CHAPTER II. 

 THE ATMOSPHERE. 



Physical Properties. — Most terrestrial plants and animals 

 live surrounded by air, and many of the processes of life are 

 directly dependent upon chemical actions in which the con^ 

 stituents of air take part. Air also plays an important part 

 in the formation of soils and in the changes which occur iij 

 their constituents. It is therefore essential that the student 

 should have some knowledge of the properties and composition 

 of the atmosphere if he is to understand the nature of the 

 themical processes which are concerned in the life and growth 

 if plants and animals. 



That air is a material substance only becomes apparent under 

 certain conditions. The space around us is apparently empty, 

 for air is invisible and seems to permit of bodies moving freely 

 within it. It is well known, however, that when a body of 

 considerable area is. moved rapidly in air great resistance is 

 ofiered, thus proving that air is a material substance. More- 

 over, by depressing a glass vessel, e.g., a beaker or tumbler, 

 mouth downwards beneath the surface of water it v/ill be 

 observed that the water only enters the vessel to a slight 

 extent, and that it is kept back by the air within the vessel. 

 These simple phenomena and many others prove the material 

 nature of air. It is quite easy, too, to show that air haii> 

 weight. If a spherical flask be provided with an india-rubber 

 stopper fitted with a short piece of glass tubing to which a 

 piece of rubber tubing and a pinch-cock is fitted, it is possible 

 \o extract a large portion of the air, either by means of an 



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