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The International Scientific Series
THE NEW PHYSICS AND ITS EVOLUTION
by
LUCIEN POINCARÉ
Inspéctéur-General de l'Instruction Publique
Being the Authorized Translation of _LA PHYSIQUE MODERNE, SON ÉVOLUTION_
New York
D. Appleton and Company
1909
Prefatory Note
M. Lucien Poincaré is one of the distinguished family of
mathematicians which has during the last few years given a
Minister of Finance to the Republic and a President to the
Académie des Sciences. He is also one of the nineteen
Inspectors-General of Public Instruction who are charged with the
duty of visiting the different universities and _lycées_ in
France and of reporting upon the state of the studies there
pursued. Hence he is in an excellent position to appreciate at
its proper value the extraordinary change which has lately
revolutionized physical science, while his official position has
kept him aloof from the controversies aroused by the discovery of
radium and by recent speculations on the constitution of matter.
M. Poincaré's object and method in writing the book are
sufficiently explained in the preface which follows; but it may
be remarked that the best of methods has its defects, and the
excessive condensation which has alone made it possible to
include the last decade's discoveries in physical science within
a compass of some 300 pages has, perhaps, made the facts here
noted assimilable with difficulty by the untrained reader. To
remedy this as far as possible, I have prefixed to the present
translation a table of contents so extended as to form a fairly
complete digest of the book, while full indexes of authors and
subjects have also been added. The few notes necessary either for
better elucidation of the terms employed, or for giving account
of discoveries made while these pages were passing through the
press, may be distinguished from the author's own by the
signature "ED."
THE EDITOR.
ROYAL INSTITUTION OF GREAT BRITAIN,
April 1907.
Author's Preface
During the last ten years so many works have accumulated in the
domain of Physics, and so many new theories have been propounded,
that those who follow with interest the progress of science, and
even some professed scholars, absorbed as they are in their own
special studies, find themselves at sea in a confusion more
apparent than real.
It has therefore occurred to me that it might be useful to write
a book which, while avoiding too great insistence on purely
technical details, should try to make known the general results
at which physicists have lately arrived, and to indicate the
direction and import which should be ascribed to those
speculations on the constitution of matter, and the discussions
on the nature of first principles, to which it has become, so to
speak, the fashion of the present day to devote oneself.
I have endeavoured throughout to rely only on the experiments in
which we can place the most confidence, and, above all, to show
how the ideas prevailing at the present day have been formed, by
tracing their evolution, and rapidly examining the successive
transformations which have brought them to their present
condition.
In order to understand the text, the reader will have no need to
consult any treatise on physics, for I have throughout given the
necessary definitions and set forth the fundamental facts.
Moreover, while strictly employing exact expressions, I have
avoided the use of mathematical language. Algebra is an admirable
tongue, but there are many occasions where it can only be used
with much discretion.
Nothing would be easier than to point out many great omissions
from this little volume; but some, at all events, are not
involuntary.
Certain questions which are still too confused have been put on
one side, as have a few others which form an important collection
for a special study to be possibly made later. Thus, as regards
electrical phenomena, the relations between electricity and
optics, as also the theories of ionization, the electronic
hypothesis, etc., have been treated at some length; but it has
not been thought necessary to dilate upon the modes of production
and utilization of the current, upon the phenomena of magnetism,
or upon all the applications which belong to the domain of
Electrotechnics.
L. POINCARÉ.
Contents
EDITOR'S PREFATORY NOTE
AUTHOR'S PREFACE
TABLE OF CONTENTS
CHAPTER I
THE EVOLUTION OF PHYSICS
Revolutionary change in modern Physics only apparent:
evolution not revolution the rule in Physical Theory--
Revival of metaphysical speculation and influence of
Descartes: all phenomena reduced to matter and movement--
Modern physicists challenge this: physical, unlike
mechanical, phenomena seldom reversible--Two schools,
one considering experimental laws imperative, the other
merely studying relations of magnitudes: both teach
something of truth--Third or eclectic school--
Is mechanics a branch of electrical science?
CHAPTER II
MEASUREMENTS
§ 1. Metrology: Lord Kelvin's view of its necessity--
Its definition
§ 2. The Measure of Length: Necessity for unit--
Absolute length--History of Standard--Description of
Standard Metre--Unit of wave-lengths preferable--The
International Metre
§ 3. The Measure of Mass: Distinction between
mass and weight--Objections to legal kilogramme
and its precision--Possible improvement
§ 4. The Measure of Time: Unit of time the
second--Alternative units proposed--Improvements in
chronometry and invar
§ 5. The Measure of Temperature: Fundamental
and derived units--Ordinary unit of temperature
purely arbitrary--Absolute unit mass of H at pressure
of 1 m. of Hg at 0° C.--Divergence of thermometric
and thermodynamic scales--Helium thermometer for low,
thermo-electric couple for high, temperatures--Lummer
and Pringsheim's improvements in thermometry.
§ 6. Derived Units and Measure of Energy:
Importance of erg as unit--Calorimeter usual means of
determination--Photometric units.
§ 7. Measure of Physical Constants: Constant of
gravitation--Discoveries of Cavendish, Vernon Boys,
Eötvös, Richarz and Krigar-Menzel--Michelson's
improvements on Fizeau and Foucault's experiments--
Measure of speed of light.
CHAPTER III
PRINCIPLES
§ 1. The Principles of Physics: The Principles of
Mechanics affected by recent discoveries--Is mass
indestructible?--Landolt and Heydweiller's experiments
--Lavoisier's law only approximately true--Curie's
principle of symmetry.
§ 2. The Principle of the Conservation of Energy:
Its evolution: Bernoulli, Lavoisier and Laplace, Young,
Rumford, Davy, Sadi Carnot, and Robert Mayer--Mayer's
drawbacks--Error of those who would make mechanics part
of energetics--Verdet's predictions--Rankine inventor
of energetics--Usefulness of Work as standard form of
energy--Physicists who think matter form of energy--
Objections to this--Philosophical value of conservation
doctrine.
§ 3. The Principle of Carnot and Clausius:
Originality of Carnot's principle that fall of
temperature necessary for production of work by heat--
Clausius' postulate that heat cannot pass from cold to
hot body without accessory phenomena--Entropy result
of this--Definition of entropy--Entropy tends to increase
incessantly--A magnitude which measures evolution
of system--Clausius' and Kelvin's deduction that
heat end of all energy in Universe--Objection to this--
Carnot's principle not necessarily referable to mechanics
--Brownian movements--Lippmann's objection to
kinetic hypothesis.
§ 4. Thermodynamics: Historical work of Massieu,
Willard Gibbs, Helmholtz, and Duhem--Willard Gibbs
founder of thermodynamic statics, Van t'Hoff its
reviver--The Phase Law--Raveau explains it without
thermodynamics.
§ 5. Atomism: Connection of subject with preceding
Hannequin's essay on the atomic hypothesis--Molecular
physics in disfavour--Surface-tension, etc., vanishes
when molecule reached--Size of molecule--Kinetic
theory of gases--Willard Gibbs and Boltzmann introduce
into it law of probabilities--Mean free path of gaseous
molecules--Application to optics--Final division of
matter.
CHAPTER IV
THE VARIOUS STATES OF MATTER
§ 1. The Statics of Fluids: Researches of Andrews,
Cailletet, and others on liquid and gaseous states--
Amagat's experiments--Van der Waals' equation--Discovery
of corresponding states--Amagat's superposed
diagrams--Exceptions to law--Statics of mixed fluids--
Kamerlingh Onnes' researches--Critical Constants--
Characteristic equation of fluid not yet ascertainable.
§ 2. The Liquefaction of Gases and Low Temperatures:
Linde's, Siemens', and Claude's methods of liquefying
gases--Apparatus of Claude described--Dewar's
experiments--Modification of electrical properties of
matter by extreme cold: of magnetic and chemical--
Vitality of bacteria unaltered--Ramsay's discovery
of rare gases of atmosphere--Their distribution in
nature--Liquid hydrogen--Helium.
§ 3. Solids and Liquids: Continuity of Solid and Liquid
States--Viscosity common to both--Also Rigidity--
Spring's analogies of solids and liquids--Crystallization
--Lehmann's liquid crystals--Their existence doubted
--Tamman's view of discontinuity between crystalline
and liquid states.
§ 4. The Deformation of Solids: Elasticity--
Hoocke's, Bach's, and Bouasse's researches--Voigt
on the elasticity of crystals--Elastic and permanent
deformations--Brillouin's states of unstable
equilibria--Duhem and the thermodynamic postulates--
Experimental confirmation--Guillaume's researches
on nickel steel--Alloys.
CHAPTER V
SOLUTIONS AND ELECTROLYTIC DISSOCIATION
§ 1. Solution: Kirchhoff's, Gibb's, Duhem's and Van
t'Hoff's researches.
§ 2. Osmosis: History of phenomenon--Traube and
biologists establish existence of semi-permeable
walls--Villard's experiments with gases--Pfeffer
shows osmotic pressure proportional to concentration--
Disagreement as to cause of phenomenon.
§ 3. Osmosis applied to Solution: Van t'Hoff's
discoveries--Analogy between dissolved body and
perfect gas--Faults in analogy.
§ 4. Electrolytic Dissociation: Van t'Hoff's and
Arrhenius' researches--Ionic hypothesis of--Fierce
opposition to at first--Arrhenius' ideas now triumphant
--Advantages of Arrhenius' hypothesis--"The ions
which react"--Ostwald's conclusions from this--Nernst's
theory of Electrolysis--Electrolysis of gases makes
electronic theory probable--Faraday's two laws--Valency--
Helmholtz's consequences from Faraday's laws.
CHAPTER VI
THE ETHER
§ 1. The Luminiferous Ether: First idea of Ether due
to Descartes--Ether must be imponderable--Fresnel shows
light vibrations to be transverse--Transverse vibrations
cannot exist in fluid--Ether must be discontinuous.
§ 2. Radiations: Wave-lengths and their
measurements--Rubens' and Lenard's researches--
Stationary waves and colour-photography--Fresnel's
hypothesis opposed by Neumann--Wiener's and Cotton's
experiments.
§ 3. The Electromagnetic Ether: Ampère's advocacy
of mathematical expression--Faraday first shows
influence of medium in electricity--Maxwell's proof
that light-waves electromagnetic--His
unintelligibility--Required confirmation of theory by Hertz.
§ 4. Electrical Oscillations: Hertz's experiments--
Blondlot proves electromagnetic disturbance propagated
with speed of light--Discovery of ether waves
intermediate between Hertzian and visible ones--Rubens'
and Nichols' experiments--Hertzian and light rays
contrasted--Pressure of light.
§ 5. The X-Rays: Röntgen's discovery--Properties
of X-rays--Not homogeneous--Rutherford and M'Clung's
experiments on energy corresponding to--Barkla's
experiments on polarisation of--Their speed that of
light--Are they merely ultra-violet?--Stokes and
Wiechert's theory of independent pulsations generally
preferred--J.J. Thomson's idea of their formation--
Sutherland's and Le Bon's theories--The N-Rays--
Blondlot's discovery--Experiments cannot be repeated
outside France--Gutton and Mascart's confirmation--
Negative experiments prove nothing--Supposed
wave-length of N-rays.
§ 6. The Ether and Gravitation: Descartes'
and Newton's ideas on gravitation--Its speed and
other extraordinary characteristics--Lesage's
hypothesis--Crémieux' experiments with drops of
liquids--Hypothesis of ether insufficient.
CHAPTER VII
WIRELESS TELEGRAPHY
§ 1. Histories of wireless telegraphy already written,
and difficulties of the subject.
§ 2. Two systems: that which uses the material media (earth,
air, or water), and that which employs ether only.
§ 3. Use of earth as return wire by Steinheil
--Morse's experiments with water of canal--Seine used as
return wire during siege of Paris--Johnson and Melhuish's
Indian experiments--Preece's telegraph over Bristol
Channel--He welcomes Marconi.
§ 4. Early attempts at transmission of messages through
ether--Experiments of Rathenau and others.
§ 5. Forerunners of ether telegraphy: Clerk Maxwell
and Hertz--Dolbear, Hughes, and Graham Bell.
§ 6. Telegraphy by Hertzian waves first suggested
by Threlfall--Crookes', Tesla's, Lodge's,
Rutherford's, and Popoff's contributions--Marconi
first makes it practicable.
§ 7. The receiver in wireless telegraphy--Varley's,
Calzecchi--Onesti's, and Branly's researches--
Explanation of coherer still obscure.
§ 8. Wireless telegraphy enters the commercial stage--
Defect of Marconi's system--Braun's, Armstrong's, Lee de
Forest's, and Fessenden's systems make use of earth--
Hertz and Marconi entitled to foremost place among
discoverers.
CHAPTER VIII
THE CONDUCTIVITY OF GASES AND THE IONS
§ 1. The Conductivity of Gases: Relations of matter to
ether cardinal problem--Conductivity of gases at first
misapprehended--Erman's forgotten researches--Giese
first notices phenomenon--Experiment with X-rays--
J.J. Thomson's interpretation--Ionized gas not obedient
to Ohm's law--Discharge of charged conductors by
ionized gas.
§ 2. The Condensation of water-vapour by Ions:
Vapour will not condense without nucleus--Wilson's
experiments on electrical condensation--Wilson and
Thomson's counting experiment--Twenty million ions
per c.cm. of gas--Estimate of charge borne by ion--
Speed of charges--Zeleny's and Langevin's
experiments--Negative ions 1/1000 of size of
atoms--Natural unit of electricity or electrons.
§ 3. How Ions are Produced: Various causes
of ionization--Moreau's experiments with alkaline
salts--Barus and Bloch on ionization by phosphorus
vapours--Ionization always result of shock.
§ 4. Electrons in Metals: Movement of
electrons in metals foreshadowed by Weber--Giese's,
Riecke's, Drude's, and J.J. Thomson's researches--Path
of ions in metals and conduction of heat--Theory of
Lorentz--Hesehus' explanation of electrification by
contact--Emission of electrons by charged body--
Thomson's measurement of positive ions.
CHAPTER IX
CATHODE RAYS AND RADIOACTIVE BODIES
§ 1. The Cathode Rays: History of discovery--Crookes'
theory--Lenard rays--Perrin's proof of negative
charge--Cathode rays give rise to X-rays--The canal
rays--Villard's researches and magneto-cathode rays--
Ionoplasty--Thomson's measurements of speed of rays--
All atoms can be dissociated.
§ 2. Radioactive Substances: Uranic rays of Niepce
de St Victor and Becquerel--General radioactivity of
matter--Le Bon's and Rutherford's comparison of uranic
with X rays--Pierre and Mme. Curie's discovery of
polonium and radium--Their characteristics--Debierne
discovers actinium.
§ 3. Radiations and Emanations of Radioactive
Bodies: Giesel's, Becquerel's, and Rutherford's
Researches--Alpha, beta, and gamma rays--Sagnac's
secondary rays--Crookes' spinthariscope--The emanation
--Ramsay and Soddy's researches upon it--Transformations
of radioactive bodies--Their order.
§ 4. Disaggregation of Matter and Atomic Energy:
Actual transformations of matter in radioactive bodies
--Helium or lead final product--Ultimate disappearance
of radium from earth--Energy liberated by radium:
its amount and source--Suggested models of radioactive
atoms--Generalization from radioactive phenomena
-Le Bon's theories--Ballistic hypothesis generally
admitted--Does energy come from without--Sagnac's
experiments--Elster and Geitel's _contra_.
CHAPTER X
THE ETHER AND MATTER
§ 1. The Relations between the Ether and Matter:
Attempts to reduce all matter to forms of ether--Emission
and absorption phenomena show reciprocal action--
Laws of radiation--Radiation of gases--Production of
spectrum--Differences between light and sound variations
show difference of media--Cauchy's, Briot's, Carvallo's
and Boussinesq's researches--Helmholtz's and
Poincaré's electromagnetic theories of dispersion.
§ 2. The Theory of Lorentz:--Mechanics fails
to explain relations between ether and matter--Lorentz
predicts action of magnet on spectrum--Zeeman's experiment
--Later researches upon Zeeman effect--
Multiplicity of electrons--Lorentz's explanation of
thermoelectric phenomena by electrons--Maxwell's and
Lorentz's theories do not agree--Lorentz's probably more
correct--Earth's movement in relation to ether.
§ 3. The Mass of Electrons: Thomson's and
Max Abraham's view that inertia of charged body due
to charge--Longitudinal and transversal mass--Speed
of electrons cannot exceed that of light--Ratio of
charge to mass and its variation--Electron simple
electric charge--Phenomena produced by its acceleration.
§ 4. New Views on Ether and Matter:
Insufficiency of Larmor's view--Ether definable
by electric and magnetic fields--Is matter all electrons?
Atom probably positive centre surrounded by
negative electrons--Ignorance concerning positive
particles--Successive transformations of matter probable
--Gravitation still unaccounted for.
CHAPTER XI
THE FUTURE OF PHYSICS
Persistence of ambition to discover supreme principle
in physics--Supremacy of electron theory at present
time--Doubtless destined to disappear like others--
Constant progress of science predicted--Immense field
open before it.
INDEX OF NAMES
INDEX OF SUBJECTS
CHAPTER I
THE EVOLUTION OF PHYSICS
The now numerous public which tries with some success to keep abreast
of the movement in science, from seeing its mental habits every day
upset, and from occasionally witnessing unexpected discoveries that
produce a more lively sensation from their reaction on social life, is
led to suppose that we live in a really exceptional epoch, scored by
profound crises and illustrated by extraordinary discoveries, whose
singularity surpasses everything known in the past. Thus we often hear
it said that physics, in particular, has of late years undergone a
veritable revolution; that all its principles have been made new, that
all the edifices constructed by our fathers have been overthrown, and
that on the field thus cleared has sprung up the most abundant harvest
that has ever enriched the domain of science.
It is in fact true that the crop becomes richer and more fruitful,
thanks to the development of our laboratories, and that the quantity
of seekers has considerably increased in all countries, while their
quality has not diminished. We should be sustaining an absolute
paradox, and at the same time committing a crying injustice, were we
to contest the high importance of recent progress, and to seek to
diminish the glory of contemporary physicists. Yet it may be as well
not to give way to exaggerations, however pardonable, and to guard
against facile illusions. On closer examination it will be seen that
our predecessors might at several periods in history have conceived,
as legitimately as ourselves, similar sentiments of scientific pride,
and have felt that the world was about to appear to them transformed
and under an aspect until then absolutely unknown.
Let us take an example which is salient enough; for, however arbitrary
the conventional division of time may appear to a physicist's eyes, it
is natural, when instituting a comparison between two epochs, to
choose those which extend over a space of half a score of years, and
are separated from each other by the gap of a century. Let us, then,
go back a hundred years and examine what would have been the state of
mind of an erudite amateur who had read and understood the chief
publications on physical research between 1800 and 1810.
Let us suppose that this intelligent and attentive spectator witnessed
in 1800 the discovery of the galvanic battery by Volta. He might from
that moment have felt a presentiment that a prodigious transformation
was about to occur in our mode of regarding electrical phenomena.
Brought up in the ideas of Coulomb and Franklin, he might till then
have imagined that electricity had unveiled nearly all its mysteries,
when an entirely original apparatus suddenly gave birth to
applications of the highest interest, and excited the blossoming of
theories of immense philosophical extent.
In the treatises on physics published a little later, we find traces
of the astonishment produced by this sudden revelation of a new world.
"Electricity," wrote the Abbé Haüy, "enriched by the labour of so many
distinguished physicists, seemed to have reached the term when a
science has no further important steps before it, and only leaves to
those who cultivate it the hope of confirming the discoveries of their
predecessors, and of casting a brighter light on the truths revealed.
One would have thought that all researches for diversifying the
results of experiment were exhausted, and that theory itself could
only be augmented by the addition of a greater degree of precision to
the applications of principles already known. While science thus
appeared to be making for repose, the phenomena of the convulsive
movements observed by Galvani in the muscles of a frog when connected
by metal were brought to the attention and astonishment of
physicists.... Volta, in that Italy which had been the cradle of the
new knowledge, discovered the principle of its true theory in a fact
which reduces the explanation of all the phenomena in question to the
simple contact of two substances of different nature. This fact became
in his hands the germ of the admirable apparatus to which its manner
of being and its fecundity assign one of the chief places among those
with which the genius of mankind has enriched physics."
Shortly afterwards, our amateur would learn that Carlisle and
Nicholson had decomposed water by the aid of a battery; then, that
Davy, in 1803, had produced, by the help of the same battery, a quite
unexpected phenomenon, and had succeeded in preparing metals endowed
with marvellous properties, beginning with substances of an earthy
appearance which had been known for a long time, but whose real nature
had not been discovered.
In another order of ideas, surprises as prodigious would wait for our
amateur. Commencing with 1802, he might have read the admirable series
of memoirs which Young then published, and might thereby have learned
how the study of the phenomena of diffraction led to the belief that
the undulation theory, which, since the works of Newton seemed
irretrievably condemned, was, on the contrary, beginning quite a new
life. A little later--in 1808--he might have witnessed the discovery
made by Malus of polarization by reflexion, and would have been able
to note, no doubt with stupefaction, that under certain conditions a
ray of light loses the property of being reflected.
He might also have heard of one Rumford, who was then promulgating
very singular ideas on the nature of heat, who thought that the then
classical notions might be false, that caloric does not exist as a
fluid, and who, in 1804, even demonstrated that heat is created by
friction. A few years later he would learn that Charles had enunciated
a capital law on the dilatation of gases; that Pierre Prevost, in
1809, was making a study, full of original ideas, on radiant heat. In
the meantime he would not have failed to read volumes iii. and iv. of
the _Mecanique celeste_ of Laplace, published in 1804 and 1805, and he
might, no doubt, have thought that before long mathematics would
enable physical science to develop with unforeseen safety.
All these results may doubtless be compared in importance with the
present discoveries. When strange metals like potassium and sodium
were isolated by an entirely new method, the astonishment must have
been on a par with that caused in our time by the magnificent
discovery of radium. The polarization of light is a phenomenon as
undoubtedly singular as the existence of the X rays; and the upheaval
produced in natural philosophy by the theories of the disintegration
of matter and the ideas concerning electrons is probably not more
considerable than that produced in the theories of light and heat by
the works of Young and Rumford.
If we now disentangle ourselves from contingencies, it will be
understood that in reality physical science progresses by evolution
rather than by revolution. Its march is continuous. The facts which
our theories enable us to discover, subsist and are linked together
long after these theories have disappeared. Out of the materials of
former edifices overthrown, new dwellings are constantly being
reconstructed.
The labour of our forerunners never wholly perishes. The ideas of
yesterday prepare for those of to-morrow; they contain them, so to
speak, _in potentia_. Science is in some sort a living organism, which
gives birth to an indefinite series of new beings taking the places of
the old, and which evolves according to the nature of its environment,
adapting itself to external conditions, and healing at every step the
wounds which contact with reality may have occasioned.
Sometimes this evolution is rapid, sometimes it is slow enough; but it
obeys the ordinary laws. The wants imposed by its surroundings create
certain organs in science. The problems set to physicists by the
engineer who wishes to facilitate transport or to produce better
illumination, or by the doctor who seeks to know how such and such a
remedy acts, or, again, by the physiologist desirous of understanding
the mechanism of the gaseous and liquid exchanges between the cell and
the outer medium, cause new chapters in physics to appear, and suggest
researches adapted to the necessities of actual life.
The evolution of the different parts of physics does not, however,
take place with equal speed, because the circumstances in which they
are placed are not equally favourable. Sometimes a whole series of
questions will appear forgotten, and will live only with a languishing
existence; and then some accidental circumstance suddenly brings them
new life, and they become the object of manifold labours, engross
public attention, and invade nearly the whole domain of science.
We have in our own day witnessed such a spectacle. The discovery of
the X rays--a discovery which physicists no doubt consider as the
logical outcome of researches long pursued by a few scholars working
in silence and obscurity on an otherwise much neglected subject--
seemed to the public eye to have inaugurated a new era in the history
of physics. If, as is the case, however, the extraordinary scientific
movement provoked by Röntgen's sensational experiments has a very
remote origin, it has, at least, been singularly quickened by the
favourable conditions created by the interest aroused in its
astonishing applications to radiography.
A lucky chance has thus hastened an evolution already taking place,
and theories previously outlined have received a singular development.
Without wishing to yield too much to what may be considered a whim of
fashion, we cannot, if we are to note in this book the stage actually
reached in the continuous march of physics, refrain from giving a
clearly preponderant place to the questions suggested by the study of
the new radiations. At the present time it is these questions which
move us the most; they have shown us unknown horizons, and towards the
fields recently opened to scientific activity the daily increasing
crowd of searchers rushes in rather disorderly fashion.
One of the most interesting consequences of the recent discoveries has
been to rehabilitate in the eyes of scholars, speculations relating to
the constitution of matter, and, in a more general way, metaphysical
problems. Philosophy has, of course, never been completely separated
from science; but in times past many physicists dissociated themselves
from studies which they looked upon as unreal word-squabbles, and
sometimes not unreasonably abstained from joining in discussions which
seemed to them idle and of rather puerile subtlety. They had seen the
ruin of most of the systems built up _a priori_ by daring
philosophers, and deemed it more prudent to listen to the advice given
by Kirchhoff and "to substitute the description of facts for a sham
explanation of nature."
It should however be remarked that these physicists somewhat deceived
themselves as to the value of their caution, and that the mistrust
they manifested towards philosophical speculations did not preclude
their admitting, unknown to themselves, certain axioms which they did
not discuss, but which are, properly speaking, metaphysical
conceptions. They were unconsciously speaking a language taught them
by their predecessors, of which they made no attempt to discover the
origin. It is thus that it was readily considered evident that physics
must necessarily some day re-enter the domain of mechanics, and thence
it was postulated that everything in nature is due to movement. We,
further, accepted the principles of the classical mechanics without
discussing their legitimacy.
This state of mind was, even of late years, that of the most
illustrious physicists. It is manifested, quite sincerely and without
the slightest reserve, in all the classical works devoted to physics.
Thus Verdet, an illustrious professor who has had the greatest and
most happy influence on the intellectual formation of a whole
generation of scholars, and whose works are even at the present day
very often consulted, wrote: "The true problem of the physicist is
always to reduce all phenomena to that which seems to us the simplest
and clearest, that is to say, to movement." In his celebrated course
of lectures at l'École Polytechnique, Jamin likewise said: "Physics
will one day form a chapter of general mechanics;" and in the preface
to his excellent course of lectures on physics, M. Violle, in 1884,
thus expresses himself: "The science of nature tends towards mechanics
by a necessary evolution, the physicist being able to establish solid
theories only on the laws of movement." The same idea is again met
with in the words of Cornu in 1896: "The general tendency should be to
show how the facts observed and the phenomena measured, though first
brought together by empirical laws, end, by the impulse of successive
progressions, in coming under the general laws of rational mechanics;"
and the same physicist showed clearly that in his mind this connexion
of phenomena with mechanics had a deep and philosophical reason, when,
in the fine discourse pronounced by him at the opening ceremony of the
Congrès de Physique in 1900, he exclaimed: "The mind of Descartes
soars over modern physics, or rather, I should say, he is their
luminary. The further we penetrate into the knowledge of natural
phenomena, the clearer and the more developed becomes the bold
Cartesian conception regarding the mechanism of the universe. There is
nothing in the physical world but matter and movement."
If we adopt this conception, we are led to construct mechanical
representations of the material world, and to imagine movements in the
different parts of bodies capable of reproducing all the
manifestations of nature. The kinematic knowledge of these movements,
that is to say, the determination of the position, speed, and
acceleration at a given moment of all the parts of the system, or, on
the other hand, their dynamical study, enabling us to know what is the
action of these parts on each other, would then be sufficient to
enable us to foretell all that can occur in the domain of nature.
This was the great thought clearly expressed by the Encyclopædists of
the eighteenth century; and if the necessity of interpreting the
phenomena of electricity or light led the physicists of last century
to imagine particular fluids which seemed to obey with some difficulty
the ordinary rules of mechanics, these physicists still continued to
retain their hope in the future, and to treat the idea of Descartes as
an ideal to be reached sooner or later.
Certain scholars--particularly those of the English School--outrunning
experiment, and pushing things to extremes, took pleasure in proposing
very curious mechanical models which were often strange images of
reality. The most illustrious of them, Lord Kelvin, may be considered
as their representative type, and he has himself said: "It seems to me
that the true sense of the question, Do we or do we not understand a
particular subject in physics? is--Can we make a mechanical model
which corresponds to it? I am never satisfied so long as I have been
unable to make a mechanical model of the object. If I am able to do
so, I understand it. If I cannot make such a model, I do not
understand it." But it must be acknowledged that some of the models
thus devised have become excessively complicated, and this
complication has for a long time discouraged all but very bold minds.
In addition, when it became a question of penetrating into the
mechanism of molecules, and we were no longer satisfied to look at
matter as a mass, the mechanical solutions seemed undetermined and the
stability of the edifices thus constructed was insufficiently
demonstrated.
Returning then to our starting-point, many contemporary physicists
wish to subject Descartes' idea to strict criticism. From the
philosophical point of view, they first enquire whether it is really
demonstrated that there exists nothing else in the knowable than
matter and movement. They ask themselves whether it is not habit and
tradition in particular which lead us to ascribe to mechanics the
origin of phenomena. Perhaps also a question of sense here comes in.
Our senses, which are, after all, the only windows open towards
external reality, give us a view of one side of the world only;
evidently we only know the universe by the relations which exist
between it and our organisms, and these organisms are peculiarly
sensitive to movement.
Nothing, however, proves that those acquisitions which are the most
ancient in historical order ought, in the development of science, to
remain the basis of our knowledge. Nor does any theory prove that our
perceptions are an exact indication of reality. Many reasons, on the
contrary, might be invoked which tend to compel us to see in nature
phenomena which cannot be reduced to movement.
Mechanics as ordinarily understood is the study of reversible
phenomena. If there be given to the parameter which represents
time,[1] and which has assumed increasing values during the duration
of the phenomena, decreasing values which make it go the opposite way,
the whole system will again pass through exactly the same stages as
before, and all the phenomena will unfold themselves in reversed
order. In physics, the contrary rule appears very general, and
reversibility generally does not exist. It is an ideal and limited
case, which may be sometimes approached, but can never, strictly
speaking, be met with in its entirety. No physical phenomenon ever
recommences in an identical manner if its direction be altered. It is
true that certain mathematicians warn us that a mechanics can be
devised in which reversibility would no longer be the rule, but the
bold attempts made in this direction are not wholly satisfactory.
[Footnote 1: I.e., the time-curve.--ED.]
On the other hand, it is established that if a mechanical explanation
of a phenomenon can be given, we can find an infinity of others which
likewise account for all the peculiarities revealed by experiment.
But, as a matter of fact, no one has ever succeeded in giving an
indisputable mechanical representation of the whole physical world.
Even were we disposed to admit the strangest solutions of the problem;
to consent, for example, to be satisfied with the hidden systems
devised by Helmholtz, whereby we ought to divide variable things into
two classes, some accessible, and the others now and for ever unknown,
we should never manage to construct an edifice to contain all the
known facts. Even the very comprehensive mechanics of a Hertz fails
where the classical mechanics has not succeeded.
Deeming this check irremediable, many contemporary physicists give up
attempts which they look upon as condemned beforehand, and adopt, to
guide them in their researches, a method which at first sight appears
much more modest, and also much more sure. They make up their minds
not to see at once to the bottom of things; they no longer seek to
suddenly strip the last veils from nature, and to divine her supreme
secrets; but they work prudently and advance but slowly, while on the
ground thus conquered foot by foot they endeavour to establish
themselves firmly. They study the various magnitudes directly
accessible to their observation without busying themselves as to their
essence. They measure quantities of heat and of temperature,
differences of potential, currents, and magnetic fields; and then,
varying the conditions, apply the rules of experimental method, and
discover between these magnitudes mutual relations, while they thus
succeed in enunciating laws which translate and sum up their labours.
These empirical laws, however, themselves bring about by induction the
promulgation of more general laws, which are termed principles. These
principles are originally only the results of experiments, and
experiment allows them besides to be checked, and their more or less
high degree of generality to be verified. When they have been thus
definitely established, they may serve as fresh starting-points, and,
by deduction, lead to very varied discoveries.
The principles which govern physical science are few in number, and
their very general form gives them a philosophical appearance, while
we cannot long resist the temptation of regarding them as metaphysical
dogmas. It thus happens that the least bold physicists, those who have
wanted to show themselves the most reserved, are themselves led to
forget the experimental character of the laws they have propounded,
and to see in them imperious beings whose authority, placed above all
verification, can no longer be discussed.
Others, on the contrary, carry prudence to the extent of timidity.
They desire to grievously limit the field of scientific investigation,
and they assign to science a too restricted domain. They content
themselves with representing phenomena by equations, and think that
they ought to submit to calculation magnitudes experimentally
determined, without asking themselves whether these calculations
retain a physical meaning. They are thus led to reconstruct a physics
in which there again appears the idea of quality, understood, of
course, not in the scholastic sense, since from this quality we can
argue with some precision by representing it under numerical symbols,
but still constituting an element of differentiation and of
heterogeneity.
Notwithstanding the errors they may lead to if carried to excess, both
these doctrines render, as a whole, most important service. It is no
bad thing that these contradictory tendencies should subsist, for this
variety in the conception of phenomena gives to actual science a
character of intense life and of veritable youth, capable of
impassioned efforts towards the truth. Spectators who see such moving
and varied pictures passing before them, experience the feeling that
there no longer exist systems fixed in an immobility which seems that
of death. They feel that nothing is unchangeable; that ceaseless
transformations are taking place before their eyes; and that this
continuous evolution and perpetual change are the necessary conditions
of progress.
A great number of seekers, moreover, show themselves on their own
account perfectly eclectic. They adopt, according to their needs, such
or such a manner of looking at nature, and do not hesitate to utilize
very different images when they appear to them useful and convenient.
And, without doubt, they are not wrong, since these images are only
symbols convenient for language. They allow facts to be grouped and
associated, but only present a fairly distant resemblance with the
objective reality. Hence it is not forbidden to multiply and to modify
them according to circumstances. The really essential thing is to
have, as a guide through the unknown, a map which certainly does not
claim to represent all the aspects of nature, but which, having been
drawn up according to predetermined rules, allows us to follow an
ascertained road in the eternal journey towards the truth.
Among the provisional theories which are thus willingly constructed by
scholars on their journey, like edifices hastily run up to receive an
unforeseen harvest, some still appear very bold and very singular.
Abandoning the search after mechanical models for all electrical
phenomena, certain physicists reverse, so to speak, the conditions of
the problem, and ask themselves whether, instead of giving a
mechanical interpretation to electricity, they may not, on the
contrary, give an electrical interpretation to the phenomena of matter
and motion, and thus merge mechanics itself in electricity. One thus
sees dawning afresh the eternal hope of co-ordinating all natural
phenomena in one grandiose and imposing synthesis. Whatever may be the
fate reserved for such attempts, they deserve attention in the highest
degree; and it is desirable to examine them carefully if we wish to
have an exact idea of the tendencies of modern physics.