EFFICIENT RESPIRATION. 179 



times in a minute, and to inhale from 15 to 40 cubic 

 inches of air at each inspiration. Sir H. Davy and 

 others rate the quantity so low as from 13 to 17 

 inches ; but most observers agree with Dr. Menzies, 

 who experimented with great care, in estimating it at 

 40 inches. The quantity, however, varies much in 

 different individuals. 



Even taking the consumpt of air at 20 inches, as 

 a very low medium, and rating the number of inspi- 

 rations at 15, it appears that, in the space of one 

 minute, no less than 300 cubic inches of air are re- 

 quired for the respiration of a single person. In 

 the same space of time, 24 cubic inches of oxygen 

 disappear, and are replaced by an equal amount of 

 carbonic acid ; so that in the course of an hour one 

 pair of lungs will, at a low estimate, vitiate the air 

 by the subtraction of no less than 1440 cubic inches 

 of oxygen, and the addition of an equal number of 

 carbonic acid, thus constituting a source of impurity 

 which cannot be safely overlooked. 



The fatal effects of breathing highly vitiated air 

 may easily be made the subject of experiment. 

 When a mouse is confined in a large and tight glass 

 jar full of air, it seems for a short time to experience 

 no inconvenience ; but in proportion as the consump- 

 tion of oxygen and the exhalation of carbonic acid 

 proceed, it begins to show symptoms of uneasiness, 

 and to pant in its breathing, as if struggling for air; 

 and in a few hours it dies, convulsed exactly as if 

 drowned or strangulated. The same results follow 

 the deprivation or vitiation of air in man and in all 

 animated beings ; and in hanging, death results not 

 from dislocation of the neck, as is often supposed, 

 but simply from the interruption to breathing pre- 

 venting the necessary changes taking place in the 

 constitution of the blood. 



The horrible fate of the Englishmen who were 

 shut up in the Black Hole of Calcutta in 1756 

 is strikingly illustrative of the destructive conse- 



