140 PHYSIOLOGY AND HEALTH. 



dead and corrupting matter which nature had so carefully 

 removed. 



310. There is another consideration in this matter. Sup- 

 posing the air were merely a passive vehicle to carry off the 

 carbonic acid gas, and had no active duty in the work, it 

 would be a natural question to ask, How much of this gas 

 can it bear away? Is there any limit to its capacity of 

 taking up and bearing off this offending matter ? Now, it is 

 well established that the air will not receive and hold an in- 

 definite quantity ; but, after having received a certain propor- 

 tion, it will receive no more. When it has arrived at this 

 point of saturation, that is, when it is so full that it can 

 receive no more, it then is useless as a vehicle to carry off 

 any more from the lungs. Bernan says * that, when the air 

 holds in solution only about three and a half per cent, of its 

 bulk of carbonic acid gas, it is unfit for respiration. 



311. The air, for this purpose, may be considered as the 

 water which the dyer would use to wash his colored cloths. 

 It is plain that, when the water is once befouled or saturated 

 with the loose coloring matter, it would take no more from 

 the cloths ; and therefore the judicious cleanser changes his 

 waters as often as they become foul ; and, whenever he can, 

 he selects a running stream, so that the water is carried 

 away as fast as it is befouled, and its place is supplied with 

 fresh and clean. 



312. Upon the same principle, the lungs cannot be thor- 

 oughly cleansed of the impurities which come to them 

 through the blood, unless the air is supplied to them fresh 

 and untainted at every respiration. For foul air, loaded with 

 carbonic acid gas, can no more cleanse the lungs, than foul 

 water can cleanse the colored cloths. 



313. The blood is relieved of its superabundant water 

 through the lungs. Tf this does not find an outlet here, and 

 if it is not carried off by the air, it must be carried back in 

 the blood to the heart and the arteries, to overload the sys- 



* Art of Warming and Ventilation. 



