58 



breathing fresh air even at low temperatures; also there may be injury 

 to furniture and wood-work, but from our present standpoint the drier 

 the room air the better. In harmony with this is the very noticeably de- 

 pressing effect of a very moist atmosphere. 



Let us now consider the numerical values concerned in these densities 

 under ordinary conditions. 



Accepting Ilallibui ton's values for the composition of fresh air and 

 expired air both in the dry condition, 



Fresh air — 



Oygen 2().1)(! per cent, by volume 



Nitrogen 79.00 per cent, by volume 



Carbon dioxide 0.04 per cent, by volume 



Expired air — 



Oxygen 1(^12 per cent, by volume 



Nitrogen 79.45 per cent, by volume 



Carbon dioxide 4 .43 per cent, by volume 



the dejisities. compared with jiydrogon at the same temperature and pres- 

 sure, are 



„ . /20.96 16\ /79.00 14\ /0.04 22\ , , .^ 



Fresh air: (^^q- x ^ ) + (-3^- x r) + (ioo Xt)= ^^'^^ 



^ . , . /1().12 10\ /79.45 14\ /4.43 22\ . . ,.^ 



Considering now the effect of moisture on tiio density of expired air. 

 the tension of aipuMius vapor, or vajior jtressure of water, is 47 milli- 

 meters of mercury at .'57° C. (9s.<>" F. t. tlici'cfore any gas saturated with 



water vapor at this temperature c()nsists of _ x — or 6.2 per cent. 



7()0 1 



water vapor and 100 -(i.2 or 93.S jM-r cent, by volume of all other constitu- 

 ents together. The compasition of expired air saturated with moisture 

 at body temiK^rature is therefore 



Oxygen in.l2x.93.S. or 15.12 per cent. l)y volume 



Nitrogen 79.45x.93S, or 74.52 per cent, by volume 



Carbon dioxide 4.43x.938, or 4.10 per cent, by volume 



Water vapor 0.20 per cent, by volume 



The densitj- of this mixture compared with hydrogen at the same tem- 

 perature and pressure, calculated as before, the density of water vapor 

 being 9, is 14.33. 



