234 EESPIEATION. 



The quantity of water contained in the expired air amounts, 

 taking the mean of the estimates of a great number of ob- 

 servers, to about 8,000 grains, or one pound in the four-and- 

 twenty hours.* 



RESPIRATION IN GASES OTHEE THAN AT^tOSPHEETC AIE. 



[ 394. With a view of obtaining still more precise informa- 

 tion regarding the changes induced in air by its assumption 

 into the lungs, experiments have been instituted on the respi- 

 ration of different kinds of gas. These experiments, however, 



* See Miiller's Physiology, by Baly, vol. i. p. 330. The statements 

 in the -text refer particularly to man ; but they also apply very closely 

 to animals which breathe by lungs, with this difference, that in cold- 

 blooded animals the quantities of oxygen absorbed, and of carbonic acid 

 eliminated, are relatively smaller. Dulong found, no matter what animal 

 he made the experiment upon, that there was rather more oxygen ab- 

 sorbed than carbonic acid evolved. The excess in graminivorous animals 

 amounts to one-tenth ; in carnivorous creatures, it was from one-fifth to 

 one-half more than the carbonic acid. Despretz observed the same thing. 

 Allen and Pepys, on the other hand, found the quantity of oxygen that 

 disappeared, and of carbonic acid that was generated, to be equal. The 

 oxygen which disappears is used up in the combustion of hydrogen, the 

 product of which is watery vapour. Treviranus and Miiller instituted 

 comparative experiments upon the respiration of some of the lower 

 animals, and the quantity of carbonic acid formed in a given time, con- 

 trasted with the weight of the animal, from which it appears that 

 mammals, for every one hundred grains of their weight, produce 0.52 of 

 cubic inch of carbonic acid in one hundred minutes ; that birds, consi- 

 dered in the same way, produce 0.97 of a cubic inch ; that amphibia (the 

 frog), still considered in the same way, produce 0.05 of a cubic inch. The 

 respiratory process performed by the medium of water is precisely the 

 same as that which goes on with the direct contact of air : the air dis- 

 solved in the water comes into contact with the blood which circulates 

 through the gills, and oxygen disappears, and carbonic acid appears as 

 usual. Water, in general, contains from five to five and a quarter per cent. 

 of its bulk of air dissolved in it this air, however, having a somewhat 

 greater relative proportion of oxygen than the air of the atmosphere, 

 oxygen being somewhat more soluble in water than nitrogen. We have 

 very admirable researches on the respiration of fishes by A. von Humboldt 

 and Provencal. The water in which the fishes were put in these experi- 

 ments contained 20,3 per cent, of air, which, in one hundred parts, con- 

 sisted of 29,8 oxygen, 66,2 nitrogen, and 4,0 carbonic acid. After having 

 been used for respiration, the water still contained 17,6 per cent, of ah*, 

 which consisted, in one hundred parts, of 2,3 oxygen, 63.9 nitrogen, and 

 33,8 carbonic acid. Here, therefore, oxygen was also absorbed, and carbonic 

 acid evolved. 



