EXHALATION OF WATERY VAPOUR. 2 1/ 



mixed together in the ratio of one volume to four, produce 

 a mixture which occupies precisely five volumes, with all 

 the properties of atmospheric air, it must result that a 

 given volume of atmosphere drawn into the lungs con- 

 tains four-fifths less weight of oxygen than an equal 

 volume composed entirely of oxygen. The greater rapidity 

 and brilliancy with which combustion goes on in an atmo- 

 sphere of oxygen than in one of common air, and the 

 increased rapidity with which the ordinary effects of 

 respiration are produced when oxygen instead of atmo- 

 spheric air is breathed, seem to leave no doubt that the 

 nitrogen with which the oxygen of the atmosphere is 

 mixed has the effect of diluting this gas, in the same sense 

 and degree as one part of alcohol is diluted when mixed 

 with four parts of water. 



It has been often discussed whether nitrogen is ever 

 absorbed by or exhaled from the lungs during respiration. 



At present, all that can be said on the subject is that, 

 under most circumstances, animals appear to expire a very 

 small quantity above that which exists in the inspired air. 

 During prolonged fasting, on the contrary, a small quantity 

 appears to be absorbed. 



4. Watery Vapour is, under ordinary circumstances, 

 always exhaled from the lungs in breathing. The quan- 

 tity emitted is, as a general rule, sufficient to saturate the 

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

 therefore, influenced by the following circumstances. First, 

 by the quantity of air respired ; 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 



