78 NATURE TEACHING. 



THE ATMOSPHERE. 



I . The atmosphere consists almost entirely 

 of two gases, oxygen and nitrogen, which rela- 

 tively compose one-fifth and four-fifths of its 

 volume. Oxygen is the substance by whose 

 agency all burning or combustion takes place 

 and which, in the breathing of animals, re- 

 moves the waste products from the blood by 

 a process of slow combustion. Nitrogen, on 

 the other hand, is an inactive gas which 

 serves to dilute the oxygen and modify the 

 rapidity and vigour of its action. In addition 

 to these two gases there are present very 

 small quantities of water- vapour and carbonic 

 acid gas or Carbon dioxide so called because 

 it is formed by the union of the two sub- 

 stances carbon and oxygen. 



2. Carbon exists in various forms, the 

 commonest being ordinary charcoal which is 

 very nearly pure carbon. All organic substan- 

 ces that is all substances which are the 

 product of life become blackened or charred 

 when strongly heated. This charring may be 

 taken as proof of the presence in them of 

 carbon. We thus recognize the truth of the 

 assertion that all organic matter contains car- 

 bon. If, however, the heating is continued still 

 further the oxygen of the air unites with the 



