Apparatus and Methods 71 



main essentially the same. If, however, there is a leakage of air out of 

 the system and a consequent replenishment of oxygen to maintain con- 

 stancy in volume, obviously there would be a loss in nitrogen. On the 

 other hand, if there is a leakage of air into the system, less oxygen will be 

 required to obtain constancy in volume, and the percentage of nitrogen 

 will continually increase. By making, at stated periods, determinations 

 of the nitrogen in the residual air, it is possible not only to detect when 

 there has been a leakage of air into or out of the system, but also from the 

 results obtained to compute easily the magnitude of this leakage. 1 This 

 principle has been used recently by Roily 2 in making experiments with a 

 small respiration apparatus. 



At this point it should be stated that throughout this discussion it is 

 considered for convenience that the air used for the determinations of oxy- 

 gen is free from water and carbon dioxide. Furthermore, since the 

 proportion of argon and the rarer gases in the atmosphere does not play 

 any role in this research, no special recognition is made of the presence of 

 0.94 per cent of argon. It is therefore assumed that the air consists only 

 of nitrogen and oxygen, and that after the absorption of the oxygen the 

 residual gas is pure nitrogen. 



While there are a large number of methods for determining the carbon 

 dioxide produced by the body, the determination of the oxygen consump- 

 tion is at best a very difficult procedure. When the Regnault-Reiset 

 type of apparatus in this laboratory has been thoroughly tested and shown 

 to be air-tight, air-analyses are unnecessary; nevertheless for long-con- 

 tinued experiments, periodical, accurate determinations of the oxygen 

 in the air residual in the chamber are important. Consequently, the 

 physiological importance of knowing the constancy or lack of constancy 

 in the composition of the air justified the study of this problem by the 

 Nutrition Laboratory. 



FUNDAMENTAL ESSENTIALS OF ACCURATE AIR-ANALYSES. 



Although the gravimetric determination of oxygen in air was especially 

 successful in the hands of Dumas and Brunner, it is too time-consuming 

 to be practicable for metabolism experimentation, and hence there has 

 been a general trend in the last 30 years toward the volumetric determina- 

 tion of oxygen by absorption with either phosphorus or potassium pyro- 

 gallate. An apparatus for the determination of oxygen in physiological 

 laboratories, to be successful and practical, should have first an efficient 

 absorbent for oxygen, i. e., the last traces of oxygen should be readily 

 absorbed; second, no by-products of the chemical reaction should be 

 given off into the residual gas, thereby increasing its volume; third, tem- 

 perature changes in the apparatus during the process of an analysis should 



1 For an elaboration of this theory and its successful application, see Atwater and 

 Benedict, loc. cit., pp. 88-89, and 93. 



2 Roily and Rosiewicz, Deutsches Archiv fur klinische Medizin, 1911, 103, p. 58. 



