113 



is breathed, the more unfit does it become for respi- 

 ration : and to this change of composition, more 

 than to the time, or the magnitude of the volume 

 of air, is the increased degree of diminution to be 

 ascribed, 



92. This will perhaps appear more striking, if 

 we attend to what happens in respiring nitrous oxide, 

 which is composed of the same elements as atmo- 

 spheric air, but contains a much larger proportion 

 of oxygen. After exhausting his lungs, Mr Davy 

 inspired 108 cubic inches of this gas, which, when 

 expired, were reduced to 99, or had lost ~ of their 

 bulk. When he made two respirations of the same 

 quantity of the oxide, the diminution was to 95, or 

 about one-eighth : and when he respired 102 cubic 

 inches of nitrous oxide, mixed with -^ of common 

 air, for half a minute, the volume of air, after the 

 seventh expiration, was reduced to 62, or had suf- 

 fered a loss equal to -^ *. Hence it appears, that 

 in the natural respiration of atmospheric air, only a 

 small diminution (85.) of its bulk takes place : that 

 this diminution increases as the air becomes vitiated 

 (91.) by repeated respirations, or is breathed in a 

 preternatural manner : and that when a gas of the 

 same elementary materials, but combined in very 

 different proportions, is substituted into the place of 

 pure atmospheric air, the diminution increases in a 

 tenfold degree. Now, the repeated breathing of the 

 same atmospheric air, has been shewn to bring on 



Researches, p. 39-1-. 416. 

 H 



