The Mechanism of Life
by Stéphane Leduc · English · Gutenberg #33862 · 337,235 chars · public domain (US)
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Title: The Mechanism of Life
Author: Stéphane Leduc
Translator: William Deane Butcher
Release date: October 15, 2010 [eBook #33862]
Language: English
Other information and formats: www.gutenberg.org/ebooks/33862
Credits: Produced by David Garcia, James Nugen, Keith Edkins and
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are listed at the end of the text.
* * * * *
THE MECHANISM OF LIFE
[Illustration: Osmotic Productions. [_Frontispiece_]
THE
MECHANISM OF LIFE
BY
DR. STEPHANE LEDUC
PROFESSEUR A L'ECOLE DE MEDECINE DE NANTES
TRANSLATED BY
W. DEANE BUTCHER
FORMERLY PRESIDENT OF THE ROENTGEN SOCIETY, AND OF THE
ELECTRO-THERAPEUTICAL SECTION OF THE ROYAL SOCIETY OF MEDICINE
"La nature a forme, et forme tous
les jours les etres les plus simples par
generation spontanee." LAMARCK.
[Illustration]
NEW YORK
REBMAN COMPANY
HERALD SQUARE BUILDING
141-145, WEST 36TH STREET
_First Impression March 1911_
_Second Impression January 1914_
_Printed in England_
* * * * *
{vii}
TRANSLATOR'S PREFACE
Professor Leduc's _Theorie Physico-chimique de la Vie et Generations
Spontanees_ has excited a good deal of attention, and not a little
opposition, on the Continent. As recently as 1907 the Academie des Sciences
excluded from its _Comptes Rendus_ the report of these experimental
researches on diffusion and osmosis, because it touched too closely on the
burning question of spontaneous generation.
As the author points out, Lamarck's early evolutionary hypothesis was
killed by opposition and neglect, and had to be reborn in England before it
obtained universal acceptance as the Darwinian Theory. Not unnaturally,
therefore, he turns for an appreciation of his work to the free air and
wide horizon of the English-speaking countries.
He has entitled his book "The Mechanism of Life," since however little we
may know of the origin of life, we may yet hope to get a glimpse of the
machinery, and perhaps even hear the whirr of the wheels in Nature's
workshop. The subject is of entrancing interest to the biologist and the
physician, quite apart from its bearing on the question of spontaneous
generation. Whatever view may be entertained by the different schools of
thought as to the nature and significance of life, all alike will welcome
this new and important contribution to our knowledge of the mechanism by
which Nature constructs the bewildering variety of her forms.
There is, I think, no more wonderful and illuminating spectacle than that
of an osmotic growth,--a crude lump of brute inanimate matter germinating
before our very eyes, putting forth bud and stem and root and branch and
leaf and fruit, with no stimulus from germ or seed, without even {viii} the
presence of organic matter. For these mineral growths are not mere
crystallizations as many suppose; they increase by intussusception and not
by accretion. They exhibit the phenomena of circulation and respiration,
and a crude sort of reproduction by budding; they have a period of vigorous
youthful growth, of old age, of death and of decay. They imitate the forms,
the colour, the texture, and even the microscopical structure of organic
growth so closely as to deceive the very elect. When we find, moreover,
that the processes of nutrition are carried on in these osmotic productions
just as in living beings, that an injury to an osmotic growth is repaired
by the coagulation of its internal sap, and that it is able to perform
periodic movements just as an animal or a plant, we are at a loss to define
any line of separation between these mineral forms and those of organic
life.
In the present volume the author has collected all the data necessary for a
complete survey of the mechanism of life, which consists essentially of
those phenomena which are exhibited at the contact of solutions of
different degrees of concentration. Whatever may be the verdict as to the
author's case for spontaneous generation, all will agree that the book is a
most brilliant and stimulating study, founded on the personal investigation
of a born experimenter.
The present volume is a translation of Dr. Leduc's French edition, but it
is more than this, the work has been translated, revised and corrected, and
in many places re-written, by the author's own hand. I am responsible only
for the English form of the treatise, and can but regret that I have been
able to reproduce so imperfectly the charm of the original.
W. DEANE BUTCHER.
EALING.
* * * * *
{ix}
PREFACE TO THE ENGLISH EDITION
C'est par l'initiative du Dr. Deane Butcher que cette ouvrage est presente
aux lecteurs anglais, a la race qui a dote l'humanite de tant de
decouvertes originales, geniales et d'une portee tres generale.
Comme un etre vivant, une idee exige pour naitre et se developper le germe
et le milieu de developpement. Il est indeniable que le peuple
anglo-americain constitue un milieu particulierement favorable a la
naissance et au developpement des idees nouvelles.
Pendant notre collaboration le Dr. Deane Butcher a ete un critique
judicieux et eclaire, tous les changements dans l'edition anglaise sont dus
a ses observations. Il s'est assimile l'ouvrage pour le traduire, et dans
beaucoup de parties, il a mis plus de clarte et de concision qu'il n'y en
avait dans le texte original.
STEPHANE LEDUC.
NANTES, 1911.
* * * * *
{xi}
TABLE OF CONTENTS
PAGE
TRANSLATOR'S PREFACE vii
AUTHOR'S PREFACE ix
INTRODUCTION xiii
I. LIFE AND LIVING BEINGS 1
II. SOLUTIONS 14
III. ELECTROLYTIC SOLUTIONS 24
IV. COLLOIDS 36
V. DIFFUSION AND OSMOSIS 43
VI. PERIODICITY 67
VII. COHESION AND CRYSTALLIZATION 78
VIII. KARYOKINESIS 89
IX. ENERGETICS 97
X. SYNTHETIC BIOLOGY 113
XI. OSMOTIC GROWTH: A STUDY IN MORPHOGENESIS 123
XII. THE PHENOMENA OF LIFE AND OSMOTIC PRODUCTIONS:
A STUDY IN PHYSIOGENESIS 147
XIII. EVOLUTION AND SPONTANEOUS GENERATION 160
* * * * *
{xiii}
INTRODUCTION
Life was formerly regarded as a phenomenon entirely separated from the
other phenomena of Nature, and even up to the present time Science has
proved wholly unable to give a definition of Life; evolution, nutrition,
sensibility, growth, organization, none of these, not even the faculty of
reproduction, is the exclusive appanage of life.
Living things are made of the same chemical elements as minerals; a living
being is the arena of the same physical forces as those which affect the
inorganic world.
Life is difficult to define because it differs from one living being to
another; the life of a man is not that of a polyp or of a plant, and if we
find it impossible to discover the line which separates life from the other
phenomena of Nature, it is in fact because no such line of demarcation
exists--the passage from animate to inanimate is gradual and insensible.
The step between a stalagmite and a polyp is less than that between a polyp
and a man, and even the trained biologist is often at a loss to determine
whether a given borderland form is the result of life, or of the inanimate
forces of the mineral world.
A living being is a transformer of matter and energy--both matter and
energy being uncreateable and indestructible, i.e. invariable in quantity.
A living being is only a current of matter and of energy, both of which
change from moment to moment while passing through the organism.
That which constitutes a living being is its form; for a living thing is
born, develops, and dies with the form and structure of its organism. This
ephemeral nature of the living being, which perishes with the destruction
of its form, is in {xiv} marked contrast to the perennial character of the
matter and the energy which circulate within it.
The elementary phenomenon of life is the contact between an alimentary
liquid and a cell. For the essential phenomenon of life is nutrition, and
in order to be assimilated all the elements of an organism must be brought
into a state of solution. Hence the study of life may be best begun by the
study of those physico-chemical phenomena which result from the contact of
two different liquids. Biology is thus but a branch of the
physico-chemistry of liquids; it includes the study of electrolytic and
colloidal solutions, and of the molecular forces brought into play by
solution, osmosis, diffusion, cohesion, and crystallization.
In this volume I have endeavoured to give as much of the science of
energetics as can be treated without the use of mathematical formulae; the
conception of entropy and Carnot's law of thermodynamics are also
discussed.
The phenomena of catalysis and of diastatic fermentation have for the first
time been brought under the general laws of energetics. This I have done by
showing that catalysis is only one instance of the general law of the
transformation of potential into kinetic energy, viz. by the intervention
of a foreign exciting and stimulating energy which may be infinitely
smaller than the energy it transforms. This conception brings life into
line with other catalytic actions, and shows us a living being as a store
of potential energy, to be set free by an external stimulus which may also
excite sensation.
In a subsequent chapter I have dealt with the rise of Synthetic Biology,
whose history and methods I have described. It is only of late that the
progress of physico-chemical science has enabled us to enter into this
field of research, the final one in the evolution of biological science.
The present work contains some of the earliest results of this synthetic
biology. We shall see how it is possible by the mere diffusion of liquids
to obtain forms which imitate with the greatest accuracy not only the
ordinary cellular tissues, but the more complicated striated structures,
such as muscle and mother-of-pearl. We shall also see how it is {xv}
possible by simple liquid diffusion to reproduce in ordered and regular
succession complicated movements like those observed in the karyokinesis of
the living cell.
The essential character of the living being is its Form. This is the only
characteristic which it retains during the whole of its existence, with
which it is born, which causes its development, and disappears with its
death. The task of synthetic biology is the recognition of those
physico-chemical forces and conditions which can produce forms and
structures analogous to those of living beings. This is the subject of the
chapter on Morphogenesis.
The last chapter deals with the doctrine of Evolution. The chain of life is
of necessity a continuous one, from the mineral at one end to the most
complicated organism at the other. We cannot allow that it is broken at any
point, or that there is a link missing between animate and inanimate
nature. Hence the theory of evolution necessarily admits the
physico-chemical nature of life and the fact of spontaneous generation.
Only thus can the evolutionary theory become a rational one, a stimulating
and fertile inspirer of research. We seek for the physico-chemical forces
which produce forms and structures analogous to those of living beings, and
phenomena analogous to those of life. We study the alterations in
environment which modify these forms, and we seek in the past history of
our planet for those natural phenomena which have brought these
physico-chemical forces into play. In this way we may find the road which
will, we hope, lead some day to the discovery of the origin and the
evolution of life upon the earth.
* * * * *
{1}
THE MECHANISM OF LIFE
CHAPTER I
LIFE AND LIVING BEINGS
Primitive man distinguished but two kinds of bodies in nature, those which
were motionless and those which were animated. Movement was for him the
expression of life. The stream, the wind, the waves, all were alive, and
each was endowed with all the attributes of life--will, sentiment, and
passion. Ancient Greek mythology is but the poetic expression of this
primitive conception.
In the evolution of the intelligence, as in that of the body, the
development of the individual is but a repetition of the development of the
race. Even now children attribute life to everything that moves. For them a
little bird still lives in the inside of a watch, and produces the
tick-tick of the wheels. In modern times, however, we have learnt that
everything in nature moves, so that motion of itself cannot be considered
as the characteristic of life.
Heraclitus aptly compares life to a flame. Aristotle says, "Life is
nutrition, growth, and decay,--having for its cause a principle which has
its end in itself, namely [Greek: entelecheia]." This principle is itself
in need of definition, and Aristotle only substitutes one unknown epithet
for another.
Bichat defined life as the ensemble of the functions which resist death.
This is to define life in terms of death,--but death is but the end of
life, and cannot be defined without first defining life. Claude Bernard
rejects all definition of life as insufficient, and incompatible with
experimental science. {2}
Some modern physiologists regard sensibility, others irritability, as the
characteristic of life, and define life as the faculty of responding, by
some sort of change, to an external stimulus. As in the case of movement,
we have found by more attentive observation that this faculty also is
universal in nature. There is no action without reaction; an elastic body
repels the body that strikes it. Every object in nature dilates with heat,
contracts with cold, and is modified by the light which it absorbs.
Everything in nature responds to exterior action by a change, and hence
this faculty cannot be the characteristic of life.
A distinguished professor of physiology was accustomed to teach that the
disproportion between action and reaction was the characteristic of life.
"Allow a gramme weight to fall on a nerve, and the muscle will raise a
weight of ten grammes. This disproportion is the characteristic of life."
But there is a much greater disproportion between action and reaction when
the friction of a match blows up a powder factory, or the turning of a
switch lights the lamps and animates the tramways and the motors of a great
city. The disproportion between action and reaction is therefore no
characteristic of life.
The essential characteristic of life is often said to be nutrition--the
phenomenon by which a living organism absorbs matter from its environment,
subjects it to chemical metamorphosis, assimilates it, and finally ejects
the destructive products of metamorphosis into the surrounding medium. But
this characteristic is also common to a great number of ordinary chemical
reactions, so that we cannot call it peculiar to life. Consider, for
instance, a fragment of calcium chloride immersed in a solution of sodium
carbonate. It absorbs the carbonic ion, incorporates it into a molecule of
calcium carbonate, and ejects the chlorine ion into the surrounding medium.
It may be argued that this is merely a chemical process, since the
substance which determines the reaction is also modified, the chloride of
calcium changing into carbonate of calcium. But every living thing is also
changing its chemical {3} constitution during every moment of its
existence,--it is this change which constitutes the process of senile
involution. The substance of the child is other than that of the ovum, and
the substance of the adult is not that of the child. Hence we cannot regard
nutrition as the exclusive characteristic of life.
Other authorities regard growth and organization as the essentials of life.
But crystals also grow. It was said that the growth of a crystal differed
from that of a living thing, in that the former grew by the addition of
material from without--the juxtaposition of bricks, as it were--while the
latter grew by intussusception, an introduction of fresh material into the
substance of the organism. A crystal, moreover, was homogeneous, while the
tissues of a living being were differentiated--such differentiation
constituting the organization. At the present time, however, we recognize
the existence of a great variety of purely physical productions, the
so-called "osmotic growths," which increase by a process of
intussusception, and develop therefrom a marvellous complexity of
organization and of form. Hence growth and organization cannot be
considered as the essential characteristics of life.
Since, then, we are totally unable to define the exact boundary which
separates life from the physical phenomena of nature, we may fairly
conclude that no such separation exists. This is in conformity with the
"law of continuity,"--the principle which asserts that all the phenomena of
nature are continuous in time and space. Classes, divisions, and
separations are all artificial, made not by nature but by man. All the
forms and phenomena of nature are united by insensible transition; it is
impossible to separate them, and in the distinction between living and
non-living things we must content ourselves with relative definitions,
which are far from being precise.
Life can only be defined as the sum of all phenomena exhibited by living
beings, and its definition thus becomes a mere corollary to the definition
of a living being.
The true definition of a living being is that it is a transformer of
energy, receiving from its environment the energy {4} which it returns to
that environment under another form. All living organisms are transformers
of energy.
A living organism is also a transformer of matter. It absorbs matter from
its environment, transforms it, and returns it to its environment in a
different chemical condition. Living things are chemical transformers of
matter.
Living beings are also transformers of form. They commence as a very simple
form, which gradually develops and becomes more complicated.
The matter of which a living organism is constituted consists essentially
of certain solutions of crystalloids and colloids. To this we may add an
osmotic membrane to contain the liquids, and a solid skeleton to support
and protect them. Finally, it would seem that a colloid of one of the
albuminoid groups is a necessary constituent of every living being.
We may say, then, that a living being is a transformer of energy and of
matter, containing certain albuminoid substances, with an evolutionary
form, the constitution of which is essentially liquid.
A living being has but a limited duration. It is born, develops, becomes
organized, declines and dies. Through all the metamorphoses of form, of
substance, and of energy, informing the whole course of its existence,
there is a certain co-ordination, a certain harmony, which is necessary for
the conservation of the individual. This harmony we call Life. Discord is
disease,--the total cessation of the harmony is Death. When the form is
profoundly altered and the substance changed, the transformation of energy
no longer follows its regular course, the organism is dead.
After death the colloids which have constituted the form of the living
thing pass from their liquid state as "sols" into their coagulated state as
"gels." The metamorphoses of form, substance, and energy still continue,
but no longer harmoniously for the conservation of the individual, but in
dis-harmony for its dissolution. Finally, the form of the individual
disappears, the substance and the energy of the living being is resolved
and dispersed into other bodies and other phenomena. {5}
The results hitherto obtained from the study of life seem but
inconsiderable when compared with the time and labour devoted to the
question. Max Verworn exclaims, "Are we on a false track? Do we ask our
questions of Nature amiss, or do we not read her answers aright?"
Each branch of science at its commencement employs only the simpler methods
of observation. It is purely descriptive. The next step is to separate the
different parts of the object studied--to dissect and to analyse. The
science has now become analytical. The final stage is to reproduce the
substances, the forms, and the phenomena which have been the subject of
investigation. The science has at last become synthetical.
Up to the present time, biology has made use only of the first two methods,
the descriptive and the analytical. The analytical method is at a grave
disadvantage in all biological investigations, since it is impossible to
separate and analyse the elementary phenomena of life. The function of an
organ ceases when it is isolated from the organism of which it forms a
part. This is the chief cause of our lack of progress in the analysis of
life.
It is only recently that we have been able to apply the synthetic method to
the study of the phenomena of life. Now that we know that a living organism
is but the arena for the transformation of energy, we may hope to reproduce
the elementary phenomena of life, by calling into play a similar
transformation of energy in a suitable medium.
Organic chemistry has already obtained numerous victories in the same
direction, and the rapid advance in the production of organic bodies by
chemical synthesis may be considered the first-fruits of synthetic biology.
A phenomenon is determined by a number of circumstances which we call its
causes, and of which it is the result. Every phenomenon, moreover,
contributes to the production of other phenomena which are called its
consequences. In order therefore to understand any phenomenon in its
entirety, we must determine all its causes both qualitatively and
quantitatively.
Phenomena succeed one another in time as consequences {6} one of another,
and thus form an uninterrupted chain from the infinite of the past into the
infinite of the future. A living being gathers from its entourage a supply
of matter and of energy, which it transforms and returns. It is part and
parcel of the medium in which it lives, which acts upon it, and upon which
it acts. The living being and the medium in which it exists are mutually
interdependent. This medium is in its turn dependent on its entourage,--and
so on from medium to medium throughout the regions of infinite space.
One of the great laws of the universe is the law of continuity in time and
space. We must not lose sight of this law when we attempt to follow the
metamorphoses of matter, of energy and of form in living beings. Evolution
is but the expression of this law of continuity, this succession of
phenomena following one another like the links of a chain, without
discontinuity through the vast extent of time and space.
The other great universal law, that of conservation, applies with equal
force to living and to inanimate things. This law asserts the
uncreateability and the indestructibility of matter and of energy. A given
quantity of matter and of energy remains absolutely invariable through all
the transformations through which it may pass.
We need not here discuss the question of the possible transformation of
matter into ether, or of ether into ponderable matter. Such a
transformation, if it exists, would have but little bearing on the
phenomena of life. Moreover, it also will probably be found to conform to
the law of conservation of energy.
In marked contrast to the permanence of matter and of energy is the
ephemeral nature of form, as exhibited by living beings. Function, since it
is but the resultant of form, is also ephemeral. All the faculties of life
are bound up with its form,--a living being is born, exists, and dies with
its form.
The phenomena of life may in certain cases slow down from their normal
rapidity and intensity, as in hibernating {7} animals, or be entirely
suspended, as in seeds. This state of suspension of life, of latent life as
it were, reminds us of a machine that has been stopped, but which retains
its form and substance unaltered, and may be started again whenever the
obstacle to its progress is removed.
During the whole course of its life a living being is intimately dependent
on its entourage. For example, the phenomena of life are circumscribed
within very narrow limits of temperature. A living organism, consisting as
it does essentially of liquid solutions, can only exist at temperatures at
which such solutions remain liquid, i.e. between 0deg C. and 100deg C.
Certain organisms, it is true, may be frozen, but their life remains in a
state of suspension so long as their substance remains solid. Since the
albuminoid substances which are a necessary component of the living
organism become coagulated at 44deg C., the manifestations of life diminish
rapidly above this temperature. The intensity of life may be said to
augment gradually as the temperature rises from 0deg to 40deg, and then to
diminish rapidly as the temperature rises above that point, becoming nearly
extinct at 60deg C.
Another condition indispensable to life is the presence of oxygen. Life,
compared by Heraclitus to a flame, is a combustion, an oxydation, for which
the presence of oxygen at a certain pressure is indispensable. There are,
it is true, certain anaerobic micro-organisms which apparently exist
without oxygen, but these in reality obtain their oxygen from the medium in
which they grow.
Life is also influenced by light, by mechanical pressure, by the chemical
composition of its entourage, and by other conditions which we do not as
yet understand. In each case the conditions which are favourable or noxious
vary with the nature of the organism, some living in air, some in fresh
water, and others in the sea.
Formerly it was supposed that the substance of a living being was
essentially different from that of the mineral world, so much so that two
distinct chemistries were in existence--organic chemistry, the study of
substances derived from bodies which had once possessed life, and inorganic
chemistry, dealing {8} with minerals, metalloids, and metals. We now know
that a living organism is composed of exactly the same elements as those
which constitute the mineral world. These are carbon, oxygen, hydrogen,
nitrogen, phosphorus, calcium, iron, sulphur, chlorine, sodium, potassium,
and one or two other elements in smaller quantity. It was formerly supposed
that the organic combinations of these elements were found only in living
organisms and could be fashioned only by vital forces. In more recent
times, however, an ever increasing number of organic substances have been
produced in the laboratory.
Organic bodies may be divided into four principal groups. (1)
_Carbohydrates_, including the sugars and the starches, all of which may be
considered as formed of carbon and water. (2) _Fats_, which may be
considered chemically as the ethers of glycerine, combinations of one
molecule of glycerine and three molecules of a fatty acid, with elimination
of water. (3) _Albuminoids_, substances whose molecules are complex,
containing nitrogen and sulphur in addition to carbon, oxygen, and
hydrogen. The albuminoid of the cell nucleus also contains phosphorus, and
the haemoglobin of the blood contains iron. (4) _Minerals_ or inorganic
elements, such as chloride of sodium, phosphate of calcium, and carbonic
acid. This group also includes water, which is the most important
constituent, since it forms more than a moiety of the substance of all
living creatures.
Woehler in 1828 accomplished the first synthesis of an organic substance,
urea, one of the products of the decomposition of albumin. Since then a
large number of organic substances have been prepared by the synthesis of
their inorganic elements. The most recent advance in this direction is that
of Emile Fischer, who has produced polypeptides having the same reactions
as the peptones, by combining a number of molecules of the amides of the
fatty acids.
In the further synthesis of organic compounds the problems we have before
us are of the same order as those already solved. There is no essential
difference between organic and inorganic chemistry; living organisms are
formed of the {9} same elements as the mineral world, and the organic
combinations of these elements may be realized in our laboratories, just as
in the laboratory of the living organism.
Not only so, but a living being only borrows for a short time those mineral
elements which, after having passed through the living organism, are
returned once again to the mineral kingdom from which they came.
All matter has life in itself--or, at any rate, all matter susceptible of
incorporation in a living cell. This life is potential while the element is
in the mineral state, and actual while the element is passing through a
living organism.
Mineral matter is changed into organic matter in its passage through a
vegetable organism. The carbonic acid produced by combustion and
respiration is absorbed by the chlorophyll of the leaves under the stimulus
of light--the oxygen of the carbonic acid being returned to the air, while
the carbon is utilized by the plant for the formation of sugar, starch,
cellulose, and fats.
Thus plants are fed in great part by their leaves, taking an important part
of their nourishment from the air, while by their roots they draw from the
earth the water, the phosphates, the mineral salts, and the nitrates
required for the formation of their albuminoid constituents. A vegetable is
a laboratory in which is carried out the process of organic synthesis by
which mineral materials are changed into organic matter. The first
synthetic reaction is the formation of a molecule of formic aldehyde,
CH_2O, by the combination of a molecule of water with an atom of carbon.
From this formic aldehyde, or formol, we may obtain all the various
carbohydrates by simple polymerization, i.e. by the association of several
molecules, with or without elimination of water. Thus two molecules of
formol form one molecule of acetic acid, 2CH_2O = C_2H_4O_2. Three
molecules of formol form a molecule of lactic acid, 3CH_2O = C_3H_6O_3. Six
molecules of formol represent glucose and levulose, 6CH_2O = C_6H_{12}O_6.
Twelve molecules of formol minus one molecule of water form saccharose,
lactose, cane sugar, and sugar of milk, 12CH_2O = C_{12}H_{22}O_{11} +
H_2O; _n_ times six {10} molecules of formol minus one molecule of water,
_n_(C_6H_{10}O_5), form starch and cellulose.
Animals derive their nourishment from vegetables either directly, or
indirectly through the flesh of herbivorous animals. The mineral matter,
rendered organic in its passage through a vegetable growth, is finally
returned by the agency of animal organisms to the mineral world again, in
the form of carbonic acid, water, urea, and nitrates. Thus vegetables may
be regarded as synthetic agents, and animals and microbes as agents of
decomposition. Here also the difference is only relative, for in certain
cases vegetables produce carbonic acid, while some animal organisms effect
synthetic combinations. Moreover, there are intermediary forms, such as
fungi, which possessing no chlorophyll are nourished like animals by
organic matter, and yet like vegetables are able to manufacture organic
matter from mineral salts.
The work of combustion begun by the animal organism is finished by the
action of micro-organisms, who complete the oxydation--the
re-mineralization of the chemical substances drawn originally from the
inorganic world by the agency of plant life.
To sum up. Vegetables obtain their nourishment from mineral substances,
which they reduce, de-oxydize, and charge with solar energy. Animal
organisms on the contrary oxydize, and micro-organisms complete the
oxydation of these substances, returning them to the mineral world as
water, carbonates, nitrates, and sulphates.
Thus matter circulates eternally from the mineral to the vegetable, from
the vegetable to the animal world, and back again. The matter which forms
our structure, which is to-day part and parcel of ourselves, has formed the
structure of an infinite number of living beings, and will continue to
pursue its endless reincarnation after our decease.
This endless cycle of life is also an endless cycle of energy. The
combination of carbon with water carried out by the agency of chlorophyll
can only take place with absorption of energy. This energy comes directly
from the sun, the red and orange light radiations being absorbed by the
chlorophyll. {11} The arrest of vegetation during the winter months is due
not so much to the lowering of temperature as to the diminution of the
radiant energy received from the sun. In the same way shade is harmful to
vegetation, since the radiant energy required for growth is prevented from
reaching the plant.
The energy radiated by the sun is accumulated and stored in the plant
tissues. Later on, animals feed on the plants and utilize this energy,
excreting the products of decomposition, _i.e._ the constituents of their
food minus the energy contained in it. Thus the whole of the energy which
animates living beings, the whole of the energy which constitutes life,
comes from the sun. To the sun also we owe all artificial heat, the energy
stored up in wood and coal. We are all of us children of the sun.
The radiant energy of the sun is transformed by plants into chemical
energy. It is this chemical energy which feeds the vital activity of
animals, who return it to the external world under the form of heat,
mechanical work, and muscular contraction, light in the glow-worm,
electricity in the electric eel.
There is a marked difference between the forms affected by organic and
inorganic substances. The forms of the mineral world are those of
crystals--geometrical forms, bounded by straight lines, planes, and regular
angles. Living organisms, on the contrary, affect forms which are less
regular--curved surfaces and rounded angles. The physical reason for this
difference in form lies in a difference of consistency, crystals being
solid, whereas living organisms are liquids or semi-liquids. The liquids of
nature, streams and clouds and dewdrops, affect the same rounded forms as
those of living organisms.
Living beings for the most part present a remarkable degree of symmetry.
Some, like radiolarians and star-fish, have a stellate form. In plants the
various organs often radiate from an axis, in such a manner that on turning
the plant about this axis the various forms are superposed thrice, four, or
more often five times in one complete revolution. It is remarkable how
often this number five recurs in the {12} divisions and parts of a living
organism. In other cases the similar parts are disposed symmetrically on
either side of a median line or plane, giving a series of homologous parts
which are not superposable.
The most important characteristic of a living being is its form. This is
implicitly admitted by naturalists, who classify animals and plants in
genera and species according to the differences and analogies of their
form.
All living beings are composed of elementary organizations called cells. In
its complete state, a cell consists of a membrane or envelope containing a
mass of protoplasm, in the centre of which is a nucleus of differentiated
protoplasm. This nucleus may in its turn contain a nucleolus. In some cases
the cell is merely a protoplasmic mass without a visible envelope, so that
a cell may be defined as essentially a mass of protoplasm provided with a
nucleus.
A living organism may consist merely of a single cell, which is able alone
to accomplish all the functions of life. Most living beings, however,
consist of a collection of innumerable cells forming a cellular association
or community. When a number of cells are thus united to constitute a single
living being, the various functions of life are divided among different
cellular groups. Certain cells become specialized for the accomplishment of
a single function, and to each function corresponds a different form of
cell. It is thus easy to recognize by their form the nerve cells, the
muscle cells which perform the function of movement, and the glandular
cells which perform the function of secretion. The cells of a living being
are microscopic in size, and it is remarkable that they never attain to any
considerable dimensions.
In order that life may be maintained in a living organism, it is necessary
that a continual supply of aliment should be brought to it, and that
certain other substances, the waste-products of combustion, should be
eliminated. In order to be absorbed and assimilated, the alimentary
substances must be presented to the living organism in a liquid or gaseous
state. Thus the essential condition necessary for the {13} maintenance of
life is the contact of a living cell with a current of liquid. The
elementary physical phenomenon of life is the contact of two different
liquids. This is the necessary condition which renders possible the
chemical exchanges and the transformations of energy which constitute life.
It is in the study of the phenomena of liquid contact and diffusion that we
may best hope to pierce the secrets of life. The physics of vital action
are the physics of the phenomena which occur in liquids, and the study of
the physics of a liquid must be the preface and the basis of all inquiry
into the nature and origin of life.
* * * * *
{14}
CHAPTER II
SOLUTIONS
We have seen that living beings are transformers of energy and of matter,
evolutionary in form and liquid in consistency; that they are solutions of
colloids and crystalloids separated by osmotic membranes to form
microscopic cells, or consisting merely of a gelatinous mass of protoplasm,
with a nucleus of slightly differentiated material. The elementary
phenomenon of life is the contact of two different solutions. This is the
initial physical phenomenon from which proceed all the other phenomena of
life in accordance with the ordinary chemical and physical laws. Thus the
basis of biological science is the study of solution and of the phenomena
which occur between two different solutions, either in immediate contact or
when separated by a membrane.
A solution is a homogeneous mixture of one or more solutes in a liquid
solvent. Before solution the solute or dissolved substance may be solid,
liquid, or gaseous.
Solutes, or substances capable of solution, may be divided into two
classes--substances which are capable of crystallization, or crystalloids;
and those which are incapable of crystallization, the colloids.
Crystalloids may be divided again into two classes, those whose solutions
are ionizable and therefore conduct electricity, chiefly salts, acids, and
bases; and those whose solutions are non-ionizable and are therefore
non-conductors. These latter are for the most part crystallizable
substances of organic origin, such as sugars, urea, etc.
Avogadro's law asserts that under similar conditions of temperature and
pressure, equal volumes of various gases {15} contain an equal number of
molecules. Under similar conditions, the molecular weights of different
substances have therefore the same ratio as the weights of equal volumes of
their vapours. Hence if we fix arbitrarily the molecular weight of any one
substance, the molecular weight of all other substances is thereby
determined. The molecular weight of hydrogen has been arbitrarily fixed as
two, and hence the molecular weight of any substance will be double its
gaseous density when compared with that of hydrogen.
_Gramme-Molecule._--A gramme-molecule is the molecular weight of a body
expressed in grammes. Occasionally for brevity a gramme-molecule is spoken
of as a "molecule." Thus we may say that the molecular weight of oxygen is
16 grammes, meaning thereby that there are the same number of molecules in
16 grammes of oxygen as there are atoms in 1 gramme of hydrogen.
_Concentration._--The concentration of a solution is the ratio between the
quantity of the solute and the quantity of the solvent. The concentration
of a solution is expressed in various ways. (_a_) The weight of solute
dissolved in 100 grammes of the solvent. (_b_) The weight of solute present
in 100 grammes of the solution. (_c_) The weight of solute dissolved in a
litre of the solvent. (_d_) The weight of solute in a litre of the
solution. The most usual method is to give the concentration as the weight
of solute dissolved in 100 grammes or in one litre of the solvent.
_Molecular Concentration._--Many of the physical and biological properties
of a solution are proportional, not to its mass or weight concentration,
but to its molecular concentration, _i.e_. to the number of
gramme-molecules of the solute contained in a litre of the solution. Many
physical properties are quite independent of the nature of the solute,
depending only on its degree of molecular concentration.
_Normal Solution._--A normal solution is one which contains one
gramme-molecule of the solute per litre. A decinormal solution contains
one-tenth of a gramme-molecule of the solute per litre, and a centinormal
solution one-hundredth of a gramme-molecule. A normal solution of urea, for
example, {16} contains 60 grammes of urea per litre, while a normal
solution of sugar contains 342 grammes of sugar per litre.
_The Dissolved Substance is a Gas._--Van t' Hoff, using the data obtained
by the botanist Pfeffer, showed that the dissolved matter in a solution
behaved exactly as if it were a gas. The analogy is complete in every
respect. Like the gaseous molecules, the molecules of a solute are mobile
with respect to one another. Like those of a gas, the molecules of a solute
tend to spread themselves equally, and to fill the whole space at their
disposal, _i.e._ the whole volume of the solution. The surface of the
solution represents the vessel containing the gas, which confines it within
definite limits and prevents further expansion.
_Osmotic Pressure._--Like the molecules of a gas, the molecules of a solute
exercise pressure on the boundaries of the space containing it. This
osmotic pressure follows exactly the same laws as gaseous pressure. It has
the same constants, and all the notions acquired by the study of gaseous
pressure are applicable to osmotic pressure. Osmotic pressure is in fact
the gaseous pressure of the molecules of the solute.
When a gas dilates and increases in volume, its temperature falls, and cold
is produced. Similarly, when a soluble substance is dissolved, it increases
in volume, and the temperature of the liquid falls. This phenomenon is well
known as a means of producing cold by a refrigerating mixture.
The phenomena of life are governed by the laws of gaseous pressure, since
all these phenomena take place in solutions. The fundamental laws of
biology are those of the distribution of substances in solution, which is
regulated by the laws of gaseous pressure, since all these laws are
applicable also to osmotic pressure.
_Boyle's Law_.--When a gas is compressed its volume is diminished. If the
pressure is doubled, the volume is reduced to one-half. The quantity V x P,
that is the volume multiplied by the pressure, is constant.
_Gay-Lussac's Law._--For a difference of temperature of a degree Centigrade
all gases dilate or contract by 1 / 273 of their volume at 0deg Centigrade.
{17}
_Dalton's Law._--In a gaseous mixture, the total pressure is equal to the
sum of the pressures which each gas would exert if it alone filled the
whole of the receptacle.
_Pressure proportional to Molecular Concentration._--The above laws are
completely independent of the chemical nature of the gas, they depend only
on the number of gaseous molecules in a given space, _i.e._ on the
molecular concentration. If we double the mass of the gas in a given space,
we double the number of molecules, and we also double the pressure,
whatever the nature of the molecules. We may also double the pressure by
compressing the molecules of a gas, or of several gases, into a space half
the original size. The molecular concentration of a gas, or of a mixture of
gases, is the ratio of the number of molecules to the volume they occupy.
The pressure of a gas or of a mixture of gases is proportional to its
molecular concentration. This is a better and a shorter way of expressing
both Boyle's law and Dalton's law.
One gramme-molecule of a gas, whatever its nature, condensed into the
volume of 1 litre, has a pressure of 22.35 atmospheres. Similarly one
gramme-molecule of a solute, whatever its nature, when dissolved in a litre
of water, has the same pressure, viz. 22.35 atmospheres.
_Absolute Zero._--According to Gay-Lussac's law, the volume of a gas
diminishes by 1 / 273 of its volume at 0deg C. for each degree fall of
temperature. Thus if the contraction is the same for all temperatures, the
volume would be reduced to zero at -273deg C. This is the absolute zero of
temperature. Temperatures measured from this point are called absolute
temperatures, and are designated by the symbol T. If _tdeg_ indicates the
Centigrade temperature above the freezing point of water, then the absolute
temperature is equal to _tdeg_ + 273deg.
_The Gaseous Constant._--Consider a mass of gas at 0deg C. under a pressure
P_o, with volume V_o. At the absolute temperature T, if the pressure be
unaltered, the volume of this gas will be V_oT / 273. Therefore the
constant PV, the product of the pressure by the volume, will be represented
by P_oV_oT / 273. {18}
At the same temperature, but under another pressure P' the gas will have a
different volume V'. Since, according to Boyle's law, PV is constant (P'V'
= P_oV_o), it will still equal P_oV_oT / 273. Therefore P_oV_o / 273 is
also constant. This quantity is called "the gaseous constant," and if we
represent it by the symbol R, we obtain the general formula PV = RT for all
gases, or PV / T = R.
Suppose, for instance, we have a gramme-molecule of a gas at 0deg C. in a
space of 1 litre. It has a pressure of 22.35 atmospheres at 0deg C., or
273deg absolute temperature. Since PV = RT, R = PV / T = 1 x 22.35 / 273 =
.0819. This number .0819 is the numerical value of the constant R for all
gases, volume being measured in litres and pressure in atmospheres.
Substances in solution behave exactly like gases, they follow the same laws
and have the same constants. All the conceptions which have been acquired
by the study of gases are applicable to solutions, and therefore to the
phenomena of life. The osmotic pressure of a solution is the force with
which the molecules of the solute, like gaseous molecules, strive to
diffuse into space, and press on the limits which confine them, the
containing vessel being represented by the surfaces of the solution.
Osmotic pressure is measured in exactly the same way as gaseous pressure.
To measure steam pressure we insert a manometer in the walls of the boiler.
In the same way we may use a manometer to measure osmotic pressure. We
attach the tube to the walls of the porous vessel, allow the solvent to
increase in volume under the pressure of the solute, and measure the rise
of the liquid in the manometer tube.
_Pfeffer's Apparatus._--Pfeffer has designed an apparatus for the
measurement of osmotic pressure. It consists of a vessel of porous
porcelain, the pores of which are filled with a colloidal solution of
ferrocyanide of copper. This forms a semi-permeable membrane which permits
the passage of water into the vessel, but prevents the passage of sugar or
of any {19} colloid. The stopper which hermetically closes the vessel is
pierced for the reception of a mercury manometer. The vessel is filled with
a solution of sugar and plunged in a bath of water. The volume of the
solution in the interior of the vessel can vary, since water passes easily
in either direction through the pores of the vessel. The boundary of the
solvent has become extensible, and its volume can increase or diminish in
accordance with the osmotic pressure of the solute. Under the pressure of
the sugar water is sucked into the vessel like air into a bellows, the
solution passes into the tube of the manometer, and raises the column of
mercury until its pressure balances the osmotic pressure of the sugar
molecules.
_Osmotic Pressure follows the Laws of Gaseous Pressure._--This osmotic
pressure is in fact gaseous pressure, and may be measured in millimetres of
mercury in just the same way. We may thus show that osmotic pressure
follows the laws of gaseous pressure as defined by Boyle, Dalton, and
Gay-Lussac. The coefficient of pressure variation for change of temperature
is the same for a solute as for a gas. The formula PV = RT is applicable to
both. The numerical value of the constant R is also the same for a solute
as for a gas. being .0819 for one gramme-molecule of either, when the
volume is expressed in litres and the pressure in atmospheres. The formula
PV = RT shows that for a given mass, with the same volume, the pressure
increases in proportion to the absolute temperature.
_Osmotic Pressure of Sugar._--A normal solution of sugar, containing 342
grammes of sugar per litre, has a pressure of 22.35 atmospheres, and it may
well be asked why such an enormous pressure is not more evident. The reason
will be found in the immense frictional resistance to diffusion. Frictional
resistance is proportional to the area of the surfaces in contact, and this
area increases rapidly with each division of the substance. When a solute
is resolved into its component molecules, its surface is enormously
increased, and therefore the friction between the molecules of the solute
and those of the solvent.
_Isotonic Solutions._--Two solutions which have the same {20} osmotic
pressure are said to be iso-osmotic or isotonic. When comparing two
solutions of different concentration, the solution with the higher osmotic
pressure is said to be hypertonic, and that with the lower osmotic pressure
hypotonic.
_Lowering of the Freezing Point._--Pure water freezes at 0deg C. Raoult
showed that the introduction of a non-ionizable substance, such as sugar or
alcohol, lowers the freezing point of a solution in proportion to the
molecular concentration of the solute. One gramme-molecule of the solute
introduced into one litre of the solution lowers its temperature of
congelation by 1.85deg C. Thus a normal solution of any non-ionizable
substance in water freezes at -1.85deg C. The measurement of this lowering
of the freezing point is called Cryoscopy, a method which is becoming of
great utility in medicine.
_Cryoscopy of Blood._--In order to determine the osmotic pressure of the
blood at 37deg C., _i.e._ 98.6deg F., the normal temperature, we proceed as
follows. On freezing the blood, we find that it congeals at -.56deg. Its
molecular concentration is therefore .56 / 1.85 = .30, or about one-third
of a gramme-molecule per litre. Its osmotic pressure at 0deg C. is
therefore .3 x 22.35 = 6.7 atmospheres. The increase of pressure with
temperature is the same as for a gas, viz. 1/273, or .00367 of its pressure
at 0deg for every degree rise of temperature. The increase of pressure at
37deg is therefore .00367 x 37 x 6.7 = .9 atmospheres. The total osmotic
pressure at 37deg is therefore 6.7 + .9 = 7.6 atmospheres.
_Rise of Boiling Point._--Water under atmospheric pressure boils at a
temperature of 100deg C. The addition of a solute whose solution does not
conduct electricity, such as sugar, causes a rise in the boiling point
proportional to the molecular concentration of that solute.
_Lowering of the Vapour Tension._--The vapour tension of a liquid is
lowered by the addition of a solute. A liquid boils at the temperature at
which its vapour tension equals that of the atmosphere. Since an aqueous
solution of sugar at atmospheric pressure does not begin to boil at 100deg
C., it is manifest that its vapour tension is then less than that of the
{21} atmosphere. The addition of a solute such as sugar, whose solution is
not ionizable, and therefore does not conduct electricity, lowers the
vapour tension of the solution in proportion to the molecular concentration
of the solute.
_Corresponding Values._--We have thus found five properties of a solution
which vary proportionally, so that from the measurement of any one of them
we can determine the corresponding values of all the others. These are--
1. The Molecular Concentration.
2. The Osmotic Pressure.
3. The Diminution of Vapour Tension.
4. The Raising of the Boiling Point.
5. The Lowering of the Freezing Point.
_Cryoscopy._--The usual method employed for the determination of the
molecular concentration and osmotic pressure of a solution is by
cryoscopy--the measurement of its temperature of congelation. A very
sensitive thermometer is used, the scale of which extends over only 5deg
and is divided into hundredths of a degree. The liquid under examination is
placed in a test tube, in which the bulb of the thermometer is plunged, and
this is supported in a second tube with an air space all round it. The
whole is then suspended to the under side of the cover of the refrigerating
vessel, which may be cooled either by filling it with a freezing mixture,
or by the evaporation of ether. During the whole of the operation the
liquid is agitated by a mechanical stirrer. The first step is to determine
the freezing point of distilled water. As the water cools the mercury
gradually descends in the stem of the thermometer till it reaches a point
below the zero mark at 0deg C. As soon as ice begins to form the mercury
rises, at first rapidly and then more slowly, reaches a maximum, and
finally descends again. This maximum reading is the true point of
congelation. The inner tube is then emptied, care being taken to leave a
few small ice crystals to serve as centres of congelation for the
subsequent experiment, thus avoiding supercooling of the solution. The
process is then repeated with the solution under examination. The
difference between {22} the two freezing points is the required "lowering
of the freezing point."
Cryoscopy is the method most used in biological research to determine
molecular concentration. It has, however, some grave defects. It
necessitates several cubic centimetres of the liquid under examination. It
gives us the constants of the solution at the temperature of freezing,
which is far below that of life. Organic liquids are easily altered and are
extremely sensible to minute differences of temperature, cryoscopy
therefore gives us no information as to the constitution of solutions under
normal conditions. It is desirable to have some other method of determining
molecular concentration and the other interdependent constants at the
normal temperature of life. A much better method, were it possible, would
be the direct determination of the vapour tension of the solutions under
normal conditions of temperature and pressure.
_Molecular Lowering of the Freezing Point._--For every substance whose
solution is not ionized and therefore does not conduct electricity, the
lowering of the freezing point is the same, viz. 1.85deg C. for each
gramme-molecule of the solute per litre of the solution.
_Determination of the Molecular Concentration._--In order to obtain the
molecular concentration of a non-ionizable substance, we have only to
determine the lowering of the freezing point. Let A be the lowering of the
freezing point of any solution. On dividing it by 1.85 (the lowering of the
freezing point for a normal solution), we obtain the number of
gramme-molecules in a litre of the solution. If n be the number of
gramme-molecules per litre, then n = A / 1.85.
_Determination of the Osmotic Pressure._--The osmotic pressure P of a
solution may be obtained by multiplying its molecular concentration n by
22.35 atmospheres. P = n x 22.35 = A / 1.85 x 22.35.
_Determination of Molecular Weight._--The lowering of the freezing point
also enables us to calculate the molecular {23} weight of any non-ionizable
solute. Thus Bouchard has been able to determine by means of cryoscopy the
mean molecular weight of the substances eliminated by the urine. A weight
_x_ of the substance is dissolved in a litre of water, and the lowering of
the freezing point is observed. The value thus found divided by 1.85 gives
us n, the number of gramme-molecules per litre. The molecular weight M may
be determined by dividing the original weight x by n.
The study of osmotic pressure was begun by the Abbe Nollet; and one of his
disciples, Parrot, at an early date thus described its importance: "It is a
force analogous in all respects to the mechanical forces, a force able to
set matter in motion, or to act as a static force in producing pressure. It
is this force which causes the circulation of heterogeneous matter in the
liquids which serve as its vehicle. It is this force which produces those
actions which escape our notice by their minuteness and bewilder us by
their results. It is for the infinitely small particles of matter what
gravitation is for heavy masses. It can displace matter in solution upwards
against gravity as easily as downwards or in a horizontal direction."
Thus the recognition of the fact that a substance in solution is really a
gas, has at a single stroke put us in possession of the laws of osmotic
pressure--laws slowly and laboriously discovered by the long series of
investigations on the pressure of gases.
Osmotic pressure plays a most important role in the arena of life. It is
found at work in all the phenomena of life. When osmotic pressure fails,
life itself ceases.
* * * * *
{24}
CHAPTER III
ELECTROLYTIC SOLUTIONS
_Solutions which conduct Electricity._--The laws of solution which we have
studied in the previous chapter apply only to those solutions, chiefly of
organic origin, which do not conduct electricity. Solutions of electrolytes
such as the ordinary salts, acids, and bases, which are ionized on
solution, give values for the various constants of solution which do not
accord with those required by theory. If, for instance, we take a
gramme-molecule of an electrolyte such as chloride of sodium, and dissolve
it in a litre of water, we find that the lowering of the freezing point is
nearly double the theoretical value of 1.85deg. The same holds good for the
osmotic pressure, and for all the constants which are proportional to the
molecular concentration of the solute. The solution behaves, in each case,
as if it contained more than one gramme-molecule of sodium chloride per
litre. It behaves, in fact, as if it contained i times the number of
molecules of solute originally introduced into it. If n be the original
number of molecules, then it will apparently contain n' = in molecules.
This law is universal for all electrolytic solutions; the theoretical value
for their concentration, osmotic pressure, and all the proportional
physical constants must be multiplied by this quantity, i = n'/n, which is
the ratio of the apparent number of the molecules present to the number
originally introduced.
A similar dissociation of the molecule is observed in the case of many
gases. The vapour of chloride of ammonium, for instance, is decomposed by
heat, and it may be shown experimentally that the increase of pressure on
heating above {25} that which theory demands, is due to an increase in the
number of the gaseous molecules present. Some of the vapour particles are
dissociated into two or more fragments, each of which plays the part of a
single molecule.
Arrhenius, in 1885, advanced the hypothesis that the apparent increase in
the number of molecules of an electrolytic solution was also due to
dissociation. This interpretation at once threw a flood of light on a
number of phenomena hitherto obscure.
_Coefficient of Dissociation._--We have seen that in order to obtain values
which accord with experiment we have to multiply the number of
gramme-molecules of the solute by the coefficient i, which is called the
Coefficient of Dissociation.
This coefficient of dissociation, i, may be found by observing the lowering
of the freezing point of a normal solution, and dividing it by 1.85. i =
t/1.85.
The coefficient of dissociation varies with the degree of concentration of
the solution, rising to a maximum when the solution is sufficiently
diluted.
If we know i, the coefficient of dissociation for a given solute, contained
in a solution of a definite concentration, we can find n', the number of
particles present in a solution containing n gramme-molecules of the solute
per litre, since n' = in. On the other hand, if from a consideration of its
freezing point and other constants we find that an electrolytic solution
appears to contain n' gramme-molecules per litre, the real number of
chemical gramme-molecules in one litre of the solution will be only n' / i
= n.
Very concentrated solutions do not conform to these laws. In this they
resemble gases, which as they approach their point of condensation tend
less and less to conform to the laws of gaseous pressure.
_Electrolysis._--If we take a solution of an acid, a salt, or a base, and
dip into it two metallic rods, one connected to the positive and the other
to the negative pole of a battery, we {26} find that the metals or metallic
radicals of the solution are liberated at the negative pole, while the acid
radicals of the salts and acids and the hydroxyl of the bases are liberated
at the positive pole. The liberated substances may either be discharged
unchanged, or they may enter into new combinations, causing a series of
secondary reactions.
_Electrolytes._--Solutions which conduct electricity are called
Electrolytes, and the conducting metallic rods dipping into the solution
are the Electrodes. Faraday gave the names of Ions to the atoms or
atom-groups liberated at either electrode. The ions liberated at the
positive electrode are the Anions, and those at the negative electrode are
the Cations. The only solutions which possess any notable degree of
electrical conductivity are the aqueous solutions of the various salts,
acids, and bases, and in these solutions only do we meet with those
phenomena of dissociation which are evidenced by anomalies of osmotic
pressure, freezing point and the like,--anomalies which show that the
solution contains a greater number of molecules than that indicated by its
molecular concentration. These anomalies are due to dissociation, the
division of some of the molecules into fragments, each of which plays the
part of a separate molecule, contributing its quota to the osmotic tension
and vapour pressure of the solution, in fact to all the phenomena which are
dependent on the degree of molecular concentration. The electrical
conductivity of a solution is therefore proved to be dependent on its
molecular dissociation.
_Arrhenius' Theory of Electrolysis._--In 1885, Arrhenius brought forward
his theory of the transport of electricity by an electrolyte. According to
this hypothesis, the electric current is carried by the ions, the positive
charges by the cations, and the negative charges by the anions. In virtue
of the attraction between charges of different sign, and repulsion between
charges of like sign, the cations are repelled by the positive charge on
the anode, and attracted by the negative charge on the cathode. Similarly
the anions are repelled by the cathode and attracted by the anode. {27}
An electrolytic solution contains three varieties of particles, positive
ions or cations, negative ions or anions, and undissociated neutral
molecules. The molecular concentration of such a solution, with the
corresponding constants, depends on the total number of these particles,
_i.e._ the sum of the ions and the undissociated neutral molecules. We may
indicate an ion by placing above it the sign of its electrical charge, one
sign for each valency. Thus Na^+ and Cl^- indicate the two ions of a salt
solution; Cu^{++} and SO_4^{--} the two ions of a solution of sulphate of
copper. A point is sometimes substituted for the + sign, and a comma for
the - sign. Thus Na^. and Cl^,; Cu^{..} and SO_4^{,,}.
My friend Dr. Lewis Jones has given a very vivid picture of the processes
which go on in an electrolytic solution when an electric current is
passing. He compares an electrolytic cell to a ballroom, in which are
gyrating a number of dancing couples, representing the neutral molecules,
and a number of isolated ladies and gentlemen representing the anions and
cations respectively. If we suppose a mirror at one end of the ballroom and
a buffet at the other, the ladies will gradually accumulate around the
mirror, and the gentlemen around the buffet. Moreover, the dancing couples
will gradually be dissociated in order to follow this movement.
_Degree of Dissociation._--The degree of dissociation is the fraction of
the molecules in the solution which have undergone dissociation. Let n be
the total number of molecules of the solute, and n" the number of
dissociated molecules. Then n" / n = a will represent the degree of
dissociation. Let k be the number of ions into which each molecule is
split. Then a = n"k / nk, _i.e._ the degree of dissociation is the ratio of
the number of ions actually present in a solution to the number which would
be present if all the molecules of the solute were dissociated.
Let n' be the total number of particles present in a solution {28}
containing n molecules, each of which is composed of k ions. Then if a is
the degree of dissociation,
n' = n - an + ank,
n' = n[1 + a (k - 1)],
n' / n = 1 + a (k - 1) = i.
We thus obtain i the coefficient of dissociation, in terms of the degree of
dissociation a and the number of ions in each molecule k.
If there is no dissociation, _i.e._ if a = 0, then n' = n, and i = 1. If
all the molecules are dissociated, a = 1, and i = k.
_Faraday's Law._--Faraday found that the quantity of electricity required
to liberate one gramme-molecule of any radical is 96.537 coulombs for each
valency of the radical.
_Electrochemical Equivalent._--The electrochemical equivalent of a radical
is the weight liberated by one coulomb of electricity. It is equal to the
molecular weight of the ion, divided by 96.537 times its valency.
_Electrolytic Conductivity._--The conductivity of an electrolyte is the
inverse of its resistance. C = 1/R.
For a given difference of potential the conductivity of an electrolyte is
proportional to the number of ions in unit volume, the electrical charge on
each ion, and the velocity of the ions.
_The specific conductivity_ [Delta] of an electrolyte is the conductivity
of a cube of the solution, each face of which is one square centimetre in
area. The _molecular conductivity_ of an electrolyte is the conductivity of
a solution containing one gramme-molecule of the substance placed between
two parallel conducting plates, one centimetre apart. The molecular
conductivity is independent of the volume occupied by the gramme-molecule
of the solute, depending only on the degree of dissociation. The molecular
conductivity U is equal to the product of V, the volume of the molecule, by
[Delta], its specific conductivity. U = V[Delta]. Whence [Delta] = U / V,
_i.e._ the specific {29} conductivity equals the molecular conductivity
divided by the volume.
The conductivity of an electrolyte is proportional to the number of ions in
a volume of the solution containing one gramme-molecule. Let M_{[infinity]}
be the conductivity for complete dissociation and M_v the molecular
conductivity at the volume V. Then
M_v / M_{[infinity]} = n"k / nk = n" / n = a,
the degree of dissociation. This is Ostwald's law, which says that the
degree of dissociation is equal to the ratio of conductivity when the
gramme-molecule occupies a volume V, to its conductivity when the solution
is so dilute that dissociation is complete. Hence the degree of
dissociation may also be determined by comparing the electrical
conductivities of two solutions of different degrees of concentration.
| -- -- -- | -- -- -- |
| SO_4 SO_4 SO_4 | SO_4 SO_4 SO_4 |
| | |
| ++ ++ ++ | ++ ++ ++ |
| Cu Cu Cu | Cu Cu Cu |
| | |
+----------------------------+--------------------------------+
FIG. 1.--Before the passage of the current.
| -- | -- -- |
| SO_4 | SO_4 SO_4 SO_4 SO_4 SO_4 |
- | | | +
| ++ | ++ ++ |
| Cu Cu Cu Cu | Cu Cu |
| | |
+---------------------------+---------------------------------+
FIG. 2.--After the passage of the current.
_Velocity of the Ions._--If the electrolytic cell is divided into two
segments by means of a porous diaphragm, we shall find after a time an
unequal distribution of the solute on the two sides. For instance, with a
solution of sulphate of copper, after the current has passed for some time
there will be a diminution of concentration in the liquid on both sides of
the diaphragm, but the loss will be very unequally divided. Two-thirds of
the loss of concentration will be on the side of the negative electrode and
only one-third on the positive side. In 1853, Hittorf gave the following
ingenious explanation of this phenomenon:-- {30}
Fig. 1 represents an electrolytic vessel containing a solution of sulphate
of copper, the vertical line indicating a porous partition separating the
vessel into two parts. Fig. 2 shows the same vessel after the passage of
the current. The acid radical has travelled twice as fast as the metal. For
each copper ion which has passed through the porous plate towards the
cathode two acid radicals have passed through it towards the anode. Three
ions have been liberated at either electrode, but in consequence of the
difference of velocity with which the positive and the negative ions have
travelled, the negative side of the vessel contains only one molecule of
copper sulphate and has lost two-thirds of its molecular concentration,
while the positive side contains two molecules of copper sulphate and has
only lost one-third of its concentration. This proves clearly that the ions
move in different directions with different velocities. Let u be the
velocity of the anions, and v the velocity of the cations. Let n be the
loss of concentration at the cathode, and 1 - n the loss of concentration
at the anode. Then
u / v = n / (1 - n),
_i.e._ the loss of concentration at the cathode is to the loss of
concentration at the anode as the velocity of the anions is to that of the
cations. Hence by measuring the loss of concentration at the two
electrodes, we have an easy means of determining the comparative velocity
of different ions.
In 1876, Kohlrausch compared the conductivity of the chlorides, bromides,
and iodides of potassium, sodium, and ammonium respectively. He found that
altering the cation did not affect the _differences_ of conductivity
between the three salts, thus showing that these differences of
conductivity were dependent on the nature of the anion only, and not on the
particular base with which it was combined. The difference of conductivity
between an iodide and a bromide, for example, is the same whether
potassium, sodium, or ammonium salts are compared. A similar experiment has
been made with a series of cations combined with various anions. The
difference of conductivity of the salts in the series is the same whichever
anion is used, _i.e._ the difference of conductivity between potassium
chloride and sodium chloride is the same as that between {31} potassium
bromide and sodium bromide. Hence we may conclude that the conductivity of
any salt is an ionic property.
Kohlrausch's law may be expressed by the formula c = d(u + v), where c is
the conductivity of the salt, d the degree of dissociation, _i.e._ the
fraction of the electrolyte broken up into ions, and u and v the velocity
of the anions and cations respectively. When all the molecules of the
electrolyte are dissociated, d = 1, and the formula becomes c_{[infinity]}
= u + v.
As we have already seen, a salt is formed by the union of a metal M with an
acid radical R. Potassium sulphate, K_2SO_4, consists of the metal K_2 and
the acid radical SO_4. Ammonium chloride, NH_4Cl, consists of the basic
radical NH_4 and the acid radical Cl. The various acids may be considered
as salts of the metal hydrogen. Thus sulphuric acid, H_2SO_4, is the
sulphate of hydrogen. Bases may be considered as salts with the hydroxyl
group, OH, replacing the acid radical. Thus potash, KOH, is the hydroxyl of
potassium. The various electrolytic combinations may be represented by the
following symbols:--
Salts = MR.
Acids = HR.
Bases = MOH.
The various chemical reactions of an electrolyte are all ionic reactions,
the chemical activity of an electrolytic solution being proportional to its
electric conductivity, _i.e._ the degree of dissociation of its ions. The
acidity of an electrolytic solution is due to the presence of the
dissociated ion H^+, and its strength is determined by the concentration of
these free hydrogen ions. Hence the greater the degree of dissociation the
stronger the acid.
The basic character of a solution is determined by the presence of the
hydroxyl radical OH^-. The greater the concentration of the hydroxyl ions,
_i.e._ the greater the dissociation, the stronger is the base.
The ions H^+ and OH^- are of special importance, since they are the ions of
water, H_2O = H^+ + OH^-. The degree of {32} dissociation of pure water is
but small. Water is, however, the most important of all the various agents
in the chemical reactions of life, since a large number of organic
substances are decomposed by water by a process of hydrolysis, and a vast
number of organic substances are but combinations of carbon with the ions
H^+ and OH^-, their diversity being due to variations in the relative
proportions and grouping.
_The Chemical, Therapeutic, and Toxic Actions of Ions._--The chemical,
therapeutic, antiseptic, and toxic actions of electrolytic solutions are
almost exclusively due to ionization. Take, for instance, a solution of
nitrate of silver in which the addition of chlorine produces a white
precipitate of chloride of silver. This precipitate occurs only when the
solution added is one such as NaCl, where the chlorine is present as the
free ion Cl^-. No such precipitate is produced in a solution of chlorate of
potassium or chloracetic acid, where the chlorine is entangled in the
complex ion ClO_3 or C_2H_3ClO_2.
Since, then, the toxic and pharmacological properties of an electrolyte
depend entirely on the ionic grouping, it behoves the physician and the
biologist to study the structure and grouping of the ions in a molecule,
rather than that of the atoms. Consider for a moment the totally different
properties of the phosphides and the phosphates. The former are extremely
toxic, while the latter are perfectly harmless. There is not the slightest
analogy between their actions on the living organism. On the other hand,
all the phosphides produce the same toxic and therapeutic effects, whatever
the cation with which they are united. Their toxic properties are derived
from the presence of the free phosphorus ion P^{---}. The phosphates
contain phosphorus in the same proportion as the phosphides, but this
phosphorus is harmlessly entangled in the complex ion PO_4^{---}, whose
properties are absolutely different from those of the ion P^{---}.
The above considerations apply equally to the chlorides and chlorates, the
iodides and iodates, the sulphides and sulphates, and in general to all
chemical salts. {33}
The question has an intimate bearing on practical pharmacology. When we
prescribe a cacodylate or an amylarsinate, we are not prescribing an
arsenical treatment whose effects can be compared with those of an
arsenide, an arsenite, or an arsenate. This fact is sufficiently indicated
by the difference in the toxic doses of the different salts. Each variety
of arsenical ion has its own special physiological and therapeutic
properties. We do not expect to obtain the results of a ferruginous
treatment from the administration of a ferrocyanide or a ferricyanide. Both
contain iron, it is true, but neither possess the properties of the cation
Fe^{+++}, but rather those of the complex anion of which they form a part.
We have already said that most of the therapeutic, toxic, and caustic
actions of an electrolyte are due to ionic action, and the substances can
therefore have no toxic action unless they are dissociated. Many of the
solvents employed in medicine, such as alcohol, glycerine, vaseline, and
chloroform dissolve the electrolytes but do not dissociate them into ions,
and these solutions therefore do not conduct electricity. Such solutions
have no therapeutic action. With the absence of dissociation all the ionic
toxic and caustic effects also disappear entirely, and only re-appear as
the water of the tissue is able slowly to effect the necessary
dissociation.
Carbolic acid dissolved in glycerine is hardly caustic and but very
slightly toxic. We have met with several instances in which a tablespoonful
of carbolized glycerine, in equal parts, has been swallowed without any ill
effect, either caustic or toxic, whereas the same dose dissolved in water
would have been fatal. This absence of dissociation has enabled the surgeon
Menciere to inject carbolic and glycerine in equal proportions into the
larger joints, the part being subsequently washed out with pure alcohol.
Thus by employing vaseline, oil, or glycerine as a solvent, and avoiding
the access of water, we are able to use electrolytic antiseptics in very
concentrated form. Their action is brought out very slowly, as the water of
the organism effects the necessary dissociation of the electrolyte. {34}
Since all chemical, toxic, and therapeutic actions are ionic, they are
proportional to the degree of ionic concentration, _i.e._ to the number of
ions in a given volume. The only point of importance, that which determines
their activity, whether chemical or therapeutic, is the degree of
ionization or dissociation. For example, all acids have the same cation
H^+. They have all identical properties, but they differ widely in the
intensity of their action. There are weak acids such as acetic acid, and
strong acids like sulphuric acid. The stronger acids are those which are
more thoroughly dissociated, and in which the ion H^+ is very concentrated;
whereas the feeble acids are but slightly dissociated, so that the ion H^+
is less concentrated.
Paul and Kroenig have shown that the bactericidal action of different salts
also varies with their degree of dissociation, _i.e._ with the
concentration of the active ions. They made a series of observations on the
bactericidal action of various salts of mercury, the bichloride, the
bibromide, and the bicyanide, on the spores of _Bacillus anthracis_. The
following results were obtained from a comparison of solutions containing 1
gramme-molecule of the salt in 64 litres of water. With the bichloride
solution, after exposure to the solution for twenty minutes, only 7
colonies of the bacillus were developed. After exposure to a similar
solution of the bibromide the number of colonies was 34. The antiseptic
action of the bichloride was therefore five times as great as that of the
bibromide. The bicyanide of mercury, however, even when four times as
concentrated, permitted the growth of an enormous number of colonies,
showing that it had no appreciable antiseptic action whatever.
Nevertheless, the proportion of Hg is the same in all the solutions, and if
there were any difference one would naturally expect that the ion Cy^-
would be more toxic than Cl^- or Br^-. The real condition which varies in
these solutions and determines their activity is the degree of
dissociation. The whole of the antiseptic property resides in the ion
Hg^{++}. This ion is very {35} concentrated in the highly dissociated
solution HgCl_2, less concentrated in the less ionized solution HgBr_2, and
exceedingly dilute in the HgCy_2, which is hardly ionized at all.
What is true of the bactericidal action of the salts of mercury is equally
true of their therapeutic effect. It is a great mistake to estimate the
medicinal activity of a solution of a salt of mercury, or indeed of any
electrolytic solution, simply by its degree of molecular concentration. The
important point is the degree of dissociation, which is the only true
measure of its activity. In the intramuscular injection of mercury salts it
is by no means a matter of indifference what salt we employ. A salt should
be used such as the bichloride or the biniodide, which is easily
dissociated. Other salts are often employed because they occasion less pain
at the site of injection; but the pain is a sign of the degree of activity
of the preparation. The pain, it is true, may be avoided by using a salt
which is less easily dissociated, or in which the mercury is bound up in a
complex ion, but by so doing we diminish the efficacy of the remedy. It is
moreover quite easy to diminish, or even entirely to suppress, the pain, by
using a very dilute solution of an active ionized salt. A one-half per
cent. or even one-quarter per cent. solution of the bichloride or biniodide
of mercury may be injected very slowly in sufficient quantity without
producing the slightest discomfort. Local action depends entirely on ionic
concentration. One drop of pure sulphuric acid will destroy the skin,
whereas the same amount if diluted in a tumblerful of water will furnish a
refreshing drink.
* * * * *
{36}
CHAPTER IV
COLLOIDS
As we have already seen, living organisms are formed essentially of
liquids. These liquids are solutions of crystallizable substances or
crystalloids, and non-crystallizable substances or colloids--a
classification which we owe to Graham.
The liquids are the most important constituents of a living organism, since
they are the seat of all the chemical and physical phenomena of life. The
junction of two liquids of different concentration is the arena in which
takes place both the chemical transformation of matter and the correlative
transformation of energy. In a former chapter we have passed in review the
class of crystalloids, we will now turn our attention to the characteristic
properties of colloids.
_Colloids._--Colloids differ from crystalloids in that they do not form
crystals from solution, being completely amorphous when in the solid state.
The solution of a colloid solidifies in the same form which it possessed in
the liquid state, the solvent being enclosed in the meshes of a sort of
network formed by the solute. This form is approximately retained even
after the water has evaporated by drying, the passage from the liquid state
of solution to the solid state being effected through a series of
intermediary states, such as a clot, coagulum, or jelly. This passage from
the state of solution into a state of jelly is called coagulation. Some
colloids, such as gelatine, coagulate with cold; while others, such as
egg-albumin, coagulate with heat. Some, like the caseine of milk, require
the addition of certain chemical substances to set up coagulation; while
still others, such as the fibrin of blood, appear to coagulate
spontaneously. The physical phenomena of {37} coagulation are still but
little understood. In some cases it is a reversible phenomenon, thus
gelatine coagulated by cold is redissolved by heat; whereas with other
colloids the process is irreversible, albumin coagulated by heat is not
redissolved on cooling.
Colloids in a state of coagulation have a vacuolar or sponge-like
structure. The solvent is imprisoned in the vacuoles of the clot, and is
expelled little by little by its retraction. Colloids diffused in water are
usually called colloidal solutions, but they are not true solutions. Such a
pseudo-solution of a colloid is called a "sol," while a colloid in a state
of coagulation is called a "gel." Colloidal solutions spread but little,
diffuse very slowly in the liquids of the body, and cannot penetrate
organic membranes.
Colloidal solutions diffuse light, unlike crystalloid solutions, which are
transparent. We all know how the trajectory of a beam of sunlight through a
darkened room is rendered visible by the particles of dust. In the same way
if a colloidal solution is illuminated by a transverse ray of light, the
light is diffused by the molecules of the colloid in semi-solution, and the
liquid appears faintly illuminated on a dark background. The light diffused
by a colloidal solution is polarized, which shows that it is reflected
light,
Siedentopf and Sigmondy have applied this principle of lateral illumination
on a dark background to the construction of the ultra-microscope. With the
aid of this instrument we may not only see, but count the particles in a
colloidal solution, which is in reality merely a pseudo-solution or
suspension, in contradistinction to the true solution of a crystalloid.
Colloidal solutions possess only a very feeble osmotic pressure. The
lowering of the freezing point and the other corresponding constants are
also quite insignificant. This arises from the fact that the molecules of a
colloid are extremely large when compared with those of a crystalloid. For
example let us take colloidal substance whose molecular weight is 2000. A
solution containing 40 grammes per litre would have an osmotic pressure
only one-fiftieth of that of a {38} solution of similar strength of a
crystalloid whose molecular weight was 40.
Not only so, but on measuring the molecular concentration, the osmotic
pressure, and the other constants of a colloidal solution, we find values
even lower than those which we should expect from a consideration of its
molecular weight. This is probably due to the tendency of a colloid to
polymerization, i.e. to form groups or associations of molecules. Suppose,
for instance, that the molecules of a colloidal solution are aggregated
into groups of ten. Since each group plays the part of a simple molecule,
the osmotic pressure will be ten times less than that corresponding to the
quantity of the solute present. Such a group of molecules is called by
Naegeli a "micella."
Similar phenomena of aggregation may be observed in the molecules of many
inorganic substances. The molecule of iodine, for example, is monatomic at
1200deg C., but becomes diatomic at the ordinary temperature. Sulphur at
860deg C. is a gas with a vapour density of 2.2, while at 500deg C. its
vapour density rises to 6.6. In both of these cases two or more molecules
of the element have been condensed into one as a result of the fall of
temperature.
We frequently find that two successive cryoscopic observations on the
freezing point of the same colloidal solution will vary. This is due to the
extreme sensitiveness of the micellae, which absorb or abandon their extra
molecules under the slightest influence. This mobility in the constitution
of the micellae appears to be one of the principal causes of the peculiar
properties of colloidal solutions.
The phenomenon of polymerization appears to be reversible. The micellae are
formed under certain conditions, and are disintegrated when these
conditions are removed. The osmotic pressure varies in the same manner,
diminishing with polymerization and augmenting with the disintegration of
the micellae. One may easily understand what an important role is played by
this alternate polymerization and disintegration in the phenomena of life.
Most colloidal substances are precipitated from their solutions by the
addition of very small quantities of electrolytic {39} solutions.
Non-electrolytic solutions do not appear to provoke this precipitation.
This is not a chemical action, for an exceedingly small quantity of an
electrolyte is able to precipitate an indefinite quantity of the colloid.
The precipitation is probably due to the electric charges carried by the
dissociated ions of the electrolytes.
When an electric current is passed through a colloid solution, the course
of the molecules of the colloid is sometimes towards the cathode and
sometimes towards the anode, according to the nature of the colloid and of
the solvent. This displacement would appear to indicate a difference of
electric potential between the molecules of the colloid and those of the
solvent. Hardy has shown that in an alkaline solution the molecules of
albumin travel towards the anode, while in an acid solution they travel
towards the cathode.
_Metallic Colloids._--Carey Lea and afterwards Crede succeeded in obtaining
silver in colloidal solution by ordinary chemical means. Professor Bredig
has introduced a more general method of obtaining a number of metals in
colloidal solutions in a state of great purity. He causes an electric arc
to pass between two rods of the metal immersed in distilled water. The
cathode is thus pulverized into a very fine powder which rests in
suspension in the liquid, constituting a colloidal solution. Bredig has in
this way prepared sols of platinum, palladium, iridium, silver, and
cadmium.
_Catalytic Properties of Colloids._--Catalysis is the property possessed by
certain bodies of initiating chemical reaction. The mass of the catalyzing
body has no definite proportion to that of the substances entering into the
reaction, and the appearance of the catalyzer is in no way altered by the
reaction.
Ostwald has shown that catalysis consists essentially in the acceleration
or retardation of chemical reactions which would take place without the
action of the catalyzer, but more slowly.
Catalytic reactions are very numerous in chemistry. The inversion of sugar
by acids, the etherization of alcohol by sulphuric acid, the decomposition
of hydrogen peroxide by {40} platinum black are all instances of catalysis.
Fermentation by means of a soluble ferment or diastase, a phenomenon which
may almost be called vital, is also a catalytic action. The action of
pepsin, of the pancreatic ferment, of zymase, and of other similar ferments
has a great analogy with the purely physical phenomenon of catalysis. The
diastases are all colloids, and so are many other catalyzers.
A catalyzer is a stimulus which excites a transformation of energy. The
catalyzer plays the same role in a chemical transformation as does the
minimal exciting force which sets free the accumulation of potential energy
previous to its transformation into kinetic energy. A catalyzer is the
friction of the match which sets free the chemical energy of the powder
magazine.
Bredig has studied the catalytic decomposition of hydrogen peroxide by
metallic colloids prepared by his electric method. He found that 1
atom-gramme of colloidal platinum gives a sensible catalytic effect when
diluted with 70 million litres of water. Caustic soda and other chemical
substances inhibit the catalytic action of colloidal platinum in the same
way as they inhibit the fermenting action of diastase. The curve of
decomposition of hydrogen peroxide by colloidal platinum may be compared
with the curve of fermentation by emulsin. Both are equally affected by the
addition of an alkali. Many other chemical and physical agents have a
similar inhibitory action on the catalysis of colloidal metals and on
diastasic fermentation. Thus a mere trace of sulphuretted hydrogen or
hydrocyanic acid will paralyse the action of a colloidal metal, just as it
does that of a ferment. This is what Bredig calls the poisoning of metallic
ferments.
We may hope that the further study of catalysis, a purely physico-chemical
phenomenon, may throw more light on the mechanism of diastasic
fermentation, which is essentially a vital reaction.
It must not be forgotten that all classification is artificial and
arbitrary, and only to be used as long as it facilitates study. This
observation is particularly applicable to the classification of substances
into crystalloids and colloids. {41} There is no sharp line between the two
groups, the passage is gradual, and it is impossible to say where one group
ends and the other begins. Many colloids such as haemoglobin are
crystallizable, and many crystallizable substances are coagulable. Many
substances appear at one time in the crystalloid state and at another time
in the colloidal state, so that instead of dividing substances into
colloids and crystalloids, we should rather consider these expressions as
denoting different phases assumed by the same substance.
In order to define clearly our various classes and divisions, we are apt to
exaggerate slight differences of properties or composition. We say that
colloids have no osmotic pressure, whereas in fact the osmotic pressure of
the colloids though feeble plays a very important part in the phenomena of
life.
So in other departments of science--a factor which is almost infinitesimal
may yet exercise a vast influence on the results. It is by infinitesimal
variations of pressure, a thousandth of a millimetre or less, that we
obtain the various degrees of penetration in the Roentgen rays.
The division into solutions and pseudo-solutions or suspensions is also an
arbitrary one. A true solution is also a suspension of the molecules of the
solute. There is no essential difference between a solution and a
suspension, but only a difference in the size of the molecules, or
agglomerations of molecules, in one case so small as to be transparent, and
in the other case just big enough to diffuse light. There are moreover many
properties common to colloidal solutions and suspensions of fine powders,
such as kaolin, mastic, charcoal, or Indian ink. These particles in
suspension are precipitated by solutions of electrolytes in a manner
similar to the coagulation of colloids.
The surface of every liquid is covered by a very thin layer, a sort of
membrane slightly differentiated from the rest of the liquid. This membrane
may be a chemical one, a pellicular precipitate like that which is formed
by the contact of two membranogenous liquids. On the other hand, the
membrane may not differ from the subjacent liquid in chemical composition,
but only in physical properties. If we {42} consider the molecules in the
middle of a liquid, each molecule is subjected to the cohesive attraction
of molecules on every side, attractions which neutralize one another. At
the surface of the liquid, however, there are quite other conditions of
equilibrium. There each molecule is drawn downwards towards the centre of
the liquid, and there is no compensating attraction in an opposite
direction. The resultant pressure is normal to the surface of the liquid,
and is mechanically equivalent to an elastic membrane which tends to
diminish the surface, and hence the volume of the liquid. We may therefore
regard this surface tension as acting the part of a veritable physical
membrane.
There is a still further differentiation of the surface of a liquid. When
the liquid is not a simple one, but complex as in a solution, we find that
the concentration of the solute is greater at the surface than in the
interior. This is the so-called phenomenon of "adsorption," which is
another cause for the production of a physical membrane covering the
surface of a liquid.
Substances in a colloidal state have a great tendency to form these
chemical or physical membranes at the point of contact between the
colloidal solute and the solvent. This is probably the reason why the
coagulum of a colloidal liquid usually presents a vacuolar or spongy
structure.
* * * * *
{43}
CHAPTER V
DIFFUSION AND OSMOSIS
_Diffusion and Osmosis._--If we place a lump of sugar in the bottom of a
glass of water, it will dissolve, and spread by slow degrees equally
throughout the whole volume of the liquid. If we pour a concentrated
solution of sulphate of copper into the bottom of a glass vessel, and
carefully pour over it a layer of clear water, the liquids, at first
sharply separated by their difference of density, will gradually mix, so as
to form a solution having exactly the same composition in all parts of the
jar. The process whereby the sugar and the copper sulphate spread uniformly
through the whole mass of the liquid in opposition to gravity is called
Diffusion. This diffusion of the solute is a phenomenon exactly analogous
to the expansion of a gas. It is the expression of osmotic pressure, or
rather of the difference of the osmotic pressure of the solute in different
parts of the vessel. The molecules of the solute move from a place where
the osmotic pressure is greater towards a position where the osmotic
pressure is less. The water molecules on the other hand pass from positions
where the osmotic pressure of the solute is less towards positions where it
is greater. As a consequence of this double circulation the osmotic
pressure tends to become equalized in all parts of the vessel.
Diffusion appears to be the fundamental physical phenomenon of life. It is
going on continually in the tissues of all living beings, and a study of
the laws of diffusion and osmosis is therefore absolutely necessary for a
just conception of vital phenomena.
_Coefficient of Diffusion._--The coefficient of diffusion has {44} been
defined by Fick as the quantity of a solute which in one second traverses
each square centimetre of the cross section of a column of liquid 1
centimetre long, between the opposite sides of which there is unit
difference of concentration. Nernst in his definition substitutes "unit
difference of osmotic pressure" for "unit difference of concentration."
Until recently it was generally believed that diffusion took place in
colloids and plasmas just as in pure water. This is, however, by no means
the case: the differences are considerable. When a solute is introduced
into a colloidal solution, the greater the concentration of the colloid the
slower will be the diffusion. This may be shown by a simple experiment.
Several glass plates are prepared, by spreading on each a solution of
gelatine of different concentration, to which a few drops of phenol
phthalein have been added. If now a drop of an alkaline solution be placed
on each plate, we can see that the drop diffuses more slowly through the
more concentrated gelatine solution, since the presence of the alkali is
rendered visible by the coloration of the phenol phthalein. A similar
demonstration may be made by allowing drops of acid to diffuse through
solutions of gelatine made slightly alkaline and coloured with phenol
phthalein. In general, we find on experiment that when similar drops of any
coloured or colouring solution are left for an equal time on plates of
gelatine of different degrees of concentration, the greater the
concentration of the gelatine the smaller will be the circle of coloration
obtained.
We may show that the rapidity of diffusion diminishes as the gelatinous
concentration increases, by another experiment. If we put side by side on
our gelatine plate a drop of sulphate of copper and another of ferrocyanide
of potassium, the point of contact of the two fluids will be sharply marked
by a line of precipitate. We find that under similar conditions the time
between the sowing of the drops and the formation of this line of
precipitate is longer when the gelatine is more concentrated.
_Osmosis._--In 1748, l'Abbe Nollet discovered that when a pig's bladder
filled with alcohol was plunged into water, the {45} water passed into the
bladder more rapidly than the alcohol passed out; the bladder became
distended, the internal pressure increased, and the liquid spirted out when
the bladder was pricked by a pin. This passage of certain substances in
solution through an animal membrane is called Osmosis, and membranes which
exhibit this property are called osmotic membranes.
_Precipitated Membranes._--In 1867, Traube of Breslau discovered that
osmotic membranes could be made artificially. Certain chemical precipitates
such as copper ferrocyanide can form membranes having properties analogous
to those of osmotic membranes. With these precipitated membranes Traube
made a number of interesting experiments. These have lately been collected
in the volume of his memoirs published by his son.
_Osmotic Membranes._--Osmotic membranes were formerly called semi-permeable
membranes, being regarded as membranes which allow water to pass through
them, but arrest the passage of the solute. This definition is inexact,
since no membrane permeable to water is absolutely impermeable to the
solutes. All we can say is that certain membranes are more permeable to
water than to the substances in solution, and are moreover very unequally
permeable to the various substances in solution. As a rule a membrane is
much more permeable to a solute whose molecule is of small dimensions.
Molecules of salt, for instance, pass through such a membrane much more
quickly than do those of sugar. The term "osmotic membrane" should
therefore in all cases replace that of "semi-permeable membrane."
Osmotic membranes behave exactly like colloids. The resistance which they
oppose to the passage of different substances varies with the nature of the
liquid or solute concerned. There is no real difference between the passage
of a solution through an osmotic membrane and its diffusion through a
colloid. The protoplasm of a living organism, being a colloid, acts exactly
like an osmotic membrane so far as regards the distribution of solutions
and substances in solution. {46}
The diffusion of molecules through a colloid, a plasma, or a membrane is
governed by laws precisely analogous to Ohm's law, which governs the
transport of electricity. The intensity or rapidity of diffusion is
proportional to the difference of osmotic pressure, and varies inversely
with the resistance.
In the case of molecular diffusion, however, the rapidity of diffusion
depends also on the size and nature of the molecules of the diffusing
substance. The theory of the resistance of the various plasmas and
membranes to diffusion has been but little understood; we can discover
hardly any reference to it in the literature of the subject.
The laws of diffusion apply equally to the diffusion of ions. Nernst has
shown that there is a difference of electric potential at the surface of
contact of two electrolytic solutions of different degrees of
concentration. Both the positive and negative ions of the more concentrated
solution pass into the less concentrated solution, but the ions of one sign
will pass more rapidly than those of the other sign, because being smaller,
they meet with less resistance.
The resistance of the medium plays a most important part in all the
phenomena of diffusion. When two solutions of different concentration come
into contact, the interchange of molecules and ions which occurs is unequal
owing to the differences in resistance. Hence both solutions become
modified not only in concentration but also in composition. It has long
been known that diffusion can cause the decomposition of certain easily
decomposed substances, and it would appear probable that diffusion is also
capable of producing new chemical combinations.
The separation of the liberated ions in consequence of the unequal
resistance which they meet with in the medium they traverse often
determines chemical reaction. This ionic separation is a fertile agent of
chemical transformation in the living organism, and may be the determinant
cause in those chemical reactions which constitute the phenomena of
nutrition.
When different liquids come into contact there are two distinct series of
phenomena, those due to osmotic pressure and those due to differences of
chemical composition. Even {47} with isotonic solutions there will be a
transfer of the solutes if these are of different chemical constitution.
Take, for instance, two isotonic solutions, one of salt and another of
sugar. When these are brought into contact there is no transference of
water from one solution to the other, but there is a transference of the
solutes. In the salt solution the osmotic pressure of the sugar is zero.
Hence the difference of osmotic pressure of the sugar in the two solutions
will cause the molecules of sugar to diffuse into the salt solution. For
the same reason the salt will diffuse into the sugar solution.
A disregard of this fact, that a solute will always pass from a solution
where its osmotic pressure is high, into one where its osmotic pressure is
low, is a frequent source of error. Thus it is said to be contrary to the
laws of osmosis that solutes should pass from the blood, with its low
osmotic pressure, into the urine, where the general osmotic pressure is
higher; the more so because in consequence of the exchange the osmotic
pressure of the urine is still further increased. Such an exchange, it is
argued, is contrary to the ordinary laws of physics, and can therefore only
be accomplished by some occult vital action. This, however, is not the
fact, as is proved by experiment.
Consider an inextensible osmotic cell containing a solution of sugar, the
walls of the cell being impermeable to sugar but permeable to salt. Let us
plunge such a cell into a solution of salt, which has a lower osmotic
pressure than the sugar solution. Since the walls of the cell are
inextensible, the quantity of water in the cell cannot increase. The salt,
however, will pass into the cell, since the osmotic pressure of the salt is
greater on the outside than on the inside, and the walls are permeable to
the molecules of salt. This passage will continue until the osmotic
pressure of the salt is equal inside and outside the cell; at the same time
the total osmotic pressure within the cell will have increased, in spite of
its being originally greater than the osmotic pressure outside.
_Plasmolysis._--We all know that a cut flower soon dries {48} up and fades.
When, however, we place the shrivelled flower in water, the contracted
protoplasm swells up again and refills the cells, which become turgid, and
the flower revives. This phenomenon is due to the fact that vegetable
protoplasm holds in solution substances like sugars and salts which have a
high osmotic pressure. Consequently water has a tendency to penetrate the
cellular walls of plants, to distend the cells and render them turgescent.
De Vries has used this phenomenon for the measurement of osmotic tension.
He employs for this purpose the turgid cells of the plant _Tradescantia
discolor_. The cells are placed under the microscope and irrigated with a
solution of nitrate of soda. On gradually increasing the concentration of
the solution there comes a moment when the protoplasmic mass is seen to
contract and to detach itself from the walls of the cell. This phenomenon,
which is known as plasmolysis, occurs at the moment when the solution of
nitrate of soda begins to abstract water from the protoplasmic juice,
_i.e._ when the osmotic tension of the nitrate of soda becomes greater than
that of the protoplasmic liquid. So long as the osmotic tension of the soda
solution is less than that of the protoplasm, there will be a tendency for
water to penetrate the cell wall and swell the protoplasm. When the osmotic
tension of the solution which bathes the cell is identical with that of the
cellular juice, there is no change in the volume of the protoplasm. In this
way we are able to determine the osmotic pressure of any solution. We have
only to dilute the solution till it has no effect on the protoplasm of the
vegetable cells. Since the osmotic tension of this protoplasm is known, we
can easily calculate the osmotic tension of the solution from the degree of
dilution required.
_Red Blood Corpuscles as Indicators of Isotony._--In 1886, Hamburger showed
that the weakest solutions of various substances which would allow the
deposition of the red blood cells, without being dilute enough to dissolve
the haemoglobin, were isotonic to one another, and also to the blood serum,
and to the contents of the blood corpuscles. This is Hamburger's method of
determining the osmotic {49} tension of a liquid. The diluted solution is
gradually increased in strength until, when a drop of blood is added to it,
the corpuscles are just precipitated, and no haemoglobin is dissolved.
_The Haematocrite._--In 1891, Hedin devised an instrument for determining
the influence of different solutions on the red blood corpuscles. This
instrument, the haematocrite, is a graduated pipette, designed to measure
the volume of the globules separated by centrifugation from a given volume
of blood under the influence of the liquid whose osmotic pressure is to be
measured. The method depends on the principle that solutions isotonic to
the blood corpuscles and to the blood serum will not alter the volume of
the blood corpuscles, whereas hypertonic solutions decrease that volume.
_Action of Solutions of Different Degrees of Concentration on Living
Cells_.--We have just seen that a living cell, whether vegetable or animal,
is not altered in volume when immersed in an isotonic solution that does
not act upon it chemically. When immersed in a hypertonic solution, it
retracts; in a slightly hypotonic solution it absorbs water and becomes
turgescent, while in a very hypotonic solution it swells up and bursts. In
a hypertonic solution the red blood cells retract and fall to the bottom of
the glass, the rapidity with which they are deposited depending on the
amount of retraction. In a hypotonic solution they swell up and burst, the
haemoglobin dissolving in the liquid and colouring it red. This is the
phenomenon of haematolysis. According to Hamburger, the serum of blood may
be considerably diluted with water before producing haematolysis.
Experimenting with the blood of the frog, he found that the globules
remained intact in size and shape when irrigated with a salt solution
containing .64 per cent. of salt, this solution being isotonic with the
frog's blood serum. On the other hand, they did not begin to lose their
haemoglobin till the proportion of salt was reduced to below .22 per cent.
Thus frog's serum may be diluted with 200 per cent. of water before
producing haematolysis. In mammals the blood corpuscles remain invariable
in a salt solution of about .9 per cent., and begin to lose their {50}
haemoglobin approximately in a .6 per cent. solution. A solution of .9 per
cent. of NaCl is therefore isotonic to the contents of the red blood
corpuscles, to the serum of the blood, and to the cells of the tissues. It
by no means follows that the cells of the blood and tissues undergo no
change when irrigated with a .9 per cent. solution of chloride of sodium.
They do not lose or gain water, it is true, and they retain their volume
and their specific gravity. But they do undergo a chemical alteration, by
the exchange of their electrolytes with those of the solution. Hamburger
has pointed out that in mammals the shape of the red corpuscles is altered
in every liquid other than the blood serum; even in the lymph of the same
animal there is a diminution of the long diameter, and an increase of the
shorter diameter, while the concave discs become more spherical.
All the cells of a living organism are extremely sensitive to slight
differences of osmotic pressure--the cells of epithelial tissue and of the
nervous system as well as the blood cells. For instance, the introduction
of too concentrated a saline solution into the nasal cavity will set up
rhinitis and destroy the terminations of the olfactory nerves. Pure water,
on the other hand, is itself a caustic. There is a spring at Gastein, in
the Tyrol, which is called the poison spring, the "Gift-Brunnen." The water
of this spring is almost absolutely pure, hence it has a tendency to
distend and burst the epithelium cells of the digestive tract, and thus
gives rise to the deleterious effects which have given it its name.
Ordinary drinking water is never pure, it contains in solution salts from
the soil and gases from the atmosphere. These give it an osmotic pressure
which prevents the deleterious effects of a strongly hypotonic liquid.
During a surgical operation it is of the first importance not to injure the
living surfaces by flooding them with strongly hypertonic or hypotonic
solutions. This precaution becomes still more important when foreign
liquids are brought into contact with the delicate cells of the large
surfaces of the serous membranes. Gardeners are well aware of the noxious
influence of a low osmotic pressure. They water the soil around the roots
of a plant, so that the water may take up {51} some of the salts from the
soil before being absorbed by the plant. Pure water poured over the heart
of a delicate plant may burst its cells owing to its low osmotic pressure.
In many medical and surgical applications, on the other hand, a low osmotic
pressure is of advantage. Thus, in order to remove the dry crusts of eczema
and impetigo, the most efficacious application is a compress of cotton wool
soaked in warm distilled water. Under the influence of such a hypotonic
solution the dry cells rapidly swell up, burst, and are dissolved.
Cooking is also very much a question of osmotic pressure. If salt is put
into the water in which potatoes and other vegetables are boiled, osmosis
is set up and a current of water passes from the vegetable cells to the
salt water. The cellular tissue of the vegetable becomes contracted and
dried, and the membranes become adherent, the vegetable loses weight and
becomes difficult of digestion, in consequence of its hard and waxy
consistency, which prevents the action of the digestive juices. Vegetables
should be cooked in soft water, and should be salted after cooking. When so
treated, a potato absorbs water, the cells swell up, the skin bursts, the
grains of starch also swell up and burst, and the pulp becomes more
friable. The digestive juice is thus able to penetrate the different parts
of the vegetable rapidly, and digestion is facilitated. Any one can easily
prove for himself that a potato boiled in salt water diminishes in weight,
whilst its weight increases when it is cooked in soft water.
The method of cryoscopy is also of considerable service in forensic
medicine. As shown by Carrara, the cryoscopy of the blood is an important
aid in determining the question whether a body found in the water was
thrown in before or after death. In the former case the concentration of
the blood will be much diminished. In certain experiments on dogs the
cryoscopic examination of the blood showed a freezing point of -.6deg C.
The dog was then drowned, when the freezing point of the blood in the left
ventricle was increased to -.29deg C., and that in the right ventricle to
-.42deg C. On the other hand, when a dog was killed before being thrown
into the water, the {52} osmotic pressure of the blood was hardly decreased
even after an immersion of 72 hours. In the case of persons or animals
drowned in sea water, a similar alteration of the point of congelation is
observed, but in the reverse direction. In this case the osmotic pressure
is raised considerably in those who are drowned, whereas no such rise is
observed in those who are thrown into the sea after death.
The circulation of the sap in plants and trees is also in great part due to
osmotic pressure. The aspiration of the water from the soil is due to the
intracellular osmotic pressure in the roots, which causes the sap to rise
in the stem of a plant as it would in the tube of a manometer. From a
knowledge of the osmotic pressure of the intracellular liquid of the roots,
we may calculate the height to which the sap can be raised in the trunk of
a tree, _i.e._ the maximum height to which the tree can possibly grow.
Suppose, for instance, the plasma of the rootlets has an osmotic pressure
of six atmospheres, corresponding to that of a 9 per cent. solution of
sugar. A pressure of six atmospheres is equal to the weight of a column of
water 6 x .76 x 13.596 = 61.95 metres high. This, then, is the maximum
height to which this osmotic pressure is able to lift the sap. That is to
say, a tree whose rootlets contain a solution of sugar of 9 per cent.
concentration, or its equivalent, can grow to a height of 62 metres.
Cryoscopy is also of great use in practical medicine, more especially for
the examination of the urine. The freezing point of urine varies from
-1.26deg C. to -2.35deg. Koryani has studied the ratio of the point of
congelation of urine to that of a solution containing an equal quantity of
chloride of sodium. He finds that the ratio (freezing point of urine) /
(freezing point of NaCl) increases when the circulation through the tubules
of the kidney is diminished.
Hans Koeppe has shown that the hydrochloric acid of the gastric juice is
produced by the osmotic exchanges between the blood and the gastric
contents. The ion Na^+ of the salt in the stomach contents exchanges with
an ion H^+ of the monobasic salts of the blood, NaHCO_3 + NaCl = HCl +
Na_2CO_3. {53}
_Influence of Muscular Contraction on the Intramuscular Osmotic
Pressure._--When a muscle is immersed in an isotonic salt solution it does
not change in weight. In a hypertonic solution it loses weight in
consequence of a loss of water, which passes from the muscle into the
solution to equalize the osmotic pressure. It gains weight in a hypotonic
solution, the water current setting towards the point of higher
concentration. It is easy, therefore, to tell whether the osmotic pressure
in a muscle is above or below that of a given solution, by observing
whether the muscle gains or loses weight when immersed in it. Thus we may
measure the osmotic pressure in a muscle by finding a salt solution in
which the muscle neither gains nor loses weight. In this way we have been
able to prove that the osmotic pressure of a tired muscle is higher than
that of the normal muscle. Our experiments were carried out on the muscles
of frogs. After having pithed the frog, one of the hind legs is removed by
a single stroke of the scissors. The leg is skinned, dried with blotting
paper, and weighed. It is then placed in a salt solution whose freezing
point is -.53deg C. At 15deg C. such a solution has an osmotic pressure of
6.6 atmospheres. We next proceed to determine the osmotic pressure of the
corresponding leg after it has been tired by muscular work. For this it is
stimulated by an intermittent faradic current passing once a second for
five minutes. The leg is then skinned, dried, weighed, and placed in the
same salt solution. After eight hours' immersion the legs are weighed
again. The following are the results of six experiments, the numbers
representing fractions of the original weight:--
Change of weight of untired leg--
After 8 hours -.000.
After 16 hours -.000.
After 24 hours -.006.
Change of weight of stimulated leg--
After 8 hours +.050.
After 16 hours +.080.
After 24 hours +.101.
{54}
This result shows that muscular work provoked by electric stimulation
noticeably increases the osmotic pressure of the muscle.
In order to discover the exact osmotic pressure in the stimulated muscles
we repeated the series of experiments, using more and more concentrated
solutions. In a solution whose freezing point was -.57deg, we obtained the
following values:--
Change of weight of untired leg--
After 8 hours -.000.
After 16 hours -.004.
After 24 hours -.006.
Change of weight of stimulated leg--
After 8 hours +.039.
After 16 hours +.072.
After 24 hours +.099.
Finally, in a solution freezing at -.72deg, _i.e._ with an osmotic pressure
at 15deg C. of 9.176 atmospheres, we obtained the following mean values for
the untired leg:--
After 8 hours -.04.
After 16 hours -.05.
After 24 hours -.05.
In this solution, freezing at -.72deg C., some of the stimulated muscles
showed no diminution in weight, while others showed a very small
diminution, and others again a slight augmentation, the maximum increase
being .085 of the initial weight. The solution is therefore practically
isotonic with the stimulated muscle.
In this case the elevation of the intramuscular osmotic pressure produced
by the electrical excitation and the muscular contractions was therefore
2.5 atmospheres, or more than 2.6 kilogrammes per square centimetre of
surface.
I made further experiments in order to discover whether the variation in
osmotic pressure depended on the duration of {55} the muscular contraction.
For this purpose I used a solution freezing at -.53deg C. and immersed in
it untired muscles, and muscles which had been electrically excited for
two, four, and six minutes respectively. The following are the results:--
Untired muscles. Muscles stimulated once a second during
2 Minutes. 4 Minutes. 6 Minutes.
.000 +.026 +.084 +.094
+.001 +.034 +.065 +.093
+.005 +.045 +.079 +.097
.000 +.037 +.070 +.095
.000 +.032 +.072 +.096
Mean of all the observations--
+.0012 +.0348 +.074 +.095
These experiments show clearly that the osmotic intramuscular pressure
rises in proportion to the duration of the electrical stimulation.
In order to determine the influence of the work accomplished by the muscle
on the elevation of the osmotic pressure, I made the following experiment.
The two hind legs of a frog were submitted to the same electrical
excitation, one leg being left at liberty, and the other being stretched by
a hundred-gramme weight, acting by a cord and pulley. After exciting them
electrically for five minutes, the legs were immersed for twenty-four hours
in a saline solution freezing at .53deg C. The free limb showed an
augmentation of .085 of the initial weight, and the stretched limb an
increase of .106 of the initial weight. It is evident, therefore, that the
osmotic pressure increases with the amount of work done by a muscle.
Briefly, then, the results of our experiments are as follow:--
1. Muscular contraction electrically produced causes an increase of the
osmotic pressure in a muscle.
2. The intramuscular osmotic pressure may reach, or even exceed, 2.5
atmospheres, or 2.6 kilogrammes per square centimetre of surface.
3. When a muscle is made to contract once a second, the {56} elevation of
the osmotic pressure increases with the number of contractions.
4. The intramuscular osmotic pressure increases with the work done by the
muscle.
5. Fatigue is caused by the increase of osmotic pressure in a contracting
muscle.
[Illustration: FIG. 3.--Fields of diffusive force.
(_a_) Monopolar field of diffusion. A drop of blood in a saline solution of
higher concentration.
(_b_) Bipolar field of diffusion. Two poles of opposite signs. On the right
a grain of salt forming a hypertonic pole of concentration, on the left a
drop of blood forming a hypotonic pole of dilution. ]
_The Field of Diffusion._--Just as Faraday introduced the conception of a
field of magnetic force and a field of electric force to explain magnetic
and electrical phenomena, so we may elucidate the phenomena of diffusion by
the conception of a field of diffusion, with centres or poles of diffusive
force. If we consider a solution as a field of diffusion, any point where
the concentration is greater than that of the rest may be considered as a
centre of force, attractive for the molecules of water, and repulsive for
the molecules of the solute. In the same way any point of less
concentration may be regarded as a centre of attraction for the molecules
of the solute, and a centre of repulsion for the molecules of water.
A field of diffusion may be monopolar or bipolar. A bipolar field has a
hypertonic pole or centre of concentration, and a hypotonic pole or centre
of dilution. By analogy with the magnetic and electric fields we may
designate the hypertonic pole as the positive pole of diffusion, and the
hypotonic as the negative pole. {57}
The positive and negative poles and the lines of force in the field of
diffusion may be illustrated by the following experiment. A thin layer of
salt water is spread over an absolutely horizontal plate of glass. If now
we take a drop of blood, or of Indian ink, and drop it carefully into the
middle of the salt solution, we shall find that the coloured particles will
travel along the lines of diffusive force, and thus map out for us a
monopolar field of diffusion, as in Fig. 3 a. Again, if we place two
similar drops side by side in a salt solution, their lines of diffusion
will repel one another, as in Fig. 4.
[Illustration: FIG. 4.--Two drops of blood in a more concentrated solution,
showing a field of diffusion between two poles of the same sign.]
Now let us put into the solution, side by side, one drop of less
concentration and another of greater concentration than the solution. The
lines of diffusion will pass from one drop to the other, diverging from the
centre of one drop and converging towards the centre of the other (Fig. 3
_b_). In this manner we are able to obtain diffusion fields analogous to
the magnetic fields between poles of the same sign and poles of opposite
signs.
The conception of poles of diffusion is of the greatest importance in
biology, throwing a flood of light on a number of phenomena, such as
karyokinesis, which have hitherto been regarded as of a mysterious nature.
It also enables us to appreciate the role played by diffusion in many other
biological phenomena. Consider, for example, a centre of anabolism in a
living organism. Here the molecules of the living protoplasm are in process
of construction, simpler molecules being united and built up to form larger
and more complex groups. As a result of this aggregation the number of
molecules in a given area is diminished, _i.e._ the concentration and the
osmotic pressure fall, producing a hypotonic centre of diffusion. We may
thus regard every centre of anabolism as a negative pole of diffusion. {58}
Consider, on the other hand, a centre of catabolism, where the molecules
are being broken up into fragments or smaller groups. The concentration of
the solution is increased, the osmotic pressure is raised, and we have a
hypertonic centre of diffusion. Every centre of catabolism is therefore a
positive pole of diffusion. Similar considerations as to the formation and
breaking up of the molecules in anabolism and catabolism apply to
polymerization.
The diffusion field has similar properties to the magnetic and the electric
field. Thus there is repulsion between poles of similar sign, and
attraction between poles of different signs. A simple experiment will show
this. A field of diffusion is made by pouring on a horizontal glass plate a
10 per cent. solution of gelatine to which 5 per cent. of salt has been
added. The gelatine being set, we place side by side on its surface two
drops, one of water, and one of a salt solution of greater concentration
than 5 per cent. We have thus two poles of diffusion of contrary signs, a
hypotonic pole at the water drop, and a hypertonic pole at the salt drop.
Diffusion immediately begins to take place through the gelatine, the drops
become elongated, advance towards one another, touch, and unite. If, on the
contrary, the two neighbouring drops are both more concentrated or both
less concentrated than the medium, they exhibit signs of repulsion as in
Fig. 4.
Diffusion not only sets up currents in the water and in the solutes, but it
also determines movements in any particles that may be in suspension, such
as blood corpuscles, particles of Indian ink, and the like. These particles
are drawn along with the water stream which passes from the hypotonic
centres or regions toward those which are hypertonic.
These considerations suggest a vast field of inquiry in biology, pathology,
and therapeutics. Inflammation, for example, is characterized by
tumefaction, turgescence of the tissues, and redness. The essence of
inflammation would appear to be destructive dis-assimilation with intense
catabolism. We have seen that a centre of catabolism is a hypertonic focus
of diffusion. Hence the osmotic pressure in an inflamed region is
increased, turgescence is produced, and {59} the current of water carries
with it the blood globules which produce the redness.
The phenomenon of agglutination may also possibly be due to osmotic
pressure, a positive centre of diffusion attracting and agglomerating the
particles held in suspension.
_Tactism and Tropism._--The phenomena of tactism and tropism may also be
partly explained by the action of these diffusion currents of particles in
suspension, these polar attractions and repulsions. In all experiments on
this subject we should take into account the possible influence of osmotic
pressure, since many of the causes of tactism or tropism also modify the
osmotic pressure at the point of action, and it is possible that this
modification is the true cause of the phenomenon. Osmotactism and
osmotropism have not as yet been sufficiently studied.
[Illustration: FIG. 5.--Liquid figures of diffusion.
The six negative poles of diffusion are coloured with Indian ink. The
positive pole in the centre is uncoloured and is formed by a drop of KNO_3
solution.]
Thus it may be said that osmotic pressure dominates all the kinetic and
dynamic phenomena of life, all those at least which are not purely
mechanical, like the movements of respiration and circulation. The study of
these vital phenomena is greatly facilitated by the conception of the field
of diffusion and poles of diffusion, and of the lines of force, which are
the trajectories of the molecules of the solutes, and the particles and
globules in suspension.
_The Morphogenic Effects of Diffusion._--Many interesting experiments may
be made showing variations of the lines of force in a field of diffusion,
and how liquids subjected only to differences of osmotic pressure diffuse
and mix with one {60} another in definite patterns. When a liquid diffuses
in another undisturbed by the influence of gravity, it produces figures of
geometric regularity, and we may thus obtain figures and forms of infinite
variety. The following is our method of procedure. A glass plate is placed
absolutely horizontal and is covered with a thin layer of water or of
saline solution. Then with a pipette we introduce into the solution, in a
regular pattern, a number of drops of liquid coloured with Indian ink. A
wonderful variety of patterns and figures may be obtained by employing
solutions of different concentration and varying the position of the drops.
[Illustration: FIG. 6.--Pattern produced in gelatine by the diffusion of
drops of concentrated solutions of nitrate of silver and bromide of
ammonium.]
Instead of the water or salt solution, we may spread on the plate a 5 or 10
per cent. solution of gelatine, containing various salts in solution. If
now we sow on this gelatine drops of various solutions which give
colorations with the salts in the gelatine, we may obtain forms of perfect
regularity, presenting most beautiful colours and contrasts. The drops, of
course, must be placed in a symmetrical pattern. In this way we may obtain
an endless number of ornamental figures.
In order to cover a lantern slide 8-1/2 cm. x 10 cm., about 5 c.c. of
gelatine is required. To this amount of gelatine we add a single drop of a
saturated solution of salicylate of sodium, and spread the liquid gelatine
evenly over the plate. When the gelatine has set, we put the plate over a
diagram, a hexagon for instance, and place a drop of ferrous sulphate
solution at each of the six angles. The drops immediately diffuse {61}
through the gelatine, and the result after a time is the production of a
beautiful purple rosette. The gelatine must be carefully covered to prevent
its drying until the diffusion is complete. The preparation may then be
dried and mounted as a lantern slide, and will give the most brilliant
effect on projection. If the gelatine has been treated with a drop of
potassium ferrocyanide solution instead of salicylate of sodium, a few
drops of FeSO_4 will give a blue pattern. Or we may treat the gelatine with
ferrocyanide of potassium and salicylate of sodium mixed, and thus obtain
an intermediary colour on the addition of FeSO_4. We may, indeed, vary
indefinitely the nature and concentration of the solution, as well as the
number and position of the drops. The results have all the charm of the
unexpected, which adds greatly to the interest of the experiment.
[Illustration: FIG. 7.--Pattern produced in gelatine by the diffusion of
drops of silver nitrate and sodium carbonate.]
These experiments are not merely a scientific toy. They show us the
possibility, hitherto unsuspected, that a vast number of the forms and
colours of nature may be the result of diffusion. Thus many of the
phenomena of life, hitherto so mysterious, present themselves to us as
merely the consequences of the diffusion of one liquid into another. One
cannot help hoping that the study of diffusion will throw still further
light on the subject.
If a number of spheres, each capable of expansion and deformation, are
produced simultaneously in a liquid, they will form polyhedra when they
expand by growth. This is the {62} precise architecture of a vast number of
living organisms and tissues, which are formed by the union of microscopic
polyhedra or cells. A section of such a polyhedral structure would appear
as a tissue of polygons. It is interesting to note that the simple process
of diffusion will produce such structures under conditions closely allied
to those which govern the development of the tissues of a living organism.
[Illustration: FIG. 8.--Pattern produced in gelatine by the diffusion of
drops of a solution of nitrate of silver and of citrate of potassium.]
We may obtain this cellular structure by a simple experiment. On a glass
plate we spread a 5 per cent. solution of pure gelatine, and when set sow
on it a number of drops of a 5 to 10 per cent. solution of ferrocyanide of
potassium. The drops must be placed at regular intervals of 5 mm. all over
the plate. When these have been allowed to diffuse and the gelatine has
dried, we obtain a preparation which exactly resembles the section of a
vegetable cellular tissue (Fig. 9). The drops have by mutual pressure
formed polygons, which appear in section as cells, with a membranous
envelope, a {63} nucleus, and a cytoplasm, which is in many cases entirely
separated from the membrane. These cells when united form a veritable
tissue, in all respects similar to the cellular structure of a living
organism.
[Illustration: FIG. 9.--Tissue of artificial cells formed by the diffusion
in gelatine of drops of potassium ferrocyanide.]
In the preparation showing artificial cells the cellular structure is not
directly visible until the gelatine has dried. One sees only a gelatinous
mass analogous to the protoplasm of a living organism. This mass is
nevertheless organized, or at least in process of organization, as we may
see by the refraction when its image is projected on the screen.
During the cell-formation, and as long as there is any difference of
concentration in the gelatine, each cell is the arena of active molecular
movement. There is a double current, as in the living cell, a stream of
water from the periphery to the centre, and of the solute from the centre
to the periphery. This molecular activity--the life of the artificial
cell--may be prolonged by appropriate nourishment, {64} _i.e._ by
continually repairing the loss of concentration at the centre of the cell.
The life of the artificial cell may also be prolonged by maintaining around
it an appropriate medium. If we prematurely dry such a preparation of
artificial cells, the molecular currents will cease, to recur again when we
restore the necessary humidity to the preparation. This to my mind gives us
a most vivid picture of the conditions of latent life in seeds and many
rotifera.
These artificial cells, like living organisms, have an evolutionary
existence. The first stage corresponds to the process of organization, the
gelatine representing the blastema, and the drop the nucleus. Thus the cell
becomes organized, forming its own cytoplasm and its own enveloping
membrane.
The second stage in the life of this artificial cell is the period during
which the metabolism of the cell is active and tends to equalize the
concentration of the liquid in the cell and in the surrounding medium.
The third stage is the period of decline. The double molecular current
gradually slows down as the difference of concentration decreases between
the cell contents and its entourage. When this equality of concentration
has become complete the molecular currents cease, the cell has terminated
its existence; it is dead. The currents of substance and of energy have
ceased to flow--the form only remains.
These artificial cells are sensible to most of the influences which affect
living organisms. Like living cells they are influenced both in their
organization and in their development by humidity, dryness, acidity, or
alkalinity. They are also greatly affected by the addition of minute
quantities of chemical substances either to the gelatinous blastema or to
the drops which represent the primary nuclei. We may in this way obtain
endless varieties, nuclei which are opaque or transparent, with or without
a nucleolus, and cells containing homogeneous cytoplasm without a nucleus.
We may also obtain cells with cytoplasm filling the whole of the cellular
cavity or separated from the cell-membrane. We may obtain {65} cells
imitating all the natural tissues, cells without a membranous envelope,
cells with thick walls adhering to one another, or cells with wide
intracellular spaces.
[Illustration: FIG. 10.--Artificial liquid cells, formed by coloured drops
of concentrated salt solution in a less concentrated salt solution.]
The forms of these artificial cells depend on the number and relative
position of the drops which represent the nuclei, and on the molecular
concentration or osmotic tension of the solution. The number of the
cellular polyhedra is determined by the number of centres of diffusion. The
magnitude of the dihedral angles, from which radiate three and occasionally
four walls, depends on the position of the hypertonic poles of diffusion.
The curvature of a surface is determined by the differences of
concentration on either side. Between isotonic solutions the surface is
plane, whilst it is curved between solutions of different osmotic
pressures, the convexity being directed towards the hypertonic solution.
[Illustration: FIG. 11.--Liquid cells with a fringe of cilia, obtained by
sowing coloured drops of concentrated salt solution in a weaker salt
solution. The contents of the cells have undergone segmentation.]
The time required for these artificial cells to grow varies from two to
twenty-four hours, according to the concentration of the gelatine, the
growth being most rapid in dilute solutions.
Similar cells may be produced in water. If we pour a thin layer of water on
a horizontal plate, and with a pipette {66} sow in it a number of drops of
salt water coloured with Indian ink, we may obtain artificial cells
composed entirely of liquid, having the same characters as those produced
in a gelatinous solution.
It is possible by liquid diffusion to produce not only ordinary cells but
ciliated cells. If we spread a layer of salt water on a horizontal glass
plate, and sow in it drops of Indian ink, artificial cells are produced by
diffusion. At the edge of the preparation there is often to be seen a sort
of fringe, analogous to the cilia of living cells (Fig. 11).
These tissues of artificial cells demonstrate the fact that inorganic
matter is able to organize itself into forms and structures analogous to
those of living organisms under the action of the simple physical forces of
osmotic pressure and diffusion. The structures thus produced have functions
which are also analogous to those of living beings, a double current of
diffusion, an evolutionary existence, and a latent vitality when desiccated
or congealed.
* * * * *
{67}
CHAPTER VI
PERIODICITY
_Periodic Precipitation._--A phenomenon is said to be periodic when it
varies in time and space and is identically reproduced at equal intervals.
We are surrounded on all sides by periodic phenomena; summer and winter,
day and night, sleep and waking, rhythm and rhyme, flux and reflux, the
movements of respiration and the beating of the heart, all are periodic.
Our first sorrows were appeased by the periodic rhythm of the cradle, and
in our later years the periodic swing of the rocking-chair and the hammock
still soothe the infirmities of old age.
Sound is a periodic movement of the atmosphere which brings to us harmony
and melody. Light consists of periodic undulations of the ether which
convey to us the beauty of form and colour. Periodic ethereal waves waft to
us the wireless message through terrestrial space and the radiant energy of
the sun and stars.
It is therefore not to be wondered at that the phenomena of diffusion are
also periodic. According to Professor Quinke of Heidelberg, the first
mention of the periodic formation of chemical precipitates must be
attributed to Runge in 1885. Since that time these precipitates have been
studied by a number of authors, and particularly by R. Liesegang of
Duesseldorf, who in 1907 published a work on the subject, entitled _On
Stratification by Diffusion_.
In 1901 I presented to the Congress of Ajaccio a number of preparations
showing concentric rings, alternately transparent and opaque, obtained by
diffusing a drop of potassium ferrocyanide solution in gelatine containing
a trace of ferric {68} sulphate. At the Congress of Rheims in 1907 I
exhibited the result of some further experiments on the same subject.
These periodic precipitates may be obtained from a great number of
different chemical substances. The following is the best method of
demonstrating the phenomenon. A glass lantern slide is carefully cleaned
and placed absolutely level. We then take 5 c.c. of a 10 per cent. solution
of gelatine and add to it one drop of a concentrated solution of sodium
arsenate. This is poured over the glass plate whilst hot, and as soon as it
is quite set, but before it can dry, we allow a drop of silver nitrate
solution containing a trace of nitric acid to fall on it from a pipette.
The drop slowly spreads in the gelatine, and we thus obtain magnificent
rings of periodic precipitates of arsenate of silver, with which any one
may easily repeat the experiments detailed in this chapter.
[Illustration: FIG. 12.--Lines of diffusion precipitate, showing the
simultaneous propagation of "undulations of different wave-length.]
_Circular Waves of Precipitation._--The wave-front of the periodic rings of
precipitates is always perpendicular to the rays of diffusion. The distance
between the rings depends on the concentration of the diffusing solution.
The greater the fall of concentration, the less is the interval between the
rings. Each ring represents an equipotential line in the field of
diffusion. These equipotential lines of diffusion give us the best and most
concrete reproduction of the mode of propagation of periodic waves in
space. They are, in fact, a visible diagram of the propagation of the waves
of light and sound. Occasionally we may observe in the gelatine the
simultaneous propagation of undulations of different wave-length, just as
we have them in the ether and the air. These diffusion wavelets {69} give
us a very beautiful representation of the simultaneous propagation of
undulations of different wave-length in the same medium.
[Illustration: FIG. 13.--Waves of diffusion refracted at a plane surface on
passing from a less concentrated into a more concentrated solution. The
refracted wave-front is flattened, the wave-length being less in the denser
medium.]
Like waves of light and sound, these waves of diffusion are refracted when
they pass from one medium into another of a different density, where they
have a different velocity. When, for instance, a diffusion wave passes from
a 5 per cent. solution of gelatine into a 10 per cent. solution, the
wave-front is retarded, the retardation being proportional to the length of
the path through the denser medium. Hence the wave-front is flattened, the
curvature of the refracted wave being less than that of the original wave
of diffusion. The contrary is the case when the wave-front passes into a
medium where its velocity is greater. The middle of the wave-front now
travels faster than the flanks, and the curvature is increased.
[Illustration: FIG. 14.--Transformation of a spherical wave-front into a
plane wave-front by a convergent diopter.]
These diffusion rings furnish us with most excellent diagrams of refraction
at a "diopter," _i.e._ a spherical surface separating two media of
different densities. Fig. 14 shows the refraction at a convergent diopter,
_i.e_. a surface where the denser medium is convex. The diffusion waves in
this case emanate from the principal focus of the diopter, and therefore
become plane on passing through the convex surface of the denser gelatine.
These periodic diffusion rings also illustrate the phenomena of colour
diffraction. Diffusion waves of different {70} wavelength are unequally
refracted by a gelatine lens. Hence rings of different wave-length which,
originating at the same spot, are at first concentric, are no longer
parallel after passing through a gelatine lens. A convergent lens which
will change the long spherical incident waves into shorter plane waves,
will transform the short incident waves into concave waves whose curvature
is opposite to that of the original waves, _i.e._ it will transform a
divergent into a convergent beam. This is an illustration of what is called
the aberration of refrangibility.
In the same way we may demonstrate the course of diffusion waves through a
gelatine prism, showing the refraction on their incidence and again on
emergence. The prism is made of a stronger gelatine solution, which is more
refractive than the gelatine around it. The waves of diffusion whilst
traversing the prism are retarded, and this retardation is greatest at the
base where the passage is longer. Hence the wave-front is tilted towards
the base of the prism, and this tilting is repeated when the wave-front
leaves the prism.
If we examine diffusion waves of different wave-length on their emergence
from the gelatine prism, we shall see that they cut one another. With a
dense prism, the wave-front of the shorter waves is more tilted towards the
base than the wave-front of the longer waves. For diffusion as for light
the shorter waves are the most refracted. Both refraction and dispersion
are due to the unequal resistances of the medium to undulatory movements of
different periodicity.
[Illustration: FIG. 15.--Diffraction of diffusion waves on passing through
a narrow aperture.]
_Diffraction._--When light traverses a minute orifice, instead {71} of
passing on in a straight line, it spreads out like a fan, forming a
diverging cone of light, just as if the orifice were itself a luminous
point. This is the phenomenon of diffraction which has hitherto been
considered incompatible with the emission theory of light. Diffusion waves
may also be made to pass through a narrow orifice, when they will behave
exactly like the waves of light. The new waves radiate from the orifice
like a fan, instead of giving a cone of waves bounded by lines passing
through the circumference of the orifice and the original centre of
radiation. Thus on passing through a small orifice diffusion waves exhibit
the phenomenon of diffraction just as light waves do.
[Illustration: FIG. 16.--Interference of diffusion waves.]
_Interference._--The phenomenon of interference may also be illustrated by
waves of diffusion. If on a gelatine plate we produce two series of
diffusion waves from two separate centres, we get at certain points an
appearance corresponding to the interference of two sets of light waves.
This appearance is best shown by sowing on the gelatine film a straight row
of drops equidistant from one another. It should be remarked that this
phenomenon of the production of circles of precipitate separated by
transparent spaces, although periodic, is not of necessity vibratory or
undulatory. It would thus appear that periodic phenomena may be propagated
through space without vibratory or oscillatory motion. If we submit to a
critical examination the various experiments which have established the
undulatory theory of light, we find that they do indeed demonstrate the
periodic nature of light, but in no wise prove that light is a vibratory
movement of the ether. {72} On the contrary, the hypothesis that light is
propagated by vibratory movements is open to many objections. Even the
Zeeman effect, although it may tend to establish the fact that light is
produced by vibratory movement, by no means proves that it is propagated in
the same manner. When the theory was accepted that the transmission of
light was periodic it was supposed that this periodic transmission could
only be vibratory or undulatory in character, since waves or vibrations
were the only periodic phenomena known at that time. We now know that there
are other means of periodic transmission which are apparently not
undulatory. The periodic precipitates produced by diffusion show us the
transmission of spherical waves through space, which follow the laws of
light, although the periodic phenomenon is apparently emissive rather than
vibratory.
It will be remembered that Newton considered light to be produced by
projectile-like particles emanating from a centre, and proceeding in
straight lines in all directions. This emission theory of light was
abandoned in favour of Huygens' undulatory theory.
It was said that the phenomena of interference and diffraction could not be
explained by the theory of emission, while the undulatory theory gave a
simple explanation. The scientific mind was unable to conceive the idea of
emission and periodicity as taking part in the same phenomenon. The savants
and thinkers who have meditated on this question have always considered the
theory of emission and that of periodicity as incompatible. Nevertheless,
we are here in presence of a phenomenon in which emission and periodicity
exist simultaneously. The molecules emanating from our drop are diffused in
straight radiating lines, and yet produce periodic precipitates which are
subject to interference and diffraction like the undulations of Huygens.
The phenomena associated with the pressure of light, the {73} discovery of
the cathode rays and the radiations of radium, together with the
introduction of the electron theory of electricity, all seem to have
brought again into greater prominence Newton's original conception of the
emissionary nature of light.
Some of the phenomena of radiation can be explained only by the emission
theory, and others by the undulatory theory of light. All these
difficulties would be solved if we admitted the hypothesis that radiating
bodies project electrons, which produce in the ether periodic waves similar
to those formed in our gelatine films by the molecules of diffusion.
These diffusion films are of the greatest possible service in the practical
teaching of optics. They place before the eye of the student a working
model as it were of the undulations of light. When projected on the screen,
they give excellent pictures of the phenomena of refraction, diffraction,
and interference, and the simultaneous propagation of undulation of
different wave-lengths, and they show in a visible manner the changes of
wave-length in media of different densities.
Diffusion waves differ greatly in length, varying from several millimetres
to 2 [mu]. Many are even shorter than this, too short to be separately
distinguished even under the highest power of the microscope, when they
give the effect of moire or mother-of-pearl.
It is easy to construct a spectroscopic grating in this way with fine lines
whose distance apart is of the order of a micron, separated by clear
spaces. Every physical laboratory may thus produce its own spectroscopic
gratings, rectilinear, circular, or of any desired form.
The most beautiful colour effects may be produced with these diffusion
gratings, as we have shown at the Congress of Rheims in 1907. We have a
considerable collection of these diffusion gratings, some with very fine
lines, giving a very extended spectrum, and others with coarser striations
which give a large number of small spectra.
This study of periodic precipitates is of the highest interest when we come
to investigate the production of colour in natural objects, such as the
wings of insects or the plumage of {74} birds. Many tissues have this lined
or striated structure and exhibit interference colours like those of the
periodic precipitates, their structure showing alternate transparent and
opaque lines, whose width is of the order of a micron. This is the
structure of muscle, and to this striated surface is also attributable many
of the most beautiful colours of nature, the gleam of tendon and
aponeurosis, the fire of scarab and beetle, the colours of the peacock, and
the iridescence of the mollusc and the pearl. The study of liquid diffusion
has given us an idea of the physical mechanism by which these striated
tissues are produced, a mechanism which up to the present time has not been
even suspected. Our experiments show how readily such striped or ruled
structures may be produced in a colloidal solution by the simple diffusion
of salts such as are found in every living organism.
[Illustration: FIG. 17.--Photomicrograph of striated structure of a
periodic precipitate of carbonate and phosphate of lime (magnified 500
times).]
To make a spectroscopic grating by diffusion we proceed as follows. We take
5 c.c. of a 10 per cent. solution of gelatine, and add to it one drop of a
concentrated solution {75} of calcium nitrate. We spread the gelatine
evenly over a plain glass lantern slide and allow it to set. After it is
set, but before it dries, we place in the centre of the slide a drop of
concentrated solution containing two parts of sodium carbonate (Na_2CO_3)
to one of dibasic sodium phosphate (Na_2HPO_4). Tribasic sodium phosphate
alone without the addition of the carbonate will also give good results. If
the phosphate solution is placed on the gelatine in the form of a drop, we
obtain circular periodic precipitates. If it is desired to make a
rectilineal grating, we deposit the phosphate solution on the gelatine in a
straight line by means of two parallel glass plates. In this way we may
obtain lines of periodic precipitation to the number of 500 to 1000 per
millimetre, forming gratings which produce most beautiful spectra.
Pearls and mother-of-pearl both owe their iridescence to a similar ruled
structure, which is developed in the living tissue of a mollusc. They are,
in fact, periodic precipitates of phosphate and carbonate of lime deposited
in the colloidal organic substance of the mollusc. They have the same
structure and the same chemical composition; they have the same physical
properties, the glow, the fire, and the brilliancy of our spectroscopic
gratings. In these experiments, indeed, we have realized the synthesis of
the pearl, not only a chemical synthesis, but the synthesis of its
structure and organism.
We have been able to make these periodic precipitates by the reaction of a
great number of chemical substances, giving a bewildering variety of form
and structure. Some of these recall the form of various organisms, and
especially of insects, as may be seen in Fig. 18.
All the phenomena of life are periodic. The movement of heart and lungs,
sleep and waking, all nervous phenomena, have a regular periodicity. It is
possible that the study of these purely physical phenomena of periodic
precipitation may give us the key to the causation of rhythm and
periodicity in living beings.
Besides this periodic precipitation there appear to be other chemical
reactions which are periodic. Professor Bredig of Heidelberg has lately
described a curious phenomenon, the {76} periodic catalysis of peroxide of
hydrogen by mercury. He thus describes his experiment: "We place in a
perfectly clean test tube a few cubic centimetres of perfectly pure
mercury. Upon this we pour 10 c.c. of a 10 per cent. solution of hydrogen
peroxide. The mercury speedily becomes covered with a thin, brilliant
bronze-coloured pellicle which reflects light. Then little by little
catalysis of the hydrogen peroxide begins, with liberation of oxygen. After
some time, from five to twenty minutes, the liberation of gas at the
surface of the mercury ceases, the cloud formed by the gas bubbles
disappears, and the bronze mirror at the surface of the mercury lights up
with the glint of silver. There is a pause of one or more seconds, and then
the catalytic action begins afresh, commencing at the edges of the mirror.
The cloud is again formed and again disappears. This beautiful and
surprising rhythmic phenomenon may continue at regular intervals for an
hour or more."
[Illustration: FIG. 18.--Articulate form produced by periodic
precipitation.]
A slight alkalinity of the liquid is necessary to start the phenomenon.
This explains the retardation at the beginning {77} of the experiment,
since the rhythmic catalysis cannot begin until the hydrogen peroxide has
dissolved a little of the glass so as to render it slightly alkaline. The
catalytic process may, however, be set going at once by adding a trace of
potassium acetate to the solution.
We may even obtain a curve giving an automatic record of the periodicity of
this catalytic action. For this purpose the oxygen given off is led to a
manometer, which registers on a revolving drum the periodic variation in
pressure. The curve thus obtained presents a remarkable resemblance to a
tracing of the pulse. The frequency and character of the undulatory curve
is modified by physical and chemical influences. Like circulation or
respiration, periodic catalysis has its poisons, and exhibits signs of
fatigue, and of paralysis by cold.
The rhythmic catalysis of Bredig produces an electrical current of action
between the mercury and the water just like that produced by the rhythmic
contraction of the heart, and this current may be registered in a similar
way by means of the Einthoven galvanometer. Thus the heart-beat may be but
an instance of rhythmic catalysis, since both produce the same phenomena,
movement, chemical action, and periodic currents. In the chapter on
physiogenesis we shall return to the study of this question and consider
another rhythmic phenomenon which is the result of osmotic growth.
* * * * *
{78}
CHAPTER VII
COHESION AND CRYSTALLIZATION
Chemical affinity is the force which holds together the different atoms in
a molecule. Cohesion is the force which holds together molecules which are
chemically similar. Although physical science distinguishes three states of
matter, solid, liquid, and gaseous, yet here as elsewhere there are no
sharp dividing lines, but rather an absolute continuity. We have in fact
many intermediate states; between liquids and gases there are the various
conditions of vapour, and between liquids and solids we get viscous,
gelatinous, and paste-like conditions. The only real difference between
solids, liquids, and gases is the intensity of the force of cohesion, which
is considerable in solids, feeble in liquids, and absent in gases.
A living organism is the arena in which are brought into play the opposing
forces of cohesion and disintegration. The study of cohesion is therefore a
vital one for the biologist, and especially cohesion under the conditions
which obtain in living beings, viz. in liquids of heterogeneous
constitution. The forces of cohesion brought into play under these
conditions may be beautifully illustrated by a simple experiment. We take a
plate of glass, well cleaned and absolutely horizontal. On it we pour a
layer of salt water, and in the middle we carefully drop a spot of Indian
ink. The drop at once begins to diffuse, and we obtain a circular figure,
like the monopolar field of diffusion already described, the rays of
diffusion radiating from the centre in all directions.
[Illustration: FIG. 19.--Muriform cohesion figure formed by a drop of
Indian ink in a solution of salt.]
If we keep the plate carefully protected from all disturbing influences,
after some ten to twenty minutes we shall see the coloured particles
returning on their path, and the centre of {79} the drop becoming more and
more black. Each line of force becomes segmented into granules, which
gradually increase in size, and approach nearer to one another and to the
centre of the drop, until it assumes the mulberry appearance shown in the
photograph (Fig. 19).
[Illustration: FIG. 20.--Seven similar drops of Indian ink diffusing in a
salt solution. Two minutes after introducing the drops.]
If we sow a number of drops of Indian ink in regular order on the surface
of a salt solution, we obtain most beautiful patterns formed by the mutual
repulsion of the drops. Figs. 20, 21, and 22 represent the successive
aspects of seven drops of Indian ink thus sown on a layer of salt solution,
and kept undisturbed long enough to allow of their evolution. Fig. 20 shows
the aspect after two minutes, when the diffusion is almost complete. In
Fig. 21, photographed after fifteen {80} minutes, the colouring matter has
almost entirely reunited to form separate granulations; whilst in Fig. 22,
taken after thirty minutes, these granulations are rearranged to form an
agglomeration around the centre of each drop.
[Illustration: FIG. 21.--The same drops 15 minutes later, showing the
granulation appearance.]
The following experiment, which is more difficult, will show the cohesive
attraction of one drop for another. A plate of glass is adjusted absolutely
horizontal, and covered as before with a layer of salt solution. On this we
sow a number of drops of the same salt solution coloured with Indian ink.
The drops must be of exactly the same concentration as the salt medium, so
as to avoid any difference of osmotic pressure between the drops and the
medium, otherwise the drops would not remain intact but would diffuse into
the solution. Since under these conditions the liquid of the medium around
the drops is perfectly symmetrical and homogeneous, it cannot exercise any
influence on the liquid of the drops.
[Illustration: FIG. 22.--The same drops after 30 minutes. The granulations
have agglomerated at the centre of the drops.]
It is otherwise, however, with the colouring matter of the {81} drops. The
particles of Indian ink may be seen passing from one drop to another, the
coloured circles become elongated towards one another, touch, and finally
unite. If, as in Fig. 23, the drops are of different size, the larger one
will have a preponderating attractive action and eat up the smaller drops.
In the figure, six small drops are placed around a large one, and the
smaller drops have begun to be deformed and to move towards the larger
drop. This central drop is also deformed, and has assumed a more or less
hexagonal form, under the influence of the attraction of the six smaller
ones. It may be noticed that the least prominent angle of the hexagon is
opposite the small drop which is farthest away from it, whilst one of the
smaller drops has already begun to be swallowed up by the large one. This
cohesion phenomenon is very slow in its action, but after an hour or two
the central drop will be found to have {82} completely absorbed the six
smaller ones, and only one large drop will remain.
[Illustration: FIG. 23.--Attraction between coloured drops in an isotonic
solution.]
_Incubation._--In the living organism we frequently find conditions similar
to those realized in this experiment, viz. very slow movements of diffusion
in liquids containing particles in suspension. In such cases the
consequences must be the same, viz. granulation and segmentation. Consider
for a moment the incubation of an egg. The heat of incubation determines a
certain amount of evaporation through the shell, with a concentration of
the liquid near the surface. As a consequence of this superficial
concentration we get segmentation of the vitellus, with the production of a
morula.
_Artificial Parthenogenesis._--The experimental parthenogenesis of Loeb and
Delage consists in plunging the egg into a liquid other than sea water, and
returning it again to its original medium. This operation will necessarily
determine slow movements of diffusion in the egg, which will give rise to
segmentation. It may be objected that segmentation is also produced by a
solution which is isotonic with sea water. Such a solution would not indeed
produce an exchange of water with the egg, but it would set up an exchange
of electrolytes, since there would be a difference of their osmotic
pressure in the egg and in the new isotonic medium. The extremely slow
movements of diffusion thus produced would be very favourable to the action
of the cohesive force on the particles in suspension, and hence to the
segmentation of the egg.
[Illustration: FIG. 24.--A circle of eight drops of Indian ink 30 minutes
after they have been sown in a salt solution. The drops have undergone
diffusion and subsequent cohesion, resulting in a reticulate structure.]
Few physical phenomena give us a deeper insight into the phenomena of life
than those which we here contemplate. There is still another experiment
which is even more convincing. On the surface of our horizontal salt
solution we sow a number of drops of a more concentrated salt solution at
equal distances around the circumference of a circle. Movements of
diffusion are thus set up in the interior of the circle, and after a time,
when this diffusion has become so slow as to be almost imperceptible, a
furrow begins to appear in the coloured mass. Then a second and third
appear, and others crossing the former break up the mass {83} into
segments. Finally the segmentation becomes complete, and the preparation
presents a muriform appearance, looking in fact something like a mulberry
(Fig. 24). If the preparation is preserved for several hours longer, we may
see the cells formed by segmentation unite around the circumference so as
to form a hollow bag corresponding to a gastrula, as shown in Fig. 25.
[Illustration: FIG. 25.--The same preparation several hours later, showing
a cellular gastrula-like structure.]
These preparations are extremely sensitive to external influences, which
renders the demonstration of cohesion phenomena difficult. I have
nevertheless on several occasions been able to project the experiment on
the screen during a lecture. The segmentation is influenced by very slight
currents of diffusion, and I have many preparations showing the
segmentation regularly distributed in various ways along radial diffusion
lines. We may in this way produce many varieties of structure lamellar,
vacuolate, or cellular, in fact {84} all the tissue structures which are
met with in living organisms. All these structures are retractile, the
retraction going on very slowly for a long time, as if the force of
cohesion continued to act in the web of the structure even after its
formation was complete. The phenomenon is a purely physical synthetic
reproduction of the phenomenon of coagulation, the cohesion figure being in
fact a retractile clot.
[Illustration: FIG. 26.--Field of crystallization of sodium chloride
(magnified 60 diameters).]
_Crystallization._--When we evaporate a solution of a crystalloid it
becomes more concentrated, slow movements of diffusion are set up, and at a
given moment agglomeration occurs, the agglomerates taking the form of
crystals. Thus crystallization may be regarded as a particular case of
conglomeration by cohesion, differing only in the regularity of the
arrangement of the molecules, which gives the geometrical form of the
crystal. Hence we can easily understand how the presence of a crystalline
fragment may facilitate the process of crystallization. Consider a liquid
in which extremely slow movements of diffusion are taking place. If the
liquid is perfectly homogeneous there will be no centre of attraction to
which the molecules may become attached. {85}
[Illustration: FIG. 27.--Field of crystallization around a crystal of
sodium chloride in process of formation.]
If, however, a crystal or other heterogeneous structure is present, it
forms a centre of cohesion which will attach any molecules that are brought
by diffusion into its sphere of attraction. We have succeeded in
photographing the arrangement of the molecules of a liquid around a crystal
in the act of formation (Fig. 26). For this purpose we add to the solution
traces of some colloidal substance, such as gelatine or gum, so as to delay
the crystallization. It may thus be shown that the molecules of the
surrounding liquid are already arranged in crystalline order for some
distance from the crystal, forming a sort of field of crystallization. The
arrangement of this regular field varies in different cases, and is more or
less complicated according to circumstances. One of the most frequent forms
is that shown in Fig. 27, which is the field around a crystal of sodium
chloride. In the centre {86} of the crystal is a square with well-marked
outline. At each corner of this square there is a straight line at right
angles to the diagonal, which will form the sides of the crystal in process
of formation. From the middle of each side arise yet other perpendiculars,
which in their turn bear other cross lines, each new line being set at
right angles to its predecessor. A later stage of crystallization is shown
in Fig. 27, where the two squares one inside the other at an angle of 45deg
are clearly indicated.
[Illustration: FIG. 28.--Three crystals of sodium chloride in process of
formation, each in the centre of a field of crystallization.]
Every crystallizable substance gives a different characteristic field of
crystallization. In 1903, at the Congress of Angers, I terminated my
address by these words: "The field of crystallization may serve to
determine the character of a substance in solution." I have subsequently
received from Carbonell y Soles of Barcelona an interesting work on this
subject, which he contributed to the International Congress of Medicine at
Madrid in 1903, entitled _Applicacion de la crystalogenia experimental a la
investigacion toxicologica de cas alcaloides_. {87}
Six years ago I received from Australia an exceedingly beautiful photograph
of a thin pellicle found in a rain gauge. My correspondent supposed that
this strange figure might have been produced under the influence of an
electric or magnetic field. I was able to assure him by return of post that
the figure was the result of the crystallization of copper sulphate in a
colloidal medium. In return I received a letter verifying this fact, and
saying that there were copper works in the neighbourhood, and the air was
filled with the dust of copper sulphate.
Living beings are but solutions of colloids and crystalloids, and their
tissues are built up by the aggregation of these solutes. We have already
seen how the forces of crystallization are modified in colloid solutions.
This force of crystallization must play an important role in the
metamorphoses of the living organism, and influence their morphology. It
may therefore be of interest to investigate some of the numberless forms of
crystallization in colloidal solutions.
[Illustration: FIG. 29.--Crystallization of sodium chloride in a colloidal
solution, giving a plant-like form.]
[Illustration: FIG. 30.--Form produced by the crystallization of chloride
of ammonium in a colloidal solution.]
Figs. 29 and 30 represent the forms produced by chloride of sodium and
chloride of ammonium respectively, in solutions of gelatine of different
degrees of concentration. Their resemblance to vegetable growth is so
remarkable that several observers on first seeing them have called them
"Fern-crystals."
I should like here to recall to your notice the work of an English
observer, Dr. E. Montgomery of St. Thomas's {88} Hospital, which was
published as long ago as 1865. This work was recently brought to my notice
by the kindness of Professor Baumler of Freiburg. He says: "Crystals are
not strangers in the organic world. Many organic compounds are able to
assume crystalline forms under certain conditions. Rainey has shown that
many shells consist of globular crystals _i.e._ of mineral substances made
to crystallize by the influence of viscid material." In this connection I
may also mention the interesting work of Otto Lehmann of Karlsruhe on
liquid crystals.
In conclusion, we may recall the words of Schwann himself, the originator
of the cell theory: "The formation of the elementary shapes of an organism
is but a crystallization of substances capable of imbibition. The organism
is but an aggregate of such imbibing crystals."
* * * * *
{89}
CHAPTER VIII
KARYOKINESIS
In 1873, Hermann Fol, writing of the eggs of Geryonia, thus describes the
phenomenon of karyokinesis: "On either side of the residue of the nucleus
there appears a concentration of plasma, thus forming two perfectly regular
star-like figures, whose rays are straight lines of granulations. There are
other curved rays which pass from one star or centre of attraction to the
other. The whole figure is extraordinarily distinct, recalling in a
striking manner the arrangement of iron filings surrounding the poles of a
magnet. Sachs' theory is that the division of the nucleus is caused by
centres of attraction, and I agree with him, not on theoretical grounds,
but because I have actually seen these centres of attraction."
Since the discovery of Hermann Fol, a great number of explanations have
been given, all of them theoretical, to account for the figures and
phenomena of karyokinesis. Many of these so-called explanations are
mechanical, while others invoke the aid of magnetism or electricity to
account for the resemblance of the figures of karyokinesis to the magnetic
or electric phantom or spectre. Among the authors who have dealt with this
question we may mention Hartog of Cork, Gallardo of Buenos Ayres, and
Rhumbler of Goettingen.
In 1904 I presented to the Grenoble Congress, and in 1906 to the Lyons
Congress, a series of photographs and preparations of experimental
karyokinesis. I showed how, in a solution analogous to that found in the
natural cell, the simple processes of liquid diffusion, without the
intervention of magnetism or electricity, may reproduce with perfect
accuracy and in their normal sequence the whole of the movements and {90}
figures which characterize the phenomenon of karyokinesis. This experiment
consists not merely in the production of a certain figure, such as is
obtained in the magnetic spectre, but in the reproduction of the movement
itself, and of all the successive forms which are seen in the natural
phenomenon. These are evolved before the eyes of the spectator in their
regular order and sequence.
I may here reproduce the text of my communication at Grenoble: "Until I
introduced the conception of a field of diffusion, there was no proper
means of studying the phenomena of diffusion, which obey the laws of a
field of force as expounded by Faraday. Moreover, no one suspected the
possibility of reproducing by liquid diffusion a spectre analogous to the
electro-magnetic phantom. Guided by this theory of a diffusion field of
force, I have been able to reproduce experimentally the figures of
karyokinesis by simple diffusion. With regard to the achromatin spindle,
Professor Hartog has shown that the two poles of the spindle are of the
same sign, and not of opposite signs as was at first supposed. In the
process of karyokinesis the two centrosomes, _i.e._ the two poles of the
achromatin spindle, repel one another. They must therefore be poles of the
same sign. An electric or magnetic spectre showing a spindle between two
poles of the same sign is unknown; such a thing would appear to be an
absolute impossibility. What is impossible in electricity and magnetism,
however, is quite possible in the artificial diffusion field; we can here
have a spindle between two poles which repel one another--that is, between
poles of the same sign. Fig. 31 is a photograph of such a spindle produced
by diffusion. On either side are two poles of concentration, which
represent the centrosomes, each pole being surrounded by a star-like
radiation. These poles being alike, repel one another. In the preparation
one may see the distance between the two poles slowly increase, the poles
gradually separating from one another just as do the centrosomes of an ovum
during karyokinesis. This preparation, then, which is produced entirely by
diffusion, presents a perfect resemblance to the achromatin spindle in
karyokinesis. {91}
[Illustration: FIG. 31.--Diffusion figure representing karyokinesis.
Achromatin spindle between two similar poles of concentration.]
"The spindle of which we give a photograph in Fig. 31 was made by placing
in salt water a drop of the same solution pigmented with blood or Indian
ink, and placing on either side of this central drop a hypertonic drop of
salt solution more lightly coloured. After diffusion had gone on for some
minutes, we obtained the figure which we have photographed. I would draw
your attention to the equatorial plane, which shows that the spindle is not
formed by lines of force passing from one pole to the other, as would be
the case between two poles of contrary sign, but by two forces acting in
opposite directions. On either side the pigment of the central drop has
been drawn towards the hypertonic centre nearest to it. In the median line,
however, the pigment is attracted in opposite directions by equal forces,
and therefore remains undisturbed, marking the position of the equatorial
plane. This observation applies equally to the equatorial plane in natural
karyokinesis, whose existence is thus readily explained.
"It is hardly necessary to insist on the fact that liquid preparations like
these are of extreme delicacy and sensitiveness, and require for their
production, and still more for their photography, the greatest care and
skill, which can only be acquired by long practice. {92}
"We are able to produce by diffusion not only the achromatin spindle, but
also the segmentation of the chromatin, and the division of the nucleus. If
in the saline solution we place a coloured isotonic drop between two
coloured hypertonic drops, all the figures and movements of karyokinesis
appear successively in their due order. The central drop, representing the
nucleus between the two lateral drops or centrosomes, first becomes
granular. Next we see what appears to be a rolled-up ribbon analogous to
the chromatin band, which soon breaks into fragments analogous to the
chromosomes. These arrange themselves around, and are gradually attracted
towards the centrosomes, where they accumulate to form two pigmented
nuclear masses. A partition then makes its appearance in the median line,
and this partition becomes continuous with the boundary of the spheres
around the centrosomes. Finally we have two cells in juxtaposition, each
with its nucleus, its protoplasm, and its enveloping membrane. I have been
able to photograph these successive stages of the segmentation of the
chromatin just as I have those of the achromatin spindle" (Fig. 32).
[Illustration: FIG. 32.--Four successive stages in the production of
artificial karyokinesis by diffusion.]
This memoir, written in 1904, clearly asserts the homopolarity of the
centrosomes, and shows that the nuclear division is the result of a bipolar
action, two poles of the same sign exerting their influence on opposite
sides of the nucleus. It also emphasizes the important fact that diffusion,
{93} and as far as we know diffusion alone, is able to produce a spindle
between homologous poles.
A glance at the photograph is enough to show that the spindle is formed
between poles of the same sign. The lines of diffusion radiate from one
centre and converge towards the other centre in curves, giving the double
convergence characteristic of a spindle. The central drop merely supplies
the necessary material, and should have a concentration but slightly less
than that of the plasma, so as not to set up its own lines of diffusion.
The photograph shows clearly that the rays of the spindle traverse the
equator without any break. It has been objected that these lines form not
so much a spindle as two hemi-spindles, but it is clear that these two
hemi-spindles are continuous and form a single sheaf of rays uniting the
two poles of concentration. This is a phenomenon entirely unknown in the
magnetic or electric fields, where two poles of the same sign, one on
either side of a pole of the contrary sign, give two separate spindles. In
a magnetic field it is impossible to make the lines emanating from one pole
converge, except to a pole of opposite sign. Hence if we admit the
homopolarity of the centrosomes, we must also admit that diffusion is the
_vera causa_ of karyokinesis, since, as I showed at the Grenoble Congress
in 1904, diffusion and diffusion alone is capable of producing a spindle
between two poles of the same sign.
_Nuclear Division._--In order to reproduce artificially the phenomena
attending the division of the nucleus, we may proceed as follows. We cover
a perfectly horizontal glass plate with a semi-saturated solution of
potassium nitrate to represent the cytoplasm of the cell. The nucleus in
the centre is reproduced by a drop of the same solution coloured by a trace
of Indian ink, the solid particles of which will represent the chromatin
granules of the nucleus. The addition of the Indian ink will have slightly
lowered the concentration of the central drop, and this is in accordance
with nature, since the osmotic pressure of the nucleus is somewhat less
than that of the plasma. We next place on either side of the drop which
represents the {94} nucleus a coloured drop of solution more concentrated
than the cytoplasm solution. The particles of Indian ink in the central
drop arrange themselves in a long coloured ribbon, apparently rolled up in
a coil, the edges of the ribbon having a beaded appearance. After a short
time the ribbon loses its beaded appearance and becomes smooth, with a
double outline, as is shown in A, Fig. 32. This coil or skein of ribbon
subsequently divides, forming a nuclear spindle, while the chromatin
substance collects together in the equatorial plane as in B, Fig. 32.
A more advanced stage of the nuclear division is shown at C, Fig. 32, where
the chromatin bands of artificial chromosomes are grouped in two conical
sheafs converging towards the two centrosomes. For some considerable time
these conical bundles remain united by fine filaments, the last vestiges of
the nuclear spindle. The final stage is that of two artificial cells in
juxtaposition, whose nuclei are formed by the original centrosomes
augmented by the chromatin bands or chromosomes (Fig. 32, D).
[Illustration: FIG. 33.--Equatorial crown produced by diffusion.]
The resemblance of these successive phenomena to those of natural
karyokinesis is of the closest. The experiment shows that diffusion is
quite sufficient to produce organic karyokinesis, and that the only
physical force required is that of osmotic pressure. If in the cytoplasm of
a cell there are two points of molecular concentration greater than that of
the general mass, the nucleus must necessarily divide with all the
phenomena which accompany karyokinesis. In nature these two centres of
positive concentration are introduced into the protoplasm of the cell by
fecundation--that is, by the entrance of the centrosomes of the sperm cell.
In certain abnormal cases the concentration may be produced in the cell
itself by the formation of two centres of catabolism or molecular
disintegration, since, as we have seen, molecular disintegration raises the
osmotic pressure. This phenomenon, namely the production of karyokinesis
from centres of catabolism, may account for the abnormal karyokinesis of
cancer cells and the like. The subject is one which would well repay
further investigation. {95}
[Illustration: FIG. 34.--A triaster produced by diffusion.]
It has been found in our experiments that in order to obtain the regular
division of the artificial nucleus represented by the intermediary drop,
the latter must have an osmotic pressure slightly below that of the plasma.
This leads to the supposition that a similar condition must obtain in the
natural cell. It may be noticed, moreover, that the grains of pigment
follow the direction of the flow of water, being carried along by the
stream. This would appear to show that the nucleus of a natural cell has
also a molecular concentration less than that of the plasma--a result
either of dehydration of the plasma, or of some diminution in the molecular
concentration of the nucleus.
Other phenomena of karyokinesis may also be closely imitated by diffusion.
For instance, in the diffusion preparation we notice at each extremity of
the equator a V-shaped figure with its apex towards the centre,
corresponding exactly to what in natural karyokinesis is called the
equatorial crown.
We may also produce diffusion figures of abnormal karyokinesis. Fig. 34
represents such a form, a triaster produced by diffusion.
Artificial karyokinesis may also be produced by hypotonic poles of
concentration--that is to say, when the central drop representing the ovum
is positive and the lateral drops representing the centrosomes are negative
with respect to the plasma. In this case, however, the resemblance to
natural karyokinesis is less perfect. {96}
Without attaching to it an importance which is not warranted by
experimental results, it is interesting to note that we have here two
methods of fertilization, hypertonic and hypotonic, _i.e._ by centrosomes
of greater concentration and by centrosomes of less concentration than that
of the plasma of the ovum, and that we have in nature two corresponding
results, viz. two different sexes. It is possible that we have in these two
methods of producing nuclear division the secret of the difference of sex.
* * * * *
{97}
CHAPTER IX
ENERGETICS
Movement is everywhere; there is no such thing as immobility; the very idea
of rest is itself an illusion. Immobility is only apparent and relative,
and disappears under closer examination. All terrestrial objects are driven
with prodigious velocity around the sun, and the dwellers on the earth's
equator travel each day around the 40,000 kilometres of its circumference.
All objects on the globe are in motion, the inanimate as well as the
living. The waters rise in vapour from the sea, float over mountain and
valley, and return down the rivers to the sea again. Still more marvellous
is the current of water which flows eternally from dew and rain, through
the sap of plants and the blood of animals to the mineral world again. The
very mountains crumble and their substance is washed down into the plains;
the winds move the air and raise the waves of the sea, whilst the strong
ocean currents are produced by variations of temperature in different
parts. This agitation, this incessant and universal motion, has been a
favourite subject of poetic contemplation. Heraclitus writes: "There is a
perpetual flow, all is one universal current; nothing remains as it was,
change alone is eternal." Ovid writes in his _Metamorphoses_: "Believe me,
nothing perishes in this vast universe, but all varies, and changes its
figure. I think that nothing endures long under the same appearance. What
was solid earth has become sea, and solid ground has issued from the bosom
of the waters."
The French poetess Mme. Ackermann has expressed the same idea in beautiful
verse:--
"Ainsi, jamais d'arret. L'immortelle matiere,
Un seul instant encore n'a pu se reposer.
La Nature ne fait, patiente ouvriere,
Que defaire et recomposer.
{98}
Tout se metamorphose entre ses mains actives;
Partout le mouvement incessant et divers,
Dans le cercle eternel des formes fugitives,
Agitant l'immense univers."
It was only towards the middle of last century that mankind in the long
search after unity in nature began to realize that all the movements of the
universe are the manifestations of a single agent, which we call energy. In
reality all the phenomena of nature may be conceived as diverse forms of
motion, and the word "energy" is the common expression applied to all the
various modes of motion in the universe. It was by the study of heat, and
more especially of thermodynamics, that we obtained our conceptions of the
science of energetics.
It was in Munich in 1798 that the English engineer Count Rumford first
observed that in the operation of boring a cannon the copper was heated to
such a degree that the shavings became red-hot. This suggested his famous
experiment, in which a heavy iron pestle was turned by horse power in a
metal mortar filled with water. The water boiled, and when more water was
added this also became heated to ebullition, and so on indefinitely.
Rumford argued that the heat thus obtained in an indefinite quantity could
not be a material substance; that motion was the only thing added to the
water without limit, and that therefore heat must be motion.
While Rumford's experiment showed the transformation of motion into heat,
the steam engine was soon afterwards to demonstrate the opposite
transformation, viz. that of heat into motion.
The actual state of our knowledge with regard to the science of energy
rests on two principles, that of Mayer and that of Carnot.
The first principle was defined by J. R. Mayer, a medical practitioner of
Heilbronn, whose work, _Bemerkungen ueber die Kraefte der unbelebten
Natur_, was published in 1842. "All physical phenomena," says Mayer,
"whether vital or chemical, are forms of motion. All these forms of motion
are susceptible of change into one another, and in all the transformations
the {99} quantity of mechanical work represented by different modes of
motion remains invariable."
The energy of a given body is the amount of transferable motion stored up
in that body, and is measured by its capacity of producing mechanical work.
Ostwald thus defines energy: "Energy is work, all that can be obtained from
work, and all that can be changed into work." Different forms of energy may
be measured in different ways, but all forms of energy can be measured
either in units of mechanical work or in units of heat, in
kilogramme-metres or foot-pounds or in calories, according as the energy in
question is transformed into mechanical work or into heat. The first
principle of energetics, the conservation of energy, may be thus expressed:
"Energy is eternal; none is ever created, and none is ever lost. The
quantity of energy in the universe is invariable, and is conserved for ever
in its integrity."
The unit by which we measure quantities of heat is the calory, the amount
of heat required to raise the temperature of one kilogramme of water one
degree Centigrade.
The practical unit of mechanical work is the kilogramme-metre, the work
required to raise the weight of one kilogramme to the height of one metre.
The theoretical unit of work is one erg, the work required to move a mass
of one gramme through one centimetre against a force of one dyne.
Joule of Manchester was the first to verify Mayer's law quantitatively. By
an experiment analogous to that of Rumford, he transformed work into heat,
arranging his apparatus so that he might measure the amount of heat
produced and the work expended. On dividing the quantity of work that had
disappeared by the quantity of heat which had been disengaged, he found
that 424 kilogramme-metres of work had been expended for each calory of
heat produced.
Hirn of Colmar measured the ratio of work to heat in the steam engine. He
found that for each calory of heat which had disappeared there were
produced 425 kilogramme-metres of work. {100}
This number 425 has therefore been accepted as representing in calories and
kilogramme-metres the transformation of work into heat, and of heat into
work.
Further measurements on the transformations of other forms of energy,
chemical energy and electrical energy, have shown that Joule's law of
equivalents is general, and that the quantity of mechanical work
represented by any form of energy remains undiminished after
transformation, whatever the nature of that transformation.
Energy presents itself to us under two forms, potential and actual.
Potential energy is slumbering energy, energy localized or locked up in the
body. In order to transform potential energy into actual energy, there is
required the intervention of an additional awakening, stimulating, or
exciting energy from without. This stimulating energy may be almost
infinitesimal in amount and bears no quantitative relation to the amount of
energy transformed. It is the small amount of work required to turn the key
which liberates an indeterminate quantity of potential energy.
Actual energy, on the other hand, is energy in movement, awake and alert,
ready to be transformed into any other form of energy without the
intervention of any such external stimulating force.
The passage of a given quantity of energy from the potential into the
actual state is effected gradually, and during the time of transformation
the sum of the actual and the potential energy remains constant.
A weight suspended by a cord possesses a quantity of potential energy equal
to the product of its weight into the height through which it can fall.
This energy is locked up in a certain space, it cannot be transformed
without the intervention of some external energy to cut the cord. During
the falling of the weight, at the middle of its path, half of this
slumbering energy has become kinetic, and is represented by the _vis viva_
of the weight, while the other half is still potential and is equivalent to
the work which the weight will accomplish during the second half of its
fall. At any moment the sum of these two energies, the sleeping and the
waking {101} energies, represents the total potential energy of the weight
before it began to fall.
So with the powder in a gun. The potential energy of the powder cannot
become actual without some stimulus, some exciting force from without to
set it free. It is the external work of pressing the trigger that liberates
the potential energy of the powder, transforming it into the actual energy
of combustion, and the kinetic energy of the projectile.
Since energy is work, and work is a function of motion, there is in reality
no such thing as energy in repose. Matter according to our modern
conception is a complex of molecules, atoms, and electrons; we conceive the
molecules of matter as always in movement, animated with cyclic or
vibratory motion, these oscillatory or rotatory movements representing the
potential energy of the body in question. Potential energy is thus the
expression of molecular motion without translation of the molecules as a
whole in space.
When this potential energy is transformed into actual energy by the
intervention of some external force, we get a current of energy, a
transference of the molecules in space. Thus, when an external force has
released the weight, the molecular orbits in the falling body change in
form, and the potential energy of the molecular motion becomes the kinetic
energy of the falling body. Similarly in the conduction of heat, the energy
of the hot body is transferred to a colder body by transmission of the
vibratory motion from molecule to molecule. So again with chemical energy,
the molecular motion of combustion may be transformed into the radiant
energy of the ethereal waves.
Actual energy may be regarded as a current of molecular motion. To make the
matter clearer, let a mass of matter be represented by a regiment of
soldiers. Then each soldier will represent an electron, a company will be
an atom, and a battalion will be a molecule. As long as the soldiers mark
time, turn, or otherwise exercise without advancing, we have simply an
accumulation of potential energy. The word of command, "March," is the
exciting force which suddenly transforms this potential into kinetic
energy. The marching {102} regiment is a representation of a body
possessing kinetic energy. Potential energy is energy confined to a certain
point in space, whereas actual energy is a current of energy, continually
changing its place or form. Energy is like water-power--potential in the
lake, actual in the waterfall or river.
Any mechanism capable of causing one form of energy to pass into another is
a transformer of energy. A steam engine is a transformer of energy,
changing caloric energy into mechanical work. An electrical machine is a
transformer of energy, converting mechanical motion into a current of
electricity, whilst an electro-motor changes the movement of electrons into
mechanical movement. Every living being, and even man himself, is but a
transformer of energy, changing the energy derived from the earth and air
and sun into mechanical motion, nervous energy, and heat.
The first law of energetics, that of the conservation of energy, is
analogous to Lavoisier's p
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