EXHALATION OF WATERY YAPOUR. 227 



tory process, is supposed to serve only mechanically, by 

 diluting the oxygen, and moderating its action upon the 

 system. This purpose, or the mode of expressing it, has 

 been denied by Liebig, on the ground that if we ^suppose 

 the nitrogen removed, the amount of oxygen in a given 

 space would not be altered. But, although it be true that, 

 if all the nitrogen of the atmosphere were removed and 

 not replaced by any other gas, the oxygen might still 

 extend over the whole space at present occupied by the 

 mixture of which the atmosphere is composed ; yet since, 

 under ordinary circumstances, oxygen and nitrogen, when 

 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 quan- 

 tity 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 



