2 16 RESPIRATION. 



ation, and thinks that 160 or 170 remain in the lungs after ordinary 

 expiration m ; for these organs are never emptied by expiration. 



" It may be asked, what are the changes which the air 

 experiences during inspiration, and which consist not in the 

 loss of elasticity, as was formerly imagined, but in the decom- 

 position of its elements. n For the atmospheric air which we 

 breathe, is a peculiar mixture of constituents, differing very much 

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

 geneous matters, such as odorous effluvia, various other besides 

 aqueous exhalations, and innumerable other substances, which are 

 generally 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. 



" In the first place, we know for certain) that, at every inspir- 

 ation (the fulness of which varies infinitely in different persons 

 of the same age, breathing placidly ), besides the quantity of 

 azotic gas being somewhat diminished P, 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 

 inspiration q ; it also contains much aqueous vapour, which is 

 condensed in a visible form by a temperature not exceeding 60 

 of Fahr." r The ordinary quantity of aqueous vapour emitted by 



m An Elementary System of Physiology, vol. ii. p. 24. sq. Dr. Thomson thinks 

 the estimate of Menzies most correct. System of Chemistry, vol. iv. 

 n " Fr. Stromeyer, Grundriss der theoreliscken Chemie, P. ii. p. 619. 



Consult, v. c. Abildgaard, Nordischen Archiv. fiir Naturkunde> &c. t. f. 

 P. i. and ii." 



P " Consult, besides, Priestley and others, especially C.H.Peaff, ib. t. iv. P. ii." 



1 " To discover how frequently an animal could breathe the same portion of the 

 different kinds of air that we have mentioned, 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 40 minutes. 



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

 minutes. 



For the third, I employed the carbonised air last expired by the second dog. 

 He died in four minutes. 



The air of the bladder, upon subsequent examination, gave the common signs 

 of carbonic acid gas. 



The instruments which I employed are described and illustrated by a plate in 

 the Medic. Biblioth. vol. i. p. 174. sq. tab. 1." 



T " J. A. De Luc, Idees sur la Me'teorologie, torn. ii. pp. 67. 229." 



