76 NATURE TEACHING 



through the agency of its leaves. In order, however, to 

 understand the various processes which go on in the 

 leaf, it is necessary to know something concerning the 

 composition of the atmosphere. 



TJte Atmosphere. 



The atmosphere consists almost entirely of two gases, 

 oxygen and nitrogen, which relatively 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, removes 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 

 substances carbon and oxygen. 



Carbon exists in various forms, the commonest being 

 ordinary charcoal, which is very nearly pure carbon. All 

 organic substances 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 recognise 

 the truth of the assertion that all organic matter contains 

 carbon. If, however, the heating is continued still 

 further, the oxygen of the air unites with the carbon, 

 forming the gas carbon dioxide, and the substance has 

 then, we usually say, " burnt away." 



The presence of carbon dioxide can readily be made 



