JULIUS ROBERT MAYER. 399 



The observation riveted his attention; he reasoned upon it, and came to the 

 conclusion that the brightness of the color was due to the fact that a less amount 

 of oxidation sufficed to keep up the temperature of the body in a hot climate 

 than in a cold one. The darkness of the venous blood he regarded as the visible 

 sign of the energy of the oxidation. 



It would be trivial to remark that accidents such as this, appealing to minds 

 prepared for them, have often led to great discoveries. Mayer's attention was 

 thereby drawn to the whole question of animal heat. Lavoisier had ascribed 

 this heat to the oxidation of the food. " One great principle," says Mayer, " of 

 the physiological theory of combustion is, that under all circumstances the same 

 amount of fuel yields, by its perfect combustion, the same amount of heat ; that 

 this law holds good even for vital processes ; and that hence the living body, 

 notwithstanding all its enigmas and wonders, is incompetent to generate heat 

 out of nothing." 



But beyond the power of generating internal heat, 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 physiological axiom that the living body can not 

 create heat out of nothing, "we are driven," says Mayer, "to the conclusion 

 that it is the total heat generated within and without that is to be regarded as 

 the true calorific effect of the matter oxidized 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 altera- 

 tion 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 subsequently 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 inevi- 

 table that a mind like his, having raised itself to clearness on this important 

 point, should push forward to consider the relationship of natural forces gener- 

 ally. At the beginning of 1842 his work had made considerable progress ; but 

 he had become physician to the town of Heilbronn, and the duties of his pro- 

 fession limited the time which he could devote to purely scientific inquiry. He 

 thought it wise, therefore, to secure himself against accident, and in the spring 

 of 1842 wrote to Liebig, asking him to publish in his "Annalen" a brief pre- 

 liminary notice of the work then accomplished. Liebig did so, and Dr. Mayer's 

 first paper is contained in the May number of the "Annalen " for 1842. 



Mayer had reached his conclusions by reflecting on the complex processes of 

 the living body ; but his first step in public was to state definitely the physical 

 principles on which his physiological deductions were to rest. He begins, there- 

 fore, with the forces of inorganic nature. He finds in the universe two systems 

 of causes which are not mutually convertible : the different kinds of matter and 

 the different forms of force. The first quality of both he affirms to be inde- 

 structibility. A force can not become nothing, nor can it arise from nothing. 

 Forces are convertible, but not destructible. In the terminology of his time, he 

 then gives clear expression to the ideas of potential and dynamic energy, illus- 



