SOURCES OF HEAT. 



379 



isodynamic with 243 grams of dry meat or 225 grams of dry syntonin, or with 

 256 grams of dextrose. According to Pfliiger, i gram of nitrogen in meat equals 

 2.79 grams of fat; i gram of animal fat equals 0.364 gram of nitrogen in meat; 

 i gram of starch equals 0.424 gram of fat or 0.154 gram of nitrogen in meat; i gram 

 of grape-sugar equals 0.390 gram of fat or 0.42 gram of nitrogen in meat; too 

 grams of vegetable albumin likewise equals 55 grams of fat or 121 grams of 

 starch or 137 grams of dextrose. 



Rubner estimates in human beings on a mixed diet the available heat -pro- 

 ducing energy for i gram of proteid at approximately 4100 calories, for i gram 

 of fat 9300 calories, for i gram of carbohydrate 4100 calories. For the dog Rubner 

 determined that i gram of nitrogen in the excreta of the fasting animal had caused 

 the production of 25,000 calories; further, that i gram of nitrogen in the excreta 

 with a meat-diet had produced 

 26,000 calories; and i gram of 

 carbon, formed from 1.3 grams 

 of fat, had yielded 12,300 calo- 

 ries. 



If it be known, therefore, 

 how many parts by weight 

 of the foregoing substances 

 a human being takes up with 

 his food during twenty-four 

 hours, the calculation can be 

 made as to how many heat- 

 units he may generate there- 

 from through oxidation. In 

 this connection the utiliza- 

 tion of the nutrient materials 

 must be taken into consider- 

 ation, in accordance with 

 which a certain, even though 

 small, percentage of the food 

 cannot be disposed of by the 

 digestive and absorptive or- 

 gans, and therefore is ex- 

 creted unused. 



Rubner found that, however 

 abundant the administration of 

 food, a larger amount of heat 

 can be shown to be produced 

 immediately on the first day of 

 feeding, as compared with the 

 preceding days of fasting. The bodily temperature under such circumstances 

 remains unaltered. The greatest amount of heat is produced as a result of 

 excessive administration of proteids, less from carbohydrates and least from fats. 



In detail the sources of heat are as follows : 



i. The transformation of chemical combinations of foods with high 

 potential energy into those of lesser or completely exhausted potential 

 energy. As the organic articles of food, exclusive of the inorganic 

 accompaniments, consist of C, H, N and O, it is especially through (a) 

 the combustion of C into CO 2 and of H into H 2 O that heat is produced. 

 In this connection it is to be noted that the combustion of i gram of 

 C into C0 2 yields 8080 heat-units, while that of i gram of H into H 2 O 

 yields 34,460 heat-units, though the C and H in the molecules of the 

 food-stuffs must not already be saturated with O. The amount of 

 necessary for this purpose is taken up in the act of respiration. There- 



FIG. 133. Water Calorimeter (after Favre and Silbermann). 



