144 ANNUAL REPORT - 
of oxygen we have 3.76 volumes of nitrogen. If, now, we let N be the 
total per cent of nitrogen found by analysis and O the per cent of oxygen 
determined, the amount of nitrogen, n,, corresponding to the amount of air 
really made useful in the combustion must be equal to N—n, or, making 
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Fig. 12. The Orsat apparatus for gas analysis. 
use of the per cent. of oxygen found, n,=N—3.760. Thus, without 
calculating the amount of air itself, we can obtain the ratio between 
the air introduced and the air used by making use of the ratio of nitrogen 
brought in and the nitrogen of the air really deprived of its oxygen, 
which is the same thing as if we compared the volumes of air proper. 
In this manner the ratio of total and useful air is obtained by 
the expression N+ (N—3.760). We can readily convert this into per cent. 
