950 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. 



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- 

 imeter. In this last-named instrument two exactly similar cham- 

 bers 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 Respiration Calorimeter. When a calorimeter is so arranged 

 that the composition of the air drawn through the apparatus for 

 ventilation can be determined, as well as the amount of heat pro- 

 duced, the apparatus becomes a respiration calorimeter. In such 

 an apparatus, if proper provision is made for analyzing the urine, 

 the feces, and the food, the total carbon and nitrogen excretion may 

 be obtained simultaneously with the heat loss. Since we may 

 calculate from the carbon and nitrogen excretion how much pro- 

 tein, fat, and carbohydrate have been burnt in the body, and since 

 the heat values of these constituents are known, it is evident that 

 we may reckon indirectly how much heat ought to be produced 

 from the combustion of so much material. This method of arriv- 

 ing at the heat production is designated indirect calorimetry. With 

 an adequate respiration calorimeter it is possible to ascertain 

 whether the results calculated by the method of indirect calorim- 

 etry really correspond with the heat obtained by direct measure- 

 ment. In the hands of good observers the correspondence is 

 very close, and gives substantial proof of the scientific belief 



* For detailed accounts of special forms of air calorimeters see Rubner, 

 "Calorimetrische Methodik " 1891; and Rosenthal, "Archiv f. Physiologic," 

 1897, p. 170. 



