THE RESPIRATION APPARATUS. 53 



fifths nitrogen, since the actual composition rarely varies sufficiently 

 to make any material difference in the calculation. 



Afterthe sample is taken, both pinchcocks are closed, the rubber bag re- 

 moved, and the glass tube again dipped in water to insure tight closure. 



In the alkaline pyrogallate method of determining oxygen it is abso- 

 lutely essential that the air sample be free from carbon dioxide. In the 

 procedure outlined above the sample is taken after the air has passed 

 through the three U tubes for the residual analysis, and consequently 

 should be free from carbon dioxide. We have frequently tested the 

 efficiency of these U tubes for removing completely the carbon dioxide 

 from an air current and have found them to be remarkably satisfactory. 

 Furthermore, it is to be remembered that the amount of residual carbon 

 dioxide is usually low, and absorption is presumably correspondingly 

 complete. It is therefore reasonable to assume that the air sample 

 is absolutely free from carbon dioxide. 



THE ANALYSIS OF AIR. 



The desirability of exact analysis of air during the progress of an 

 experiment with the respiration apparatus has been emphasized on 

 page 12. The methods and apparatus used thus far in this work are 

 essentially those outlined previously for analyzing oxygen, and reference 

 is made in the following description to the illustration previously given 

 (fig. 16). 



But in the analysis of air certain refinements of the method described 

 are necessary. The chief of these is an accurate observation of changes 

 in temperature of the gas between the time of the first and final readings- 

 While a variation in temperature of several tenths of a degree could not 

 have any appreciable effect on the small volume of residual nitrogen 

 obtained in the analyses of oxygen, in air analyses, where the residual 

 nitrogen amounts to about 80 cc., fluctuations in temperature will be 

 accompanied by marked fluctuations in the residual volume. Further- 

 more, fluctuations in barometric pressure, although seldom occurring 

 during the actual process of analysis, might affect perceptibly the 

 percentage of nitrogen. 



The important role played by temperature fluctuations necessitates 

 the use of a thermometer graduated in tenths of a degree centigrade, 

 and read with a reading glass to 0.01. This thermometer is placed in 

 the water-jacket surrounding burette B 2 . To insure a more equable 

 temperature of the gas in the burettes, provision is made for stirring 

 the water in the water-jackets. A slow stream of air is forced through 

 two fine jets at the bottom of the water-jackets, openings in the corks 

 in the top allowing for the free escape of air. As the air bubbles 

 through the long column of water, the water is very completely stirred. 



