_Historical Sketch of Second Law of Thermodynamics_— *Carnot’s perfect heat-engine, *106 117 *Sir W. Thomson’s definition of absolute temperature, *107 118 *Melting point of ice lowered by pressure, *108 120 *Sir W. Thomson’s rectification of Carnot’s reasoning, *109-*110 120 *Professor J. Clerk-Maxwell’s demons, *111-*113 122 Degradation of energy, 114 126 Future of the physical universe, 114-115 126 Past of the physical universe, 116 128 CHAPTER IV. MATTER AND ETHER. Inquiry regarding structure and material of the universe, 117 129 _Various hypotheses regarding matter_— (1.) Greek notion of the _Atom_, 118 130 Speculations of Lucretius, 119-130 131 (2.) Theory of Boscovich (centres of force), 131 137 (3.) Theory of infinite divisibility, 132 138 (4.) Vortex-atom theory, 133-134 139 Remarks on these theories, 135-136 141 Relative quantity of matter associated with energy, 137-138 142 _Universal gravitation_— Is a weak force, 139 144 Two ways of accounting for it, 140 145 Le Sage’s hypothesis, 141-142 146 _The Ethereal medium_— Its principal properties apparently incongruous, 143 148 Analogy of Professor Stokes, 144 149 Distortion and displacement of ether, 145 149 Inferior limit of its density, 146 150 Its supposed imperfect transparency, 147 151 Remarks on ether, 148 153 Remarks on the speculations of this chapter, 149-150 154 Modification of the vortex-ring hypothesis, 151-152 155 Possible disappearance of the visible universe, 153 157 CHAPTER V. DEVELOPMENT. Nature of inquiry stated, 154 158 _Chemical development_— Changes in lists of elementary substances, 155 159 Prout’s speculations, 156 160 Experiments of M. Stas, 156 160 Family groups, 157 161 Mr. Lockyer’s speculations, 158-159 161