24 THE CONSERVATION OF ENERGY. PART i. 



equivalent to the dominant chemical action in the 

 battery. The two electricities, positive and negative, 

 are developed in equal proportions, which may be com- 

 bined so as to produce many changes in their respective 

 relations, yet the sum of the energy of the one kind can 

 never be made in the smallest degree either to exceed 

 or to come short of the sum of the other. 



The mechanical energy of machinery or working power 

 is exhausted by the very act of working, and cannot be 

 restored except by the action of other forces. In clock- 

 work, the weight must sink to move the wheel, and 

 when the weight is down, the store of energy is gone, 

 and can only be restored by raising the weight through 

 the expenditure of energy in the human arm, and the 

 expenditure of human energy must be restored by food 

 and rest. The heat given off from the bodies of men 

 and animals is restored by the combustion of the oxygen 

 inhaled during respiration and the carbon of the food, 

 and the light and heat given out by the combustion of 

 fuel, whether in the form of coal or wood, is compen- 

 sated by the light and heat of the sun stored up in 

 living vegetables. It is this equivalent for force or 

 energy which prevails in every department of nature 

 that constitutes the universal and invariable law of 

 the Conservation of Energy, ' a principle in physics as 

 large and sure as that of the indestructibility of matter 

 or the invariability of gravity. No hypothesis should 

 be admitted nor any assertion of a fact credited, that 

 denies this principle. No view should be inconsistent 

 or incompatible with it. Many of our hypotheses in the 

 present state of science may not comprehend it, and 

 may be unable to suggest its consequences, but none 

 should oppose or contradict it.' 1 Thus, 'there is a 

 definite store of energy in the universe, and every natural 

 change or technical work is produced by a part only of 



1 Professor Faraday. 



