Section III.-CALORIFIC VALUE OF FOODS 



The Animal as a Heat Engine.— Just as an engine may 

 be regarded as a means of converting the fuel supplied into 

 work done, so a food fed to a horse may be also regarded in the 

 same light, and the food fed to a milk-producing or fattening 

 beast may be also regarded from the energy point of view. 

 Energy is usually represented in terms of calories. The 

 calorie adopted in theoretical considerations is the amount of 

 heat necessary to raise the temperature of one gramme of 

 water one degree Centigrade. In practical, big-scale work it 

 is preferable to employ a unit looo times that size, and to 

 define this large Calorie as the amount of heat required to 

 raise the temperature of i kilogramme of water i® Cent. On 

 such a scale, the complete combustion of earth nut oil 

 would give 8-8 Calories, wheat gluten 5*8 Calories, starch 

 4*1 Calories, and urea 2*5 Calories. In the animal body the 

 final products of the decomposition of the foods differ from 

 those obtained in the steel bomb used for determining heat 

 equivalents, owing to the fact that the nitrogen is not given 

 off as elementary nitrogen, but is given off in the form of 

 urea. As the amount of nitrogen in urea is nearly three 

 times as great as that in the ordinary albuminoid or protein, 

 one part of protein may be assumed to produce one-third 

 of a part of urea, giving a loss of 2 5-^3 Calories, and, 

 therefore, the 5-8 Calories from wheat gluten would only 

 produce about 5 Calories in the animal body, because the 

 fractional part would represent the loss due to producing 

 urea instead of nitrogen. No such deduction, of course, 

 has to be made for the carbohydrates or oils. The calories 

 evolved in the consumption of a food, therefore, needs 

 two deductions to be made from them. Firstly the 



