ANIMAL HEAT 



447 



of the absolute amount of heat retained or lost by the animal above or 

 below the initial temperature, as well as that absorbed by the materials of 

 the apparatus in these various instances, the water equivalent of the tissues of 

 the animal and the materials of the calorimeter must be obtained, and then 

 added to or subtracted from, as the case may be, the amount of water in the 

 calorimeter, and the amount thus obtained multiplied by its rise in temperature. 



When means are taken to collect for purposes of analysis, the urine and 

 the feces and also the expired air, the apparatus is designated a respira- 

 tion calorimeter. The nitrogen in the urine and feces and the carbon dioxid 

 in the expired air permit of the determination of the amounts of protein 

 and fat metabolized. The ventilation of the chamber is accomplished by 

 some form of aspirating 

 apparatus. 



In properly conducted 

 experiments in which the 

 sources of error are reduced 

 to a minimum there is a 

 very close correspondence 

 between the total physio- 

 logic heat value of the food 

 and the amount collected 

 by the calorimeter. Thus, 

 in an experiment detailed 

 by Rubner, a dog was 

 given during twelve days 

 228.06 grams of protein and 

 340.4 grams of fat, the 

 physical heat value of which 

 was estimated at 4429 Cal- 

 ories. The urine and feces 

 during this period were col- 

 lected and their heat value 

 determined, which amounted to 305 Calories. The heat which theoretic- 

 ally therefore, should have been produced was 4124 Calories. During the 

 experiment the calorimeter actually absorbed 3958 Calories, a difference 

 between the theoretic and experimental results of 166 Calories; thus of the 

 total energy liberated about 96 per cent, appeared as heat. 



Calorimetric experiments on human beings corresponding in general 

 to those made on dogs and other animals, have been made possible in 

 recent years by the employment of the ingenious and finely constructed 

 respiration calorimeters devised by Atwater, Benedict and their co-workers. 

 With these forms of apparatus the same close correspondence between the 

 amount of heat actually absorbed by the calorimeter and the amount which 

 theoretically would be liberated by the oxidation of the food, has been found 

 to hold true. Inasmuch as the calorimeter of Benedict differs in some re- 

 spects from those hitherto employed, a brief description of the principle on 

 which it is constructed may not be inappropriate. The calorimeter chambers, 

 though sufficiently large to accommodate a human being, vary in size and 

 general arrangement in accordance with the character of the experiment. 

 The construction of each however is essentially the same. 



FIG. 200. WATER CALORIMETER OF DULONG. D 

 and D'. Tubes for the entrance and exit of air. T and T'. 

 Thermometers for ascertaining the temperature of the 

 water. S. A mechanic contrivance for stirring the water 

 for the purpose of distributing the absorbed heat uni- 

 formly. To prevent the escape of heat with the expired 

 air, the tube D' is wound many times in the water-space 

 beneath the animal cage. 



