178 RESPIRATION. 



tory process, is supposed to serve only mechanically, by dilut- 

 ing 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 re- 

 moved, 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 vol- 

 umes, 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 bril- 

 liancy with which combustion goes on in an atmosphere 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 atmospheric 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 ab- 

 sorbed 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 Vapor is, under ordinary circumstances, always 

 exhaled from the lungs in breathing. The quantity 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 vapor 

 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 ex- 

 pired air; for the higher this is, the greater will be the quantity 

 of watery vapor 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 



