59 



Comparing then the densities of dry fresli air and expired air sat- 

 urated witli moisture, both at 37° C. (98.6° F.), we find them to be 14.42 

 and 14.33 respectively, the addition of the moisture liaving a greater effect 

 in decreasing the density than the replacement of part of the oxygen by 

 carbon dioxide in increasing it, if the inspired air is dry. 



Talcing into account such differences in temperature as are likely to 

 occur between the inspired and the expired air, we find that since the 

 density of any gas or mixture of gases is proportional to the absolute 

 temperature, a density of 14.42 for dry fresh air at 37° C, or 310° abso- 

 lute, becomes at 20° C. or 293° al)S(ilute. O^y^x^^ '^r ^•'"'•26, so that the 



relative densities of dry fresh air at 20° C. (68° F.), and ordinary ex- 

 haled air (at 37° C), are 15.26 and 14.42. The difference between these 

 figures, which is favorable to the automatic removal of respiration pro- 

 ducts from the level of respiration, decreases with any increase in temjver- 

 ature of the fresh air. A density of 14.42 at 37° C. becomes 14.33 at 39'' 



C, for ( 



14 42 310\ 



x—p j or 312° absolute is 39° C. ; therefore dry fresh air 



would have at 39° C. (102° F.), the same density as ordinary expired air 

 (saturated with moistui-e and at 37° C), and at 39° C. the automatic 

 upward removal of respiration products due to difference in density ceases. 

 Having considered the case of perfectly dry fresh air, let us take the 

 other extreme of fresh air saturated with molstui*e at certain temperatures. 

 The tension of aqueous vapor at 30° and 3r>° C. is respectively 32 and 42 

 millimeters of mercury, so, by reasoning similar t;) that on page 58, the 

 composition of fresli air saturated with moisture at these temperatures is 



At 30° C— 



Oxygen 20.90 x .958, or 20.08 per cent, by volume 



Nitrogen 79 . 00 x . 958, or 75 . 68 per cent, by volume 



Carbon dioxide 0.04 x .958, or 0.04 per cent, by volume 



Water vapor 4 . 20 per cent, by volume 



At 35° C— 



Oxygen 20.96x .945, or 19.81 per cent, by volume 



Nitrogen 79.00x.945, or 74 . 05 per cent, by volume 



Carbon dioxide 0.04x .945. or 0.04 per cent, by volume 



Water vapor 5.50 per cent, by volume 



