CH. XLII.] 



CALOR1METRY 



631 



which the body does no work, and neither gains nor loses in weight, 

 resolves itself into the following problem : Does the heat given out 

 by the body equal that which would be given out by the complete 

 combustion of the various food substances, minus that given out by 

 the complete combustion of the excreta ? 



The data necessary for settling such a question are determined 

 by the process known as calorimetry. 



The Bomb Calorimeter. The heat of combustion of any of the 

 food substances or of the excreta is determined by placing a known 

 weight of the substance in 

 question (A, fig. 393) within 

 a bomb (B) immersed in a 

 known volume of water ; the 

 water is at air-temperature 

 in a brass vessel (E), en- 

 closed within an ebonite 

 casing (F), which acts as a 

 non-conductor of heat. The 

 bomb is connected with a 

 cylinder of oxygen at high 

 pressure; and the sub- 

 stance A is ignited by an 

 electric spark by means of 

 the wires D. The pro- 

 ducts of combustion pass 

 out through the spiral tube 

 C, and on their journey give 

 off their heat to the water. 

 If the heat of combustion 

 of gases or volatile liquids is 

 to be determined, a special 

 form of burner is introduced 

 at the opening at the bottom 

 of the bomb. When the 

 combustion is complete the 

 rise of temperature of the 

 water is observed by the 

 thermometer T. During the 

 combustion, the water is kept in movement by the stirrer S, which 

 is worked by a small motor. The rise of temperature multiplied by 

 the weight of the water gives the amount of heat expressed in calories, 

 1 calorie or heat-unit being the quantity of heat necessary to raise 

 1 gramme of water 1 C. 



Any given oxidation will always produce the same amount of 

 heat. Thus, if we oxidise a gramme of carbon, a known amount of 



FIG. 393. Diagram of Bomb Calorimeter. 

 (After Thomsen.) 



