39 



TEXT-BOOK OF PHYSIOLOGY. 



general use consist essentially of two metallic boxes placed one within 

 the other, though separated by a space sufficiently large to hold a 

 definite amount of water (Fig. 180). The animal is placed in the 

 inner box, which is also provided with tubes for the entrance of fresh 

 and the exit of expired air. The heat radiated is absorbed by the 

 water and its temperature raised. To prevent loss by radiation and to 

 render it independent of changes in the surrounding temperature the 

 calorimeter is surrounded by a poorly conducting material, such as 

 wool. The temperature of the animal is taken at the beginning and 

 the end of the experiment. If the temperature of the animal remains 

 the same at the end of the experiment, then the heat absorbed by the 



water represents the 

 amount produced by the 

 animal. If, on the con- 

 trary, the temperature of 

 the animal rises or falls, 

 the number of calories 

 so retained or lost must 

 be added to or sub- 

 tracted from the amount 

 absorbed by the calor- 

 imeter. In the determi- 

 nation 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 appara- 

 tus in these various in- 

 stances, the water equiv- 

 alent 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 ob- 

 tained multiplied by its rise in temperature. In properly conducted 

 experiments in which the sources of error are reduced to a minimum 

 there is a very close correspondence between the total physiologic 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 proteid and 340.4 grams of fat the physical heat 

 value of which was estimated at 4429 calories. The urine and feces 

 during this period were collected and their heat value determined, 

 which amounted to 305 calories. The heat which theoretically 



FIG. 180. WATER CALORIMETER OF DULONG. D 

 and D'. Tubes for the entrance and exit of 

 air. T and T'. Thermometers for ascertain- 

 ing the temperature of the water. S. A 

 mechanic contrivance for stirring the water for 

 the purpose of distributing the absorbed heat 

 uniformly. 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. 



