CHANGES OF AIR IN RESPIRATION. 221 



The changes produced by respiration on the atmospheric 

 air are, that, i, it is warmed ; 2, its carbonic acid is in- 

 creased ; 3, its oxygen is diminished ; 4, its watery vapour 

 is increased. 



1. The expired air, heated by its contact with the in- 

 terior of the lungs, is (at least in most climates) hotter 

 than the inspired air. Its temperature varies beween 97 

 and 99 1, the lower temperature being observed when 

 the air has remained but a short time in the lungs, rather 

 than when it is inhaled at a very low temperature ; for 

 whatever the temperature when inhaled may be, the air 

 nearly acquires that of the blood before it is expelled from 

 the chest. 



2. The carbonic acid in respired air is always increased; 

 but the quantity exhaled in a given time is subject to 

 change from various circumstances. From every volume 

 of air inspired, about 4^ per cent, of oxygen are abstracted; 

 while a rather smaller quantity of carbonic acid is added 

 in its place. It may be stated, as a general average de- 

 duced from the results of experiments by Valentin and 

 Brunner, that, under ordinary circumstances, the quantity 

 of carbonic acid exhaled into the air breathed by a healthy 

 adult man amounts to 1346 cubic inches, or about 636 

 grains, per hour. According to this estimate, which cor- 

 responds very closely with the one furnished by Sir. H. 

 Davy, and does not widely differ from those obtained by 

 Allen and Pepys, Lavoisier, and Dr. Ed. Smith, the weight 

 of carbon excreted from the lungs is about 173 grains per 

 hour, or rather more than 8 ounces in the course of twenty- 

 four hours. Discrepancies in the results obtained by dif- 

 ferent experimenters may be due to the variations to 

 which the exhalation of carbonic acid is liable in different 

 circumstances ; for even in health the quantity varies accord- 

 ing to age, sex, diversities in the respiratory movements, 

 external temperature, the degree of purity of the respired 

 air, and other circumstances. Each of these deserves a 



