8 LUCALITY AND FOKEST VEGETATION. 



Section I. — Tiik Atmosphere. 



The earth is surrounded by gaseous bodies, which move 

 with it, and collectively are termed the atmosphere. Owing 

 to the weight of its component parts, the atmosphere is densest 

 close to the surface of the earth, and l)ecomes tliiimer with 

 increasing distance from the earth passing gradually into space. 



The atmosphere consists essentially of the following 

 substances : — 



(1.) Oxygen and Nitrogen. 



(2.) Carbon dioxide. 



(3.) Water, in various conditions. 



(4.) Solid bodies. 



(5.) Ammonia and Nitric Acid. 



1. O.rijficii (iinl Xili'Dficii. 



The chief constituents of the atmosphere are 21 parts of 

 oxygen and 79 parts of nitrogen in a mechanical mixture, as 

 well as small quantities of argon, helion, and other substances. 

 The latter have quite lately been discovered, and it would 

 be beyond the scope of this book to discuss them licrc. No 

 chemical process is required to separate oxygen from nitrogen ; 

 as a matter of fact, all porous bodies possess the faculty of 

 taking oxygen from the atmosphere, without entering into a 

 chemical combination with it. Amongst such bodies are the 

 soil, leaves, bark, and roots of plants. Until a comparatively 

 late date it was believed that plants could not take nitrogen 

 direct from the air. It has now been proved tliat certain 

 plants, including various forest trees, can do this, and further 

 investigation will doubtlessly lead to additional discoveries in 

 this respect. 



Although tlie leaves of plants take up oxygen, tliey exhale 

 greater quantities of it under the effect of light ; the latter is 

 the result of the decomposition of carbon dioxide by the 

 leaves, which retain the carbon and surrender the oxygen. 

 Thus, plants are powerful agents in the productit)n of oxygen. 



