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

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 move-

ment-Modern physicists challenge this: physical, un-

like 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?

MEASUREMENTS .

CHAPTER II

§ 1. Metrology: Lord Kelvin's view of its necessity-Its

definition (pp. 19-21). § 2. The Measure of Length:

Necessity for unit-Absolute length-History of Stand-

ard-Description of Standard Metre-Unit of wave-

lengths preferable - The International Metre (pp.

21-30). § 3. The Measure of Mass: Distinction be-

tween mass and weight-Objections to legal kilo-

gramme and its precision-Possible improvement (pp.

31-33). § 4. The Measure of Time: Unit of time the

second-Alternative units proposed-Improvements in

chronometry and invar (pp. 34-37). § 5. The Measure

1-18

. 19-50

=

of Temperature: Fundamental and derived units-Or-
dinary 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 im
provements in thermometry (pp. 37-42). § 6. Derived
Units and Measure of Energy: Importance of erg as
unit - Calorimeter usual means of determination
Photometric units (pp. 42-45). § 7. Measure of Phy-
sical 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 (pp.
45-50).

PRINCIPLES

-

CHAPTER III

§1. The Principles of Physics: The Principles of

Mechanics affected by recent discoveries-Is mass

indestructible? Landolt and Heydweiller's experi-

ments-Lavoisier's law only approximately true-

Curie's principle of symmetry (pp. 51-55). § 2. The

Principle of the Conservation of Energy: Its evolution :

Bernoulli, Lavoisier and Laplace, Young, Rumford,

Davy, Sadi Carnot, and Robert Mayer-Mayer's draw-

backs-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 (pp. 55-72). §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 in

crease incessantly-A magnitude which measures evolu-

tion 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 (pp. 72-87). § 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 (pp. 87-92). § 5.

Atomism: Connection of subject with preceding-

PAGES

51-104

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 (pp. 87-104).

-

PAGES

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-Dis-

covery 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

(pp. 105-117). § 2. The Liquefaction of Gases and

Low Temperatures: Linde's, Siemens', and Claude's

methods of liquefying gases-Apparatus of Claude de-

scribed-Dewar's experiments-Modification of electrical

properties of matter by extreme cold: of magnetic and

chemical-Vitality of bacteria unaltered-Ramsay's dis-

covery of rare gases of atmosphere-Their distribution

in nature-Liquid hydrogen-Helium (pp. 117-126).

§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 (pp. 126-134). § 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 un-

stable equilibria - Duhem and the thermodynamic

postulates Experimental confirmation — Guillaume's

researches on nickel steel-Alloys (pp. 135-142).

CHAPTER V

-

SOLUTIONS AND ELECTROLYTIC DISSOCIATION

105-142

143-168

§ 1. Solution: Kirchhoff's, Gibb's, Duhem's and Van

t'Hoff's researches (pp. 143-146). § 2. Osmosis: His-

tory 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 pheno-

menon (pp. 146-151). § 3. Osmosis applied to Solution:

Van t'Hoff's discoveries-Analogy between dissolved

body and perfect gas-Faults in analogy (pp. 151-155).

§ 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 elec-
tronic theory probable--Faraday's two laws-Valency—
Helmholtz's consequences from Faraday's laws (pp.
155-168).

THE ETHER

CHAPTER VI

PAGES

169-207

§ 1. The Luminiferous Ether: First idea of Ether due

to Descartes-Ether must be imponderable-Fresnel

shows light vibrations to be transverse-
e-Transverse

vibrations cannot exist in fluid-Ether must be discon-

tinuous (pp. 169-175). § 2. Radiations: Wave-lengths

and their measurements-Rubens' and Lenard's re-

searches Stationary waves and colour-photography-

Fresnel's hypothesis opposed by Neumann-Wiener's

and Cotton's experiments (pp. 175-182). § 3. The

Electromagnetic Ether: Ampère's advocacy of mathe-

matical expression-Faraday first shows influence of

medium in electricity-Maxwell's proof that light-waves

electromagnetic-His unintelligibility-Required confir-

mation of theory by Hertz (pp. 182-189). § 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' ex-

periments-Hertzian and light rays contrasted--Pres-

of light (pp. 189-194). § 5. The X-Rays:

Röntgen's discovery-Properties of X-rays-Not homo-

geneous

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 (pp.

sure

-