SECT. XXXII. ELECTRO-INDUCTION. 329 



at rest ; but the instant the machine is set in motion a reaction 

 takes place in the intensity of the current, causing a diminution 

 in the quantity of heat, because the heat that disappears is con- 

 verted into the mechanical force exerted by the engine. 



Mr. Joule's experiments prove that, whenever a current of 

 electricity is generated by a magneto-electric machine, the quan- 

 tity of heat evolved by that current has a constant relation to the 

 power required to work the machine ; and on the other hand, 

 whenever an engine is worked by a Voltaic battery, that the 

 power developed is at the expense of the calorific force of the 

 battery for a given consumption of zinc, the mechanical effect 

 produced having a fixed relation to the heat lost in the Voltaic 

 current. The obvious conclusion Mr. Joule draws from these 

 experiments is, that heat and mechanical power are convertible 

 into one another, and it becomes evident, therefore, that heat is 

 either the vis viva or living force of ponderable particles, or a 

 state of attraction and repulsion capable of generating vis viva 

 (N. 222). 



