RESPIRATION. 139 



307. When the dyer has determined what strength of dye 

 will give the due color to his cloths, he adds fresh coloring 

 matter as often as one piece has weakened it, in order to 

 keep up the dye to its full strength, and to give to each suc- 

 cessive piece of cloth the same hue ; for, if the dye be 

 weakened, it will give a weaker color. He would not, there- 

 fore, continue to dip his cloths in it, after it is reduced, be 

 cause there was some coloring matter left; nor would he 

 think of exhausting all the power of the dye, unless he was 

 satisfied to produce a duller shade. 



308. Precisely analogous to this is the effect of the air 

 in purifying the blood of its corrupting carbon. The stronger 

 the air, that is, the greater the proportion of its oxygen, 

 the more effectually will this carbon be carried away ; the 

 weakened air must produce a weak effect, artd take away less 

 of the impurities. Air, therefore, which has been breathed 

 once or more, having lost a certain part of its oxygen, must 

 be, in that proportion, unfit to do the work of respiration. 

 If we breathe pure oxygen, or air too strongly oxygenated, 

 that is, air containing more than twenty-two per cent, of this 

 gas, the carbon would be taken from the blood faster than 

 it could be spared, and the body would be wasted. If we 

 breathe air containing less than twenty-one or twenty-two per 

 cent, of oxygen, it will not carry the carbon off so fast as is 

 required. It is only by breathing air of the natural strength 

 that this work is best performed, and the carbon carried 

 away neither too rapidly nor too slowly. Air, therefore, 

 should be breathed once, and once only. We need a fresh 

 draft of air at every inspiration, as much as the dyer needs a 

 fresh dye at every coloring. 



309. Air, when it has been breathed, not only loses its 

 oxygen, which is its active principle, but it is loaded more 

 or less with carbonic acid gas, which is increased by every 

 respiration. Therefore, when we breathe air over and over, 

 we not only breathe a weaker gas, but a fouler one; we 

 receive back into the lungs, and into the vital system, that 



