CHAPTER III.

THE PRESENT PHYSICAL UNIVERSE.

Definition of the term ‘Physical Universe’, 92 99

It contains something else besides _matter_ or _stuff_, 93 100

Grounds of our belief in an external universe, 94 101

These in accordance with our definition of the laws of the universe (Art. 54), 95 102

Meaning of conservation, 96 103

[Use and Abuse of the Term _Force_,] 97 104

Conservation of _Momentum_, 97 105

Conservation of _Moment of Momentum_, 97 106

Conservation of _Vis Viva_, 97 107

Definition of _Energy_, 98-99 108

Newton’s second interpretation of his Third Law, 99-100 108

Friction changes work into heat, 101 110

Historical sketch of the theory of energy, 102-103 112

Transformability of energy constitutes its use, 104 115

Case where energy is useless, 105 116

_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