60 



The densities of these mixtures compared with hydrogen at the same 

 temperature, say 37° C, are respectively 14.20 and 14.11, calculated as 

 l^efore, while ordinary exhaled air has the density 14.33 compared with 

 the same standard (hydrogen at 37° C). Imagining these mixtures cooled 

 down to 30° and 35° C, respectively, their densities become 14.53 and 

 14.20, calculated as before from the absolute temperatures. By interpo- 

 lation we find that if densities 14.53 and 14.20 correspond to temperatures 

 "0° and 35° C, 14.33 corresponds to approximately 33° C. ; therefore if 

 fresh air is saturated with moisture it has at about 33° C. the same den- 

 sity as ordinary exhaled air (saturated with moisture and at 37° C), 

 therefore at 33° C. (91° F.) the useful upward movement of expired air 

 ceases if the surrounding air is saturated with moisture. 



A certain temperature between 33° and 39° C. corresponds to each 

 degree of saturation with moisture. 



It has been shown that under all ordinary conditions of ventilation the 

 products of respiration move upwards; that this upward movement, by 

 which the harmful products are removed from the level of respiration, is 

 assisted by a low vmnn teini>erature. and liy dryness of the air of the room; 

 also, that the fresh air has the same density as expired air (sjiturated 

 with moisture and at body teniiMM-ature) at 33° C. or 91° F. if the fresh 

 air is saturated with moisture, at 39° C. or 102° F. if perfectly dry, and 

 at temperatures intermediate between these with dKTerent degrees of 

 raoistness. 



Purdue UnivcrsH]/, 



LaFayette, Indiana, 



November, 1911. 



