228 RESPIRATION. 



expired air or very nearly so. Its absolute amount is, 

 therefore, influenced by the following circumstances. First, 

 by the volume of air expired ; for the greater this is, the 

 greater also will be the quantity of moisture exhaled. 

 Secondly, by the quantity of watery vapour contained in 

 the air previous to its being inspired ; because the greater 

 this is, the less will be the amount required to complete 

 the saturation of the air. Thirdly, by the temperature of 

 the expired air : for the higher this is, the greater will be 

 the quantity of watery vapour required to saturate the 

 air. Fourthly, by the length of time which each volume 

 of inspired air is allowed to remain in the lungs ; for it 

 seems probable that, although during ordinary respiration 

 the expired air is always saturated with watery vapour, 

 yet when respiration is performed very rapidly the air has 

 scarcely time to be raised to the highest temperature, or be 

 fully charged with moisture ere it is expelled. 



For ordinary cases, however, it may be held that the 

 expired air is saturated with watery vapour, and hence is 

 derivable a means of estimating the quantity exhaled in 

 any given time : namely, by subtracting the quantity con- 

 tained in the air inspired from the quantity which (at the 

 barometric pressure) would saturate the same air at the 

 temperature of expiration, which is ordinarily about 99. 

 And, on the other hand, if the quantity of watery vapour 

 in the expired air be estimated, the quantity of air itself 

 may from it be determined, being as much as that quantity 

 of watery vapour would saturate at the ascertained tem- 

 perature and barometric pressure. 



The quantity of water exhaled from the lungs in twenty- 

 four hours ranges (according to the various modifying cir- 

 cumstances already mentioned) from about 6 to 27 ounces, 

 the ordinary quantity being about 9 or 10 ounces. Some 

 of this is probably formed by the combination of the excess 

 of oxygen absorbed in the lungs with the hydrogen of the 

 blood ; but the far larger proportion of it must be the mere 



