ENERGY IN GENERAL. 8l 



of the metamorphosis of mechanical into thermal 

 energy. 



The Mechanical Equivalent of Heat, — The inter- 

 pretation assumes a remarkable character of precision, 

 which at once strikes the mind when physics applies 

 to these transformations the almost absolute accuracy 

 of its measurements. We then find that the rate of 

 exchange is invariable. Transformations of heat into 

 motion, and of motion into heat, take place according 

 to a rigorous numerical law, which brings into exact 

 correspondence the quantity of each. Mechanical 

 effect is estimated, as we have seen, by work, that is in 

 kilogrammetres. Heat is measured in calories, the 

 calorie being the quantity of heat necessary to raise 

 from o°C to i°C a kilogramme of water (Calorie) or 

 one gramme of water (calorie). It is found that 

 whatever may be the bodies and the phenomena 

 which serve as intermediaries for carrying out this 

 transformation, we must always expend 425 kilo- 

 grammetres to create a Calorie, or expend 0*00234 

 Calories to create a kilogrammetre. The number 425 

 is the mechanical equivalent of the Calorie, or, as is 

 incorrectly stated, of the heat It is this constant fact 

 which constitutes the principle of the equivalence of heat 

 and of mechanical zvork. 



§ 5. Chemical Energy. 



We cannot yet actually measure chemical activity 

 directly, but we know that chemical action may give 

 rise to all other phenomenal modalities. It is their 

 most ordinary source, and it is to it that industries 

 appeal to obtain heat, electricity, and mechanical 



