42 



THE ANIMAL AS A MACHINE. 



In 1856-57 Hirn experimented with men engaged in 

 regular work and at rest, and found that when at rest 

 they produced a quantity of heat almost exactly, if not 

 precisely, proportional to the amount of oxidation, as 

 measured by the quantity of oxygen absorbed by them 

 and exhaled in carbonic acid. 



This was not precisely the case when they were at 

 work. The principle of equivalence of energies then 

 takes effect, and the measure of all the energy pro- 

 duced by oxidation is found in the sum of the heat 

 discharged from the system and that energy of work 

 which stands for the parts converted into dynamic 

 forms. 



Hirn found that the quantity of heat generated by 

 the human body at rest, whether that of men of mid- 

 dle age, or youth of either sex approaching maturity, 

 was substantially the same under the same circum- 

 stances : about 5 calories per gramme of oxygen in- 

 spired and exhaled as a minimum, 5.2 as a maximum, 

 and usually the latter figure. The differences may be 

 ascribed to variations in observations, rather than to 

 *" real differences of fact. Precisely the same quantities 

 of air were measured as exhaled as were measured as 

 inhaled, in all cases. A singular and significant fact 

 was, however, discoverable in the results, as reported 

 by Hirn : The quantity of heat produced per unit of 

 oxygen absorbed and converted into compounds ex- 

 ceeds by a third the amount computed upon the basis 

 of the experiments of Favre and Silbermann. This 

 result Avould seem to indicate other sources of heat 

 than combustion with oxygen. It may be due to 



* L'Equivalent mecanique de la Chaleur. G. A. Hirn, 1S5S. 



