EESPIEATION OF KTTROGEN. 235 



almost necessarily extended to the consideration of the effects 

 which breathing different gases produced upon the organism, 

 as well as to the changes which the gases suffered in the 

 process. We shall therefore here consider the two together. 

 During healthy respiration, the atmospheric air that supplies 

 the lungs is constantly changed. If this renewal of the air is 

 not provided for, but the same air is breathed over and over 

 again, the circumstances attending respiration are altered. 

 In the same proportion, for example, as the oxygenous con- 

 tents of the air dimmish, and the carbonaceous contents in- 

 crease, less and less oxygen is absorbed, less and less carbonic 

 acid is evolved ; and when the air comes to have a certain 

 proportion of carbonic acid mixed with it, which, from the 

 experiments of Allen and Pepys, appears to be ten per cent., 

 no more carbonic acid is formed, and the elastic fluid no 

 longer suffices for respiration, although it still contains some- 

 thing like ten per cent, of oxygen. A little oxygen, indeed, 

 continues to disappear, but the respiration becomes laborious, 

 and cannot be carried on without imminent risk of suffocation to 

 any of the higher animals. This is the source of the oppressive 

 sensation experienced when many persons, crowded together in 

 a limited space, continue to breathe the same atmosphere. In 

 pure oxygen gas respiration goes on as readily as in atmospheric 

 air, but a feeling of uneasiness and of exhaustion is soon ex- 

 perienced. The changes produced in the gas are of the same 

 nature as when the common atmospheric air is breathed 

 oxygen disappears, and carbonic acid is engendered ; the 

 quantity of the latter, according to Allen and Pepys, being, 

 however, greater than under ordinary respiration it amounts, 

 instead of eight per cent., to between eleven and twelve per 

 cent. The same experimenters also found that nitrogen gas 

 was evolved during -the respiration of oxygen gas. Nitrous 

 oxyde gas (consisting of sixty-four nitrogen, thirty-six oxygen), 

 like oxygen, will support life for a time, but it produces a pe- 

 culiar intoxicating effect upon the economy. A portion of 

 the gas is dissolved by the blood, which assumes a purple red 

 colour ; and the face and hands, in consequence of this 

 change, acquire a livid and cadaverous hue. Nitrogen and traces 

 of carbonic acid are found in the expired nitrous oxyde gas. 

 Pure nitrogen, although it can be taken readily into the lungs, 

 and is not at all poisonous, is quite incompetent to support- 



