CALORIMETRY. 827 



resultant may be recorded upon smoked paper, as indicated in the 

 figure.* 



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- 

 teid, 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 

 that in the living body the energy liberated as heat or as heat 

 and work is all contained in potential form in the foodstuffs 

 eaten. By means of the respiration calorimeter we can obtain a 

 balance between the energy income and outgo of the body as well 

 as between the material income and outgo, that is, the carbon and 

 nitrogen equilibrium. The most complete and elaborate form of 

 respiration calorimeter used is that devised by Atwater and Rosa for 

 experiments upon man.f By means of this apparatus many inter- 

 esting and important experiments have been made upon the nu- 

 trition of man under special conditions. Such results as the follow- 

 ing have been obtained (Atwater and Benedict) in the case of a man 

 who, while in the apparatus, did much muscular work on a bicycle 

 ergometer: 



Income : Potential energy of material metabolized in body = 5459 Cal. 



} Ener gy given off from the body as heat 4833 Cal. 



\ Heat equivalent of muscular work 602 Cal. 



5435 Cal. 5435 Cal. 



Experimental error 24 Cal. 



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

 " Calorimetrische Methodik," 1891; and Rosenthal, "Archiv f. Physiol- 

 ogic, " 1897, p. 170. 



f See Atwater and Rosa, Bulletin 63, United States Department of Agri- 

 culture, 1899; and for recent improvements, Atwater and Benedict, 1905. 



