potash = potassium chlorate
carbonic acid = carbon dioxide
Further details and more examples are at the end of the e-text.
Each Volume had its own table of contents. They have been merged for
this e-text, but the Vol. II title page was retained. Some Conversations
were renumbered between the 4th and 5th edition, resulting in the
apparent disappearance of Conversations XI and XII.
Typographical errors are listed at the end of the text.]
* * * * *
* * * *
* * * * *
CONVERSATIONS
ON
CHEMISTRY;
In Which
The Elements Of That Science
Are
_Familiarly Explained_
And
Illustrated By Experiments.
IN TWO VOLUMES.
_The Fifth Edition, revised, corrected,_
_and considerably enlarged._
VOL. I.
ON SIMPLE BODIES.
_London:_
Printed For Longman, Hurst, Rees, Orme, and Brown,
Paternoster-Row.
1817.
Printed by A. Strahan, Printers-Street, London.
ADVERTISEMENT.
_The Author, in this fifth edition, has endeavoured to give an account
of the principal discoveries which have been made within the last four
years in Chemical Science, and of the various important applications,
such as the gas-lights, and the miner’s-lamp, to which they have given
rise. But in regard to doctrines or principles, the work has undergone
no material alteration._
_London_, _July_, 1817.
PREFACE.
In venturing to offer to the public, and more particularly to the female
sex, an Introduction to Chemistry, the author, herself a woman,
conceives that some explanation may be required; and she feels it the
more necessary to apologise for the present undertaking, as her
knowledge of the subject is but recent, and as she can have no real
claims to the title of chemist.
On attending for the first time experimental lectures, the author found
it almost impossible to derive any clear or satisfactory information
from the rapid demonstrations which are usually, and perhaps
necessarily, crowded into popular courses of this kind. But frequent
opportunities having afterwards occurred of conversing with a friend on
the subject of chemistry, and of repeating a variety of experiments, she
became better acquainted with the principles of that science, and began
to feel highly interested in its pursuit. It was then that she
perceived, in attending the excellent lectures delivered at the Royal
Institution, by the present Professor of Chemistry, the great advantage
which her previous knowledge of the subject, slight as it was, gave her
over others who had not enjoyed the same means of private instruction.
Every fact or experiment attracted her attention, and served to explain
some theory to which she was not a total stranger; and she had the
gratification to find that the numerous and elegant illustrations, for
which that school is so much distinguished, seldom failed to produce on
her mind the effect for which they were intended.
Hence it was natural to infer, that familiar conversation was, in
studies of this kind, a most useful auxiliary source of information; and
more especially to the female sex, whose education is seldom calculated
to prepare their minds for abstract ideas, or scientific language.
As, however, there are but few women who have access to this mode of
instruction; and as the author was not acquainted with any book that
could prove a substitute for it, she thought that it might be useful for
beginners, as well as satisfactory to herself, to trace the steps by
which she had acquired her little stock of chemical knowledge, and to
record, in the form of dialogue, those ideas which she had first derived
from conversation.
But to do this with sufficient method, and to fix upon a mode of
arrangement, was an object of some difficulty. After much hesitation,
and a degree of embarrassment, which, probably, the most competent
chemical writers have often felt in common with the most superficial,
a mode of division was adopted, which, though the most natural, does not
always admit of being strictly pursued--it is that of treating first of
the simplest bodies, and then gradually rising to the most intricate
compounds.
It is not the author’s intention to enter into a minute vindication of
this plan. But whatever may be its advantages or inconveniences, the
method adopted in this work is such, that a young pupil, who should
occasionally recur to it, with a view to procure information on
particular subjects, might often find it obscure or unintelligible; for
its various parts are so connected with each other as to form an
uninterrupted chain of facts and reasonings, which will appear
sufficiently clear and consistent to those only who may have patience to
go through the whole work, or have previously devoted some attention to
the subject.
It will, no doubt, be observed, that in the course of these
Conversations, remarks are often introduced, which appear much too acute
for the young pupils, by whom they are supposed to be made. Of this
fault the author is fully aware. But, in order to avoid it, it would
have been necessary either to omit a variety of useful illustrations, or
to submit to such minute explanations and frequent repetitions, as would
have rendered the work tedious, and therefore less suited to its
intended purpose.
In writing these pages, the author was more than once checked in her
progress by the apprehension that such an attempt might be considered by
some, either as unsuited to the ordinary pursuits of her sex, or
ill-justified by her own recent and imperfect knowledge of the subject.
But, on the one hand, she felt encouraged by the establishment of those
public institutions, open to both sexes, for the dissemination of
philosophical knowledge, which clearly prove that the general opinion no
longer excludes women from an acquaintance with the elements of science;
and, on the other, she flattered herself that whilst the impressions
made upon her mind, by the wonders of Nature, studied in this new point
of view, were still fresh and strong, she might perhaps succeed the
better in communicating to others the sentiments she herself
experienced.
The reader will soon perceive, in perusing this work, that he is often
supposed to have previously acquired some slight knowledge of natural
philosophy, a circumstance, indeed, which appears very desirable. The
author’s original intention was to commence this work by a small tract,
explaining, on a plan analogous to this, the most essential rudiments of
that science. This idea she has since abandoned; but the manuscript was
ready, and might, perhaps, have been printed at some future period, had
not an elementary work of a similar description, under the tide of
“Scientific Dialogues,” been pointed out to her, which, on a rapid
perusal, she thought very ingenious, and well calculated to answer its
intended object.
Contents Of
_The First Volume_.
ON SIMPLE BODIES.
CONVERSATION I.
Page
ON THE GENERAL PRINCIPLES OF CHEMISTRY. 1
Connexion between Chemistry and Natural Philosophy. --Improved State
of modern Chemistry. --Its use in the Arts. --The general Objects of
Chemistry. --Definition of Elementary Bodies. --Definition of
Decomposition. --Integrant and Constituent Particles. --Distinction
between Simple and Compound Bodies. --Classification of Simple
Bodies. --Of Chemical Affinity, or Attraction of Composition.
--Examples of Composition and Decomposition.
CONVERSATION II.
ON LIGHT AND HEAT. 26
Light and Heat capable of being separated. --Dr. Herschel’s
Experiments. --Phosphorescence. --Of Caloric. --Its two
Modifications. --Free Caloric. --Of the three different States of
Bodies, solid, fluid, and aeriform. --Dilatation of solid Bodies.
--Pyrometer. --Dilatation of Fluids. --Thermometer. --Dilatation of
Elastic Fluids. --Air Thermometer. --Equal Diffusion of Caloric.
--Cold a Negative Quality. --Professor Prevost’s Theory of the
Radiation of Heat. --Professor Pictet’s Experiments on the Reflexion
of Heat. --Mr. Leslie’s Experiments on the Radiation of Heat.
CONVERSATION III.
CONTINUATION OF THE SUBJECT. 70
Of the different Power of Bodies to conduct Heat. --Attempt to
account for this Power. --Count Rumford’s Theory of the
non-conducting Power of Fluids. --Phenomena of Boiling. --Of
Solution in general. --Solvent Power of Water. --Difference between
Solution and Mixture. --Solvent Power of Caloric. --Of Clouds, Rain,
Dr. Wells’ theory of Dew, Evaporation, &c. --Influence of
Atmospherical Pressure on Evaporation. --Ignition.
CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT
AND LATENT HEAT. 122
Of Specific Heat. --Of the different Capacities of Bodies for Heat.
--Specific Heat not perceptible by the Senses. --How to be
ascertained. --Of Latent Heat. --Distinction between Latent and
Specific Heat. --Phenomena attending the Melting of Ice and the
Formation of Vapour. --Phenomena attending the Formation of Ice, and
the Condensation of Elastic Fluids. --Instances of Condensation, and
consequent Disengagement of Heat, produced by Mixtures, by the
Slaking of Lime. --General Remarks on Latent Heat. --Explanation of
the Phenomena of Ether boiling, and Water freezing, at the same
Temperature. --Of the Production of Cold by Evaporation.
--Calorimeter. --Meteorological Remarks.
CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160
Of Positive and Negative Electricity. --Galvani’s Discoveries.
--Voltaic Battery. --Electrical Machine. --Theory of Voltaic
Excitement.
CONVERSATION VI.
ON OXYGEN AND NITROGEN. 181
The Atmosphere composed of Oxygen and Nitrogen in the State of Gas.
--Definition of Gas. --Distinction between Gas and Vapour. --Oxygen
essential to Combustion and Respiration. --Decomposition of the
Atmosphere by Combustion. --Nitrogen Gas obtained by this Process.
--Of Oxygenation in general. --Of the Oxydation of Metals. --Oxygen
Gas obtained from Oxyd of Manganese. --Description of a Water-Bath
for collecting and preserving Gases. --Combustion of Iron Wire in
Oxygen Gas. --Fixed and volatile Products of Combustion. --Patent
Lamps. --Decomposition of the Atmosphere by Respiration.
--Recomposition of the Atmosphere.
CONVERSATION VII.
ON HYDROGEN. 214
Of Hydrogen. --Of the Formation of Water by the Combustion of
Hydrogen. --Of the Decomposition of Water. --Detonation of Hydrogen
Gas. --Description of Lavoisier’s Apparatus for the formation of
Water. --Hydrogen Gas essential to the Production of Flame.
--Musical Tones produced by the Combustion of Hydrogen Gas within a
Glass Tube. --Combustion of Candles explained. --Gas lights.
--Detonation of Hydrogen Gas in Soap Bubbles. --Air Balloons.
--Meteorological Phenomena ascribed to Hydrogen Gas. --Miner’s Lamp.
[Transcriber’s Note:
The final two pages of the Table of Contents for Volume I were
missing; everything after “Decomposition of Water” was supplied
from earlier and later editions, compared against the body text.
The section marked “Diamond” (Conv. IX) was called “Diamond is
Carbon(e) in a state of perfect purity” in the 4th edn., “Diamond”
alone in later editions.]
CONVERSATION VIII.
ON SULPHUR AND PHOSPHORUS. 256
Natural History of Sulphur. --Sublimation. --Alembic. --Combustion
of Sulphur in Atmospheric Air. --Of Acidification in general.
--Nomenclature of the Acids. --Combustion of Sulphur in Oxygen Gas.
--Sulphuric Acid. --Sulphurous Acid. --Decomposition of Sulphur.
--Sulphurated Hydrogen Gas. --Harrogate, or Hydro-sulphurated
Waters. --Phosphorus. --History of its Discovery. --Its Combustion
in Oxygen Gas. --Phosphoric Acid. --Phosphorus Acid. --Eudiometer.
--Combination of Phosphorus with Sulphur. --Phosphorated Hydrogen
Gas. --Nomenclature of Binary Compounds. --Phosphoret of Lime
burning under Water.
CONVERSATION IX.
ON CARBON. 282
Method of obtaining pure Charcoal. --Method of making common
Charcoal. --Pure Carbon not to be obtained by Art. --Diamond.
--Properties of Carbon. --Combustion of Carbon. --Production of
Carbonic Acid Gas. --Carbon susceptible of only one Degree of
Acidification. --Gaseous Oxyd of Carbon. --Of Seltzer Water and
other Mineral Waters. --Effervescence. --Decomposition of Water by
Carbon. --Of Fixed and Essential Oils. --Of the Combustion of Lamps
and Candles. --Vegetable Acids. --Of the Power of Carbon to revive
Metals.
CONVERSATION X.
ON METALS. 314
Natural History of Metals. --Of Roasting, Smelting, &c. --Oxydation
of metals by the Atmosphere. --Change of Colours produced by
different degrees of Oxydation. --Combustion of Metals. --Perfect
Metals burnt by Electricity only. --Some Metals revived by Carbon
and other Combustibles. --Perfect Metals revived by Heat alone. --Of
the Oxydation of certain Metals by the Decomposition of Water. Power
of Acids to promote this Effect. --Oxydation of Metals by Acids.
--Metallic Neutral Salts. --Previous oxydation of the Metal
requisite. --Crystallisation. --Solution distinguished from
Dissolution. --Five metals susceptible of acidification. --Meteoric
Stones. --Alloys, Soldering, Plating, &c. --Of Arsenic, and of the
caustic Effects of Oxygen. --Of Verdigris, Sympathetic Ink, &c. --Of
the new Metals discovered by Sir H. Davy.
Contents Of
_The Second Volume_.
ON COMPOUND BODIES.
CONVERSATION XIII.
Page
ON THE ATTRACTION OF COMPOSITION. 1
Of the laws which regulate the Phenomena of the Attraction of
Composition. --1. It takes place only between Bodies of a different
Nature. --2. Between the most minute Particles only. --3. Between 2,
3, 4, or more Bodies. --Of Compound or Neutral Salts. --4. Produces
a Change of Temperature. --5. The Properties which characterise
Bodies in their separate State, destroyed by Combination. --6. The
Force of Attraction estimated by that which is required by the
Separation of the Constituents. --7. Bodies have amongst themselves
different Degrees of Attraction. --Of simple elective and double
elective Attractions. --Of quiescent and divellent Forces. --Law of
definite Proportions. --Decomposition of Salts by Voltaic
Electricity.
CONVERSATION XIV.
ON ALKALIES. 19
Of the Composition and general Properties of the Alkalies. --Of
Potash. --Manner of preparing it. --Pearlash. --Soap. --Carbonat of
Potash. --Chemical Nomenclature. --Solution of Potash. --Of Glass.
--Of Nitrat of Potash or Saltpetre. --Effect of Alkalies on
Vegetable Colours. --Of Soda. --Of Ammonia or Volatile Alkali.
--Muriat of Ammonia. --Ammoniacal Gas. --Composition of Ammonia.
--Hartshorn and Sal Volatile. --Combustion of Ammoniacal Gas.
CONVERSATION XV.
ON EARTHS. 44
Composition of the Earths. --Of their Incombustibility. --Form the
Basis of all Minerals. --Their Alkaline Properties. --Silex; its
Properties and Uses in the Arts. --Alumine; its Uses in Pottery, &c.
--Alkaline Earths. --Barytes. --Lime; its extensive chemical
Properties and Uses in the Arts. --Magnesia. --Strontian.
CONVERSATION XVI.
ON ACIDS. 69
Nomenclature of the Acids. --Of the Classification of Acids. --1st
Class --Acids of simple and known Radicals, or Mineral Acids.
--2d Class --Acids of double Radicals, or Vegetable Acids.
--3d Class --Acids of triple Radicals or Animal Acids. --Of the
Decomposition of Acids of the 1st Class by Combustible bodies.
CONVERSATION XVII.
OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF
OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE SULPHATS
AND PHOSPHATS. 80
Of the Sulphuric Acid. --Combustion of Animal or Vegetable Bodies by
this Acid. --Method of preparing it. -- The Sulphurous Acid obtained
in the Form of Gas. --May be obtained from Sulphuric Acid. --May be
reduced to Sulphur. --Is absorbable by Water. --Destroys Vegetable
Colours. --Oxyd of Sulphur. --Of Salts in general. --Sulphats.
--Sulphat of Potash, or Sal Polychrest. --Cold produced by the
melting of Salts. --Sulphat of Soda, or Glauber’s Salt. --Heat
evolved during the Formation of Salts. --Crystallisation of Salts.
--Water of Crystallisation. --Efflorescence and Deliquescence of
Salts. --Sulphat of Lime, Gypsum or Plaister of Paris. --Sulphat of
Magnesia. --Sulphat of Alumine, or Alum. --Sulphat of Iron. --Of
Ink. --Of the Phosphoric and Phosphorous Acids. --Phosphorus
obtained from Bones. --Phosphat of Lime.
CONVERSATION XVIII.
OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF
OXYGEN WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND
CARBONATS. 100
Nitrogen susceptible of various Degrees of Acidification. --Of the
Nitric Acid. --Its Nature and Composition discovered by
Mr. Cavendish. --Obtained from Nitrat of Potash. --Aqua Fortis.
--Nitric Acid may be converted into Nitrous Acid. --Nitric Oxyd Gas.
--Its Conversion into Nitrous Acid Gas. --Used as an Eudiometrical
Test. --Gaseous Oxyd of Nitrogen, or exhilarating Gas, obtained from
Nitrat of Ammonia. --Its singular Effects on being respired.
--Nitrats. --Of Nitrat of Potash, Nitre or Saltpetre. --Of
Gunpowder. --Causes of Detonation. --Decomposition of Nitre.
--Deflagration. --Nitrat of Ammonia. --Nitrat of Silver. --Of the
Carbonic Acid. --Formed by the Combustion of Carbon. --Constitutes a
component Part of the Atmosphere. --Exhaled in some Caverns.
--Grotto del Cane. --Great Weight of this Gas. --Produced from
calcareous Stones by Sulphuric Acid. --Deleterious Effects of this
Gas when respired. --Sources which keep up a Supply of this Gas in
the Atmosphere. --Its Effects on Vegetation. --Of the Carbonats of
Lime; Marble, Chalk, Shells, Spars, and calcareous Stones.
CONVERSATION XIX.
ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS;
AND ON MURIATS. 131
On the Boracic Acid. --Its Decomposition by Sir H. Davy. --Its Basis
Boracium. --Its Recomposition. --Its Uses in the Arts. --Borax or
Borat of Soda. --Of the Fluoric Acid. --Obtained from Fluor;
corrodes Siliceous Earth; its supposed Composition. --Fluorine; its
supposed Basis. --Of the Muriatic Acid. --Obtained from Muriats.
--Its gaseous Form. --Is absorbable by Water. --Its Decomposition.
--Is susceptible of a stronger Degree of Oxygenation. --Oxygenated
Muriatic Acid. --Its gaseous Form and other Properties. --Combustion
of Bodies in this Gas. --It dissolves Gold. --Composition of Aqua
Regia. --Oxygenated Muriatic Acid destroys all Colours. --Sir H.
Davy’s Theory of the Nature of Muriatic and Oxymuriatic Acid.
--Chlorine. --Used for Bleaching and for Fumigations. --Its
offensive Smell, &c. --Muriats. --Muriat of Soda, or common Salt.
--Muriat of Ammonia. --Oxygenated Muriat of Potash. --Detonates with
Sulphur, Phosphorus, &c. --Experiment of burning Phosphorus under
Water by means of this Salt and of Sulphuric Acid.
CONVERSATION XX.
ON THE NATURE AND COMPOSITION OF VEGETABLES. 162
Of organised Bodies. --Of the Functions of Vegetables. --Of the
Elements of Vegetables. --Of the Materials of Vegetables. --Analysis
of Vegetables. --Of Sap. --Mucilage, or Gum. --Sugar. --Manna, and
Honey. --Gluten. --Vegetable Oils. --Fixed Oils, Linseed, Nut, and
Olive Oils. --Volatile Oils, forming Essences and Perfumes.
--Camphor. --Resins and Varnishes. --Pitch, Tar, Copal, Mastic, &c.
--Gum Resins. --Myrrh, Assafœtida, &c. --Caoutchouc, or Gum Elastic.
--Extractive colouring Matter; its Use in the Arts of Dyeing and
Painting. --Tannin; its Use in the Art of preparing Leather. --Woody
Fibre. --Vegetable Acids. --The Alkalies and Salts contained in
Vegetables.
CONVERSATION XXI.
ON THE DECOMPOSITION OF VEGETABLES. 202
Of Fermentation in general. --Of the Saccharine Fermentation, the
Product of which is Sugar. --Of the Vinous Fermentation, the Product
of which is Wine. --Alcohol, or Spirit of Wine. --Analysis of Wine
by Distillation. --Of Brandy, Rum, Arrack, Gin, &c. --Tartrit of
Potash, or Cream of Tartar. --Liqueurs. --Chemical Properties of
Alcohol. --Its Combustion. --Of Ether. --Of the Acetous
Fermentation, the Product of which is Vinegar. --Fermentation of
Bread. --Of the Putrid Fermentation, which reduces Vegetables to
their Elements. --Spontaneous Succession of these Fermentations.
--Of Vegetables said to be petrified. --Of Bitumens: Naphtha,
Asphaltum, Jet, Coal, Succin, or Yellow Amber. --Of Fossil Wood,
Peat, and Turf.
CONVERSATION XXII.
HISTORY OF VEGETATION. 243
Connexion between the Vegetable and Animal Kingdoms. --Of Manures.
--Of Agriculture. --Inexhaustible Sources of Materials for the
Purposes of Agriculture. --Of sowing Seed. --Germination of the
Seed. --Function of the Leaves of Plants. --Effects of Light and Air
on Vegetation. --Effects of Water on Vegetation. --Effects of
Vegetation on the Atmosphere. --Formation of Vegetable Materials by
the Organs of Plants. --Vegetable Heat. --Of the Organs of Plants.
--Of the Bark, consisting of Epidermis, Parenchyma, and Cortical
Layers. --Of Alburnum, or Wood. --Leaves, Flowers, and Seeds.
--Effects of the Season on Vegetation. --Vegetation of Evergreens in
Winter.
CONVERSATION XXIII.
ON THE COMPOSITION OF ANIMALS. 276
Elements of Animals. --Of the principal Materials of Animals, viz.
--Gelatine, Albumen, Fibrine, Mucus. --Of Animal Acids. --Of Animal
Colours, Prussian Blue, Carmine, and Ivory Black.
CONVERSATION XXIV.
ON THE ANIMAL ECONOMY. 297
Of the principal Animal Organs. --Of Bones, Teeth, Horns, Ligaments,
and Cartilage. --Of the Muscles, constituting the Organs of Motion.
--Of the Vascular System, for the Conveyance of Fluids. --Of the
Glands, for the Secretion of Fluids. --Of the Nerves, constituting
the Organs of Sensation. --Of the Cellular Substance which connects
the several Organs. --Of the Skin.
CONVERSATION XXV.
ON ANIMALISATION, NUTRITION, AND RESPIRATION. 314
Digestion. --Solvent Power of the Gastric Juice. --Formation of a
Chyle. --Its Assimilation, or Conversion into Blood. --Of
Respiration. --Mechanical Process of Respiration. --Chemical Process
of Respiration. --Of the Circulation of the Blood. --Of the
Functions of the Arteries, the Veins, and the Heart. --Of the Lungs.
--Effects of Respiration on the Blood.
CONVERSATION XXVI.
ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS. 336
Of the Analogy of Combustion and Respiration. --Animal Heat evolved
in the Lungs. --Animal Heat evolved in the Circulation. --Heat
produced by Fever. --Perspiration. --Heat produced by Exercise.
--Equal Temperature of Animals at all Seasons. --Power of the Animal
Body to resist the Effects of Heat. --Cold produced by Perspiration.
--Respiration of Fish and of Birds. --Effects of Respiration on
Muscular Strength. --Of several Animal Products, viz. Milk, Butter,
and Cheese; Spermaceti; Ambergris; Wax; Lac; Silk; Musk; Civet;
Castor. --Of the putrid Fermentation. --Conclusion.
CONVERSATIONS
ON
CHEMISTRY.
CONVERSATION I.
ON THE GENERAL PRINCIPLES OF CHEMISTRY.
MRS. B.
As you have now acquired some elementary notions of NATURAL PHILOSOPHY,
I am going to propose to you another branch of science, to which I am
particularly anxious that you should devote a share of your attention.
This is CHEMISTRY, which is so closely connected with Natural
Philosophy, that the study of the one must be incomplete without some
knowledge of the other; for, it is obvious that we can derive but a very
imperfect idea of bodies from the study of the general laws by which
they are governed, if we remain totally ignorant of their intimate
nature.
CAROLINE.
To confess the truth, Mrs. B., I am not disposed to form a very
favourable idea of chemistry, nor do I expect to derive much
entertainment from it. I prefer the sciences which exhibit nature on a
grand scale, to those that are confined to the minutiæ of petty details.
Can the studies which we have lately pursued, the general properties of
matter, or the revolutions of the heavenly bodies, be compared to the
mixing up of a few insignificant drugs? I grant, however, there may be
entertaining experiments in chemistry, and should not dislike to try
some of them: the distilling, for instance, of lavender, or rose
water . . . . . .
MRS. B.
I rather imagine, my dear Caroline, that your want of taste for
chemistry proceeds from the very limited idea you entertain of its
object. You confine the chemist’s laboratory to the narrow precincts of
the apothecary’s and perfumer’s shops, whilst it is subservient to an
immense variety of other useful purposes. Besides, my dear, chemistry is
by no means confined to works of art. Nature also has her laboratory,
which is the universe, and there she is incessantly employed in chemical
operations. You are surprised, Caroline, but I assure you that the most
wonderful and the most interesting phenomena of nature are almost all of
them produced by chemical powers. What Bergman, in the introduction to
his history of chemistry, has said of this science, will give you a more
just and enlarged idea of it. The knowledge of nature may be divided, he
observes, into three periods. The first was that in which the attention
of men was occupied in learning the external forms and characters of
objects, and this is called _Natural History_. In the second, they
considered the effects of bodies acting on each other by their
mechanical power, as their weight and motion, and this constitutes the
science of _Natural Philosophy_. The third period is that in which the
properties and mutual action of the elementary parts of bodies was
investigated. This last is the science of CHEMISTRY, and I have no doubt
you will soon agree with me in thinking it the most interesting.
You may easily conceive, therefore, that without entering into the
minute details of practical chemistry, a woman may obtain such a
knowledge of the science as will not only throw an interest on the
common occurrences of life, but will enlarge the sphere of her ideas,
and render the contemplation of nature a source of delightful
instruction.
CAROLINE.
If this is the case, I have certainly been much mistaken in the notion I
had formed of chemistry. I own that I thought it was chiefly confined to
the knowledge and preparation of medicines.
MRS. B.
That is only a branch of chemistry which is called Pharmacy; and, though
the study of it is certainly of great importance to the world at large,
it belongs exclusively to professional men, and is therefore the last
that I should advise you to pursue.
EMILY.
But, did not the chemists formerly employ themselves in search of the
philosopher’s stone, or the secret of making gold?
MRS. B.
These were a particular set of misguided philosophers, who dignified
themselves with the name of Alchemists, to distinguish their pursuits
from those of the common chemists, whose studies were confined to the
knowledge of medicines.
But, since that period, chemistry has undergone so complete a
revolution, that, from an obscure and mysterious art, it is now become a
regular and beautiful science, to which art is entirely subservient. It
is true, however, that we are indebted to the alchemists for many very
useful discoveries, which sprung from their fruitless attempts to make
gold, and which, undoubtedly, have proved of infinitely greater
advantage to mankind than all their chimerical pursuits.
The modern chemists, instead of directing their ambition to the vain
attempt of producing any of the original substances in nature, rather
aim at analysing and imitating her operations, and have sometimes
succeeded in forming combinations, or effecting decompositions, no
instances of which occur in the chemistry of Nature. They have little
reason to regret their inability to make gold, whilst, by their
innumerable inventions and discoveries, they have so greatly stimulated
industry and facilitated labour, as prodigiously to increase the
luxuries as well as the necessaries of life.
EMILY.
But, I do not understand by what means chemistry can facilitate labour;
is not that rather the province of the mechanic?
MRS. B.
There are many ways by which labour may be rendered more easy,
independently of mechanics; but even the machine, the most wonderful in
its effects, the Steam-engine, cannot be understood without the
assistance of chemistry. In agriculture, a chemical knowledge of the
nature of soils, and of vegetation, is highly useful; and, in those arts
which relate to the comforts and conveniences of life, it would be
endless to enumerate the advantages which result from the study of this
science.
CAROLINE.
But, pray, tell us more precisely in what manner the discoveries of
chemists have proved so beneficial to society?
MRS. B.
That would be an injudicious anticipation; for you would not comprehend
the nature of such discoveries and useful applications, as well as you
will do hereafter. Without a due regard to method, we cannot expect to
make any progress in chemistry. I wish to direct your observations
chiefly to the chemical operations of Nature; but those of Art are
certainly of too high importance to pass unnoticed. We shall therefore
allow them also some share of our attention.
EMILY.
Well, then, let us now set to work regularly. I am very anxious to
begin.
MRS. B.
The object of chemistry is to obtain a knowledge of the intimate nature
of bodies, and of their mutual action on each other. You find therefore,
Caroline, that this is no narrow or confined science, which comprehends
every thing material within our sphere.
CAROLINE.
On the contrary, it must be inexhaustible; and I am a loss to conceive
how any proficiency can be made in a science whose objects are so
numerous.
MRS. B.
If every individual substance were formed of different materials, the
study of chemistry would, indeed, be endless; but you must observe that
the various bodies in nature are composed of certain elementary
principles, which are not very numerous.
CAROLINE.
Yes; I know that all bodies are composed of fire, air, earth, and water;
I learnt that many years ago.
MRS. B.
But you must now endeavour to forget it. I have already informed you
what a great change chemistry has undergone since it has become a
regular science. Within these thirty years especially, it has
experienced an entire revolution, and it is now proved, that neither
fire, air, earth, nor water, can be called elementary bodies. For an
elementary body is one that has never been decomposed, that is to say,
separated into other substances; and fire, air, earth, and water, are
all of them susceptible of decomposition.
EMILY.
I thought that decomposing a body was dividing it into its minutest
parts. And if so, I do not understand why an elementary substance is not
capable of being decomposed, as well as any other.
MRS. B.
You have misconceived the idea of _decomposition_; it is very different
from mere _division_. The latter simply reduces a body into parts, but
the former separates it into the various ingredients, or materials, of
which it is composed. If we were to take a loaf of bread, and separate
the several ingredients of which it is made, the flour, the yeast, the
salt, and the water, it would be very different from cutting or
crumbling the loaf into pieces.
EMILY.
I understand you now very well. To decompose a body is to separate from
each other the various elementary substances of which it consists.
CAROLINE.
But flour, water, and other materials of bread, according to our
definition, are not elementary substances?
MRS. B.
No, my dear; I mentioned bread rather as a familiar comparison, to
illustrate the idea, than as an example.
The elementary substances of which a body is composed are called the
_constituent_ parts of that body; in decomposing it, therefore, we
separate its constituent parts. If, on the contrary, we divide a body by
chopping it to pieces, or even by grinding or pounding it to the finest
powder, each of these small particles will still consist of a portion of
the several constituent parts of the whole body: these are called the
_integrant_ parts; do you understand the difference?
EMILY.
Yes, I think, perfectly. We _decompose_ a body into its _constituent_
parts; and _divide_ it into its _integrant_ parts.
MRS. B.
Exactly so. If therefore a body consists of only one kind of substance,
though it may be divided into its integrant parts, it is not possible to
decompose it. Such bodies are therefore called _simple_ or _elementary_,
as they are the elements of which all other bodies are composed.
_Compound bodies_ are such as consist of more than one of these
elementary principles.
CAROLINE.
But do not fire, air, earth, and water, consist, each of them, but of
one kind of substance?
MRS. B.
No, my dear; they are every one of them susceptible of being separated
into various simple bodies. Instead of four, chemists now reckon upwards
of forty elementary substances. The existence of most of these is
established by the clearest experiments; but, in regard to a few of
them, particularly the most subtle agents of nature, _heat_, _light_,
and _electricity_, there is yet much uncertainty, and I can only give
you the opinion which seems most probably deduced from the latest
discoveries. After I have given you a list of the elementary bodies,
classed according to their properties, we shall proceed to examine each
of them separately, and then consider them in their combinations with
each other.
Excepting the more general agents of nature, heat, light, and
electricity, it would seem that the simple form of bodies is that of a
metal.
CAROLINE.
You astonish me! I thought the metals were only one class of minerals,
and that there were besides, earths, stones, rocks, acids, alkalies,
vapours, fluids, and the whole of the animal and vegetable kingdoms.
MRS. B.
You have made a tolerably good enumeration, though I fear not arranged
in the most scientific order. All these bodies, however, it is now
strongly believed, may be ultimately resolved into metallic substances.
Your surprise at this circumstance is not singular, as the decomposition
of some of them, which has been but lately accomplished, has excited the
wonder of the whole philosophical world.
But to return to the list of simple bodies--these being usually found in
combination with oxygen, I shall class them according to their
properties when so combined. This will, I think, facilitate their future
investigation.
EMILY.
Pray what is oxygen?
MRS. B.
A simple body; at least one that is supposed to be so, as it has never
been decomposed. It is always found united with the negative
electricity. It will be one of the first of the elementary bodies whose
properties I shall explain to you, and, as you will soon perceive, it is
one of the most important in nature; but it would be irrelevant to enter
upon this subject at present. We must now confine our attention to the
enumeration and classification of the simple bodies in general. They may
be arranged as follows:
CLASS I.
_Comprehending the imponderable agents, viz._
HEAT or CALORIC,
LIGHT,
ELECTRICITY.
CLASS II.
_Comprehending agents capable of uniting with inflammable bodies, and in
most instances of effecting their combustion._
OXYGEN,
CHLORINE,
IODINE.*
[Footnote *: It has been questioned by some eminent chemists,
whether these two last agents should not be classed among the
inflammable bodies, as they are capable of combining with oxygen,
as well as with inflammable bodies. But they seem to be more
distinctly characterised by their property of supporting
combustion than by any other quality.]
CLASS III.
_Comprehending bodies capable of uniting with oxygen, and, forming with
it various compounds. This class may be divided as follows:_
DIVISION 1.
HYDROGEN, _forming_ water.
DIVISION 2.
_Bodies forming acids._
NITROGEN, _forming_ nitric acid.
SULPHUR, _forming_ sulphuric acid.
PHOSPHORUS, _forming_ phosphoric acid.
CARBON, _forming_ carbonic acid.
BORACIUM, _forming_ boracic acid.
FLUORIUM, _forming_ fluoric acid.
MURIATIUM, _forming_ muriatic acid.
DIVISION 3.
_Metallic bodies forming alkalies._
POTASSIUM, _forming_ potash.
SODIUM, _forming_ soda.
AMMONIUM, _forming_ ammonia.
DIVISION 4.
_Metallic bodies forming earths._
CALCIUM, _or metal forming_ lime.
MAGNIUM, _forming_ magnesia.
BARIUM, _forming_ barytes.
STRONTIUM, _forming_ strontites.
SILICIUM, _forming_ silex.
ALUMIUM, _forming_ alumine.
YTTRIUM, _forming_ yttria.
GLUCIUM, _forming_ glucina.
ZIRCONIUM, _forming_ zirconi.*
[Footnote *: Of all these earths, three or four only have as yet
been distinctly decomposed.]
DIVISION 5.
_Metals, either naturally metallic, or yielding their oxygen to carbon
or to heat alone._
_Subdivision 1._
_Malleable Metals._
GOLD,
PLATINA,
PALLADIUM,
SILVER*
MERCURY†
TIN,
COPPER,
IRON,
LEAD,
NICKEL,
ZINC.
[Footnote *: These first four metals have commonly been
distinguished by the appellation of perfect or noble metals, on
account of their possessing the characteristic properties of
ductility, malleability, inalterability, and great specific
gravity, in an eminent degree.]
[Footnote †: Mercury, in its liquid state, cannot, of course,
be called a malleable metal. But when frozen, it possesses a
considerable degree of malleability.]
_Subdiv. 2._
_Brittle Metals._
ARSENIC,
BISMUTH,
ANTIMONY,
MANGANESE,
TELLURIUM,
COBALT,
TUNGSTEN,
MOLYBDENUM,
TITANIUM,
CHROME,
URANIUM,
COLUMBIUM _or_ TANTALIUM,
IRIDIUM,
OSMIUM,
RHODIUM.*
[Footnote *: These last four or five metallic bodies are placed
under this class for the sake of arrangement, though some of their
properties have not been yet fully investigated.]
CAROLINE.
Oh, what a formidable list! You will have much to do to explain it,
Mrs. B.; for I assure you it is perfectly unintelligible to me, and I
think rather perplexes than assists me.
MRS. B.
Do not let that alarm you, my dear; I hope that hereafter this
classification will appear quite clear, and, so far from perplexing you,
will assist you in arranging your ideas. It would be in vain to attempt
forming a division that would appear perfectly clear to a beginner: for
you may easily conceive that a chemical division being necessarily
founded on properties with which you are almost wholly unacquainted, it
is impossible that you should at once be able to understand its meaning
or appreciate its utility.
But, before we proceed further, it will be necessary to give you some
idea of chemical attraction, a power on which the whole science depends.
_Chemical Attraction_, or the _Attraction of Composition_, consists in
the peculiar tendency which bodies of a different nature have to unite
with each other. It is by this force that all the compositions, and
decompositions, are effected.
EMILY.
What is the difference between chemical attraction, and the attraction
of cohesion, or of aggregation, which you often mentioned to us, in
former conversations?
MRS. B.
The attraction of cohesion exists only between particles of the _same_
nature, whether simple or compound; thus it unites the particles of a
piece of metal which is a simple substance, and likewise the particles
of a loaf of bread which is a compound. The attraction of composition,
on the contrary, unites and maintains, in a state of combination,
particles of a _dissimilar_ nature; it is this power that forms each of
the compound particles of which bread consists; and it is by the
attraction of cohesion that all these particles are connected into a
single mass.
EMILY.
The attraction of cohesion, then, is the power which unites the
integrant particles of a body: the attraction of composition that which
combines the constituent particles. Is it not so?
MRS. B.
Precisely: and observe that the attraction of cohesion unites particles
of a similar nature, without changing their original properties; the
result of such an union, therefore, is a body of the same kind as the
particles of which it is formed; whilst the attraction of composition,
by combining particles of a dissimilar nature, produces compound bodies,
quite different from any of their constituents. If, for instance, I pour
on the piece of copper, contained in this glass, some of this liquid
(which is called nitric acid), for which it has a strong attraction,
every particle of the copper will combine with a particle of acid, and
together they will form a new body, totally different from either the
copper or the acid.
Do you observe the internal commotion that already begins to take place?
It is produced by the combination of these two substances; and yet the
acid has in this case to overcome not only the resistance which the
strong cohesion of the particles of copper opposes to their combination
with it, but also to overcome the weight of the copper, which makes it
sink to the bottom of the glass, and prevents the acid from having such
free access to it as it would if the metal were suspended in the liquid.
EMILY.
The acid seems, however, to overcome both these obstacles without
difficulty, and appears to be very rapidly dissolving the copper.
MRS. B.
By this means it reduces the copper into more minute parts than could
possibly be done by any mechanical power. But as the acid can act only
on the surface of the metal, it will be some time before the union of
these two bodies will be completed.
You may, however, already see how totally different this compound is
from either of its ingredients. It is neither colourless, like the acid,
nor hard, heavy, and yellow like the copper. If you tasted it, you would
no longer perceive the sourness of the acid. It has at present the
appearance of a blue liquid; but when the union is completed, and the
water with which the acid is diluted is evaporated, the compound will
assume the form of regular crystals, of a fine blue colour, and
perfectly transparent*. Of these I can shew you a specimen, as I have
prepared some for that purpose.
[Footnote *: These crystals are more easily obtained from a
mixture of sulphuric with a little nitric acid.]
CAROLINE.
How very beautiful they are, in colour, form, and transparency!
EMILY.
Nothing can be more striking than this example of chemical attraction.
MRS. B.
The term _attraction_ has been lately introduced into chemistry as a
substitute for the word _affinity_, to which some chemists have
objected, because it originated in the vague notion that chemical
combinations depended upon a certain resemblance, or relationship,
between particles that are disposed to unite; and this idea is not only
imperfect, but erroneous, as it is generally particles of the most
dissimilar nature, that have the greatest tendency to combine.
CAROLINE.
Besides, there seems to be no advantage in using a variety of terms to
express the same meaning; on the contrary it creates confusion; and as
we are well acquainted with the term Attraction in natural philosophy,
we had better adopt it in chemistry likewise.
MRS. B.
If you have a clear idea of the meaning, I shall leave you at liberty to
express it in the terms you prefer. For myself, I confess that I think
the word Attraction best suited to the general law that unites the
integrant particles of bodies; and Affinity better adapted to that which
combines the constituent particles, as it may convey an idea of the
preference which some bodies have for others, which the term _attraction
of composition_ does not so well express.
EMILY.
So I think; for though that preference may not result from any
relationship, or similitude, between the particles (as you say was once
supposed), yet, as it really exists, it ought to be expressed.
MRS. B.
Well, let it be agreed that you may use the terms _affinity_, _chemical
attraction_ and _attraction of composition_, indifferently, provided you
recollect that they have all the same meaning.
EMILY.
I do not conceive how bodies can be decomposed by chemical attraction.
That this power should be the means of composing them, is very obvious;
but that it should, at the same time, produce exactly the contrary
effect, appears to me very singular.
MRS. B.
To decompose a body is, you know, to separate its constituent parts,
which, as we have just observed, cannot be done by mechanical means.
EMILY.
No: because mechanical means separate only the integrant particles; they
act merely against the attraction of cohesion, and only divide a
compound into smaller parts.
MRS. B.
The decomposition of a body is performed by chemical powers. If you
present to a body composed of two principles, a third, which has a
greater affinity for one of them than the two first have for each other,
it will be decomposed, that is, its two principles will be separated by
means of the third body. Let us call two ingredients, of which the body
is composed, A and B. If we present to it another ingredient C, which
has a greater affinity for B than that which unites A and B, it
necessarily follows that B will quit A to combine with C. The new
ingredient, therefore, has effected a decomposition of the original body
A B; A has been left alone, and a new compound, B C, has been formed.
EMILY.
We might, I think, use the comparison of two friends, who were very
happy in each other’s society, till a third disunited them by the
preference which one of them gave to the new-comer.
MRS. B.
Very well. I shall now show you how this takes place in chemistry.
Let us suppose that we wish to decompose the compound we have just
formed by the combination of the two ingredients, copper and nitric
acid; we may do this by presenting to it a piece of iron, for which the
acid has a stronger attraction than for copper; the acid will,
consequently, quit the copper to combine with the iron, and the copper
will be what the chemists call _precipitated_, that is to say, it will
be thrown down in its separate state, and reappear in its simple form.
In order to produce this effect, I shall dip the blade of this knife
into the fluid, and, when I take it out, you will observe, that, instead
of being wetted with a bluish liquid, like that contained in the glass,
it will be covered with a thin coat of copper.
CAROLINE.
So it is really! but then is it not the copper, instead of the acid,
that has combined with the iron blade?
MRS. B.
No; you are deceived by appearances: it is the acid which combines with
the iron, and, in so doing, deposits or precipitates the copper on the
surface of the blade.
EMILY.
But, cannot three or more substances combine together, without any of
them being precipitated?
MRS. B.
That is sometimes the case; but, in general, the stronger affinity
destroys the weaker; and it seldom happens that the attraction of
several substances for each other is so equally balanced as to produce
such complicated compounds.
CAROLINE.
But, pray, Mrs. B., what is the cause of the chemical attraction of
bodies for each other? It appears to me more extraordinary or unnatural,
if I may use the expression, than the attraction of cohesion, which
unites particles of a similar nature.
MRS. B.
Chemical attraction may, like that of cohesion or gravitation, be one of
the powers inherent in matter which, in our present state of knowledge,
admits of no other satisfactory explanation than an immediate reference
to a divine cause. Sir H. Davy, however, whose important discoveries
have opened such improved views in chemistry, has suggested an
hypothesis which may throw great light upon that science. He supposes
that there are two kinds of electricity, with one or other of which all
bodies are united. These we distinguish by the names of _positive_ and
_negative_ electricity; those bodies are disposed to combine, which
possess opposite electricities, as they are brought together by the
attraction which these electricities have for each other. But, whether
this hypothesis be altogether founded on truth or not, it is impossible
to question the great influence of electricity in chemical combinations.
EMILY.
So, that we must suppose that the two electricities always attract each
other, and thus compel the bodies in which they exist to combine?
CAROLINE.
And may not this be also the cause of the attraction of cohesion?
MRS. B.
No, for in particles of the same nature the same electricities must
prevail, and it is only the different or opposite electric fluids that
attract each other.
CAROLINE.
These electricities seem to me to be a kind of chemical spirit, which
animates the particles of bodies, and draws them together.
EMILY.
If it is known, then, with which of the electricities bodies are united,
it can be inferred which will, and which will not, combine together?
MRS. B.
Certainly. --I should not omit to mention, that some doubts have been
entertained whether electricity be really a material agent, or whether
it might not be a power inherent in bodies, similar to, or, perhaps
identical with, attraction.
EMILY.
But what then would be the electric spark which is visible, and must
therefore be really material?
MRS. B.
What we call the electric spark, may, Sir H. Davy says, be merely the
heat and light, or fire produced by the chemical combinations with which
these phenomena are always connected. We will not, however, enter more
fully on this important subject at present, but reserve the principal
facts which relate to it to a future conversation.
Before we part, however, I must recommend you to fix in your memory the
names of the simple bodies, against our next interview.
CONVERSATION II.
ON LIGHT AND HEAT OR CALORIC.
CAROLINE.
We have learned by heart the names of all the simple bodies which you
have enumerated, and we are now ready to enter on the examination of
each of them successively. You will begin, I suppose, with LIGHT?
MRS. B.
Respecting the nature of light we have little more than conjectures. It
is considered by most philosophers as a real substance, immediately
emanating from the sun, and from all luminous bodies, from which it is
projected in right lines with prodigious velocity. Light, however, being
imponderable, it cannot be confined and examined by itself; and
therefore it is to the effects it produces on other bodies, rather than
to its immediate nature, that we must direct our attention.
The connection between light and heat is very obvious; indeed, it is
such, that it is extremely difficult to examine the one independently of
the other.
EMILY.
But, is it possible to separate light from heat; I thought they were
only different degrees of the same thing, fire?
MRS. B.
I told you that fire was not now considered as a simple element. Whether
light and heat be altogether different agents, or not, I cannot pretend
to decide; but, in many cases, light may be separated from heat. The
first discovery of this was made by a celebrated Swedish chemist,
Scheele. Another very striking illustration of the separation of heat
and light was long after pointed out by Dr. Herschell. This philosopher
discovered that these two agents were emitted in the rays of the sun,
and that heat was less refrangible than light; for, in separating the
different coloured rays of light by a prism (as we did some time ago),
he found that the greatest heat was beyond the spectrum, at a little
distance from the red rays, which, you may recollect, are the least
refrangible.
EMILY.
I should like to try that experiment.
MRS. B.
It is by no means an easy one: the heat of a ray of light, refracted by
a prism, is so small, that it requires a very delicate thermometer to
distinguish the difference of the degree of heat within and without the
spectrum. For in this experiment the heat is not totally separated from
the light, each coloured ray retaining a certain portion of it, though
the greatest part is not sufficiently refracted to fall within the
spectrum.
EMILY.
I suppose, then, that those coloured rays which are the least
refrangible, retain the greatest quantity of heat?
MRS. B.
They do so.
EMILY.
Though I no longer doubt that light and heat can be separated, Dr.
Herschell’s experiment does not appear to me to afford sufficient proof
that they are essentially different; for light, which you call a simple
body, may likewise be divided into the various coloured rays.
MRS. B.
No doubt there must be some difference in the various coloured rays.
Even their chemical powers are different. The blue rays, for instance,
have the greatest effect in separating oxygen from bodies, as was found
by Scheele; and there exist also, as Dr. Wollaston has shown, rays more
refrangible than the blue, which produce the same chemical effect, and,
what is very remarkable, are invisible.
EMILY.
Do you think it possible that heat may be merely a modification of
light?
MRS. B.
That is a supposition which, in the present state of natural philosophy,
can neither be positively affirmed nor denied. Let us, therefore,
instead of discussing theoretical points, be contented with examining
what is known respecting the chemical effects of light.
Light is capable of entering into a kind of transitory union with
certain substances, and this is what has been called phosphorescence.
Bodies that are possessed of this property, after being exposed to the
sun’s rays, appear luminous in the dark. The shells of fish, the bones
of land animals, marble, limestone, and a variety of combinations of
earths, are more or less powerfully phosphorescent.
CAROLINE.
I remember being much surprised last summer with the phosphorescent
appearance of some pieces of rotten wood, which had just been dug out of
the ground; they shone so bright that I at first supposed them to be
glow-worms.
EMILY.
And is not the light of a glow-worm of a phosphorescent nature?
MRS. B.
It is a very remarkable instance of phosphorescence in living animals;
this property, however, is not exclusively possessed by the glow-worm.
The insect called the lanthorn-fly, which is peculiar to warm climates,
emits light as it flies, producing in the dark a remarkably sparkling
appearance. But it is more common to see animal matter in a dead state
possessed of a phosphorescent quality; sea fish is often eminently so.
EMILY.
I have heard that the sea has sometimes had the appearance of being
illuminated, and that the light is supposed to proceed from the spawn of
fishes floating on its surface.
MRS. B.
This light is probably owing to that or some other animal matter. Sea
water has been observed to become luminous from the substance of a fresh
herring having been immersed in it; and certain insects, of the Medusa
kind, are known to produce similar effects.
But the strongest phosphorescence is produced by chemical compositions
prepared for the purpose, the most common of which consists of oyster
shells and sulphur, and is known by the name of Canton’s Phosphorus.
EMILY.
I am rather surprised, Mrs. B., that you should have said so much of the
light emitted by phosphorescent bodies without taking any notice of that
which is produced by burning bodies.
MRS. B.
The light emitted by the latter is so intimately connected with the
chemical history of combustion, that I must defer all explanation of it
till we come to the examination of that process, which is one of the
most interesting in chemical science.
Light is an agent capable of producing various chemical changes. It is
essential to the welfare both of the animal and vegetable kingdoms; for
men and plants grow pale and sickly if deprived of its salutary
influence. It is likewise remarkable for its property of destroying
colour, which renders it of great consequence in the process of
bleaching.
EMILY.
Is it not singular that light, which in studying optics we were taught
to consider as the source and origin of colours, should have also the
power of destroying them?
CAROLINE.
It is a fact, however, that we every day experience; you know how it
fades the colours of linens and silks.
EMILY.
Certainly. And I recollect that endive is made to grow white instead of
green, by being covered up so as to exclude the light. But by what means
does light produce these effects?
MRS. B.
This I cannot attempt to explain to you until you have obtained a
further knowledge of chemistry. As the chemical properties of light can
be accounted for only in their reference to compound bodies, it would be
useless to detain you any longer on this subject; we may therefore pass
on to the examination of heat, or caloric, with which we are somewhat
better acquainted.
HEAT and LIGHT may be always distinguished by the different sensations
they produce, _Light_ affects the sense of sight; _Caloric_ that of
feeling; the one produces _Vision_, the other the sensation of _Heat_.
Caloric is found to exist in a variety of forms or modifications, and I
think it will be best to consider it under the two following heads, viz.
1. FREE OR RADIANT CALORIC.
2. COMBINED CALORIC.
The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE;
it comprehends all heat which is perceptible to the senses, and affects
the thermometer.
EMILY.
You mean such as the heat of the sun, of fire, of candles, of stoves; in
short, of every thing that burns?
MRS. B.
And likewise of things that do not burn, as, for instance, the warmth of
the body; in a word, all heat that is _sensible_, whatever may be its
degree, or the source from which it is derived.
CAROLINE.
What then are the other modifications of caloric? It must be a strange
kind of heat that cannot be perceived by our senses.
MRS. B.
None of the modifications of caloric should properly be called _heat_;
for heat, strictly speaking, is the sensation produced by caloric, on
animated bodies; this word, therefore, in the accurate language of
science, should be confined to express the sensation. But custom has
adapted it likewise to inanimate matter, and we say _the heat of an
oven_, _the heat of the sun_, without any reference to the sensation
which they are capable of exciting.
It was in order to avoid the confusion which arose from thus confounding
the cause and effect, that modern chemists adopted the new word
_caloric_, to denote the principle which produces heat; yet they do not
always, in compliance with their own language, limit the word _heat_ to
the expression of the sensation, since they still frequently employ it
in reference to the other modifications of caloric which are quite
independent of sensation.
CAROLINE.
But you have not yet explained to us what these other modifications of
caloric are.
MRS. B.
Because you are not acquainted with the properties of free caloric, and
you know that we have agreed to proceed with regularity.
One of the most remarkable properties of free caloric is its power of
_dilating_ bodies. This fluid is so extremely subtle, that it enters and
pervades all bodies whatever, forces itself between their particles, and
not only separates them, but frequently drives them asunder to a
considerable distance from each other. It is thus that caloric dilates
or expands a body so as to make it occupy a greater space than it did
before.
EMILY.
The effect it has on bodies, therefore, is directly contrary to that of
the attraction of cohesion; the one draws the particles together, the
other drives them asunder.
MRS. B.
Precisely. There is a continual struggle between the attraction of
aggregation, and the expansive power of caloric; and from the action of
these two opposite forces, result all the various forms of matter, or
degrees of consistence, from the solid, to the liquid and aëriform
state. And accordingly we find that most bodies are capable of passing
from one of these forms to the other, merely in consequence of their
receiving different quantities of caloric.
CAROLINE.
That is very curious; but I think I understand the reason of it. If a
great quantity of caloric is added to a solid body, it introduces itself
between the particles in such a manner as to overcome, in a considerable
degree, the attraction of cohesion; and the body, from a solid, is then
converted into a fluid.
MRS. B.
This is the case whenever a body is fused or melted; but if you add
caloric to a liquid, can you tell me what is the consequence?
CAROLINE.
The caloric forces itself in greater abundance between the particles of
the fluid, and drives them to such a distance from each other, that
their attraction of aggregation is wholly destroyed: the liquid is then
transformed into vapour.
MRS. B.
Very well; and this is precisely the case with boiling water, when it is
converted into steam or vapour, and with all bodies that assume an
aëriform state.
EMILY.
I do not well understand the word aëriform?
MRS. B.
Any elastic fluid whatever, whether it be merely vapour or permanent
air, is called aëriform.
But each of these various states, solid, liquid, and aëriform, admit of
many different degrees of density, or consistence, still arising
(chiefly at least) from the different quantities of caloric the bodies
contain. Solids are of various degrees of density, from that of gold, to
that of a thin jelly. Liquids, from the consistence of melted glue, or
melted metals, to that of ether, which is the lightest of all liquids.
The different elastic fluids (with which you are not yet acquainted) are
susceptible of no less variety in their degrees of density.
EMILY.
But does not every individual body also admit of different degrees of
consistence, without changing its state?
MRS. B.
Undoubtedly; and this I can immediately show you by a very simple
experiment. This piece of iron now exactly fits the frame, or ring, made
to receive it; but if heated red hot, it will no longer do so, for its
dimensions will be so much increased by the caloric that has penetrated
into it, that it will be much too large for the frame.
The iron is now red hot; by applying it to the frame, we shall see how
much it is dilated.
EMILY.
Considerably so indeed! I knew that heat had this effect on bodies, but
I did not imagine that it could be made so conspicuous.
MRS. B.
By means of this instrument (called a Pyrometer) we may estimate, in the
most exact manner, the various dilatations of any solid body by heat.
The body we are now going to submit to trial is this small iron bar;
I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by
lighting the three lamps beneath it: when the bar expands, it increases
in length as well as thickness; and, as one end communicates with this
wheel-work, whilst the other end is fixed and immoveable, no sooner does
it begin to dilate than it presses against the wheel-work, and sets in
motion the index, which points out the degrees of dilatation on the
dial-plate.
[Illustration: Plate I. Vol. I. p. 38.
Fig. 1. Pyrometer.
A.A Bar of Metal.
1.2.3 Lamps burning.
B.B Wheel work.
C Index.
Fig. 2
A.A Glass tubes with bulbs.
B.B Glasses of water in which they are immersed.]
EMILY.
This is, indeed, a very curious instrument; but I do not understand the
use of the wheels: would it not be more simple, and answer the purpose
equally well, if the bar, in dilating, pressed against the index, and
put it in motion without the intervention of the wheels?
MRS. B.
The use of the wheels is merely to multiply the motion, and therefore
render the effect of the caloric more obvious; for if the index moved no
more than the bar increased in length, its motion would scarcely be
perceptible; but by means of the wheels it moves in a much greater
proportion, which therefore renders the variations far more conspicuous.
By submitting different bodies to the test of the pyrometer, it is found
that they are far from dilating in the same proportion. Different metals
expand in different degrees, and other kinds of solid bodies vary still
more in this respect. But this different susceptibility of dilatation is
still more remarkable in fluids than in solid bodies, as I shall show
you. I have here two glass tubes, terminated at one end by large bulbs.
We shall fill the bulbs, the one with spirit of wine, the other with
water. I have coloured both liquids, in order that the effect may be
more conspicuous. The spirit of wine, you see, dilates by the warmth of
my hand as I hold the bulb.
EMILY.
It certainly does, for I see it is rising into the tube. But water, it
seems, is not so easily affected by heat; for scarcely any change is
produced on it by the warmth of the hand.
MRS. B.
True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.)
and you will see both liquids rise in the tubes; but the spirit of wine
will ascend highest.
CAROLINE.
How rapidly it expands! Now it has nearly reached the top of the tube,
though the water has hardly begun to rise.
EMILY.
The water now begins to dilate. Are not these glass tubes, with liquids
rising within them, very like thermometers?
MRS. B.
A thermometer is constructed exactly on the same principle, and these
tubes require only a scale to answer the purpose of thermometers: but
they would be rather awkward in their dimensions. The tubes and bulbs of
thermometers, though of various sizes, are in general much smaller than
these; the tube too is hermetically closed, and the air excluded from
it. The fluid most generally used in thermometers is mercury, commonly
called quicksilver, the dilatations and contractions of which correspond
more exactly to the additions, and subtractions, of caloric, than those
of any other fluid.
CAROLINE.
Yet I have often seen coloured spirit of wine used in thermometers.
MRS. B.
The expansions and contractions of that liquid are not quite so uniform
as those of mercury; but in cases in which it is not requisite to
ascertain the temperature with great precision, spirit of wine will
answer the purpose equally well, and indeed in some respects better, as
the expansion of the latter is greater, and therefore more conspicuous.
This fluid is used likewise in situations and experiments in which
mercury would be frozen; for mercury becomes a solid body, like a piece
of lead or any other metal, at a certain degree of cold: but no degree
of cold has ever been known to freeze spirit of wine.
A thermometer, therefore, consists of a tube with a bulb, such as you
see here, containing a fluid whose degrees of dilatation and contraction
are indicated by a scale to which the tube is fixed. The degree which
indicates the boiling point, simply means that, when the fluid is
sufficiently dilated to rise to this point, the heat is such that water
exposed to the same temperature will boil. When, on the other hand, the
fluid is so much condensed as to sink to the freezing point, we know
that water will freeze at that temperature. The extreme points of the
scales are not the same in all thermometers, nor are the degrees always
divided in the same manner. In different countries philosophers have
chosen to adopt different scales and divisions. The two thermometers
most used are those of Fahrenheit, and of Reaumur; the first is
generally preferred by the English, the latter by the French.
EMILY.
The variety of scale must be very inconvenient, and I should think
liable to occasion confusion, when French and English experiments are
compared.
MRS. B.
The inconvenience is but very trifling, because the different gradations
of the scales do not affect the principle upon which thermometers are
constructed. When we know, for instance, that Fahrenheit’s scale is
divided into 212 degrees, in which 32° corresponds with the freezing
point, and 212° with the point of boiling water: and that Reaumur’s is
divided only into 80 degrees, in which 0° denotes the freezing point,
and 80° that of boiling water, it is easy to compare the two scales
together, and reduce the one into the other. But, for greater
convenience, thermometers are sometimes constructed with both these
scales, one on either side of the tube; so that the correspondence of
the different degrees of the two scales is thus instantly seen. Here
is one of these scales, (PLATE II. Fig. 1.) by which you can at
once perceive that each degree of Reaumur’s corresponds to 2¼ of
Fahrenheit’s division. But I believe the French have, of late, given
the preference to what they call the centigrade scale, in which the
space between the freezing and the boiling point is divided into 100
degrees.
[Illustration: Plate II. Vol. I. p. 42.
Fig. 1. Thermometer.
Fahrenheit’s Scale.
Reaumur’s Scale.
Boiling point of Water
Freezing point of Water
Fig. 2. Differential Thermometer.]
CAROLINE.
That seems to me the most reasonable division, and I cannot guess
why the freezing point is called 32°, or what advantage is derived
from it.
MRS. B.
There really is no advantage in it; and it originated in a mistaken
opinion of the instrument-maker, Fahrenheit, who first constructed these
thermometers. He mixed snow and salt together, and produced by that
means a degree of cold which he concluded was the greatest possible, and
therefore made his scale begin from that point. Between that and boiling
water he made 212 degrees, and the freezing point was found to be at
32°.
EMILY.
Are spirit of wine, and mercury, the only liquids used in the
construction of thermometers?
MRS. B.
I believe they are the only liquids now in use, though some others, such
as linseed oil, would make tolerable thermometers: but for experiments
in which a very quick and delicate test of the changes of temperature is
required, air is the fluid sometimes employed. The bulb of air
thermometers is filled with common air only, and its expansion and
contraction are indicated by a small drop of any coloured liquor, which
is suspended within the tube, and moves up and down, according as the
air within the bulb and tube expands or contracts. But in general, air
thermometers, however sensible to changes of temperature, are by no
means accurate in their indications.
I can, however, show you an air thermometer of a very peculiar
construction, which is remarkably well adapted for some chemical
experiments, as it is equally delicate and accurate in its indications.
CAROLINE.
It looks like a double thermometer reversed, the tube being bent, and
having a large bulb at each of its extremities. (PLATE II. Fig. 2.)
EMILY.
Why do you call it an air thermometer; the tube contains a coloured
liquid?
MRS. B.
But observe that the bulbs are filled with air, the liquid being
confined to a portion of the tube, and answering only the purpose of
showing, by its motion in the tube, the comparative dilatation or
contraction of the air within the bulbs, which afford an indication of
their relative temperature. Thus if you heat the bulb A, by the warmth
of your hand, the fluid will rise towards the bulb B, and the contrary
will happen if you reverse the experiment.
But if, on the contrary, both tubes are of the same temperature, as is
the case now, the coloured liquid, suffering an equal pressure on each
side, no change of level takes place.
CAROLINE.
This instrument appears, indeed, uncommonly delicate. The fluid is set
in motion by the mere approach of my hand.
MRS. B.
You must observe, however, that this thermometer cannot indicate the
temperature of any particular body, or of the medium in which it is
immersed; it serves only to point out the _difference_ of temperature
between the two bulbs, when placed under different circumstances. For
this reason it has been called _differential_ thermometer. You will see
by-and-bye to what particular purposes this instrument applies.
EMILY.
But do common thermometers indicate the exact quantity of caloric
contained either in the atmosphere, or in any body with which they are
in contact?
MRS. B.
No: first, because there are other modifications of caloric which do not
affect the thermometer; and, secondly, because the temperature of a
body, as indicated by the thermometer, is only relative. When, for
instance, the thermometer remains stationary at the freezing point, we
know that the atmosphere (or medium in which it is placed, whatever it
may be) is as cold as freezing water; and when it stands at the boiling
point, we know that this medium is as hot as boiling water; but we do
not know the positive quantity of heat contained either in freezing or
boiling water, any more than we know the real extremes of heat and cold;
and consequently we cannot determine that of the body in which the
thermometer is placed.
CAROLINE.
I do not quite understand this explanation.
MRS. B.
Let us compare a thermometer to a well, in which the water rises to
different heights, according as it is more or less supplied by the
spring which feeds it: if the depth of the well is unfathomable, it must
be impossible to know the absolute quantity of water it contains; yet we
can with the greatest accuracy measure the number of feet the water has
risen or fallen in the well at any time, and consequently know the
precise quantity of its increase or diminution, without having the least
knowledge of the whole quantity of water it contains.
CAROLINE.
Now I comprehend it very well; nothing appears to me to explain a thing
so clearly as a comparison.
EMILY.
But will thermometers bear any degree of heat?
MRS. B.
No; for if the temperature were much above the highest degree marked on
the scale of the thermometer, the mercury would burst the tube in an
attempt to ascend. And at any rate, no thermometer can be applied to
temperatures higher than the boiling point of the liquid used in its
construction, for the steam, on the liquid beginning to boil, would
burst the tube. In furnaces, or whenever any very high temperature is to
be measured, a pyrometer, invented by Wedgwood, is used for that
purpose. It is made of a certain composition of baked clay, which has
the peculiar property of contracting by heat, so that the degree of
contraction of this substance indicates the temperature to which it has
been exposed.
EMILY.
But is it possible for a body to contract by heat? I thought that heat
dilated all bodies whatever.
MRS. B.
This is not an exception to the rule. You must recollect that the bulk
of the clay is not compared, whilst hot, with that which it has when
cold; but it is from the change which the clay has undergone by _having
been_ heated that the indications of this instrument are derived. This
change consists in a beginning fusion which tends to unite the particles
of clay more closely, thus rendering it less pervious or spongy.
Clay is to be considered as a spongy body, having many interstices or
pores, from its having contained water when soft. These interstices are
by heat lessened, and would by extreme heat be entirely obliterated.
CAROLINE.
And how do you ascertain the degrees of contraction of Wedgwood’s
pyrometer?
MRS. B.
The dimensions of a piece of clay are measured by a scale graduated on
the side of a tapered groove, formed in a brass ruler; the more the clay
is contracted by the heat, the further it will descend into the narrow
part of the tube.
Before we quit the subject of expansion, I must observe to you that, as
liquids expand more readily than solids, so elastic fluids, whether air
or vapour, are the most expansible of all bodies.
It may appear extraordinary that all elastic fluids whatever, undergo
the same degree of expansion from equal augmentations of temperature.
EMILY.
I suppose, then, that all elastic fluids are of the same density?
MRS. B.
Very far from it; they vary in density, more than either liquids or
solids. The uniformity of their expansibility, which at first may appear
singular, is, however, readily accounted for. For if the different
susceptibilities of expansion of bodies arise from their various degrees
of attraction of cohesion, no such difference can be expected in elastic
fluids, since in these the attraction of cohesion does not exist, their
particles being on the contrary possessed of an elastic or repulsive
power; they will therefore all be equally expanded by equal degrees of
caloric.
EMILY.
True; as there is no power opposed to the expansive force of caloric in
elastic bodies, its effect must be the same in all of them.
MRS. B.
Let us now proceed to examine the other properties of free caloric.
Free caloric always tends to diffuse itself equally, that is to say,
when two bodies are of different temperatures, the warmer gradually
parts with its heat to the colder, till they are both brought to the
same temperature. Thus, when a thermometer is applied to a hot body, it
receives caloric; when to a cold one, it communicates part of its own
caloric, and this communication continues until the thermometer and the
body arrive at the same temperature.
EMILY.
Cold, then, is nothing but a negative quality, simply implying the
absence of heat.
MRS. B.
Not the total absence, but a diminution of heat; for we know of no body
in which some caloric may not be discovered.
CAROLINE.
But when I lay my hand on this marble table I feel it _positively_ cold,
and cannot conceive that there is any caloric in it.
MRS. B.
The cold you experience consists in the loss of caloric that your hand
sustains in an attempt to bring its temperature to an equilibrium with
the marble. If you lay a piece of ice upon it, you will find that the
contrary effect will take place; the ice will be melted by the heat
which it abstracts from the marble.
CAROLINE.
Is it not in this case the air of the room, which being warmer than the
marble, melts the ice?
MRS. B.
The air certainly acts on the surface which is exposed to it, but the
table melts that part with which it is in contact.
CAROLINE.
But why does caloric tend to an equilibrium? It cannot be on the same
principle as other fluids, since it has no weight?
MRS. B.
Very true, Caroline, that is an excellent objection. You might also,
with some propriety, object to the term _equilibrium_ being applied to a
body that is without weight; but I know of no expression that would
explain my meaning so well. You must consider it, however, in a
figurative rather than a literal sense; its strict meaning is an _equal
diffusion_. We cannot, indeed, well say by what power it diffuses itself
equally, though it is not surprising that it should go from the parts
which have the most to those which have the least. This subject is best
explained by a theory suggested by Professor Prevost of Geneva, which is
now, I believe, generally adopted.
According to this theory, caloric is composed of particles perfectly
separate from each other, every one of which moves with a rapid velocity
in a certain direction. These directions vary as much as imagination can
conceive, the result of which is, that there are rays or lines of these
particles moving with immense velocity in every possible direction.
Caloric is thus universally diffused, so that when any portion of space
happens to be in the neighbourhood of another, which contains more
caloric, the colder portion receives a quantity of calorific rays from
the latter, sufficient to restore an equilibrium of temperature. This
radiation does not only take place in free space, but extends also to
bodies of every kind. Thus you may suppose all bodies whatever
constantly radiating caloric: those that are of the same temperature
give out and absorb equal quantities, so that no variation of
temperature is produced in them; but when one body contains more free
caloric than another, the exchange is always in favour of the colder
body, until an equilibrium is effected; this you found to be the case
when the marble table cooled your hand, and again when it melted the
ice.
CAROLINE.
This reciprocal radiation surprises me extremely; I thought, from what
you first said, that the hotter bodies alone emitted rays of caloric
which were absorbed by the colder; for it seems unnatural that a hot
body should receive any caloric from a cold one, even though it should
return a greater quantity.
MRS. B.
It may at first appear so, but it is no more extraordinary than that a
candle should send forth rays of light to the sun, which, you know, must
necessarily happen.
CAROLINE.
Well, Mrs. B--, I believe that I must give up the point. But I wish I
could _see_ these rays of caloric; I should then have greater faith in
them.
MRS. B.
Will you give no credit to any sense but that of sight? You may feel the
rays of caloric which you receive from any body of a temperature higher
than your own; the loss of the caloric you part with in return, it is
true, is not perceptible; for as you gain more than you lose, instead of
suffering a diminution, you are really making an acquisition of caloric.
It is, therefore, only when you are parting with it to a body of a lower
temperature, that you are sensible of the sensation of cold, because you
then sustain an absolute loss of caloric.
EMILY.
And in this case we cannot be sensible of the small quantity of heat we
receive in exchange from the colder body, because it serves only to
diminish the loss.
MRS. B.
Very well, indeed, Emily. Professor Pictet, of Geneva, has made some
very interesting experiments, which prove not only that caloric radiates
from all bodies whatever, but that these rays may be reflected,
according to the laws of optics, in the same manner as light. I shall
repeat these experiments before you, having procured mirrors fit for the
purpose; and it will afford us an opportunity of using the differential
thermometer, which is particularly well adapted for these experiments.
--I place an iron bullet, (PLATE III. Fig. 1.) about two inches in
diameter, and heated to a degree not sufficient to render it luminous,
in the focus of this large metallic concave mirror. The rays of heat
which fall on this mirror are reflected, agreeably to the property of
concave mirrors, in a parallel direction, so as to fall on a similar
mirror, which, you see, is placed opposite to the first, at the distance
of about ten feet; thence the rays converge to the focus of the second
mirror, in which I place one of the bulbs of this thermometer. Now,
observe in what manner it is affected by the caloric which is reflected
on it from the heated bullet. --The air is dilated in the bulb which we
placed in the focus of the mirror, and the liquor rises considerably in
the opposite leg.
[Illustration: Plate III. Vol. I. p. 54
Mr. Pictet’s Apparatus for the Reflection of Heat.
Fig. 1.
A.A. & B.B Concave mirrors fixed on stands.
C Heated Bullet placed in the focus of the mirror A.
D Thermometer, with its bulb placed in the focus of the mirror B.
1.2.3.4 Rays of Caloric radiating from the bullet & falling on the
mirror A.
5.6.7.8 The same rays reflected from the mirror A to the mirror B.
9.10.11.12 The same rays reflected by the mirror B to the
Thermometer.]
EMILY.
But would not the same effect take place, if the rays of caloric from
the heated bullet fell directly on the thermometer, without the
assistance of the mirrors?
MRS. B.
The effect would in that case be so trifling, at the distance at which
the bullet and the thermometer are from each other, that it would be
almost imperceptible. The mirrors, you know, greatly increase the
effect, by collecting a large quantity of rays into a focus; place your
hand in the focus of the mirror, and you will find it much hotter there
than when you remove it nearer to the bullet.
EMILY.
That is very true; it appears extremely singular to feel the heat
diminish in approaching the body from which it proceeds.
CAROLINE.
And the mirror which produces so much heat, by converging the rays, is
itself quite cold.
MRS. B.
The same number of rays that are dispersed over the surface of the
mirror are collected by it into the focus; but, if you consider how
large a surface the mirror presents to the rays, and, consequently, how
much they are diffused in comparison to what they are at the focus,
which is little more than a point, I think you can no longer wonder that
the focus should be so much hotter than the mirror.
The principal use of the mirrors in this experiment is, to prove that
the calorific emanation is reflected in the same manner as light.
CAROLINE.
And the result, I think, is very conclusive.
MRS. B.
The experiment may be repeated with a wax taper instead of the bullet,
with a view of separating the light from the caloric. For this purpose a
transparent plate of glass must be interposed between the mirrors; for
light, you know, passes with great facility through glass, whilst the
transmission of caloric is almost wholly impeded by it. We shall find,
however, in this experiment, that some few of the calorific rays pass
through the glass together with the light, as the thermometer rises a
little; but, as soon as the glass is removed, and a free passage left to
the caloric, it will rise considerably higher.
EMILY.
This experiment, as well as that of Dr. Herschell’s, proves that light
and heat may be separated; for in the latter experiment the separation
was not perfect, any more than in that of Mr. Pictet.
CAROLINE.
I should like to repeat this experiment, with the difference of
substituting a cold body instead of the hot one, to see whether cold
would not be reflected as well as heat.
MRS. B.
That experiment was proposed to Mr. Pictet by an incredulous philosopher
like yourself, and he immediately tried it by substituting a piece of
ice in the place of the heated bullet.
CAROLINE.
Well, Mrs. B., and what was the result?
MRS. B.
That we shall see; I have procured some ice for the purpose.
EMILY.
The thermometer falls considerably!
CAROLINE.
And does not that prove that cold is not merely a _negative_ quality,
implying simply an inferior degree of heat? The cold must be _positive_,
since it is capable of reflection.
MRS. B.
So it at first appeared to Mr. Pictet; but upon a little consideration
he found that it afforded only an additional proof of the reflection of
heat: this I shall endeavour to explain to you.
According to Mr. Prevost’s theory, we suppose that all bodies whatever
radiate caloric; the thermometer used in these experiments therefore
emits calorific rays in the same manner as any other substance. When its
temperature is in equilibrium with that of the surrounding bodies, it
receives as much caloric as it parts with, and no change of temperature
is produced. But when we introduce a body of a lower temperature, such
as a piece of ice, which parts with less caloric than it receives, the
consequence is, that its temperature is raised, whilst that of the
surrounding bodies is proportionally lowered.
EMILY.
If, for instance, I was to bring a large piece of ice into this room,
the ice would in time be melted, by absorbing caloric from the general
radiation which is going on throughout the room; and as it would
contribute very little caloric in return for what is absorbed, the room
would necessarily be cooled by it.
MRS. B.
Just so; and as in consequence of the mirrors, a more considerable
exchange of rays takes place between the ice and the thermometer, than
between these and any of the surrounding bodies, the temperature of the
thermometer must be more lowered than that of any other adjacent object.
CAROLINE.
I confess I do not perfectly understand your explanation.
MRS. B.
This experiment is exactly similar to that made with the heated bullet:
for, if we consider the thermometer as the hot body (which it certainly
is in comparison to the ice), you may then easily understand that it is
by the loss of the calorific rays which the thermometer sends to the
ice, and not by any cold rays received from it, that the fall of the
mercury is occasioned: for the ice, far from emitting rays of cold,
sends forth rays of caloric, which diminish the loss sustained by the
thermometer.
Let us say, for instance, that the radiation of the thermometer towards
the ice is equal to 20, and that of the ice towards the thermometer to
10: the exchange in favour of the ice is as 20 is to 10, or the
thermometer absolutely loses 10, whilst the ice gains 10.
CAROLINE.
But if the ice actually sends rays of caloric to the thermometer, must
not the latter fall still lower when the ice is removed?
MRS. B.
No; for the space that the ice occupied, admits rays from all the
surrounding bodies to pass through it; and those being of the same
temperature as the thermometer, will not affect it, because as much heat
now returns to the thermometer as radiates from it.
CAROLINE.
I must confess that you have explained this in so satisfactory a manner,
that I cannot help being convinced now that cold has no real claim to
the rank of a positive being.
MRS. B.
Before I conclude the subject of radiation I must observe to you that
different bodies, (or rather surfaces,) possess the power of radiating
caloric in very different degrees.
Some very curious experiments have been made by Mr. Leslie on this
subject, and it was for this purpose that he invented the differential
thermometer; with its assistance he ascertained that black surfaces
radiate most, glass next, and polished surfaces the least of all.
EMILY.
Supposing these surfaces, of course, to be all of the same temperature.
MRS. B.
Undoubtedly. I will now show you the very simple and ingenious
apparatus, by means of which he made these experiments. This cubical tin
vessel or canister, has each of its sides externally covered with
different materials; the one is simply blackened; the next is covered
with white paper; the third with a pane of glass, and in the fourth the
polished tin surface remains uncovered. We shall fill this vessel with
hot water, so that there can be no doubt but that all its sides will be
of the same temperature. Now let us place it in the focus of one of the
mirrors, making each of its sides front it in succession. We shall begin
with the black surface.
CAROLINE.
It makes the thermometer which is in the focus of the other mirror rise
considerably. Let us turn the paper surface towards the mirror. The
thermometer falls a little, therefore of course this side cannot emit or
radiate so much caloric as the blackened side.
EMILY.
This is very surprising; for the sides are exactly of the same size, and
must be of the same temperature. But let us try the glass surface.
MRS. B.
The thermometer continues falling, and with the plain surface it falls
still lower; these two surfaces therefore radiate less and less.
CAROLINE.
I think I have found out the reason of this.
MRS. B.
I should be very happy to hear it, for it has not yet (to my knowledge)
been accounted for.
CAROLINE.
The water within the vessel gradually cools, and the thermometer in
consequence gradually falls.
MRS. B.
It is true that the water cools, but certainly in much less proportion
than the thermometer descends, as you will perceive if you now change
the tin surface for the black one.
CAROLINE.
I was mistaken certainly, for the thermometer rises again now that the
black surface fronts the mirror.
MRS. B.
And yet the water in the vessel is still cooling, Caroline.
EMILY.
I am surprised that the tin surface should radiate the least caloric,
for a metallic vessel filled with hot water, a silver teapot, for
instance, feels much hotter to the hand than one of black earthen ware.
MRS. B.
That is owing to the different power which various bodies possess for
_conducting_ caloric, a property which we shall presently examine. Thus,
although a metallic vessel feels warmer to the hand, a vessel of this
kind is known to preserve the heat of the liquid within, better than one
of any other materials; it is for this reason that silver teapots make
better tea than those of earthen ware.
EMILY.
According to these experiments, light-coloured dresses, in cold weather,
should keep us warmer than black clothes, since the latter radiate so
much more than the former.
MRS. B.
And that is actually the case.
EMILY.
This property, of different surfaces to radiate in different degrees,
appears to me to be at variance with the equilibrium of caloric; since
it would imply that those bodies which radiate most, must ultimately
become coldest.
Suppose that we were to vary this experiment, by using two metallic
vessels full of boiling water, the one blackened, the other not; would
not the black one cool the first?
CAROLINE.
True; but when they were both brought down to the temperature of the
room, the interchange of caloric between the canisters and the other
bodies of the room being then equal, their temperatures would remain the
same.
EMILY.
I do not see why that should be the case; for if different surfaces of
the same temperature radiate in different degrees when heated, why
should they not continue to do so when cooled down to the temperature of
the room?
MRS. B.
You have started a difficulty, Emily, which certainly requires
explanation. It is found by experiment that the power of absorption
corresponds with and is proportional to that of radiation; so that under
equal temperatures, bodies compensate for the greater loss they sustain
in consequence of their greater radiation by their greater absorption;
so that if you were to make your experiment in an atmosphere heated like
the canisters, to the temperature of boiling water, though it is true
that the canisters would radiate in different degrees, no change of
temperature would be produced in them, because they would each absorb
caloric in proportion to their respective radiation.
EMILY.
But would not the canisters of boiling water also absorb caloric in
different degrees in a room of the common temperature?
MRS. B.
Undoubtedly they would. But the various bodies in the room would not, at
a lower temperature, furnish either of the canisters with a sufficiency
of caloric to compensate for the loss they undergo; for, suppose the
black canister to absorb 400 rays of caloric, whilst the metallic one
absorbed only 200; yet if the former radiate 800, whilst the latter
radiates only 400, the black canister will be the first cooled down to
the temperature of the room. But from the moment the equilibrium of
temperature has taken place, the black canister, both receiving and
giving out 400 rays, and the metallic one 200, no change of temperature
will take place.
EMILY.
I now understand it extremely well. But what becomes of the surplus of
calorific rays, which good radiators emit and bad radiators refuse to
receive; they must wander about in search of a resting-place?
MRS. B.
They really do so; for they are rejected and sent back, or, in other
words, _reflected_ by the bodies which are bad radiators of caloric; and
they are thus transmitted to other bodies which happen to lie in their
way, by which they are either absorbed or again reflected, according as
the property of reflection, or that of absorption, predominates in these
bodies.
CAROLINE.
I do not well understand the difference between radiating and reflecting
caloric, for the caloric that is reflected from a body proceeds from it
in straight lines, and may surely be said to radiate from it?
MRS. B.
It is true that there at first appears to be a great analogy between
_radiation_ and _reflection_, as they equally convey the idea of the
transmission of caloric.
But if you consider a little, you will perceive that when a body
_radiates_ caloric, the heat which it emits not only proceeds from, but
has its origin in the body itself. Whilst when a body _reflects_
caloric, it parts with none of its own caloric, but only reflects that
which it receives from other bodies.
EMILY.
Of this difference we have very striking examples before us, in the tin
vessel of water, and the concave mirrors; the first radiates its own
heat, the latter reflect the heat which they receive from other bodies.
CAROLINE.
Now, that I understand the difference, it no longer surprises me that
bodies which radiate, or part with their own caloric freely, should not
have the power of transmitting with equal facility that which they
receive from other bodies.
EMILY.
Yet no body can be said to possess caloric of its own, if all caloric is
originally derived from the sun.
MRS. B.
When I speak of a body radiating its own caloric, I mean that which it
has absorbed and incorporated either immediately from the sun’s rays, or
through the medium of any other substance.
CAROLINE.
It seems natural enough that the power of absorption should be in
opposition to that of reflection, for the more caloric a body receives,
the less it will reject.
EMILY.
And equally so that the power of radiation should correspond with that
of absorption. It is, in fact, cause and effect; for a body cannot
radiate heat without having previously absorbed it; just as a spring
that is well fed flows abundantly.
MRS. B.
Fluids are in general very bad radiators of caloric; and air neither
radiates nor absorbs caloric in any sensible degree.
We have not yet concluded our observations on free caloric. But I shall
defer, till our next meeting, what I have further to say on this
subject. I believe it will afford us ample conversation for another
interview.
CONVERSATION III.
CONTINUATION OF THE SUBJECT.
MRS. B.
In our last conversation, we began to examine the tendency of caloric to
restore an equilibrium of temperature. This property, when once well
understood, affords the explanation of a great variety of facts which
appeared formerly unaccountable. You must observe, in the first place,
that the effect of this tendency is gradually to bring all bodies that
are in contact to the same temperature. Thus, the fire which burns in
the grate, communicates its heat from one object to another, till every
part of the room has an equal proportion of it.
EMILY.
And yet this book is not so cold as the table on which it lies, though
both are at an equal distance from the fire, and actually in contact
with each other, so that, according to your theory, they should be
exactly of the same temperature.
CAROLINE.
And the hearth, which is much nearer the fire than the carpet, is
certainly the colder of the two.
MRS. B.
If you ascertain the temperature of these several bodies by a
thermometer (which is a much more accurate test than your feeling), you
will find that it is exactly the same.
CAROLINE.
But if they are of the same temperature, why should the one feel colder
than the other?
MRS. B.
The hearth and the table feel colder than the carpet or the book,
because the latter are not such good _conductors of heat_ as the former.
Caloric finds a more easy passage through marble and wood, than through
leather and worsted; the two former will therefore absorb heat more
rapidly from your hand, and consequently give it a stronger sensation of
cold than the two latter, although they are all of them really of the
same temperature.
CAROLINE.
So, then, the sensation I feel on touching a cold body, is in proportion
to the rapidity with which my hand yields its heat to that body?
MRS. B.
Precisely; and, if you lay your hand successively on every object in the
room, you will discover which are good, and which are bad conductors of
heat, by the different degrees of cold you feel. But, in order to
ascertain this point, it is necessary that the several substances should
be of the same temperature, which will not be the case with those that
are very near the fire, or those that are exposed to a current of cold
air from a window or door.
EMILY.
But what is the reason that some bodies are better conductors of heat
than others?
MRS. B.
This is a point not well ascertained. It has been conjectured that a
certain union or adherence takes place between the caloric and the
particles of the body through which it passes. If this adherence be
strong, the body detains the heat, and parts with it slowly and
reluctantly; if slight, it propagates it freely and rapidly. The
conducting power of a body is therefore, inversely, as its tendency to
unite with caloric.
EMILY.
That is to say, that the best conductors are those that have the least
affinity for caloric.
MRS. B.
Yes; but the term affinity is objectionable in this case, because, as
that word is used to express a chemical attraction (which can be
destroyed only by decomposition), it cannot be applicable to the slight
and transient union that takes place between free caloric and the bodies
through which it passes; an union which is so weak, that it constantly
yields to the tendency which caloric has to an equilibrium. Now you
clearly understand, that the passage of caloric, through bodies that are
good conductors, is much more rapid than through those that are bad
conductors, and that the former both give and receive it more quickly,
and therefore, in a given time, more abundantly, than bad conductors,
which makes them feel either hotter or colder, though they may be, in
fact, both of the same temperature.
CAROLINE.
Yes, I understand it now; the table, and the book lying upon it, being
really of the same temperature, would each receive, in the same space of
time, the same quantity of heat from my hand, were their conducting
powers equal; but as the table is the best conductor of the two, it will
absorb the heat from my hand more rapidly, and consequently produce a
stronger sensation of cold than the book.
MRS. B.
Very well, my dear; and observe, likewise, that if you were to heat the
table and the book an equal number of degrees above the temperature of
your body, the table, which before felt the colder, would now feel the
hotter of the two; for, as in the first case it took the heat most
rapidly from your hand, so it will now impart heat most rapidly to it.
Thus the marble table, which seems to us colder than the mahogany one,
will prove the hotter of the two to the ice; for, if it takes heat more
rapidly from our hands, which are warmer, it will give out heat more
rapidly to the ice, which is colder. Do you understand the reason of
these apparently opposite effects?
EMILY.
Perfectly. A body which is a good conductor of caloric, affords it a
free passage; so that it penetrates through that body more rapidly than
through one which is a bad conductor; and consequently, if it is colder
than your hand, you lose more caloric, and if it is hotter, you gain
more than with a bad conductor of the same temperature.
MRS. B.
But you must observe that this is the case only when the conductors are
either hotter or colder than your hand; for, if you heat different
conductors to the temperature of your body, they will all feel equally
warm, since the exchange of caloric between bodies of the same
temperature is equal. Now, can you tell me why flannel clothing, which
is a very bad conductor of heat, prevents our feeling cold?
CAROLINE.
It prevents the cold from penetrating . . . . . . . .
MRS. B.
But you forget that cold is only a negative quality.
CAROLINE.
True; it only prevents the heat of our bodies from escaping so rapidly
as it would otherwise do.
MRS. B.
Now you have explained it right; the flannel rather keeps in the heat,
than keeps out the cold. Were the atmosphere of a higher temperature
than our bodies, it would be equally efficacious in keeping their
temperature at the same degree, as it would prevent the free access of
the external heat, by the difficulty with which it conducts it.
EMILY.
This, I think, is very clear. Heat, whether external or internal, cannot
easily penetrate flannel; therefore in cold weather it keeps us warm;
and if the weather was hotter than our bodies, it would keep us cool.
MRS. B.
The most dense bodies are, generally speaking, the best conductors of
heat; probably because the denser the body the greater are the number of
points or particles that come in contact with caloric. At the common
temperature of the atmosphere a piece of metal will feel much colder
than a piece of wood, and the latter than a piece of woollen cloth; this
again will feel colder than flannel; and down, which is one of the
lightest, is at the same time one of the warmest bodies.
CAROLINE.
This is, I suppose, the reason that the plumage of birds preserves them
so effectually from the influence of cold in winter?
MRS. B.
Yes; but though feathers in general are an excellent preservative
against cold, down is a kind of plumage peculiar to aquatic birds, and
covers their chest, which is the part most exposed to the water; for
though the surface of the water is not of a lower temperature than the
atmosphere, yet, as it is a better conductor of heat, it feels much
colder, consequently the chest of the bird requires a warmer covering
than any other part of its body. Besides, the breasts of aquatic birds
are exposed to cold not only from the temperature of the water, but also
from the velocity with which the breast of the bird strikes against it;
and likewise from the rapid evaporation occasioned in that part by the
air against which it strikes, after it has been moistened by dipping
from time to time into the water.
If you hold a finger of one hand motionless in a glass of water, and at
the same time move a finger of the other hand swiftly through water of
the same temperature, a different sensation will be soon perceived in
the different fingers.
Most animal substances, especially those which Providence has assigned
as a covering for animals, such as fur, wool, hair, skin, &c. are bad
conductors of heat, and are, on that account, such excellent
preservatives against the inclemency of winter, that our warmest apparel
is made of these materials.
EMILY.
Wood is, I dare say, not so good a conductor as metal, and it is for
that reason, no doubt, that silver teapots have always wooden handles.
MRS. B.
Yes; and it is the facility with which metals conduct caloric that made
you suppose that a silver pot radiated more caloric than an earthen one.
The silver pot is in fact hotter to the hand when in contact with it;
but it is because its conducting power more than counterbalances its
deficiency in regard to radiation.
We have observed that the most dense bodies are in general the best
conductors; and metals, you know, are of that class. Porous bodies, such
as the earths and wood, are worse conductors, chiefly, I believe, on
account of their pores being filled with air; for air is a remarkably
bad conductor.
CAROLINE.
It is a very fortunate circumstance that air should be a bad conductor,
as it tends to preserve the heat of the body when exposed to cold
weather.
MRS. B.
It is one of the many benevolent dispensations of Providence, in order
to soften the inclemency of the seasons, and to render almost all
climates habitable to man.
In fluids of different densities, the power of conducting heat varies no
less remarkably; if you dip your hand into this vessel full of mercury,
you will scarcely conceive that its temperature is not lower than that
of the atmosphere.
CAROLINE.
Indeed I know not how to believe it, it feels so extremely cold. --But
we may easily ascertain its true temperature by the thermometer. --It is
really not colder than the air;--the apparent difference then is
produced merely by the difference of the conducting power in mercury and
in air.
MRS. B.
Yes; hence you may judge how little the sense of feeling is to be relied
on as a test of the temperature of bodies, and how necessary a
thermometer is for that purpose.
It has indeed been doubted whether fluids have the power of conducting
caloric in the same manner as solid bodies. Count Rumford, a very few
years since, attempted to prove, by a variety of experiments, that
fluids, when at rest, were not at all endowed with this property.
CAROLINE.
How is that possible, since they are capable of imparting cold or heat
to us; for if they did not conduct heat, they would neither take it
from, nor give it to us?
MRS. B.
Count Rumford did not mean to say that fluids would not communicate
their heat to solid bodies; but only that heat does not pervade fluids,
that is to say, is not transmitted from one particle of a fluid to
another, in the same manner as in solid bodies.
EMILY.
But when you heat a vessel of water over the fire, if the particles of
water do not communicate heat to each other, how does the water become
hot throughout?
MRS. B.
By constant agitation. Water, as you have seen, expands by heat in the
same manner as solid bodies; the heated particles of water, therefore,
at the bottom of the vessel, become specifically lighter than the rest
of the liquid, and consequently ascend to the surface, where, parting
with some of their heat to the colder atmosphere, they are condensed,
and give way to a fresh succession of heated particles ascending from
the bottom, which having thrown off their heat at the surface, are in
their turn displaced. Thus every particle is successively heated at the
bottom, and cooled at the surface of the liquid; but as the fire
communicates heat more rapidly than the atmosphere cools the succession
of surfaces, the whole of the liquid in time becomes heated.
CAROLINE.
This accounts most ingeniously for the propagation of heat upwards. But
suppose you were to heat the upper surface of a liquid, the particles
being specifically lighter than those below, could not descend: how
therefore would the heat be communicated downwards?
MRS. B.
If there were no agitation to force the heated surface downwards, Count
Rumford assures us that the heat would not descend. In proof of this he
succeeded in making the upper surface of a vessel of water boil and
evaporate, while a cake of ice remained frozen at the bottom.
CAROLINE.
That is very extraordinary indeed!
MRS. B.
It appears so, because we are not accustomed to heat liquids by their
upper surface; but you will understand this theory better if I show you
the internal motion that takes place in liquids when they experience a
change of temperature. The motion of the liquid itself is indeed
invisible from the extreme minuteness of its particles; but if you mix
with it any coloured dust, or powder, of nearly the same specific
gravity as the liquid, you may judge of the internal motion of the
latter by that of the coloured dust it contains. --Do you see the small
pieces of amber moving about in the liquid contained in this phial?
CAROLINE.
Yes, perfectly.
MRS. B.
We shall now immerse the phial in a glass of hot water, and the motion
of the liquid will be shown, by that which it communicates to the amber.
EMILY.
I see two currents, the one rising along the sides of the phial, the
other descending in the centre: but I do not understand the reason of
this.
MRS. B.
The hot water communicates its caloric, through the medium of the phial,
to the particles of the fluid nearest to the glass; these dilate and
ascend laterally to the surface, where, in parting with their heat, they
are condensed, and in descending, form the central current.
CAROLINE.
This is indeed a very clear and satisfactory experiment; but how much
slower the currents now move than they did at first?
MRS. B.
It is because the circulation of particles has nearly produced an
equilibrium of temperature between the liquid in the glass and that in
the phial.
CAROLINE.
But these communicate laterally, and I thought that heat in liquids
could be propagated only upwards.
MRS. B.
You do not take notice that the heat is imparted from one liquid to the
other, through the medium of the phial itself, the external surface of
which receives the heat from the water in the glass, whilst its internal
surface transmits it to the liquid it contains. Now take the phial out
of the hot water, and observe the effect of its cooling.
EMILY.
The currents are reversed; the external current now descends, and the
internal one rises. --I guess the reason of this change:-- the phial
being in contact with cold air instead of hot water, the external
particles are cooled instead of being heated; they therefore descend and
force up the central particles, which, being warmer, are consequently
lighter.
MRS. B.
It is just so. Count Rumford hence infers that no alteration of
temperature can take place in a fluid, without an internal motion of its
particles, and as this motion is produced only by the comparative levity
of the heated particles, heat cannot be propagated downwards.
But though I believe that Count Rumford’s theory as to heat being
incapable of pervading fluids is not strictly correct, yet there is, no
doubt, much truth in his observation, that the communication is
materially promoted by a motion of the parts; and this accounts for the
cold that is found to prevail at the bottom of the lakes in Switzerland,
which are fed by rivers issuing from the snowy Alps. The water of these
rivers being colder, and therefore more dense than that of the lakes,
subsides to the bottom, where it cannot be affected by the warmer
temperature of the surface; the motion of the waves may communicate this
temperature to some little depth, but it can descend no further than the
agitation extends.
EMILY.
But when the atmosphere is colder than the lake, the colder surface of
the water will descend, for the very reason that the warmer will not.
MRS. B.
Certainly: and it is on this account that neither a lake, nor any body
of water whatever, can be frozen until every particle of the water has
risen to the surface to give off its caloric to the colder atmosphere;
therefore the deeper a body of water is, the longer will be the time it
requires to be frozen.
EMILY.
But if the temperature of the whole body of water be brought down to the
freezing point, why is only the surface frozen?
MRS. B.
The temperature of the whole body is lowered, but not to the freezing
point. The diminution of heat, as you know, produces a contraction in
the bulk of fluids, as well as of solids. This effect, however, does not
take place in water below the temperature of 40 degrees, which is 8
degrees above the freezing point. At that temperature, therefore, the
internal motion, occasioned by the increased specific gravity of the
condensed particles, ceases; for when the water at the surface no longer
condenses, it will no longer descend, and leave a fresh surface exposed
to the atmosphere: this surface alone, therefore, will be further
exposed to its severity, and will soon be brought down to the freezing
point, when it becomes ice, which being a bad conductor of heat,
preserves the water beneath a long time from being affected by the
external cold.
CAROLINE.
And the sea does not freeze, I suppose, because its depth is so great,
that a frost never lasts long enough to bring down the temperature of
such a great body of water to 40 degrees?
MRS. B.
That is one reason why the sea, as a large mass of water, does not
freeze. But, independently of this, salt water does not freeze till it
is cooled much below 32 degrees, and with respect to the law of
condensation, salt water is an exception, as it condenses even many
degrees below the freezing point. When the caloric of fresh water,
therefore, is imprisoned by the ice on its surface, the ocean still
continues throwing off heat into the atmosphere, which is a most signal
dispensation of Providence to moderate the intensity of the cold in
winter.
CAROLINE.
This theory of the non-conducting power of liquids, does not, I suppose,
hold good with respect to air, otherwise the atmosphere would not be
heated by the rays of the sun passing through it?
MRS. B.
Nor is it heated in that way. The pure atmosphere is a perfectly
transparent medium, which neither radiates, absorbs, nor conducts
caloric, but transmits the rays of the sun to us without in any way
diminishing their intensity. The air is therefore not more heated, by
the sun’s rays passing through it, than diamond, glass, water, or any
other transparent medium.
CAROLINE.
That is very extraordinary! Are glass windows not heated then by the sun
shining on them?
MRS. B.
No; not if the glass be perfectly transparent. A most convincing proof
that glass transmits the rays of the sun without being heated by them is
afforded by the burning lens, which by converging the rays to a focus
will set combustible bodies on fire, without its own temperature being
raised.
EMILY.
Yet, Mrs. B., if I hold a piece of glass near the fire it is almost
immediately warmed by it; the glass therefore must retain some of the
caloric radiated by the fire? Is it that the solar rays alone pass
freely through glass without paying tribute? It seems unaccountable that
the radiation of a common fire should have power to do what the sun’s
rays cannot accomplish.
MRS. B.
It is not because the rays from the fire have more power, but rather
because they have less, that they heat glass and other transparent
bodies. It is true, however, that as you approach the source of heat the
rays being nearer each other, the heat is more condensed, and can
produce effects of which the solar rays, from the great distance of
their source, are incapable. Thus we should find it impossible to roast
a joint of meat by the sun’s rays, though it is so easily done by
culinary heat. Yet caloric emanated from burning bodies, which is
commonly called _culinary heat_, has neither the intensity nor the
velocity of solar rays. All caloric, we have said, is supposed to
proceed originally from the sun; but after having been incorporated with
terrestrial bodies, and again given out by them, though its nature is
not essentially altered, it retains neither the intensity nor the
velocity with which it first emanated from that luminary; it has
therefore not the power of passing through transparent mediums, such as
glass and water, without being partially retained by those bodies.
EMILY.
I recollect that in the experiment on the reflection of heat, the glass
skreen which you interposed between the burning taper and the mirror,
arrested the rays of caloric, and suffered only those of light to pass
through it.
CAROLINE.
Glass windows, then, though they cannot be heated by the sun shining on
them, may be heated internally by a fire in the room? But, Mrs. B.,
since the atmosphere is not warmed by the solar rays passing through it,
how does it obtain heat; for all the fires that are burning on the
surface of the earth would contribute very little towards warming it?
EMILY.
The radiation of heat is not confined to burning bodies: for all bodies,
you know, have that property; therefore, not only every thing upon the
surface of the earth, but the earth itself, must radiate heat; and this
terrestrial caloric, not having, I suppose, sufficient power to traverse
the atmosphere, communicates heat to it.
MRS. B.
Your inference is extremely well drawn, Emily; but the foundation on
which it rests is not sound; for the fact is, that terrestrial or
culinary heat, though it cannot pass through the denser transparent
mediums, such as glass or water, without loss, traverses the atmosphere
completely: so that all the heat which the earth radiates, unless it
meet with clouds or any foreign body to intercept its passage, passes
into the distant regions of the universe.
CAROLINE.
What a pity that so much heat should be wasted!
MRS. B.
Before you are tempted to object to any law of nature, reflect whether
it may not prove to be one of the numberless dispensations of Providence
for our good. If all the heat which the earth has received from the sun,
since the creation had been accumulated in it, its temperature by this
time would, no doubt, have been more elevated than any human being could
have borne.
CAROLINE.
I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at
such a high temperature, might have scorched our feet, we should always
have had a cool refreshing air to breathe, since the radiation of the
earth does not heat the atmosphere.
EMILY.
The cool air would have afforded but very insufficient refreshment,
whilst our bodies were exposed to the burning radiation of the earth.
MRS. B.
Nor should we have breathed a cool air; for though it is true that heat
is not communicated to the atmosphere by radiation, yet the air is
warmed by contact with heated bodies, in the same manner as solids or
liquids. The stratum of air which is immediately in contact with the
earth is heated by it; it becomes specifically lighter and rises, making
way for another stratum of air which is in its turn heated and carried
upwards; and thus each successive stratum of air is warmed by coming in
contact with the earth. You may perceive this effect in a sultry day, if
you attentively observe the strata of air near the surface of the earth;
they appear in constant agitation, for though it is true the air is
itself invisible, yet the sun shining on the vapours floating in it,
render them visible, like the amber dust in the water. The temperature
of the surface of the earth is therefore the source from whence the
atmosphere derives its heat, though it is communicated neither by
radiation, nor transmitted from one particle of it to another by the
conducting power; but every particle of air must come in contact with
the earth in order to receive heat from it.
EMILY.
Wind then by agitating the air should contribute to cool the earth and
warm the atmosphere, by bringing a more rapid succession of fresh strata
of air in contact with the earth, and yet in general wind feels cooler
than still air?
MRS. B.
Because the agitation of the air carries off heat from the surface of
our bodies more rapidly than still air, by occasioning a greater number
of points of contact in a given time.
EMILY.
Since it is from the earth and not the sun that the atmosphere receives
its heat, I no longer wonder that elevated regions should be colder than
plains and valleys; it was always a subject of astonishment to me, that
in ascending a mountain and approaching the sun, the air became colder
instead of being more heated.
MRS. B.
At the distance of about a hundred million of miles, which we are from
the sun, the approach of a few thousand feet makes no sensible
difference, whilst it produces a very considerable effect with regard to
the warming the atmosphere at the surface of the earth.
CAROLINE.
Yet as the warm air rises from the earth and the cold air descends to
it, I should have supposed that heat would have accumulated in the upper
regions of the atmosphere, and that we should have felt the air warmer
as we ascended?
MRS. B.
The atmosphere, you know, diminishes in density, and consequently in
weight, as it is more distant from the earth; the warm air, therefore,
rises only till it meets with a stratum of air of its own density; and
it will not ascend into the upper regions of the atmosphere until all
the parts beneath have been previously heated. The length of summer even
in warm climates does not heat the air sufficiently to melt the snow
which has accumulated during the winter on very high mountains, although
they are almost constantly exposed to the heat of the sun’s rays, being
too much elevated to be often enveloped in clouds.
EMILY.
These explanations are very satisfactory; but allow me to ask you one
more question respecting the increased levity of heated liquids. You
said that when water was heated over the fire, the particles at the
bottom of the vessel ascended as soon as heated, in consequence of their
specific levity: why does not the same effect continue when the water
boils, and is converted into steam? and why does the steam rise from the
surface, instead of the bottom of the liquid?
MRS. B.
The steam or vapour does ascend from the bottom, though it seems to
arise from the surface of the liquid. We shall boil some water in this
Florence flask, (PLATE IV. Fig. 1.) in order that you may be well
acquainted with the process of ebullition;--you will then see, through
the glass, that the vapour rises in bubbles from the bottom. We shall
make it boil by means of a lamp, which is more convenient for this
purpose than the chimney fire.
[Illustration: Plate IV. Vol. I. p. 84.
Fig. 1. Pneumatic Pump.
Ether evaporated & water frozen in the air pump.
A Phial of Ether.
B Glass vessel containing water.
C.C Thermometers one in the Ether, the other in the water.
Fig. 2. Boiling water in a flask over a Patent lamp.]
EMILY.
I see some small bubbles ascend, and a great many appear all over the
inside of the flask; does the water begin to boil already?
MRS. B.
No; what you now see are bubbles of air, which were either dissolved in
the water, or attached to the inner surface of the flask, and which,
being rarefied by the heat, ascend in the water.
EMILY.
But the heat which rarefies the air inclosed in the water must rarefy
the water at the same time; therefore, if it could remain stationary in
the water when both were cold, I do not understand why it should not
when both are equally heated?
MRS. B.
Air being much less dense than water, is more easily rarefied; the
former, therefore, expands to a great extent, whilst the latter
continues to occupy nearly the same space; for water dilates
comparatively but very little without changing its state and becoming
vapour. Now that the water in the flask begins to boil, observe what
large bubbles rise from the bottom of it.
EMILY.
I see them perfectly; but I wonder that they have sufficient power to
force themselves through the water.
CAROLINE.
They _must_ rise, you know, from their specific levity.
MRS. B.
You are right, Caroline; but vapour has not in all liquids (when brought
to the degree of vaporization) the power of overcoming the pressure of
the less heated surface. Metals, for instance, mercury excepted,
evaporate only from the surface; therefore no vapour will ascend from
them till the degree of heat which is necessary to form it has reached
the surface; that is to say, till the whole of the liquid is brought to
a state of ebullition.
EMILY.
I have observed that steam, immediately issuing from the spout of a
teakettle, is less visible than at a further distance from it; yet it
must be more dense when it first evaporates, than when it begins to
diffuse itself in the air.
MRS. B.
When the steam is first formed, it is so perfectly dissolved by caloric,
as to be invisible. In order however to understand this, it will be
necessary for me to enter into some explanation respecting the nature of
SOLUTION. Solution takes place whenever a body is melted in a fluid. In
this operation the body is reduced to such a minute state of division by
the fluid, as to become invisible in it, and to partake of its fluidity;
but in common solutions this happens without any decomposition, the body
being only divided into its integrant particles by the fluid in which it
is melted.
CAROLINE.
It is then a mode of destroying the attraction of aggregation.
MRS. B.
Undoubtedly. --The two principal solvent fluids are _water_, and
_caloric_. You may have observed that if you melt salt in water, it
totally disappears, and the water remains clear, and transparent as
before; yet though the union of these two bodies appears so perfect, it
is not produced by any chemical combination; both the salt and the water
remain unchanged; and if you were to separate them by evaporating the
latter, you would find the salt in the same state as before.
EMILY.
I suppose that water is a solvent for solid bodies, and caloric for
liquids?
MRS. B.
Liquids of course can only be converted into vapour by caloric. But the
solvent power of this agent is not at all confined to that class of
bodies; a great variety of solid substances are dissolved by heat: thus
metals, which are insoluble in water, can be dissolved by intense heat,
being first fused or converted into a liquid, and then rarefied into an
invisible vapour. Many other bodies, such as salt, gums, &c. yield to
either of these solvents.
CAROLINE.
And that, no doubt, is the reason why hot water will melt them so much
better than cold water?
MRS. B.
It is so. Caloric may, indeed, be considered as having, in every
instance, some share in the solution of a body by water, since water,
however low its temperature may be, always contains more or less
caloric.
EMILY.
Then, perhaps, water owes its solvent power merely to the caloric
contained in it?
MRS. B.
That, probably, would be carrying the speculation too far; I should
rather think that water and caloric unite their efforts to dissolve a
body, and that the difficulty or facility of effecting this, depend both
on the degree of attraction of aggregation to be overcome, and on the
arrangement of the particles which are more or less disposed to be
divided and penetrated by the solvent.
EMILY.
But have not all liquids the same solvent power as water?
MRS. B.
The solvent power of other liquids varies according to their nature, and
that of the substances submitted to their action. Most of these
solvents, indeed, differ essentially from water, as they do not merely
separate the integrant particles of the bodies which they dissolve, but
attack their constituent principles by the power of chemical attraction,
thus producing a true decomposition. These more complicated operations
we must consider in another place, and confine our attention at present
to the solutions by water and caloric.
CAROLINE.
But there are a variety of substances which, when dissolved in water,
make it thick and muddy, and destroy its transparency.
MRS. B.
In this case it is not a solution, but simply a mixture. I shall show
you the difference between a solution and a mixture, by putting some
common salt into one glass of water, and some powder of chalk into
another; both these substances are white, but their effect on the water
will be very different.
CAROLINE.
Very different indeed! The salt entirely disappears and leaves the water
transparent, whilst the chalk changes it into an opaque liquid like
milk.
EMILY.
And would lumps of chalk and salt produce similar effects on water?
MRS. B.
Yes, but not so rapidly; salt is, indeed, soon melted though in a lump;
but chalk, which does not mix so readily with water, would require a
much greater length of time; I therefore preferred showing you the
experiment with both substances reduced to powder, which does not in any
respect alter their nature, but facilitates the operation merely by
presenting a greater quantity of surface to the water.
I must not forget to mention a very curious circumstance respecting
solutions, which is, that a fluid is not nearly so much increased in
bulk by holding a body in solution, as it would by mere mixture with the
body.
CAROLINE.
That seems impossible; for two bodies cannot exist together in the same
space.
MRS. B.
Two bodies may, by condensation, occupy less space when in union than
when separate, and this I can show you by an easy experiment.
This phial, which contains some salt, I shall fill with water, pouring
it in quickly, so as not to dissolve much of the salt; and when it is
quite full I cork it. --If I now shake the phial till the salt is
dissolved, you will observe that it is no longer full.
CAROLINE.
I shall try to add a little more salt. --But now, you see, Mrs. B., the
water runs over.
MRS. B.
Yes; but observe that the last quantity of salt you put in remains solid
at the bottom, and displaces the water; for it has already melted all
the salt it is capable of holding in solution. This is called the point
of _saturation_; and the water in this case is said to be _saturated_
with salt.
EMILY.
I think I now understand the solution of a solid body by water
perfectly: but I have not so clear an idea of the solution of a liquid
by caloric.
MRS. B.
It is probably of a similar nature; but as caloric is an invisible
fluid, its action as a solvent is not so obvious as that of water.
Caloric, we may conceive, dissolves water, and converts it into vapour
by the same process as water dissolves salt; that is to say, the
particles of water are so minutely divided by the caloric as to become
invisible. Thus, you are now enabled to understand why the vapour of
boiling water, when it first issues from the spout of a kettle, is
invisible; it is so, because it is then completely dissolved by caloric.
But the air with which it comes in contact, being much colder than the
vapour, the latter yields to it a quantity of its caloric. The particles
of vapour being thus in a great measure deprived of their solvent,
gradually collect, and become visible in the form of steam, which is
water in a state of imperfect solution; and if you were further to
deprive it of its caloric, it would return to its original liquid state.
CAROLINE.
That I understand very well. If you hold a cold plate over a tea-urn,
the steam issuing from it will be immediately converted into drops of
water by parting with its caloric to the plate; but in what state is the
steam, when it becomes invisible by being diffused in the air?
MRS. B.
It is not merely diffused, but is again dissolved by the air.
EMILY.
The air, then, has a solvent power, like water and caloric?
MRS. B.
This was formerly believed to be the case. But it appears from more
recent enquiries that the solvent power of the atmosphere depends solely
upon the caloric contained in it. Sometimes the watery vapour diffused
in the atmosphere is but imperfectly dissolved, as is the case in the
formation of clouds and fogs; but if it gets into a region sufficiently
warm, it becomes perfectly invisible.
EMILY.
Can any water dissolve in the atmosphere without its being previously
converted into vapour by boiling?
MRS. B.
Unquestionably; and this constitutes the difference between
_vaporization_ and _evaporation_. Water, when heated to the boiling
point, can no longer exist in the form of water, and must necessarily be
converted into vapour or steam, whatever may be the state and
temperature of the surrounding medium; this is called vaporization. But
the atmosphere, by means of the caloric it contains, can take up a
certain portion of water at any temperature, and hold it in a state of
solution. This is simply evaporation. Thus the atmosphere is continually
carrying off moisture from the surface of the earth, until it is
saturated with it.
CAROLINE.
That is the case, no doubt, when we feel the atmosphere damp.
MRS. B.
On the contrary, when the moisture is well dissolved it occasions no
humidity: it is only when in a state of imperfect solution and floating
in the atmosphere, in the form of watery vapour, that it produces
dampness. This happens more frequently in winter than in summer; for the
lower the temperature of the atmosphere, the less water it can dissolve;
and in reality it never contains so much moisture as in a dry hot
summer’s day.
CAROLINE.
You astonish me! But why, then, is the air so dry in frosty weather,
when its temperature is at the lowest?
EMILY.
This, I conjecture, proceeds not so much from the moisture being
dissolved, as from its being frozen; is not that the case?
MRS. B.
It is; and the freezing of the watery vapour which the atmospheric heat
could not dissolve, produces what is called a hoar frost; for the
particles descend in freezing, and attach themselves to whatever they
meet with on the surface of the earth.
The tendency of free caloric to an equilibrium, together with its
solvent power, are likewise connected with the phenomena of rain, of
dew, &c. When moist air of a certain temperature happens to pass through
a colder region of the atmosphere, it parts with a portion of its heat
to the surrounding air; the quantity of caloric, therefore, which served
to keep the water in a state of vapour, being diminished, the watery
particles approach each other, and form themselves into drops of water,
which being heavier than the atmosphere, descend to the earth. There are
also other circumstances, and particularly the variation in the weight
of the atmosphere, which may contribute to the formation of rain. This,
however, is an intricate subject, into which we cannot more fully enter
at present.
EMILY.
In what manner do you account for the formation of dew?
MRS. B.
Dew is a deposition of watery particles or minute drops from the
atmosphere, precipitated by the coolness of the evening.
CAROLINE.
This precipitation is owing, I suppose, to the cooling of the
atmosphere, which prevents its retaining so great a quantity of watery
vapour in solution as during the heat of the day.
MRS. B.
Such was, from time immemorial, the generally received opinion
respecting the cause of dew; but it has been very recently proved by a
course of ingenious experiments of Dr. Wells, that the deposition of dew
is produced by the cooling of the surface of the earth, which he has
shown to take place previously to the cooling of the atmosphere; for on
examining the temperature of a plot of grass just before the dew-fall,
he found that it was considerably colder than the air a few feet above
it, from which the dew was shortly after precipitated.
EMILY.
But why should the earth cool in the evening sooner than the atmosphere?
MRS. B.
Because it parts with its heat more readily than the air; the earth is
an excellent radiator of caloric, whilst the atmosphere does not possess
that property, at least in any sensible degree. Towards evening,
therefore, when the solar heat declines, and when after sunset it
entirely ceases, the earth rapidly cools by radiating heat towards the
skies; whilst the air has no means of parting with its heat but by
coming into contact with the cooled surface of the earth, to which it
communicates its caloric. Its solvent power being thus reduced, it is
unable to retain so large a portion of watery vapour, and deposits those
pearly drops which we call dew.
EMILY.
If this be the cause of dew, we need not be apprehensive of receiving
any injury from it; for it can be deposited only on surfaces that are
colder than the atmosphere, which is never the case with our bodies.
MRS. B.
Very true; yet I would not advise you for this reason to be too
confident of escaping all the ill effects which may arise from exposure
to the dew; for it may be deposited on your clothes, and chill you
afterwards by its evaporation from them. Besides, whenever the dew is
copious, there is a chill in the atmosphere which it is not always safe
to encounter.
CAROLINE.
Wind, then, must promote the deposition of dew, by bringing a more rapid
succession of particles of air in contact with the earth, just as it
promotes the cooling of the earth and warming of the atmosphere during
the heat of the day?
MRS. B.
Yes; provided the wind be unattended with clouds, for these
accumulations of moisture not only prevent the free radiation of the
earth towards the upper regions, but themselves radiate towards the
earth; under these circumstances much less dew is formed than on fine
clear nights, when the radiation of the earth passes without obstacle
through the atmosphere to the distant regions of space, whence it
receives no caloric in exchange. The dew continues to be deposited
during the night, and is generally most abundant towards morning, when
the contrast between the temperature of the earth and that of the air is
greatest. After sunrise the equilibrium of temperature between these two
bodies is gradually restored by the solar rays passing freely through
the atmosphere to the earth; and later in the morning the temperature of
the earth gains the ascendency, and gives out caloric to the air by
contact, in the same manner as it receives it from the air during the
night. --Can you tell me, now, why a bottle of wine taken fresh from the
cellar (in summer particularly), will soon be covered with dew; and even
the glasses into which the wine is poured will be moistened with a
similar vapour?
EMILY.
The bottle being colder than the surrounding air, must absorb caloric
from it; the moisture therefore which that air contained becomes
visible, and forms the dew which is deposited on the bottle.
MRS. B.
Very well, Emily. Now, Caroline, can you inform me why, in a warm room,
or close carriage, the contrary effect takes place; that is to say, that
the inside of the windows is covered with vapour?
CAROLINE.
I have heard that it proceeds from the breath of those within the room
or the carriage; and I suppose it is occasioned by the windows which,
being colder than the breath, deprive it of part of its caloric, and by
this means convert it into watery vapour.
MRS. B.
You have both explained it extremely well. Bodies attract dew in
proportion as they are good radiators of caloric, as it is this quality
which reduces their temperature below that of the atmosphere; hence we
find that little or no dew is deposited on rocks, sand, water; while
grass and living vegetables, to which it is so highly beneficial,
attract it in abundance--another remarkable instance of the wise and
bountiful dispensations of Providence.
EMILY.
And we may again observe it in the abundance of dew in summer, and in
hot climates, when its cooling effects are so much required; but I do
not understand what natural cause increases the dew in hot weather?
MRS. B.
The more caloric the earth receives during the day, the more it will
radiate afterwards, and consequently the more rapidly its temperature
will be reduced in the evening, in comparison to that of the atmosphere.
In the West-Indies especially, where the intense heat of the day is
strongly contrasted with the coolness of the evening, the dew is
prodigiously abundant. During a drought, the dew is less plentiful, as
the earth is not sufficiently supplied with moisture to be able to
saturate the atmosphere.
CAROLINE.
I have often observed, Mrs. B., that when I walk out in frosty weather,
with a veil over my face, my breath freezes upon it. Pray what is the
reason of that?
MRS. B.
It is because the cold air immediately seizes on the caloric of your
breath, and, by robbing it of its solvent, reduces it to a denser fluid,
which is the watery vapour that settles on your veil, and there it
continues parting with its caloric till it is brought down to the
temperature of the atmosphere, and assumes the form of ice.
You may, perhaps, have observed that the breath of animals, or rather
the moisture contained in it, is visible in damp weather, or during a
frost. In the former case, the atmosphere being over-saturated with
moisture, can dissolve no more. In the latter, the cold condenses it
into visible vapour; and for the same reason, the steam arising from
water that is warmer than the atmosphere, becomes visible. Have you
never taken notice of the vapour rising from your hands after having
dipped them into warm water?
CAROLINE.
Frequently, especially in frosty weather.
MRS. B.
We have already observed that pressure is an obstacle to evaporation:
there are liquids that contain so great a quantity of caloric, and whose
particles consequently adhere so slightly together, that they may be
rapidly converted into vapour without any elevation of temperature,
merely by taking off the weight of the atmosphere. In such liquids, you
perceive, it is the pressure of the atmosphere alone that connects their
particles, and keeps them in a liquid state.
CAROLINE.
I do not well understand why the particles of such fluids should be
disunited and converted into vapour, without any elevation of
temperature, in spite of the attraction of cohesion.
MRS. B.
It is because the degree of heat at which we usually observe these
fluids is sufficient to overcome their attraction of cohesion. Ether is
of this description; it will boil and be converted into vapour, at the
common temperature of the air, if the pressure of the atmosphere be
taken off.
EMILY.
I thought that ether would evaporate without either the pressure of the
atmosphere being taken away, or heat applied; and that it was for that
reason so necessary to keep it carefully corked up?
MRS. B.
It is true it will evaporate, but without ebullition; what I am now
speaking of is the vaporization of ether, or its conversion into vapour
by boiling. I am going to show you how suddenly the ether in this phial
will be converted into vapour, by means of the air-pump. --Observe with
what rapidity the bubbles ascend, as I take off the pressure of the
atmosphere.
CAROLINE.
It positively boils: how singular to see a liquid boil without heat!
MRS. B.
Now I shall place the phial of ether in this glass, which it nearly
fits, so as to leave only a small space, which I fill with water; and in
this state I put it again under the receiver. (PLATE IV. Fig. 1.)* You
will observe, as I exhaust the air from it, that whilst the ether boils,
the water freezes.
[Footnote *: Two pieces of thin glass tubes, sealed at one end,
might answer this purpose better. The experiment, however, as here
described, is difficult, and requires a very nice apparatus. But
if, instead of phials or tubes, two watch-glasses be used, water
may be frozen almost instantly in the same manner. The two glasses
are placed over one another, with a few drops of water interposed
between them, and the uppermost glass is filled with ether. After
working the pump for a minute or two, the glasses are found to
adhere strongly together, and a thin layer of ice is seen between
them.]
CAROLINE.
It is indeed wonderful to see water freeze in contact with a boiling
fluid!
EMILY.
I am at a loss to conceive how the ether can pass to the state of vapour
without an addition of caloric. Does it not contain more caloric in a
state of vapour, than in a state of liquidity?
MRS. B.
It certainly does; for though it is the pressure of the atmosphere which
condenses it into a liquid, it is by forcing out the caloric that
belongs to it when in an aëriform state.
EMILY.
You have, therefore, two difficulties to explain, Mrs. B. --First, from
whence the ether obtains the caloric necessary to convert it into vapour
when it is relieved from the pressure of the atmosphere; and, secondly,
what is the reason that the water, in which the bottle of ether stands,
is frozen?
CAROLINE.
Now, I think, I can answer both these questions. The ether obtains the
addition of caloric required, from the water in the glass; and the loss
of caloric, which the latter sustains, is the occasion of its freezing.
MRS. B.
You are perfectly right; and if you look at the thermometer which I have
placed in the water, whilst I am working the pump, you will see that
every time bubbles of vapour are produced, the mercury descends; which
proves that the heat of the water diminishes in proportion as the ether
boils.
EMILY.
This I understand now very well; but if the water freezes in consequence
of yielding its caloric to the ether, the equilibrium of heat must, in
this case, be totally destroyed. Yet you have told us, that the exchange
of caloric between two bodies of equal temperature, was always equal;
how, then, is it that the water, which was originally of the same
temperature as the ether, gives out caloric to it, till the water is
frozen, and the ether made to boil?
MRS. B.
I suspected that you would make these objections; and, in order to
remove them, I enclosed two thermometers in the air-pump; one which
stands in the glass of water, the other in the phial of ether; and you
may see that the equilibrium of temperature is not destroyed; for as the
thermometer descends in the water, that in the ether sinks in the same
manner; so that both thermometers indicate the same temperature, though
one of them is in a boiling, the other in a freezing liquid.
EMILY.
The ether, then, becomes colder as it boils? This is so contrary to
common experience, that I confess it astonishes me exceedingly.
CAROLINE.
It is, indeed, a most extraordinary circumstance. But pray, how do you
account for it?
MRS. B.
I cannot satisfy your curiosity at present; for before we can attempt to
explain this apparent paradox, it is necessary to become acquainted with
the subject of LATENT HEAT: and that, I think, we must defer till our
next interview.
CAROLINE.
I believe, Mrs. B., that you are glad to put off the explanation; for it
must be a very difficult point to account for.
MRS. B.
I hope, however, that I shall do it to your complete satisfaction.
EMILY.
But before we part, give me leave to ask you one question. Would not
water, as well as ether, boil with less heat, if deprived of the
pressure of the atmosphere?
MRS. B.
Undoubtedly. You must always recollect that there are two forces to
overcome, in order to make a liquid boil or evaporate; the attraction of
aggregation, and the weight of the atmosphere. On the summit of a high
mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat
is required to make water boil, than in the plain, where the weight of
the atmosphere is greater.* Indeed if the weight of the atmosphere be
entirely removed by means of a good air-pump, and if water be placed in
the exhausted receiver, it will evaporate so fast, however cold it
maybe, as to give it the appearance of boiling from the surface. But
without the assistance of the air-pump, I can show you a very pretty
experiment, which proves the effect of the pressure of the atmosphere in
this respect.
Observe, that this Florence flask is about half full of water, and the
upper half of invisible vapour, the water being in the act of boiling.
--I take it from the lamp, and cork it carefully--the water, you see,
immediately ceases boiling. --I shall now dip the flask into a bason of
cold water.†
[Footnote *: On the top of Mount Blanc, water boiled when heated
only to 187 degrees, instead of 212 degrees.]
[Footnote †: The same effect may be produced by wrapping a cold
wet linen cloth round the upper part of the flask. In order to
show how much the water cools whilst it is boiling, a thermometer,
graduated on the tube itself, may be introduced into the bottle
through the cork.]
CAROLINE.
But look, Mrs. B., the hot water begins to boil again, although the cold
water must rob it more and more of its caloric! What can be the reason
of that?
MRS. B.
Let us examine its temperature. You see the thermometer immersed in it
remains stationary at 180 degrees, which is about 30 degrees below the
boiling point. When I took the flask from the lamp, I observed to you
that the upper part of it was filled with vapour; this being compelled
to yield its caloric to the cold water, was again condensed into water--
What, then, filled the upper part of the flask?
EMILY.
Nothing; for it was too well corked for the air to gain admittance, and
therefore the upper part of the flask must be a vacuum.
MRS. B.
The water below, therefore, no longer sustains the pressure of the
atmosphere, and will consequently boil at a much lower temperature.
Thus, you see, though it had lost many degrees of heat, it began boiling
again the instant the vacuum was formed above it. The boiling has now
ceased, the temperature of the water being still farther reduced; if it
had been ether, instead of water, it would have continued boiling much
longer, for ether boils, under the usual atmospheric pressure, at a
temperature as low as 100 degrees; and in a vacuum it boils at almost
any temperature; but water being a more dense fluid, requires a more
considerable quantity of caloric to make it evaporate quickly, even when
the pressure of the atmosphere is removed.
EMILY.
What proportion of vapour can the atmosphere contain in a state of
solution?
MRS. B.
I do not know whether it has been exactly ascertained by experiment; but
at any rate this proportion must vary, both according to the temperature
and the weight of the atmosphere; for the lower the temperature, and the
greater the pressure, the smaller must be the proportion of vapour that
the atmosphere can contain.
To conclude the subject of free caloric, I should mention _Ignition_, by
which is meant that emission of light which is produced in bodies at a
very high temperature, and which is the effect of accumulated caloric.
EMILY.
You mean, I suppose, that light which is produced by a burning body?
MRS. B.
No: ignition is quite independent of combustion. Clay, chalk, and indeed
all incombustible substances, may be made red hot. When a body burns,
the light emitted is the effect of a chemical change which takes place,
whilst ignition is the effect of caloric alone, and no other change than
that of temperature is produced in the ignited body.
All solid bodies, and most liquids, are susceptible of ignition, or, in
other words, of being heated so as to become luminous; and it is
remarkable that this takes place pretty nearly at the same temperature
in all bodies, that is, at about 800 degrees of Fahrenheit’s scale.
EMILY.
But how can liquids attain so high a temperature, without being
converted into vapour?
MRS. B.
By means of confinement and pressure. Water confined in a strong iron
vessel (called Papin’s digester) can have its temperature raised to
upwards of 400 degrees. Sir James Hall has made some very curious
experiments on the effects of heat assisted by pressure; by means of
strong gun-barrels, he succeeded in melting a variety of substances
which were considered as infusible: and it is not unlikely that, by
similar methods, water itself might be heated to redness.
EMILY.
I am surprised at that: for I thought that the force of steam was such
as to destroy almost all mechanical resistance.
MRS. B.
The expansive force of steam is prodigious; but in order to subject
water to such high temperatures, it is prevented by confinement from
being converted into steam, and the expansion of heated water is
comparatively trifling. --But we have dwelt so long on the subject of
free caloric, that we must reserve the other modifications of that agent
to our next meeting, when we shall endeavour to proceed more rapidly.
CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT.
MRS. B.
We are now to examine the other modifications of caloric.
CAROLINE.
I am very curious to know of what nature they can be; for I have no
notion of any kind of heat that is not perceptible to the senses.
MRS. B.
In order to enable you to understand them, it will be necessary to enter
into some previous explanations.
It has been discovered by modern chemists, that bodies of a different
nature, heated to the same temperature, do not contain the same quantity
of caloric.
CAROLINE.
How could that be ascertained? Have you not told us that it is
impossible to discover the absolute quantity of caloric which bodies
contain?
MRS. B.
True; but at the same time I said that we were enabled to form a
judgment of the proportions which bodies bore to each other in this
respect. Thus it is found that, in order to raise the temperature of
different bodies the same number of degrees, different quantities of
caloric are required for each of them. If, for instance, you place a
pound of lead, a pound of chalk, and a pound of milk, in a hot oven,
they will be gradually heated to the temperature of the oven; but the
lead will attain it first, the chalk next, and the milk last.
CAROLINE.
That is a natural consequence of their different bulks; the lead being
the smallest body, will be heated soonest, and the milk, which is the
largest, will require the longest time.
MRS. B.
That explanation will not do, for if the lead be the least in bulk, it
offers also the least surface to the caloric, the quantity of heat
therefore which can enter into it in the same space of time is
proportionally smaller.
EMILY.
Why, then, do not the three bodies attain the temperature of the oven at
the same time?
MRS. B.
It is supposed to be on account of the different capacity of these
bodies for caloric.
CAROLINE.
What do you mean by the capacity of a body for caloric?
MRS. B.
I mean a certain disposition of bodies to require more or less caloric
for raising their temperature to any degree of heat. Perhaps the fact
may be thus explained:
Let us put as many marbles into this glass as it will contain, and pour
some sand over them--observe how the sand penetrates and lodges between
them. We shall now fill another glass with pebbles of various forms--you
see that they arrange themselves in a more compact manner than the
marbles, which, being globular, can touch each other by a single point
only. The pebbles, therefore, will not admit so much sand between them;
and consequently one of these glasses will necessarily contain more sand
than the other, though both of them be equally full.
CAROLINE.
This I understand perfectly. The marbles and the pebbles represent two
bodies of different kinds, and the sand the caloric contained in them;
it appears very plain, from this comparison, that one body may admit of
more caloric between its particles than another.
MRS. B.
You can no longer be surprised, therefore, that bodies of a different
capacity for caloric should require different proportions of that fluid
to raise their temperatures equally.
EMILY.
But I do not conceive why the body that contains the most caloric should
not be of the highest temperature; that is to say, feel hot in
proportion to the quantity of caloric it contains?
MRS. B.
The caloric that is employed in filling the capacity of a body, is not
free caloric; but is imprisoned as it were in the body, and is therefore
imperceptible: for we can feel only the caloric which the body parts
with, and not that which it retains.
CAROLINE.
It appears to me very extraordinary that heat should be confined in a
body in such a manner as to be imperceptible.
MRS. B.
If you lay your hand on a hot body, you feel only the caloric which
leaves it, and enters your hand; for it is impossible that you should be
sensible of that which remains in the body. The thermometer, in the same
manner, is affected only by the free caloric which a body transmits to
it, and not at all by that which it does not part with.
CAROLINE.
I begin to understand it: but I confess that the idea of insensible heat
is so new and strange to me, that it requires some time to render it
familiar.
MRS. B.
Call it insensible caloric, and the difficulty will appear much less
formidable. It is indeed a sort of contradiction to call it heat, when
it is so situated as to be incapable of producing that sensation. Yet
this modification of caloric is commonly called SPECIFIC HEAT.
CAROLINE.
But it certainly would have been more correct to have called it
_specific caloric_.
EMILY.
I do not understand how the term _specific_ applies to this modification
of caloric?
MRS. B.
It expresses the relative quantity of caloric which different _species_
of bodies of the same weight and temperature are capable of containing.
This modification is also frequently called _heat of capacity_, a term
perhaps preferable, as it explains better its own meaning.
You now understand, I suppose, why the milk and chalk required a longer
portion of time than the lead to raise their temperature to that of the
oven?
EMILY.
Yes: the milk and chalk having a greater capacity for caloric than the
lead, a greater proportion of that fluid became insensible in those
bodies: and the more slowly, therefore, their temperature was raised.
CAROLINE.
But might not this difference proceed from the different conducting
powers of heat in these three bodies, since that which is the best
conductor must necessarily attain the temperature of the oven first?
MRS. B.
Very well observed, Caroline. This objection would be insurmountable, if
we could not, by reversing the experiment, prove that the milk, the
chalk, and the lead, actually absorbed different quantities of caloric,
and we know that if the different time they took in heating, proceeded
merely from their different conducting powers, they would each have
acquired an equal quantity of caloric.
CAROLINE.
Certainly. But how can you reverse this experiment?
MRS. B.
It may be done by cooling the several bodies to the same degree in an
apparatus adapted to receive and measure the caloric which they give
out. Thus, if you plunge them into three equal quantities of water, each
at the same temperature, you will be able to judge of the relative
quantity of caloric which the three bodies contained, by that, which, in
cooling, they communicated to their respective portions of water: for
the same quantity of caloric which they each absorbed to raise their
temperature, will abandon them in lowering it; and on examining the
three vessels of water, you will find the one in which you immersed the
lead to be the least heated; that which held the chalk will be the next;
and that which contained the milk will be heated the most of all. The
celebrated Lavoisier has invented a machine to estimate, upon this
principle, the specific heat of bodies in a more perfect manner; but I
cannot explain it to you, till you are acquainted with the next
modification of caloric.
EMILY.
The more dense a body is, I suppose, the less is its capacity for
caloric?
MRS. B.
This is not always the case with bodies of different nature; iron, for
instance, contains more specific heat than tin, though it is more dense.
This seems to show that specific heat does hot merely depend upon the
interstices between the particles; but, probably, also upon some
peculiar constitution of the bodies which we do not comprehend.
EMILY.
But, Mrs. B., it would appear to me more proper to compare bodies by
_measure_, rather than by _weight_, in order to estimate their specific
heat. Why, for instance, should we not compare _pints_ of milk, of
chalk, and of lead, rather than _pounds_ of those substances; for equal
weights may be composed of very different quantities?
MRS. B.
You are mistaken, my dear; equal weight must contain equal quantities of
matter; and when we wish to know what is the relative quantity of
caloric, which substances of various kinds are capable of containing
under the same temperature, we must compare equal weights, and not equal
bulks of those substances. Bodies of the same weight may undoubtedly be
of very different dimensions; but that does not change their real
quantity of matter. A pound of feathers does not contain one atom more
than a pound of lead.
CAROLINE.
I have another difficulty to propose. It appears to me, that if the
temperature of the three bodies in the oven did not rise equally, they
would never reach the same degree; the lead would always keep its
advantage over the chalk and milk, and would perhaps be boiling before
the others had attained the temperature of the oven. I think you might
as well say that, in the course of time, you and I should be of the same
age?
MRS. B.
Your comparison is not correct, Caroline. As soon as the lead reached
the temperature of the oven, it would remain stationary; for it would
then give out as much heat as it would receive. You should recollect
that the exchange of radiating heat, between two bodies of equal
temperature, is equal: it would be impossible, therefore, for the lead
to accumulate heat after having attained the temperature of the oven;
and that of the chalk and milk therefore would ultimately arrive at the
same standard. Now I fear that this will not hold good with respect to
our ages, and that, as long as I live, I shall never cease to keep my
advantage over you.
EMILY.
I think that I have found a comparison for specific heat, which is very
applicable. Suppose that two men of equal weight and bulk, but who
required different quantities of food to satisfy their appetites, sit
down to dinner, both equally hungry; the one would consume a much
greater quantity of provisions than the other, in order to be equally
satisfied.
MRS. B.
Yes, that is very fair; for the quantity of food necessary to satisfy
their respective appetites, varies in the same manner as the quantity of
caloric requisite to raise equally the temperature of different bodies.
EMILY.
The thermometer, then, affords no indication of the specific heat of
bodies?
MRS. B.
None at all: no more than satiety is a test of the quantity of food
eaten. The thermometer, as I have repeatedly said, can be affected only
by free caloric, which alone raises the temperature of bodies.
But there is another mode of proving the existence of specific heat,
which affords a very satisfactory illustration of that modification.
This, however, I did not enlarge upon before, as I thought it might
appear to you rather complicated. --If you mix two fluids of different
temperatures, let us say the one at 50 degrees, and the other at 100
degrees, of what temperature do you suppose the mixture will be?
CAROLINE.
It will be no doubt the medium between the two, that is to say, 75
degrees.
MRS. B.
That will be the case if the two bodies happen to have the same capacity
for caloric; but if not, a different result will be obtained. Thus, for
instance, if you mix together a pound of mercury, heated at 50 degrees,
and a pound of water heated at 100 degrees, the temperature of the
mixture, instead of being 75 degrees, will be 80 degrees; so that the
water will have lost only 12 degrees, whilst the mercury will have
gained 38 degrees; from which you will conclude that the capacity of
mercury for heat is less than that of water.
CAROLINE.
I wonder that mercury should have so little specific heat. Did we not
see it was a much better conductor of heat than water?
MRS. B.
And it is precisely on that account that its specific heat is less. For
since the conductive power of bodies depends, as we have observed
before, on their readiness to receive heat and part with it, it is
natural to expect that those bodies which are the worst conductors
should absorb the most caloric before they are disposed to part with it
to other bodies. But let us now proceed to LATENT HEAT.
CAROLINE.
And pray what kind of heat is that?
MRS. B.
It is another modification of combined caloric, which is so analogous to
specific heat, that most chemists make no distinction between them; but
Mr. Pictet, in his Essay on Fire, has so clearly discriminated them,
that I am induced to adopt his view of the subject. We therefore call
_latent heat_ that portion of insensible caloric which is employed in
changing the state of bodies; that is to say, in converting solids into
liquids, or liquids; into vapour. When a body changes its state from
solid to liquid, or from liquid to vapour, its expansion occasions a
sudden and considerable increase of capacity for heat, in consequence of
which it immediately absorbs a quantity of caloric, which becomes fixed
in the body which it has transformed; and, as it is perfectly concealed
from our senses, it has obtained the name of _latent_ heat.
CAROLINE.
I think it would be much more correct to call this modification latent
caloric instead of latent heat, since it does not excite the sensation
of heat.
MRS. B.
This modification of heat was discovered and named by Dr. Black long
before the French chemists introduced the term caloric, and we must not
presume to alter it, as it is still used by much better chemists than
ourselves. And, besides, you are not to suppose that the nature of heat
is altered by being variously modified: for if latent heat and specific
heat do not excite the same sensations as free caloric, it is owing to
their being in a state of confinement, which prevents them from acting
upon our organs; and consequently, as soon as they are extricated from
the body in which they are imprisoned, they return to their state of
free caloric.
EMILY.
But I do not yet clearly see in what respect latent heat differs from
specific heat; for they are both of them imprisoned and concealed in
bodies.
MRS. B.
Specific heat is that which is employed in filling the capacity of a
body for caloric, in the state in which this body actually exists; while
latent heat is that which is employed only in effecting a change of
state, that is, in converting bodies from a solid to a liquid, or from a
liquid to an aëriform state. But I think that, in a general point of
view, both these modifications might be comprehended under the name of
_heat of capacity_, as in both cases the caloric is equally engaged in
filling the capacities of bodies.
I shall now show you an experiment, which I hope will give you a clear
idea of what is understood by latent heat.
The snow which you see in this phial has been cooled by certain chemical
means (which I cannot well explain to you at present), to 5 or 6 degrees
below the freezing point, as you will find indicated by the thermometer
which is placed in it. We shall expose it to the heat of a lamp, and you
will see the thermometer gradually rise, till it reaches the freezing
point----
EMILY.
But there it stops, Mrs. B., and yet the lamp burns just as well as
before. Why is not its heat communicated to the thermometer?
CAROLINE.
And the snow begins to melt, therefore it must be rising above the
freezing point?
MRS. B.
The heat no longer affects the thermometer, because it is wholly
employed in converting the ice into water. As the ice melts, the caloric
becomes _latent_ in the new-formed liquid, and therefore cannot raise
its temperature; and the thermometer will consequently remain
stationary, till the whole of the ice be melted.
CAROLINE.
Now it is all melted, and the thermometer begins to rise again.
MRS. B.
Because the conversion of the ice into water being completed, the
caloric no longer becomes latent; and therefore the heat which the water
now receives raises its temperature, as you find the thermometer
indicates.
EMILY.
But I do not think that the thermometer rises so quickly in the water as
it did in the ice, previous to its beginning to melt, though the lamp
burns equally well?
MRS. B.
That is owing to the different specific heat of ice and water. The
capacity of water for caloric being greater than that of ice, more heat
is required to raise its temperature, and therefore the thermometer
rises slower in the water than in the ice.
EMILY.
True; you said that a solid body always increased its capacity for heat
by becoming fluid; and this is an instance of it.
MRS. B.
Yes, and the latent heat is that which is absorbed in consequence of the
greater capacity which the water has for heat, in comparison to ice.
I must now tell you a curious calculation founded on that consideration.
I have before observed to you that though the thermometer shows us the
comparative warmth of bodies, and enables us to determine the same point
at different times and places, it gives us no idea of the absolute
quantity of heat in any body. We cannot tell how low it ought to fall by
the privation of all heat, but an attempt has been made to infer it in
the following manner. It has been found by experiment, that the capacity
of water for heat, when compared with that of ice, is as 10 to 9, so
that, at the same temperature, ice contains one tenth of caloric less
than water. By experiment also it is observed, that in order to melt
ice, there must be added to it as much heat, as would, if it did not
melt it, raise its temperature 140 degrees. This quantity of heat is
therefore absorbed when the ice, by being converted into water, is made
to contain one-ninth more caloric than it did before. Therefore 140
degrees is a ninth part of the heat contained in ice at 30 degrees; and
the point of zero, or the absolute privation of heat, must consequently
be 1260 degrees below 32 degrees.
This mode of investigating so curious a question is ingenious, but its
correctness is not yet established by similar calculations for other
bodies. The points of absolute cold, indicated by this method in various
bodies, are very remote from each other; it is however possible, that
this may arise from some imperfection in the experiments.
CAROLINE.
It is indeed very ingenious--but we must now attend to our present
experiment. The water begins to boil, and the thermometer is again
stationary.
MRS. B.
Well, Caroline, it is your turn to explain the phenomenon.
CAROLINE.
It is wonderfully curious! The caloric is now busy in changing the water
into steam, in which it hides itself, and becomes insensible. This is
another example of latent heat, producing a change of form. At first it
converted a solid body into a liquid, and now it turns the liquid into
vapour!
MRS. B.
You see, my dear, how easily you have become acquainted with these
modifications of insensible heat, which at first appeared so
unintelligible. If, now, we were to reverse these changes, and condense
the vapour into water, and the water into ice, the latent heat would
re-appear entirely, in the form of free caloric.
EMILY.
Pray do let us see the effect of latent heat returning to its free
state.
MRS. B.
For the purpose of showing this, we need simply conduct the vapour
through this tube into this vessel of cold water, where it will part
with its latent heat and return to its liquid form.
EMILY.
How rapidly the steam heats the water!
MRS. B.
That is because it does not merely impart its free caloric to the water,
but likewise its latent heat. This method of heating liquids, has been
turned to advantage, in several economical establishments. The
steam-kitchens, which are getting into such general use, are upon the
same principle. The steam is conveyed through a pipe in a similar
manner, into the several vessels which contain the provisions to be
dressed, where it communicates to them its latent caloric, and returns
to the state of water. Count Rumford makes great use of this principle
in many of his fire-places: his grand maxim is to avoid all unnecessary
waste of caloric, for which purpose he confines the heat in such a
manner, that not a particle of it shall unnecessarily escape; and while
he economises the free caloric, he takes care also to turn the latent
heat to advantage. It is thus that he is enabled to produce a degree of
heat superior to that which is obtained in common fire-places, though he
employs less fuel.
EMILY.
When the advantages of such contrivances are so clear and plain,
I cannot understand why they are not universally used.
MRS. B.
A long time is always required before innovations, however useful, can
be reconciled with the prejudices of the vulgar.
EMILY.
What a pity it is that there should be a prejudice against new
inventions; how much more rapidly the world would improve, if such
useful discoveries were immediately and universally adopted!
MRS. B.
I believe, my dear, that there are as many novelties attempted to be
introduced, the adoption of which would be prejudicial to society, as
there are of those which would be beneficial to it. The well-informed,
though by no means exempt from error, have an unquestionable advantage
over the illiterate, in judging what is likely or not to prove
serviceable; and therefore we find the former more ready to adopt such
discoveries as promise to be really advantageous, than the latter, who
having no other test of the value of a novelty but time and experience,
at first oppose its introduction. The well-informed, however, are
frequently disappointed in their most sanguine expectations, and the
prejudices of the vulgar, though they often retard the progress of
knowledge, yet sometimes, it must be admitted, prevent the propagation
of error. --But we are deviating from our subject.
We have converted steam into water, and are now to change water into
ice, in order to render the latent heat sensible, as it escapes from the
water on its becoming solid. For this purpose we must produce a degree
of cold that will make water freeze.
CAROLINE.
That must be very difficult to accomplish in this warm room.
MRS. B.
Not so much as you think. There are certain chemical mixtures which
produce a rapid change from the solid to the fluid state, or the
reverse, in the substances combined, in consequence of which change
latent heat is either extricated or absorbed.
EMILY.
I do not quite understand you.
MRS. B.
This snow and salt, which you see me mix together, are melting rapidly;
heat, therefore, must be absorbed by the mixture, and cold produced.
CAROLINE.
It feels even colder than ice, and yet the snow is melted. This is very
extraordinary.
MRS. B.
The cause of the intense cold of the mixture is to be attributed to the
change from a solid to a fluid state. The union of the snow and salt
produces a new arrangement of their particles, in consequence of which
they become liquid; and the quantity of caloric, required to effect this
change, is seized upon by the mixture wherever it can be obtained. This
eagerness of the mixture for caloric, during its liquefaction, is such,
that it converts part of its own free caloric into latent heat, and it
is thus that its temperature is lowered.
EMILY.
Whatever you put in this mixture, therefore, would freeze?
MRS. B.
Yes; at least any fluid that is susceptible of freezing at that
temperature. I have prepared this mixture of salt and snow for the
purpose of freezing the water from which you are desirous of seeing the
latent heat escape. I have put a thermometer in the glass of water that
is to be frozen, in order that you may see how it cools.
CAROLINE.
The thermometer descends, but the heat which the water is now losing, is
its _free_, not its _latent_ heat.
MRS. B.
Certainly; it does not part with its latent heat till it changes its
state and is converted into ice.
EMILY.
But here is a very extraordinary circumstance! The thermometer is fallen
below the freezing point, and yet the water is not frozen.
MRS. B.
That is always the case previous to the freezing of water when it is in
a state of rest. Now it begins to congeal, and you may observe that the
thermometer again rises to the freezing point.
CAROLINE.
It appears to me very strange that the thermometer should rise the very
moment that the water freezes; for it seems to imply that the water was
colder before it froze than when in the act of freezing.
MRS. B.
It is so; and after our long dissertation on this circumstance, I did
not think it would appear so surprising to you. Reflect a little, and I
think you will discover the reason of it.
CAROLINE.
It must be, no doubt, the extrications of latent heat, at the instant
the water freezes, that raises the temperature.
MRS. B.
Certainly; and if you now examine the thermometer, you will find that
its rise was but temporary, and lasted only during the disengagement of
the latent heat--now that all the water is frozen it falls again, and
will continue to fall till the ice and mixture are of an equal
temperature.
EMILY.
And can you show us any experiments in which liquids, by being mixed,
become solid, and disengage latent heat?
MRS. B.
I could show you several; but you are not yet sufficiently advanced to
understand them well. I shall, however, try one, which will afford you a
striking instance of the fact. The fluid which you see in this phial
consists of a quantity of a certain salt called _muriat of lime_,
dissolved in water. Now, if I pour into it a few drops of this other
fluid, called _sulphuric acid_, the whole, or very nearly the whole,
will be instantaneously converted into a solid mass.
EMILY.
How white it turns! I feel the latent heat escaping, for the bottle is
warm, and the fluid is changed to a solid white substance like chalk!
CAROLINE.
This is, indeed, the most curious experiment we have seen yet. But pray
what is that white vapour that ascends from the mixture?
MRS. B.
You are not yet enough of a chemist to understand that. --But take care,
Caroline, do not approach too near it, for it has a very pungent smell.
I shall show you another instance similar to that of the water, which
you observed to become warmer as it froze. I have in this phial a
solution of a salt called sulphat of soda or Glauber’s salt, made very
strong, and corked up when it was hot, and kept without agitation till
it became cold, as you may feel the phial is. Now when I take out the
cork and let the air fall upon it, (for being closed when boiling, there
was a vacuum in the upper part) observe that the salt will suddenly
crystallize. . . .
CAROLINE.
Surprising! how beautifully the needles of salt have shot through the
whole phial!
MRS. B.
Yes, it is very striking--but pray do not forget the object of the
experiment. Feel how warm the phial has become by the conversion of part
of the liquid into a solid.
EMILY.
Quite warm I declare! this is a most curious experiment of the
disengagement of latent heat.
MRS. B.
The slakeing of lime is another remarkable instance of the extrication
of latent heat. Have you never observed how quick-lime smokes when water
is poured upon it, and how much heat it produces?
CAROLINE.
Yes; but I do not understand what change of state takes place in the
lime that occasions its giving out latent heat; for the quick-lime,
which is solid, is (if I recollect right) reduced to powder, by this
operation, and is, therefore, rather expanded than condensed.
MRS. B.
It is from the water, not the lime, that the latent heat is set free.
The water incorporates with, and becomes solid in the lime; in
consequence of which, the heat, which kept it in a liquid state, is
disengaged, and escapes in a sensible form.
CAROLINE.
I always thought that the heat originated in the lime. It seems very
strange that water, and cold water too, should contain so much heat.
EMILY.
After this extrication of caloric, the water must exist in a state of
ice in the lime, since it parts with the heat which kept it liquid.
MRS. B.
It cannot properly be called ice, since ice implies a degree of cold, at
least equal to the freezing point. Yet as water, in combining with lime,
gives out more heat than in freezing, it must be in a state of still
greater solidity in the lime, than it is in the form of ice; and you may
have observed that it does not moisten or liquefy the lime in the
smallest degree.
EMILY.
But, Mrs. B., the smoke that rises is white; if it was only pure caloric
which escaped, we might feel, but could not see it.
MRS. B.
This white vapour is formed by some of the particles of lime, in a state
of fine dust, which are carried off by the caloric.
EMILY.
In all changes of state, then, a body either absorbs or disengages
latent heat?
MRS. B.
You cannot exactly say _absorbs latent heat_, as the heat becomes latent
only on being confined in the body; but you may say, generally, that
bodies, in passing from a solid to a liquid form, or from the liquid
state to that of vapour, absorb heat; and that when the reverse takes
place, heat is disengaged.*
[Footnote *: This rule, if not universal, admits of very few
exceptions.]
EMILY.
We can now, I think, account for the ether boiling, and the water
freezing in vacuo, at the same temperature.†
[Footnote †: See page 102.]
MRS. B.
Let me hear how you explain it.
EMILY.
The latent heat, which the water gave out in freezing, was immediately
absorbed by the ether, during its conversion into vapour; and therefore,
from a latent state in one liquid, it passed into a latent state in the
other.
MRS. B.
But this only partly accounts for the result of the experiment; it
remains to be explained why the temperature of the ether, while in a
state of ebullition, is brought down to the freezing temperature of the
water. --It is because the ether, during its evaporation, reduces its
own temperature, in the same proportion as that of the water, by
converting its free caloric into latent heat: so that, though one liquid
boils, and the other freezes, their temperatures remain in a state of
equilibrium.
EMILY.
But why does not water, as well as ether, reduce its own temperature by
evaporating?
MRS. B.
The fact is that it does, though much less rapidly than ether. Thus, for
instance, you may often have observed, in the heat of summer, how much
any particular spot may be cooled by watering, though the water used for
that purpose be as warm as the air itself. Indeed so much cold may be
produced by the mere evaporation of water, that the inhabitants of
India, by availing themselves of the most favourable circumstances for
this process which their warm climate can afford, namely, the cool of
the night, and situations most exposed to the night breeze, succeed in
causing water to freeze, though the temperature of the air be as high as
60 degrees. The water is put into shallow earthen trays, so as to expose
an extensive surface to the process of evaporation, and in the morning,
the water is found covered with a thin cake of ice, which is collected
in sufficient quantity to be used for purposes of luxury.
CAROLINE.
How delicious it must be to drink liquids so cold in those tropical
climates! But, Mrs. B., could we not try that experiment?
MRS. B.
If we were in the country, I have no doubt but that we should be able to
freeze water, by the same means, and under similar circumstances. But we
can do it immediately, upon a small scale, in this very room, in which
the thermometer stands at 70 degrees. For this purpose we need only
place some water in a little cup under the receiver of the air-pump
(PLATE V. fig. 1.), and exhaust the air from it. What will be the
consequence, Caroline?
[Illustration: Plate V. Vol. I. page 138.
Fig. 1.
The air-pump & receiver for Mr. Leslie’s experiment.
C a saucer with sulphuric Acid.
B a glass or earthen cup containing Water.
D a stand for the cup with its legs made of Glass.
A a Thermometer.
Fig. 2. Dr. Wollaston’s Cryophorus.
Fig. 5. Dr. Marcet’s mode of using the Cryophorus.
Fig. 3. & 4. the different parts of Fig. 5. seen separate.]
CAROLINE.
Of course the water will evaporate more quickly, since there will no
longer be any atmospheric pressure on its surface: but will this be
sufficient to make the water freeze?
MRS. B.
Probably not, because the vapour will not be carried off fast enough;
but this will be accomplished without difficulty if we introduce into
the receiver (fig. 1.), in a saucer, or other large shallow vessel, some
strong sulphuric acid, a substance which has a great attraction for
water, whether in the form of vapour, or in the liquid state. This
attraction is such that the acid will instantly absorb the moisture as
it rises from the water, so as to make room for the formation of fresh
vapour; this will of course hasten the process, and the cold produced
from the rapid evaporation of the water, will, in a few minutes, be
sufficient to freeze its surface.* We shall now exhaust the air from the
receiver.
[Footnote *: This experiment was first devised by Mr. Leslie, and
has since been modified in a variety of forms.]
EMILY.
Thousands of small bubbles already rise through the water from the
internal surface of the cup; what is the reason of this?
MRS. B.
These are bubbles of air which were partly attached to the vessel, and
partly diffused in the water itself; and they expand and rise in
consequence of the atmospheric pressure being removed.
CAROLINE.
See, Mrs. B.; the thermometer in the cup is sinking fast; it has already
descended to 40 degrees!
EMILY.
The water seems now and then violently agitated on the surface, as if it
was boiling; and yet the thermometer is descending fast!
MRS. B.
You may call it _boiling_, if you please, for this appearance is, as
well as boiling, owing to the rapid formation of vapour; but here, as
you have just observed, it takes place from the surface, for it is only
when heat is applied to the bottom of the vessel that the vapour is
formed there. --Now crystals of ice are actually shooting all over the
surface of the water.
CAROLINE.
How beautiful it is! The surface is now entirely frozen--but the
thermometer remains at 32 degrees.
MRS. B.
And so it will, conformably with our doctrine of latent heat, until the
whole of the water is frozen; but it will then again begin to descend
lower and lower, in consequence of the evaporation which goes on from
the surface of the ice.
EMILY.
This is a most interesting experiment; but it would be still more
striking if no sulphuric acid were required.
MRS. B.
I will show you a freezing instrument, contrived by Dr. Wollaston, upon
the same principle as Mr. Leslie’s experiment, by which water may be
frozen by its own evaporation alone, without the assistance of sulphuric
acid.
This tube, which, as you see (PLATE V. fig. 2.), is terminated at each
extremity by a bulb, one of which is half full of water, is internally
perfectly exhausted of air; the consequence of this is, that the water
in the bulb is always much disposed to evaporate. This evaporation,
however, does not proceed sufficiently fast to freeze the water; but if
the empty ball be cooled by some artificial means, so as to condense
quickly the vapour which rises from the water, the process may be thus
so much promoted as to cause the water to freeze in the other ball. Dr.
Wollaston has called this instrument _Cryophorus_.
CAROLINE.
So that cold seems to perform here the same part which the sulphuric
acid acted in Mr. Leslie’s experiment?
MRS. B.
Exactly so; but let us try the experiment.
EMILY.
How will you cool the instrument? You have neither ice nor snow.
MRS. B.
True: but we have other means of effecting this.* You recollect what an
intense cold can be produced by the evaporation of ether in an exhausted
receiver. We shall inclose the bulb in this little bag of fine flannel
(fig. 3.), then soke it in ether, and introduce it into the receiver of
the air-pump. (Fig. 5.) For this purpose we shall find it more
convenient to use a cryophorus of this shape (fig. 4.), as its elongated
bulb passes easily through a brass plate which closes the top of the
receiver. If we now exhaust the receiver quickly, you will see, in less
than a minute, the water freeze in the other bulb, out of the receiver.
[Footnote *: This mode of making the experiment was proposed, and
the particulars detailed, by Dr. Marcet, in the 34th vol. of
Nicholson’s Journal, page 119.]
EMILY.
The bulb already looks quite dim, and small drops of water are
condensing on its surface.
CAROLINE.
And now crystals of ice shoot all over the water. This is, indeed,
a very curious experiment!
MRS. B.
You will see, some other day, that, by a similar method, even
quicksilver may be frozen. --But we cannot at present indulge in any
further digression.
Having advanced so far on the subject of heat, I may now give you an
account of the calorimeter, an instrument invented by Lavoisier, upon
the principles just explained, for the purpose of estimating the
specific heat of bodies. It consists of a vessel, the inner surface of
which is lined with ice, so as to form a sort of hollow globe of ice, in
the midst of which the body, whose specific heat is to be ascertained,
is placed. The ice absorbs caloric from this body, till it has brought
it down to the freezing point; this caloric converts into water a
certain portion of the ice which runs out through an aperture at the
bottom of the machine; and the quantity of ice changed to water is a
test of the quantity of caloric which the body has given out in
descending from a certain temperature to the freezing point.
CAROLINE.
In this apparatus, I suppose, the milk, chalk, and lead, would melt
different quantities of ice, in proportion to their different capacities
for caloric?
MRS. B.
Certainly: and thence we are able to ascertain, with precision, their
respective capacities for heat. But the calorimeter affords us no more
idea of the absolute quantity of heat contained in a body, than the
thermometer; for though by means of it we extricate both the free and
combined caloric, yet we extricate them only to a certain degree, which
is the freezing point; and we know not how much they contain of either
below that point.
EMILY.
According to the theory of latent heat, it appears to me that the
weather should be warm when it freezes, and cold in a thaw: for latent
heat is liberated from every substance that it freezes, and such a large
supply of heat must warm the atmosphere; whilst, during a thaw, that
very quantity of free heat must be taken from the atmosphere, and return
to a latent state in the bodies which it thaws.
MRS. B.
Your observation is very natural; but consider that in a frost the
atmosphere is so much colder than the earth, that all the caloric which
it takes from the freezing bodies is insufficient to raise its
temperature above the freezing point; otherwise the frost must cease.
But if the quantity of latent heat extricated does not destroy the
frost, it serves to moderate the suddenness of the change of temperature
of the atmosphere, at the commencement both of frost, and of a thaw. In
the first instance, its extrication diminishes the severity of the cold;
and, in the latter, its absorption moderates the warmth occasioned by a
thaw: it even sometimes produces a discernible chill, at the breaking up
of a frost.
CAROLINE.
But what are the general causes that produce those sudden changes in the
weather, especially from hot to cold, which we often experience?
MRS. B.
This question would lead us into meteorological discussions, to which I
am by no means competent. One circumstance, however, we can easily
understand. When the air has passed over cold countries, it will
probably arrive here at a temperature much below our own, and then it
must absorb heat from every object it meets with, which will produce a
general fall of temperature.
CAROLINE.
But pray, now that we know so much of the effects of heat, will you
inform us whether it is really a distinct body, or, as I have heard,
a peculiar kind of motion produced in bodies?
MRS. B.
As I before told you, there is yet much uncertainty as to the nature of
these subtle agents. But I am inclined to consider heat not as mere
motion, but as a separate substance. Late experiments too appear to make
it a compound body, consisting of the two electricities, and in our next
conversation I shall inform you of the principal facts on which that
opinion is founded.
CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY.
MRS. B.
Before we proceed further it will be necessary to give you some account
of certain properties of electricity, which have of late years been
discovered to have an essential connection with the phenomena of
chemistry.
CAROLINE.
It is ELECTRICITY, if I recollect right, which comes next in our list of
simple substances?
MRS. B.
I have placed electricity in that list, rather from the necessity of
classing it somewhere, than from any conviction that it has a right to
that situation, for we are as yet so ignorant of its intimate nature,
that we are unable to determine, not only whether it is simple or
compound, but whether it is in fact a material agent; or, as Sir H. Davy
has hinted, whether it may not be merely a property inherent in matter.
As, however, it is necessary to adopt some hypothesis for the
explanation of the discoveries which this agent has enabled us to make,
I have chosen the opinion, at present most prevalent, which supposes the
existence of two kinds of electricity, distinguished by the names of
_positive_ and _negative_ electricity.
CAROLINE.
Well, I must confess, I do not feel nearly so interested in a science in
which so much uncertainty prevails, as in those which rest upon
established principles; I never was fond of electricity, because,
however beautiful and curious the phenomena it exhibits may be, the
theories, by which they were explained, appeared to me so various, so
obscure and inadequate, that I always remained dissatisfied. I was in
hopes that the new discoveries in electricity had thrown so great a
light on the subject, that every thing respecting it would now have been
clearly explained.
MRS. B.
That is a point which we are yet far from having attained. But, in spite
of the imperfection of our theories, you will be amply repaid by the
importance and novelty of the subject. The number of new facts which
have already been ascertained, and the immense prospect of discovery
which has lately been opened to us, will, I hope, ultimately lead to a
perfect elucidation of this branch of natural science; but at present
you must be contented with studying the effects, and in some degree
explaining the phenomena, without aspiring to a precise knowledge of the
remote cause of electricity.
You have already obtained some notions of electricity: in our present
conversation, therefore,
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