METABOLISM. 



339 



THE PRODUCTION OF HEAT. 



The heat evolved by the complete oxidation of a food-stuff can be 

 ascertained by means of the bomb calorimeter (fig. 127), which consists 

 of a metal bomb, A, through the top of which pass two wires, h and k', 

 connected by a strip of soft iron wire ; one gram of the substance, 

 e.g. dextrose, which is to undergo combustion, is placed in contact with 

 the iron wire. The bomb is filled with oxygen from an oxygen cylinder, 

 under a pressure of 7 to 8 atmospheres, and is enclosed in a bath, C, 

 containing a known volume of 

 water, which is surrounded by 

 an air and a water jacket, D. 

 When a current is passed 

 through the wires the soft iron 

 fuses and ignites the sugar, 

 which is rapidly oxidised to 

 carbonic acid and water. The 

 heat evolved in this process raises 

 the temperature of the water in 

 which the bomb is placed ; and 

 if the temperature of the water 

 before and after the combustion 

 is observed, the amount of heat 

 evolved can be calculated, and 

 is expressed in calories. A large 

 calorie is the amount of heat re- 

 quired to raise 1000 grams of 

 water 1 C. (A small calorie is 

 the amount of heat necessary to 

 raise the temperature of 1 gram 

 of water 1 C. ; this measurement is now seldom used.) If the volume 

 of water in such an experiment is 1 litre and the rise of temperature 

 is 4'l C., 1 gram of sugar when fully oxidised gives out 4'1 calories; 

 and this amount, which is spoken of as the calorie value of dextrose, 

 is constant. The average calorie value of fat, carbohydrate, and protein 

 is shown in the following table : 



Fat, 1 gram = 9 -3 calories. 

 Carbohydrate, 1 gram = 4*1 calories. 

 Protein, 1 gram = 4'l calories. 



The calorie value of protein when completely oxidised in the calori- 

 meter is 5*6 ; but protein is not fully oxidised in the body, its nitrogen 



FIG. 127. Bomb calorimeter. 



T = thermometer; E^air jacket; B = stripof 

 iron wire. 



