THE COPLEY MEDALIST OF 1870 447 



experiments an equivalent of 770 foot-pounds, a figure 

 remarkably near the one now accepted. A detached state- 

 ment regarding the origin and convertibility of animal heat 

 strikingly illustrates the penetration of Mr. Joule, and his 

 mastery of principles, at the period now referred to. A 

 friend had mentioned to him Haller'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 expe- 

 rienced. " 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 Association 

 paper equally illustrates his insight and precision regard- 

 ing the nature of chemical and latent heat. "I had," he 

 writes, "endeavored to prove that when two atoms com- 

 bine 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 attraction of affinity, but rather the 

 mechanical force expended by the atoms in falling toward 

 one another, which determines the intensity of the cur- 



