176 RES PI BAT I ON. 



often the same air may be respired, even if until it will no longer 

 sustain life, it does not become charged with more than ten 

 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 rendered 

 obvious ; for even when the air is not completely irrespirable, 

 yet in the same proportion as it is already charged with car- 

 bonic acid, does the further extrication of that gas from the 

 lungs suffer hindrance. 



/. Hygrometric State of Atmosphere. Lehmann's observations 

 have shown that the amount of carbonic acid exhaled is con- 

 siderably influenced by the degree of moisture of the atmo- 

 sphere, 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 circum- 

 stances. 



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

 by fasting it is diminished : and, according to Regnault and 

 Reiset, it is greater when animals are fed on farinaceous food 

 than when fed on meat. Spirituous drinks, especially when 

 taken on an empty stomach, are generally believed to pro- 

 duce an immediate and marked diminution in the quantity 

 of this gas exhaled. Recent observations by Dr. Edward 

 Smith, however, furnish some singular results on this subject. 

 Dr. Smith found, for example, that the effects produced by 

 spirituous drinks depend much on the kind of drink taken. 

 Pure alcohol tended rather to increase than to lessen respira- 

 tory changes, and the amount, therefore, of carbonic acid ex- 

 pired : rum, ale, and porter, also sherry, had very similar effects. 

 On the other hand, brandy, whisky, and gin, particularly the 

 latter, almost always lessened the respiratory changes, and 

 consequently the amount of carbonic acid exhaled. 



i. Exercise and Sleep. Bodily exercise, in moderation, in- 

 creases the quantity to about one-third more than it is during 

 rest ; and for about an hour after exercise, the volume of the 

 air expired in the minute is increased about 118 cubic inches ; 

 and the quantity of carbonic acid about 7.8 cubic inches per 

 minute. Violent exercise, such as full labor on the tread- 

 wheel, still further increases, according to Dr. E. Smith, the 

 amount of the acid exhaled. 



