§ 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.