COMMON THINGS AIR. 



It sometimes issues from the earth, being evolved in some sub- 

 terraneous process. Examples of this are presented in the 

 case of the celebrated Grotto del Cane in Italy, and at Pyrmont, 

 in Westphalia. The former place takes its name from the cruel 

 and now useless experiment of showing that a dog let down 

 into it dies. 



31. Carbonic acid is so much heavier than air, that it may be 

 decanted like a liquid from one vessel to another. It is, however, 

 a mistake to suppose that in consequence of its relative weight, 

 it will permanently sink to the lowest strata of the atmosphere 

 on which it happens to be placed, on the same principle that 

 water would sink to the bottom of oil. Gases in general are 

 subject to a physical law, in virtue of which they mingle one 

 with another when they are in contact, and become at length 

 uniformly diffused through each other, notwithstanding these 

 differences of weight. 



A small proportion of this gas is always diffused through the 

 atmosphere, being the produce of innumerable natural processes 

 which take place on the surface of the earth. This is not to be 

 regarded, however, as a constituent of common air, any more than 

 the mud of the Mississippi or the Tiber, or the salt of the ocean, is 

 to be considered as a constituent part of pure water. 



32. Carbonic acid is evolved in large quantities by respiration. 

 The oxygen, which forms one-fifth part of the common air which 

 is inspired in respiration, is absorbed by the blood before it enters 

 the arterial system, and the same blood on issuing from the 

 venous system dismisses a corresponding quantity of carbonic 

 acid, which is expired at the mouth and nostrils. Thus, while 

 the air inspired is a mixture of azote and oxygen, the air expired 

 is a mixture of azote and carbonic acid. 



The effect, therefore, of respiration on the surrounding air is 

 precisely the same as that of a lamp or candle. In both cases the 

 oxygen constituent disappears, and is replaced by carbonic acid. 



It is evident, therefore, that it is the oxygen constituent of 

 common air which is the means of supporting animal life by a 

 specific effect which it produces upon the blood which absorbs it, 

 and which carries it through the arterial and venous systems, 

 where it is converted into carbonic acid, and discharges a variety 

 of functions necessary to the maintenance of life. 



33. It is for this reason that oxygen is often called VITAL AIK. 



34. In apartments or buildings where persons are crowded 

 together in large numbers, more especially when they are illu- 

 minated by artificial light, there is therefore an enormous and 

 rapid production of this noxious gas, as well by respiration as 

 by the lamps, candles, or gas-burners used for illumination. 



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