272 Biological Chemistry. 



into account only the metabolism of matter. The utilization 

 can also be dealt with from the point of view of energetics. 

 If a substance is burnt in oxygen so as to yield the 

 end products, a certain amount of heat is evolved which 

 can be measured by carrying out the combustion in a 

 calorimetric bomb. This is a closed platinum-lined appa- 

 ratus into which a weighed amount of the substance under 

 investigation is introduced together with an excess of 

 oxygen under pressure of several atmospheres. The 

 whole is then sunk in the water of a calorimeter, and 

 the combustion is initiated by heating electrically a small 

 piece of iron wire placed in contact with the solid. (A 

 separate measurement should be made of the heat evolved 

 by the combustion of the wire.) The heat- value of a food- 

 stuff is usually measured in large " calories," one such unit 

 being the amount of heat necessary to raise 1 kilogram 

 of water from 1C. By measuring, therefore, the rise 

 of temperature of the water in the calorimeter produced 

 by the combustion of the weighed amount of foodstuff, its 

 caloric value can be calculated. The following represent 

 the caloric values of the chief classes of foods : 



1 gramme protein . . . . 4-1 calories 



1 gramme fat 9-3 ,, 



1 gramme carbohydrate . .4-1 



These numbers assume that complete combustion has taken 

 place. In the animal body this is not the case, as the 

 nitrogen is eliminated in the form of urea and other organic 

 compounds and not as nitrogen. Furthermore, certain of 

 the carbohydrates, such as cellulose, undergo only very 

 incomplete combustion, and are eliminated almost unchanged 

 in the faeces. Some nitrogenous matter and fat also pass 

 out in the faeces. Hence a measurement of the utilization 

 of the foodstuff can be made by determining the caloric 

 value of the solids of the excreta (urine and fasces) and 



