246 FUNCTIONS OF NUTRITION, 



When fresh and in the healthy condition, the organic ingredient of 

 the expired breath is not offensive and appears to have no unwhole- 

 some qualities. It is only when accumulated in undue quantity, and 

 allowed to stagnate in the atmosphere, that its disagreeable properties 

 become manifest. It appears to be distinct in character for each species 

 of animal ; and as it is liable to be absorbed and retained by porous 

 materials, such as wood, plaster, or woven fabrics, its odor remains 

 perceptible in any small inclosure or transportation-car in which such 

 animals have been confined. It has not been isolated in any case in 

 sufficient quantity to determine its exact composition. 



Vitiation of Air by Continued Respiration. It appears from the 

 foregoing that the air, when discharged in expiration, has been deteri- 

 orated by the loss of oxygen, and by the addition of matters derived 

 from the lungs. Under ordinary conditions, the deteriorated air is at 

 once diffused in the surrounding atmosphere, rising to a higher level 

 on account of its increased temperature, and dispersed by the aerial 

 currents ; so that a fresh supply, of normal constitution, is taken into 

 the lungs with each inspiration. But when breathing is carried on in 

 a limited space, the air necessarily becomes vitiated ; and this effect is 

 produced with greater rapidity, the smaller the volume of the air and 

 the larger the number of men or animals using it for respiration. 



The vitiation of the air by respiration is, accordingly, the result of 

 several changes taking place at the same time, and its effects are due 

 to all these alterations combined. 



So far as regards immediate danger to life, the diminution of oxygen 

 is the most important change in the vitiated air, when carried to a 

 sufficient extent. We have already seen that for man and mammalians, 

 the air is completely irrespirable when its proportion of oxygen is 

 diminished to 10 per cent. In these experiments, however, the exhaled 

 carbonic acid was removed, as fast as produced, by the action of an 

 alkaline solution, so that the air remained in a state of purity except 

 for its loss of oxygen. But if the products of respiration be allowed 

 to accumulate at the same time, the loss of oxygen is more quickly felt. 

 In the experiments of Leblanc, a dog and a pigeon, breathing in a con- 

 fined space, were both reduced to extremities when the air was con- 

 taminated with 30 per cent, of carbonic acid, though still containing 

 16 per cent, of oxygen. 



The second element in the vitiation of the respired air is the presence 

 of carbonic acid. The effect of this gas, as produced by respiration, 

 cannot be ascertained from that of carbonic acid alone. A man or an 

 animal, suddenly introduced into an atmosphere of pure carbonic acid, 

 dies at once by suffocation. But this result is not caused by the in- 

 fluence of carbonic acid. It is due to the absence of oxygen ; and 

 death would take place as promptly in an atmosphere of nitrogen or 

 any other indifferent gas. It may be said that, in general, for birds 

 and small mammalians, the atmosphere becomes incapable of sup- 

 porting life when, in addition to its normal proportion of oxygen, it 



