216 PHYSIOLOGICAL CHEMISTRY 



ment consists in observing any change in the temperature of the 

 patient or the apparatus. The sum of these three factors will 

 give the total heat given off by the subject. 



The circulating air is in a closed system. The carbon dioxide 

 is removed by passing the air through an absorption apparatus, 

 and its amount can be determined by weighing. Oxygen is sup- 

 plied from an oxygen retort, so that the amount used by the 

 subject can be determined accurately by the loss of weight of 

 the retort, considered in connection with any change in the 

 composition of the circulating air. 



By means of this calorimeter, accurate measurements may be 

 made of the heat produced in the body, and the amount of car- 

 bon dioxide and water produced. Results of the most extra- 

 ordinary accuracy have been obtained by Atwater, Benedict, 

 Lusk, du Bois, and others, who have worked with apparatus 

 constructed on this general principle. 



One of the most important and far reaching results obtained 

 in this way is that the law of conservation of energy holds for 

 the animal body. The animal body can neither create nor 

 destroy energy. In a series of experiments representing an 

 exchange of over 590,000 calories, the difference between the 

 theoretical calculation and the amount actually measured was 

 only 50.1 calories, an error of only about 0.1%. Thus an animal 

 works on the same principle as an engine, a candle or an alcohol 

 flame. The heat which it produces comes from the oxidation of 

 organic substances, either those of the food, or those of the body 

 tissues. 



In calculating the amount of heat which will be produced in 

 the body by a given foodstuff, it must be borne in mind that 

 only the portion of the food which is absorbed will be available 

 for burning in the cells, and also that if the food is not com- 

 pletely burned to carbon dioxide and water, this fact must be 

 taken into account. The carbohydrates ordinarily are com- 

 pletely burned to C0 2 and H 2 0, as are also the fats. The pro- 

 teins, however, are not completely oxidized, for uric acid, urea, 

 and other substances of protein origin are excreted in the urine. 



