VI. CALORIMETRIC MEASUREMENTS 193 



weight of the bottle containing concentrated siiUuric acid through 

 which the air from the chamber bubbles, as indicated in Figure 4. 

 Under normal conditions the latent heat of water evaporation 

 amounts to about one-fifth of the total heat given off by man. Its 

 share of the total heat loss increases with increasing environmental 

 temperature, and when the environment is at body temperature all 

 the heat produced has to be given off as heat of evaporation. The air- 

 circulating system of the At water-Rosa apparatus permits the meas- 

 urement of carbon dioxide production and of oxygen consumption 

 as well as water evaporation, as indicated in Figure 4. 



7. Bomb Caloriiiietry 



The energy content of substances used as fuel for furnaces or 

 organisms is measured by the heat evolved when these substances are 

 combusted. Aside from measuring fuel values, however, heats of 

 combustion serve as the most common bases for calculating other im- 

 portant data for thermochemistry and thermodynamics, such as heat 

 of reaction and free energy. Combustion calorimetry is therefore 

 the type of heat measurement most widely used in bioenergetics. 

 Among combustion calorimeters, in turn, the instrument best known 

 and most widely used is the bomb calorimeter, invented by Berthelot 

 in 1869. 



Figure 5 is a schematic sketch of a simple bomb calorimeter. The sample, 

 F, to be combusted is weighed into a little dish, which is subsequently sus- 

 pended in the center of the bomb, B. For ignition, one end of a thin iron 

 wire, /, is clamped to an electrically insulated plug in the wall of the bomb; 

 the other end of the wire is directly connected to the wall of the bomb. In 

 many laboratories a weighed cotton thread is hung over the ignition wire for 

 more reliable ignition of the sample. 



The bomb is then closed — as a rule by a heavy steel cap screwed on a lead 

 gasket, and filled with oxygen to a pressure of 30 atmospheres through valve 

 V. The bomb is then placed into the calorimeter pail, which contains a 

 weighed amount, usually 2 to 3 kg., of water, W. This pail, in turn, is in- 

 stalled inside the calorimeter jacket, /, which is filled with water to provide 

 a large heat capacity and consequent uniformity of the temperature sur- 

 rounding the calorimeter water, W. The jacket may be covered with an in- 

 sulating layer to shield the inside from sudden changes in room temperature. 

 This precaution is unnecessary when calorimetric measurements are carried 

 out in a temperature-controlled room. A good stirrer, S, is essential. A 

 cover. C, to prevent errors from water evaporation is very desirable. The 

 temperature of the calorimeter water is measured by a thermometer, T, 



