MTKodKN AND AIR 229 



contains a mixture of several gases and vapours. Some of them are 

 met with in it in nearly constant proportions, whilst others, 011 the 

 contrary, are very variable in their amount. The chief component 

 parts of air, placed in the order of their relative amounts, are the 

 following : nitrogen, oxygen, aqueous vapour, carbonic anhydride, nitric 

 acid, salts of ammonia, ozone, hydrogen peroxide, and complex nitro- 

 genous substances. Besides these, air generally contains water, as spray ^ 

 drops, and snow, and particles of solids, perhaps of cosmic origin in 

 certain instances but in the majority of cases proceeding from the 

 mechanical translation of solid particles from one locality to another by 

 the wind. These small solid and liquid particles (having a large sur- 

 face and little weight) hang in air as solid matter hangs in turbid 

 water ; they often settle on the surface of the earth, but the air is never 

 -entirely free from them, because they are never in a state of complete 

 rest. Then, air not unfrequently contains incidental traces of various 

 substances, as everyone knows by experience. These incidental sub- 

 stances sometimes belong to the order of those which act injuriously 

 (miasmas), the germs of lower organisms for instance, of moulds and 

 to the class of carriers of infectious diseases. 



In the air of the diverse countries of the earth, at different longitudes 

 and at different altitudes above its surface, on the ocean or on the dry 

 land in a word, in the air of most diverse localities of the earth the 

 oxygen and nitrogen are everywhere in a constant ratio. This is, 

 moreover, self-evident from the fact that the air constantly diffuses 

 (intermixes in virtue of the internal movement of the gaseous particles) 

 and is put in a state of movement and intermixed by the wind, and 

 therefore it is equalised in its composition over the entire surface of the 



is so represented in science is based on the fact that only the two above-named com- 

 ponents are met with in air in a constant quantity, whilst the others are variable. The 

 solid impurities may be separated from air required for chemical or physical research 

 by simple filtration through a long layer of cotton- wool placed in a tube. Organic im- 

 purities are removed by passing the air through a solution of potassium permanganate. 

 The carbonic anhydride contained in air is absorbed by alkalis best of all, soda lime, 

 which in a dry state in porous lumps absorbs it with exceeding rapidity and complete- 

 ness. Aqueous vapour is removed by passing the air over calcium chloride, strong sul- 

 phuric acid, or phosphoric anhydride. Air thus purified is accepted as containing only 

 nitrogen and oxygen, although in reality it still contains a certain quantity of hydrogen 

 .and hydrocarbons, from which it may be purified by passing over copper oxide heated to 

 redness. The copper oxide then oxidises the hydrogen and hydrocarbons it burns them, 

 forming water and carbonic anhydride, which may be removed as above described. Such 

 purified air differs in many respects from ordinary air. Thus, for instance, it does not 

 support plant life. When it is said that in the determination of the density of gases the 

 weight of air is taken as unity, then it is understood to be such air, containing only 

 nitrogen and oxygen. It is a litre of such air that weighs 1'298 grams at and 700 mm. 

 pressure at Jong. 4S 3 , and T294 grams at St. Petersburg. 



