CHEMICAL CHANGE AND HEAT PRODUCTION. 837 



The heat of combustion of food may be determined in three ways 

 (1) by direct estimation with a calorimeter, (2) by calculation from the 

 oxygen necessary for oxidation, and (3) by measurement of the heat 

 produced by the combustion of the food inside the animal body. Such 

 determinations have been made by Rubner, 1 and the following table is the 

 isodynamic value of 100 grins, of fat estimated by these three modes : 



It is to be noted that, with the exception of proteid, all food substances 

 give too low a value for the heat of combustion when it is calculated from 

 the equivalents of oxygen necessary for combustion. The calculation of the 

 heat of combustion from the oxygen necessary for oxidation gives results 

 which are not exact. 



The value of these calculations in the estimation of the heat pro- 

 duced in a living body will be seen by comparing the results with those 

 obtained by direct determination with the calorimeter. The following 

 are Vierordt's 2 calculations for the heat production of an adult man in 

 twenty-four hours : 



(a) Calculation, according to Dulong's principle, from the heat of combus- 

 tion of carbon and hydrogen. 



An adult man consumes in twenty-four hours 



251-4 grms. C. x 8,080 

 12-56 H. x 34,460 



Total heat production 



= 2,031,312 calories. 

 - 432,818 



= 2,464,130 



1 Ztschr.f. Biol, Mlinchen, 1883, Bd. xix. S. 386. 

 3 "Grundriss der Physiol.," S. 281. 



