THE COPLEY MEDALIST OF 1870. 425 



the penetration of Mr. Joule, and his mastery of prin- 

 ciples, at the period now referred to. A friend had 

 mentioned to him nailer's hypothesis, that animal heat 

 might arise from the friction of the blood in the veins 

 and arteries. ' It is unquestionable,' writes Mr. Joule, 

 ' that heat is produced by such friction; but it must be 

 understood that the mechanical force expended in the 

 friction is a part of the force of affinity which causes 

 the venous blood to unite with oxygen, so that the 

 whole heat of the system must still be referred to the 

 chemical changes. But if the animal were engaged in 

 turning a piece of machinery, or in ascending a moun- 

 tain, I apprehend that in proportion to the muscular 

 effort put forth for the purpose, a diminution of the 

 heat evolved in the system by a given chemical action 

 would be experienced.' The italics in this memorable 

 passage, written, it is to be remembered, in 1843, are 

 Mr. Joule's own. 



The concluding paragraph of this British Associa- 

 tion paper equally illustrates his insight and precision, 

 regarding the nature of chemical and latent heat. ' I 

 had,' he writes, ' endeavoured to prove that when two 

 atoms combine together, the heat evolved is exactly 

 that which would have been evolved by the electrical 

 current due to the chemical action taking place, and is 

 therefore proportional to the intensity of the chemical 

 force causing the atoms to combine. I now venture to 

 state more explicitly, that it is not precisely the attrac- 

 tion of affinity, but rather the mechanical force ex- 

 pended by the atoms in falling towards one another, 

 which determines the intensity of the current, and, 

 consequently, the quantity of heat evolved; so that we 

 have a simple hypothesis by which we may explain why 

 heat is evolved so freely in the combination of gases, 

 and by which indeed we may account " latent heat " as 



