§ 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.