224 RESPIRATION. 



e. Purity of the Respired Air. The average quantity of 

 carbonic acid given out by the lungs constitutes about 4-48 

 per cent, of the expired air ; but if the air which is breathed 

 be previously impregnated with carbonic acid (as is the case 

 when the same air is frequently respired), then the quan- 

 tity of carbonic acid exhaled becomes much less. This is 

 shown by the results of two experiments performed by 

 Allen and Pepys. In one, in which fresh air was taken 

 in at each respiration, thirty-two cubic inches of carbonic 

 acid were exhaled in a minute ; whilst in the other, in 

 which the same air was respired repeatedly, the quantity 

 of carbonic acid emitted in the same time was only 9-5 cubic 

 inches. They found also that, however often the same air 

 may be respired, even if until it will no longer sustain life, 

 it does not become charged with more than 10 per cent, of 

 carbonic acid. The necessity of a constant supply of fresh 

 air, by means of ventilation, through rooms in which many 

 persons are breathing together, or in which, from any 

 other source, much carbonic acid is evolved, is thus ren- 

 dered obvious; for even when the air is not completely 

 irrespirable, yet in the same proportion as it is already 

 charged with carbonic acid, does the further extrication of 

 that gas from the lungs suffer hindrance. 



/. Hygrometric State of Atmosphere. Lehmann's obser- 

 vations have shown that the amount of carbonic acid 

 exhaled is considerably influenced by the degree of mois- 

 ture of the atmosphere, much more being given off when 

 the air is moist than when it is dry. 



g. Period of the Day. The period of day seems to exercise 

 a slight influence on the amount of carbonic acid exhaled 

 in a given time, though beyond the fact that the quantity 

 exhaled is much less by night, we are scarcely yet in a 

 position to state that variations in the amount exhaled 

 occur at uniform periods of the day, independently of the 

 influence of other circumstances. 



h. Food. By the use of food the quantity is increased, 



