98 PHYSIOLOGY AND TEMPERANCE. 



provided against this by diluting the oxygen with nitrogen, 

 an inert gas. These gases exist in the air in the propor- 

 tion of one part of oxygen to four parts of nitrogen. There 

 is also of necessity, since animals are constantly breathing it 

 out, a trace of carbonic acid gas. 



There exists a well-known chemical law amongst gases that 

 when separated only by a thin moist animal membrane, they 

 will commingle. Such an arrangement is found in the lungs 

 a very thin membrane or partition, on the one side of 

 which are the gases of the air and on the other the gases 

 in the impure blood. The process of exchange is therefore 

 easily understood. The blood gives up its carbonic acid, its 

 watery vapor with the organic matter dissolved in it and 

 a trace of ammonia, while it receives in return a new 

 supply of oxygen, which is carried to the heart and thence 

 distributed to the tissues. The air, in parting with the 

 required oxygen, receives from the blood its elements of 

 impurity, and these escape with the expired air. Carbonic 

 acid gas will not support combustion, that is, a lighted 

 candle will go out if placed in a jar filled with this gas. 

 The same will happen if we breathe into a jar and dip in 

 a lighted taper, hence we know this gas is present in the 

 breath. Watery vapor can be detected by breathing on a 

 mirror or any highly polished substance. It collects more 

 readily in a cool room; indeed, in very cold weather it 

 condenses so rapidly "that we can see our own breath as 

 we walk in the open air. The organic matter that escapes 

 by the lungs rapidly changes and becomes putrid. Breathe 

 into a jar, close it and put it aside. In a few hours it will 

 have a very rank smell, owing to the presence of decomposed 

 organic matter. 



JL-& The Effects of Impure Air. It may be taken for 

 granted generally that anything the body casts off would, if 

 retained, injure the system. Ke-brea thing the same air over 



