84 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



to 1.25 liters and therefore influences markedly the introduction of oxygen. 

 All four of the factors, therefore (barometric pressure, temperature, residual 

 carbon dioxide, and residual water- vapor), affect noticeably the oxygen 

 determination. 



CONTROL OF RESIDUAL ANALYSES. 



Of the three factors to be determined in the residual air, the oxygen 

 (which is most important from the standpoint of the relative weight to be 

 placed upon the analysis) unfortunately can not be directly determined 

 without great difficulty. Furthermore, any errors in the analysis may be 

 very greatly multiplied by the known errors involved in the determination 

 of the true volume of the air in the chamber as a result of the difficulties 

 in obtaining the average temperature of the air. Believing that the method 

 of analysis as outlined above should be controlled as far as possible by 

 other independent methods, we were able to compare the carbon dioxide as 

 determined by the soda-lime method with that obtained by the extremely 

 accurate method used by Sonden and Pettersson. An apparatus for the 

 determination of carbon dioxide and oxygen on the Pettersson principle 

 has been devised by Sonden and constructed for us by Grave, of Stockholm. 



In the control experiments, the air leaving the mercury valve D (fig. 30, 

 page 66) was caused to pass through a T-tube, one arm of which connected 

 directly with the sampling pipette of the Sonden gas-analysis apparatus, 

 the other arm connecting with the U-tubes for residual analyses. By lower- 

 ing and raising the mercury reservoir on the gas-analysis apparatus, a 

 sample of air could be drawn into the apparatus for analysis. The results 

 of the analysis were expressed on the basis of moist air in volume per 

 cents rather than by weight, as is done with the soda-lime method. Hence 

 in comparison it was necessary to convert the weights to volume, and 

 during this process the errors due to not correcting for temperature and 

 barometer are made manifest. However, the important point to be noted 

 is that whatever fluctuations in composition of the residual air were noted 

 by the soda-lime method, similar fluctuations of a corresponding size were 

 recorded by the volumetric analysis with the Sonden apparatus. Under 

 these conditions, therefore, we believe that the gravimetric method out- 

 lined above is sufficiently satisfactory, so far as the carbon-dioxide content 

 is concerned, for ordinary work where there are no wide variations in the 

 composition of the air from period to period. 



NITROGEN ADMITTED WITH THE OXYGEN. 



It is impossible to obtain in the market absolutely chemically pure oxy- 

 gen. All the oxygen that we have thus far been able to purchase contains 

 nitrogen and, in some instances, measurable amounts of water-vapor and 

 carbon dioxide. The better grade of oxygen, that prepared from liquid air, 

 is practically free from carbon dioxide and water-vapor, but it still contains 



