826 



NUTRITION AND HEAT REGULATION. 



to reduce it to so much water, and this product is multiplied by the 

 difference in body temperature at the beginning and the end of the 

 experiment. The product is obtained in calories and is subtracted 

 from the amount of heat lost, as determined by the calorimeter, to 

 obtain the amount of heat produced. If, on the contrary, the ani- 

 mal's temperature has risen during the experiment the body has 

 produced more heat than it has dissipated. The increase may be 

 determined as above by multiplying the weight of the animal, the 

 specific heat of the body, and the difference in temperature. This 

 amount added to the heat lost gives the heat produced. 



Most recent 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- 



Fig. 267. D'Arsonval's differential calorimeter. 



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

 rimeter. In this last-named instrument two exactly similar chambers 

 are constructed ; one contains the animal while the other serves as a 

 dummy. These two chambers are balanced against each other, 

 the air space in the dummy being heated by immersion in a bath or 

 by burning hydrogen in the interior. As these sources of heat are 

 known and can be controlled, it is evident that if the dummy is 

 made to balance exactly the chamber containing the animal the 

 amount of heat given off in each is the same. The principle of the 

 differential calorimeter is represented in Fig. 267, which gives a 

 schema of the form originally employed by d 'Arson val; 8 and 8' 

 represent the two calorimeters, in one of which the animal is placed 

 while the other acts as dummy. Each is double walled and the 

 air spaces are connected by tubes, 10 and 10', to small gasometers, 

 4, 4', suspended in water and hung on opposite sides of a balance. 

 The movements of these gasometers antagonize each other, and the 



