CALCULATION OF RESULTS. 85 



nitrogen, and hence with every liter of oxygen admitted there is a slight 

 amount of nitrogen added. This amount can readily be found from the 

 gasometric analysis of the oxygen and from the well-known relation between 

 the weight and the volume of nitrogen the weight can be accurately found. 

 This addition of nitrogen played a very important role in the calculation 

 of the oxygen consumption as formerly employed. As is seen later, a much 

 abbreviated form of calculation is now in use in which the nitrogen admitted 

 with the oxygen does not influence the calculation of the residual oxygen. 



REJECTION OF AIR. 



In long-continued experiments, where there is a possibility of a notice- 

 able diminution in the percentage of oxygen in the chamber — a diminu- 

 tion caused either by a marked fall in barometer, which expands the air 

 inside of the chamber and permits admission of less oxygen than would 

 otherwise be required, or by the use of oxygen containing a high percentage 

 of nitrogen, thus continually increasing the amount of nitrogen present 

 in the system — it is highly probable that there may be such an accumula- 

 tion of nitrogen as to render it advisable to provide for the admission of a 

 large amount of oxygen to restore the air to approximately normal condi- 

 tions. In rest experiments of short duration this is never necessary. 

 The procedure by which such a restoration of oxygen percentage is accom- 

 plished has already been discussed elsewhere.* It involves the rejection 

 of a definite amount of air by allowing it to pass into the room through 

 the gas-meter and then making proper corrections for the composition of 

 this air, deducting the volume of oxygen in it from the excess volume of 

 oxygen introduced and correcting the nitrogen residual in order to deter- 

 mine the oxygen absorption during the period in which the air has been 

 rejected. 



INTERCHANGE OF AIR IN THE FOOD-APERTURE. 



The volume of air in the food-aperture between the two glass doors is 

 approximately 5.3 liters. When the door on the inside is opened and the 

 material placed in the food-aperture and the outer door is subsequently 

 opened, there is by diffusion a passage outward of air of the composition of 

 the air inside of the chamber, and the food-aperture is now filled with room 

 air. When the inner door is again opened this room air enters the chamber 

 and is replaced by air of the same composition as that in the chamber. It 

 is seen, then, that there may theoretically be an interchange of air here 

 which may have an influence on the results. In severe work experiments, 

 where the amount of carbon dioxide in the air is enormously increased, 

 such interchange doubtless does take place in measurable amounts and 

 correction should undoubtedly be made. In ordinary rest experiments, 



•Atwater and Benedict: Carnegie Institution of Washington Publication No. 

 42, p. 77. 



