

CHAP. XXIX.] INHALATION OF OXYGEN. 409 



carbonic acid in the expired air is less. Thus the per centage of this 

 gas bears a certain proportion to the frequency of the respirations, 

 supposing their bulk to remain the same. 



If the respiratory movements are suspended for a short time, 

 the per centage of carbonic acid in the expired air becomes in- 

 creased. The total quantity expired is, on the other hand, some- 

 what diminished; showing that this increased per centage, in a 

 given quantity of air, does not compensate for the smaller 

 proportion of air entering the lungs under these circumstances. 



Allen and Pepys found, that when the same air was breathed 

 more than once, the proportion of carbonic acid underwent a 

 considerable increase. Air breathed nine or ten times, contained 

 9*5 per cent, of carbonic acid; but if the air was breathed over 

 again as often as possible, the per centage of this gas could not 

 be increased above 10. Mr. Coathupe obtained as much as 12' 75 

 per cent., of carbonic acid from air in which animals had been 

 placed until they were suffocated. 



The per centage of carbonic acid varies also at different periods 

 of time during the same respiration. By taking the average of 

 twenty-one experiments, Vierordt found, that while the proportion 

 of carbonic acid in the entire expiration amounted to 4'48 per 

 cent., the first half contained 3'72, and the last half 5*44 per cent. 

 It has been estimated, that the air from the air-cells contains as 

 much as 5'83 per cent, of carbonic acid, or about 1*3 per cent, 

 more than the air of an ordinary expiration. 



Amount of Oxygen Inhaled. The quantity of oxygen introduced 

 into the system in respiration, is always greater than is required 

 to form the amount of carbonic acid eliminated during this process. 

 This surplus quantity no doubt is employed in oxydizing other 

 substances in the organism besides the carbon ; such, for instance, 

 as sulphur and phosphorus, which are eliminated in the urine in the 

 form of sulphuric and phosphoric acids. Valentin and Brunner 

 found that the proportions of these gases approximated very closely 

 to their diffusive volumes; for the quantities obtained by direct 

 experiment, and by calculation, differed very slightly. Oxygen 

 being the lighter gas, a larger quantity is required to replace the 

 carbonic acid; 81 parts of the latter will require 95 of the former 

 to replace it according to Graham's law, that the diffusion volume 

 of different gases varies inversely as the square root of the density, 

 or about one volume of absorbed oxygen corresponds to '85 of 

 exhaled carbonic acid. The experiments of Dulong and Despretz, 



VOL. II, E E 



