59 



Compaiiiii;: then the densities of dry fresh air and expired air sat- 

 urated with moisture, hoth at o7° C. (98.6° F. ), we find tlieni to he 14.42 

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

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

 carbon dioxide in increasing it, if tlie 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 propoii:ional 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° absolute, O^^x^) ^r ^-'•2<5, so that the 



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

 haled air (at 37° C), are 15.2G 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 temi)er- 

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



(14 42 310\ 

 — '■ — X — I or 312° absolute is 39° C. ; therefore drv fresh air 

 14.33 1 / 



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



(saturated with moisture 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 moisture at certain temperatures. 



The tension of aqueous vapor at 30° and 35° C. is respectively 32 and 42 



millimeters of mercury, so, liy reasoning similar tn that on page 58, the 



composition of fresh air saturated with moisture at tliese temperatures is 



At 30° C— 



Oxygen 20.96 x .958, or 20.08 per cent, by volume 



Nitrogen 79.00x .958, or 75.68 i>er cent, liy 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 20.90 x .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 



