954 NUTRITION AND HEAT REGULATION. 



electrical current. The bomb is immersed in water and the heat 

 given off raises the temperature of the water a certain number 

 of degrees centigrade. The weight of water being known, the 

 amount of heat is easily expressed in calories. For the purpose 

 of measuring the heat given off by living animals two principal 

 forms of calorimeter are used, each form having a number of 

 modifications. These two forms are the water calorimeter and the 

 air calorimeter. The water calorimeter was the form used in the 

 first experiments on record (Crawford, 1779). In principle it 

 consists of a double-walled box with a known weight of water 

 between the walls. The animal is placed in the inner box and the 

 heat given off is absorbed by the water. Knowing the weight of 

 the water and how much its temperature is raised, the data are at 

 hand for determining the number of calories given off during the 

 experiment. One form of this variety of calorimeter, used in this 

 country by Reichert, is shown in Fig. 300. It consists of two con- 

 centric boxes of metal with the space between them filled with 

 water. The animal is placed in the inner box (A). The two 

 boxes are inclosed in a large wooden box, the space between 

 the metal and wooden boxes being filled with shavings (SH). 

 The object of this outer box is to prevent radiation of heat from 

 the metal boxes. The tubes EN and EX, which lead into the 

 interior chamber containing the animal, are for the entrance 

 and exit of the ventilating air. A thermometer is placed in 

 each to determine the heat carried off by the air. The ther- 

 mometer, CT, measures the temperature of the water, and S is 

 a stirrer to keep the water well mixed and thus insure a uniform 

 temperature. When the animal is placed in the apparatus the 

 heat given off warms not only the water, but also the metal ; so 

 that to determine the total heat the weight of metal must be re- 

 duced to an equivalent amount of water by multiplying its weight 

 by its specific heat, or, a more simple method, the calorimetric equiv- 

 alent of the apparatus is determined, — that is, the actual amount of 

 heat necessary to raise the temperature of the apparatus, water and 

 metal, one degree. This value is obtained by burning in the appa- 

 ratus a known weight of some substance (alcohol, hydrogen) whose 

 heat of combustion is known. The calorimeter used in this way 

 measures directly the amount of heat given off from the animal 

 during the period of observation. 



Many investigators have used some form of air calorimeter. 

 An air calorimeter consists essentially of a double-walled chamber 

 or box with air between the walls. The animal is placed in the 

 inner box and the heat given off is measured by the expansion of 

 the air between the walls. Many different forms are used, prefer- 

 ence being given to some modification of the differential air calor- 



