EXCHANGE OF MATERIAL. 471 



In adults weighing about 70 kilos about 30 to 35 grms. urea per 

 day are eliminated, or 0.5 gnu. per kilo. At about 15 years of age 

 the destruction of proteids per kilo is about the same as in adults. 

 The relatively greater metabolism of proteids in young individuals 

 is explained partly by the fact that the metabolism of material in 

 general is more active in young animals, and partly by the fact that 

 the young animal is as a rule poorer in fat than the grown ones. 



V. Potential Energy and the Relative Nutritive Value of 

 Various Organic Foods. 



With the organic foods the organism receives a supply of poten 

 tial energy which is converted into living force in the body. This 

 potential energy of the various foods may be represented by the 

 amount of heat which is set free in their combustion. This quan- 

 tity of heat, expressed in calories, if we consider the calorie as that 

 quantity of heat which is necessary to raise 1 grm. of water from 

 C. to 1 0., is the following for 1 grin, of the substance. 



TABLE XV. 



Calories in Calories in 



Dry Substance. Ash-free Substance. 



Proteids (in meat) 5754 5778 ) 



Muscle 5345 5656 [ RUBNER. 



Fat (pig-fat, melt, pt. -f 43) 9423 ) 



Grape-sugar 36921 



Milk-sugar 3877 I STOHMANN 



Cane-sugar 3959 ^TOHMANN. 



Starch 4116 J 



Fat and carbohydrates are completely burnt in the body, and we 

 can therefore consider their combustion equivalent as a measure of 

 the living force developed by them within the organism. The pro- 

 teids act differently. They are only incompletely burnt, and they 

 yield certain products of decomposition which, leaving the body 

 with the excreta, still represent a certain amount of potential 

 energy which is lost for the body. The heat of combustion of the 

 proteids is smaller within the organism than outside of it, and they 

 must therefore be specially determined. For this purpose RUBNER 

 fed a dog on washed meat. He subtracted from the heat of com- 



