430 FRAGMENTS OF SCIENCE. 



the animal organism can also generate heat outside of 

 itself. A blacksmith, for example, by hammering can 

 heat a nail, and a savage by friction can warm wood to 

 its point of ignition. Now, unless we give up the physio- 

 logical axiom that the living body cannot create heat 

 out of nothing, c we are driven,' says Mayer, 'to the 

 conclusion that it is the toted heat generated within and 

 without that is to be regarded as the true calorific 

 effect of the matter oxidised in the body.' 



From this, again, he inferred that the heat generated 

 externally must stand in a fixed relation to the work 

 expended in its production. For, supposing the organic 

 processes to remain the same ; if it were possible, by the 

 mere alteration of the apparatus, to generate different 

 amounts of heat by the same amount of work, it would 

 follow that the oxidation of the same amount of material 

 would sometimes yield a less, sometimes a greater, 

 quantity of heat. * Hence,' says Mayer, * that a fixed 

 relation subsists between heat and work, is a postulate 

 of the physiological theory of combustion.' 



This is the simple and natural account, given subse- 

 quently by Mayer himself, of the course of thought 

 started by his observation in Java. But the conviction 

 once formed, that an unalterable relation subsists between 

 work and heat, it was inevitable that Mayer should seek 

 to express it numerically. It was also inevitable that a 

 mind like his, having raised itself to clearness on this 

 important point, should push forward to consider the 

 relationship of natural forces generally. At the begin- 

 ning of 1842 his work had made considerable progress ; 

 but he had become physician to the town of Heilbronn, 

 and the duties of his profession limited the time which 

 he could devote to purely scientific enquiry. He thought 

 it wise, therefore, to secure himself against accident, and 

 in the spring of 1842 wrote to Liebig, asking him to 



