82 CALOEIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



The total volume of air which contains water-vapor is not far from 1,360 

 liters; consequently multiplying the weight of water in the sample by 136 

 gives the total amount of water in the chamber and the piping. The 

 volume of air containing carbon dioxide is that contained in the chamber 

 and piping to the first sulphuric-acid vessel plus 16 liters of air above the 

 sulphuric acid and connections in the first porcelain vessel, and in order 

 to obtain the amount of carbon dioxide from the sample it is only necessary 

 to multiply the weight of carbon dioxide in the sample by 137.6. 



Since in the calculation of the total amount of residual oxygen volumes 

 rather than weights of gases are used, it is our custom to convert the 

 weights of carbon dioxide and water-vapor in the chamber to volumes by 

 multiplying by the well-known factors. The determination of oxygen 

 depends upon the knowledge of the true rather than the apparent volume 

 of air in the system, and consequently the apparent volume must be reduced 

 to standard conditions of temperature and pressure each time the calcula- 

 tion is made. To this end, the total volume of air in the inclosed circuit 

 (including that in the tension-equalizer, amounting to 1,400 liters in all) 

 is reduced to 0° and 760 millimeters by the usual methods of computation. 

 The total volume of air (which may be designated as V) includes the 

 volumes of carbon dioxide, water-vapor, oxygen, and nitrogen. From the 

 calculations mentioned above, the volumes of water-vapor and carbon di- 

 oxide have been computed, and deducting the sum of these from the reduced 

 volume of air gives the volume of oxygen plus nitrogen. If the volume of 

 nitrogen is known, obviously the volume of oxygen can be found. 



At the beginning of the experiment, it is assumed that the chamber is 

 filled with ordinary air. By calculating the amount of nitrogen in the 

 chamber at the start as four-fifths of the total amount, no great error is 

 introduced. In many experiments actual analyses of the air have been 

 made at the moment of the beginning of the experiment The important 

 thing to bear in mind is that having once sealed the chamber and closed 

 it tightly, no nitrogen can enter other than that admitted with the oxygen, 

 and hence the residual amount of nitrogen remains unaltered save for this 

 single exception. If care is taken to keep an accurate record of the amount 

 of nitrogen admitted with the oxygen, the nitrogen residual in the chamber 

 at any given time is readily computed. While from an absolute mathe- 

 matical standpoint the accuracy of this computation can be questioned, 

 here again we are seeking an accurate record of differences rather than an 

 absolute amount, and whether we assume the volume of the air in the 

 chamber to contain 20.4 per cent of oxygen or 21.6 per cent is a matter of 

 indifference. It is of importance only to note the increases in the amount 

 of nitrogen, since these increases represent decrease in the residual oxygen 

 and it is with the changes in the residual oxygen that we particularly have 

 to do. 



