JMJ'OltTANT ALLIANCE. 57 



azote, and 21 pails oxygen. There is about one and a Imlf per 

 cent, of watery vapour, and a very small fraction per cent, of 

 carbonic acid gas, but they need not be considered for our 

 present purpose. This propoi'tion of 71) parts nitrogen to 21 of 

 oxygen is found in all free air, and is the exact proportion 

 necessary for the welfare of the lungs in all warm blooded 

 animals. 



A very sliglit increase iu the quantity of oxygen would 

 injuriously increase the animal fire, whilst a slight diminution 

 would depress and extinguish vigour, warmth, and liie. Nature 

 has accordingly made w^ouderful provision to prevent any 

 alteration in these proportions, by endowing both gases with a 

 peculiar constitution, wliich seems to give them almost an 

 intelligent determination to unite together exactly in that 

 proportion. Unless absolutely shut up, neither of these gases 

 will rest in any other proportion ; the nitrogen will seek oxygen, 

 or the oxygen will seek nitrogen, until the natural proportion is 

 restored, so that the free open air is always right, and only shut 

 up air can be much wrong. This remarkable law of affinity, so 

 essential to aninial welfare, and which appears to be quite peculiar 

 to the atmospheric constitution, is so strong that it entirely 

 overpowers the usual laws of gravity, and prevents the heavy 

 oxygen when free from seeking a lower level than the light 

 nitrogen, as other liquids or gases would do. This law causes 

 just as much heavy oxygen to be found at the highest distance 

 from the earth that man has ever reached, as at the level of the 

 sea. 



112. — There is another important practical result from this 

 law, which must be considered. To avoid technical expressions 

 we are sometimes driven to use not very accurate ones, that 

 will convey our meaning to the general reader. We may here 

 say that the unwillingness of these gases to part with each other, 

 or to depart from the natural porportion in which they unite, 

 increases as that proportion becomes more disturbed ; so that 

 whilst the pure air freely gives up five per cent, of its oxygen to 

 the blood in the lungs, it is very unwilling to give up another 

 five per cent., and absolutely refuses to give up anything beyond 



