CALCULATION OF RESULTS. 77 



for sugar, carbohydrates, bread, crackers, cereals, etc., 1.5 ; for books, 

 papers, underclothes, etc., i.o ; for milk, cream, butter, drinking water, 

 cereal coffee, beef tea, i .o. The specific gravity of both urine and feces 

 is taken as i.o. 



The calculations of volume are made by dividing the weight of 

 material by the specific gravity. Thus, inasmuch as the interchange 

 through the food aperture should be known for each experimental 

 period, it is our custom to add together the weights of all the glass 

 entering the food aperture during the period in question and then 

 divide the total weight by the specific gravity, 2.6. In a similar man- 

 ner the total weight of sugar, carbohydrates, bread, cereals, etc. , is found 

 and this value divided by the specific gravity, 1.5. The volume of the 

 materials leaving the respiration chamber during this same experimental 

 period is likewise found, and the difference between the two volumes, 

 i. e. , the volume of the material entering the chamber and the volume 

 of the material leaving the chamber, is taken as representing the volume 

 of air either removed from or added to the air in the closed circuit. If 

 the volume of material entering the chamber is larger than the volume 

 of material leaving it, this difference in volume is subtracted from the 

 air in the system, and if, on the other hand, the volume of material 

 leaving the chamber is greater than that entering the chamber, the 

 volume of air equivalent to this difference is added to the total volume 

 of air in the system. This correction in volume is made for every ex- 

 perimental period. (See blank on p. 84.) 



.ADDITION OF NITROGEN WITH THE OXYGEN. 



As oxygen is admitted to the system, there is a continuous addition 

 of nitrogen which accumulates in the ventilating air-circuit. The 

 amount thus admitted is calculated very exactly, as has been shown on 

 page 34. The application of this correction in the volumes will be 

 discussed when the total amount of nitrogen in the system is considered. 

 (See p. 88.) 



THE REJECTION OF AIR. 



Since the amount of nitrogen in the closed air-circuit accumulates 

 as a result of its admission as an impurity in the oxygen, it becomes 

 necessary to reject air from time to time, replacing it with pure oxygen, 

 i. e., oxygen containing from 2.5 to 8 per cent nitrogen, to keep up 

 the normal percentage of oxygen in the main air current. In order to 

 know exactly the proportions of oxygen and nitrogen rejected, it is 

 desirable, theoretically at least, to make an analysis of a sample of the 

 air rejected. In practice, however, we have been in the habit of re- 



