Nov., 1931] FOODS IN A COLLEGE COMMUNITY 7 



temperature control especiall.y of the oxy-calorimeter that we wish to 

 emphasize in making this brief description of the equipment. 



Principle of the oxy-calorimeter. In the oxy-calorimeter a weighed 

 portion, usually 2 grams, of air-dry material is burned in a confined at- 

 mosphere rich in oxygen. The carbon dioxide produced by this combus- 

 tion is absorbed by ventilating the combustion chamber and accessory 

 parts with a rotary blower and forcing the gases of combustion through 

 soda-lime to absorb the carbon dioxide. The air, now carbon-dioxide 

 free but still rich in oxygen, is returned to the combustion chamber, pass- 

 ing by a connection to a spirometer, which has a capacity of about 6 

 liters. As a result of the combustion there is a notable reduction in the 

 volume of ox.vgen. This reduction in volume is immediately apparent 

 in the lowered level of the spirometer bell. By reading the level of this 

 bell before and after the combustion one can obtain a measure of the 

 reduction in the volume of air in the closed system, which is a measure 

 of the apparent volume of oxygen consumed. Approximately one liter 

 of oxygen is absorbed in the combustion of each gram of dry substance 

 of ordinary mixed foods. Precise calculations with corrections for bar- 

 ometric pressure, tension of aqueous vapor and, above all, temperature 

 give the true contraction in the volume of oxygen at 0°C. (dry) and 

 760 mm. absorbed in the combustion of the weight of the substance used. 

 Since the caloric value of a liter of oxygen is known to be not far from 

 5 calories when mixed foods are burned, the energy content of the sub- 

 stance under consideration can be readily computed. 



The main parts of the oxj^-calorimeter in the form as ordinarily used 

 consist of a combustion chamber, a ventilating device, a vessel for ab- 

 sorbing carbon dioxide, and an expansion chamber. Strictly speaking, 

 this latter should be called a contraction chamber, for we are dealing 

 with a closed volume of oxygen, and provision for contraction as a result 

 of the process of combustion must be made. Although usually the con- 

 traction chamber is a spirometer, in the effort to have the principle of 

 the oxy-calorimeter applicable in as many laboratories as possible the 

 combustion chamber and the ventilating device have been adapted to 

 other types of respiration apparatus, such as the student respiration ap- 

 paratus^ and the field respiration apparatus,^ in both of which models the 

 expansion chamber is formed by a thin rubber bathing-cap attached over 

 a container in which the soda-lime is placed. 



Liberation of heat. Obviously when a dry substance is burned in 

 pure or nearly pure oxygen there is a tremendous liberation of heat in 

 the combustion chamber. Most of this heat is dissipated as the air 

 passes from the combustion chamber to the carbon-dioxide absorbing 

 vessel, but here again a considerable amount of heat is developed by the 

 chemical reaction taking place in the absorption of carbon dioxide by 



^Benedict, F. G., and C. G. Benedict, Boston Med. and Surg. Journ., 1923, 

 188, p. 567; ibid., Skand. Arch. f. Physiol., 1923, 44, p. 87. 



^Benedict, F. G., Boston Med. and Surg. Journ., 1927, 197, p. 1161; ibid., 

 Chinese Journ. Physiol., Report Series No. 1, 1928, p. 39; ibid., Abderhalden s 

 Handb. d. biolog. Arbeitsmethoden, 1929, Abt. IV, Tell 13, p. 1. 



