QUANTIVALENCY OF HEAT AND MOTION. 103 



ment of heat being spread over a period of many minutes, 

 there is no great manifestation of temperature at any one 

 instant. The quantity of heat is, however, the same in both 

 cases. One gramme of hydrogen in combining with oxygen, 

 whether quickly or slowly, will always evolve 34 units of 

 heat; and one gramme of carbon in combining with oxygen, 

 whether quickly or slowly, will always evolve 8 units of 

 heat, The slow oxidation of so much carbon and hydrogen 

 in the human body, therefore, will always produce its due 

 amount of heat, or an equivalent in some other form of 

 energy ; for while the latent force liberated by the combustion of 

 the carbon and hydrogen of fat is expressed solely in the form of 

 heat, the combustion of an equal quantity of the carbon and 

 hydrogen of voluntary muscle is expressed chiefly in the form of 

 motion. You may remember that I referred in my last lecture 

 to the equivalency subsisting between heat and motion to the 

 circumstance that so much heat was convertible into so much 

 motion. Accordingly, when we burn or oxidise the carbo-hydro- 

 gen of animal tissue, instead of getting the heat-force which was 

 exerted in separating this carbo -hydrogen from oxygen remani- 

 fested in the form of heat, we have it, under certain circumstances, 

 manifested in the form of motion. 



(no.) Let us then proceed to consider what are the quanti- 

 tative relations subsisting between heat and motion. We have 

 taken as our unit of heat the quantity of heat absorbed or 

 evolved by one gramme of water in rising or falling through one 

 centigrade degree of temperature. Now, this is found by experi- 

 ment to be the exact quantity of heat generated by collision with 

 the earth of a kilogramme weight falling from a height of 424 

 metres. So that the mechanical force of a kilogramme weight 

 which has fallen through 424 metres, or, in other words, the 

 mechanical force necessary to lift a kilogramme weight to the 

 height of 424 metres, is equal to, interchangeable for, and con- 

 vertible into the heat-force evolved or absorbed by a kilogramme 

 of water in changing its temperature one degree. Or the arrest 

 of a unit of motion would raise the temperature* of a kilogramme 



