OF RESPIRATION. 89 



148. It may be asked what are the changes which 

 the air experiences during inspiration, and which con- 

 sist not in the loss of elasticity, as was formerly 

 imagined, but in the decomposition of its elements.* 

 For the atmospheric air which we breathe, is a singular 

 mixture of constituents, differing very much in their 

 nature from each other; and, not to mention hetero- 

 geneous matters, such as odorous effluvia, various 

 exhalations, and innumerable others which are gene- 

 rally present, is always impregnated with aqueous 

 vapour, electric and magnetic matter, and generally 

 with carbonic acid gas; and is itself composed of 

 unequal parts of two aeriform fluids, viz. 79 of azotic 

 gas, and 21 of oxygen gas in 100. 



149. In the first place we know for certain, that at 

 every inspiration (the fulness of which varies infinitely 

 in different men of the same age, breathing placidly f), 

 besides the quantity of azotic gas being somewhat 

 diminished,^: the oxygen gas is in a great measure 

 converted into carbonic acid gas, or fixed air; so that 

 the air of expiration, if collected, instantly extinguishes 

 flame and live coals, precipitates lime from lime water, 

 and is specifically heavier than atmospheric air, and 

 rendered unfit for respiration ; § it also contains much 



* Fr. Sromeycr, Grundiss der theoretischen Cliemie. P. ii. p. 619. 



f Consult for instance Abildgaard, Nordisch. Archiv. fur Naturhunde, Sfi: 

 T. 1. P. i.andii. 



X Consult, besides Priestley and others, especially C. H. PeafF, ib. T. iv. P. ii. 



§ To discover how frequently an animal could breathe the same portion of air, 

 I took three dogs equal in size and strength, and to the trachea of the first, by 

 means of a tube, I tied a bladder, containing about 20 cubic inches of oxygen 

 gas. He died in 11 minutes. 



For the second, the bladder was filled with atmospheric air. He died in 

 six minutes. For 



