I 1 IIIK IT. A NT. 



the power of uniting with many substances, forming 

 comounds which are different from either of their 



eoii-tituents alone. Thus: oxygen unites with iron 

 and forms oxide of iron or //v//-/W, which does not 

 n-emble tin- grey metallic iron nor the gas oxygen ; 

 oxvtren unites with carbon and forms carbonic acid, 

 which is an invisible gas, but not at all like pure oxy- 

 iren ; oxygen combines with hydrogen and forms 

 water. All water, ice, steam, etc., are composed of 

 these two irases. "\Ve know this because we can arti- 

 1i> ;.;!lv decompose, or separate, all water, and obtain 

 as a result simply oxygen and hydrogen, or we can 

 combine these two gases and thus form pure water ; 

 oxvgen combines with nitrogen and forms nitric 

 acid. These chemical changes and combinations 

 take place only under certain circumstances, which, 

 BO far as they affect our subject, will be considered 

 in the following pages. 



As the atmospheric elements of plants are ob- 

 tained from matters existing in the atmosphere which 

 surrounds our globe, we will examine its constitution. 



CHAPTER II. 



TIIH ATMOSPHERE AND ITS CARBON. 



A i MOSPHERIC air is composed of oxygen and nitrogen. 

 Their proportions are, one part of oxygen to four 

 parts of nitrogen. Oxygen is the active agent in the 

 combustion, decay, and decomposition of organized 



