108 CHEMISTRY. 



snow, or hail. So, likewise, if you dissolve as much as pos- 

 sible of alum in hot water, and then let it cool, the water 

 can not then hold as much alum in solution, and some of it 

 will be separated and deposited. 



138. How the Air is Freed from Impurities. Impurities 

 that rise in the air and become mingled with it become dif- 

 fused widely, as the air is so continually in motion. If they 

 were not thus diluted and dissipated they would do great 

 harm to health, especially in cities. The falling rain is the 

 chief means of ridding the air of them. Water is here, as 

 every where, the grand purifier. The shower-bath which 

 the air receives whenever it rains brings down most of these 

 impurities, as it does the nitric acid formed by the lightning 

 ( 132), and mingles them with the earth, where they are 

 used in vegetation. 



139. Proofs that the Air is a Mixture. You are now pre- 

 pared to appreciate fully the proofs that air is a mixture. It 

 was the prevalent doctrine, even for a long time after Priest- 

 ley and Scheele had by their discoveries placed chemistry 

 upon a rational basis, that air is a compound. This opinion 

 was based chiefly upon the fact that the proportions of the 

 ingredients are always the same in all free air. Then, be- 

 sides, it was thought that if the air were merely a mixture 

 of the gases composing it, they would be very prone to obey 

 the influence of gravity, the oxygen taking its place under 

 the nitrogen, and the carbonic anhydride under the oxygen. 

 The disposition of gases to mingle together ( 120) had not 

 then been demonstrated and illustrated, or this ground of the 

 doctrine would have been abandoned. At the present time 

 all chemists regard the air as a mixture, and the proofs are 

 briefly these : The ingredients of the air are separated from 

 each other too easily to warrant the belief that it is a chem- 

 ical compound. Then again, though the composition of all 

 free air is always the same, the proportions of the ingredients 



