48 ATMOSPHERIC AIR. 



lungs, and is absorbed into the blood: it appears 

 essentially necessary to vitality. 



An animal can only live for a limited time in a 

 given portion of air. If a mouse or a bird be con- 

 fined under a glass that is closed, they will soon 

 die ; a candle, also, will burn only a short time. In 

 crowded rooms, wher,e there is not a free circula- 

 tion of air, the oxygen is diminished by the respi- 

 ration, of so many persons, and the air is rendered 

 unhealthy. The lights, also, are observed to burn 

 dim, and contribute much to exhaust the oxygen. 

 This points out the importance of ventilating 

 all kinds of apartments, but particularly public 

 places. 



It has been found that in 100 parts by measure 

 of atmospheric air, there are 21 parts of oxygen 

 gas and 79 of nitrogen gas. 



From the property of oxygen as being essential 

 to respiration and animal life, it had been thought 

 that the salubrity of air must depend upon the 

 quantity of oxygen which it contained ; but, al- 

 though the airs of various places have been ex- 

 amined, as that of towns, prisons, the country, 

 tops of hills, the ocean, &c., it appeared that the 

 proportion of oxygen did not sensibly differ in them 

 all. The healthiness of certain airs, therefore, 

 must depend upon some other circumstances. 



Although the great mass of the atmosphere is to 

 be considered as consisting of oxygen and hydro- 

 gen, yet it contains a small quantity of many other 

 gases, and also water, and a variety of exhalations 

 and substances dissolved in it. It always contains 

 a portion of carbonic acid gas, perhaps 1 part in 

 1000,for alkalies become effervescent when exposed 

 to it, and lime water acquires a pellicle on being 



