14 THE PLANT. 



the power of uniting with many substances, fonning 

 compounds which are different from either of their 

 constituents aloii'^. Thus : oxygen unites with iron. 

 aud forms oxide o' iron or iron-mist, whicli does not 

 resemble tlie grey nietaUic iron nor the gas oxygen ; 

 oxvgen unites with ^-Arbon and forms carbonic acid, 

 wliich is an invisible ^as, but not at all like pure oxy- 

 gen ; oxygen combin-^s? with hydrogen and forms 

 water. All water, ice, "^team, etc., are composed of 

 these two gases. We kn-^w this because we can arti- 

 ficially decompose, or separate, all water, and obtain 

 as a result simply oxygen ?vd hydrogen, or we can 

 combine these two gases and Ihns form pure water ; 

 oxygen combines with nitrogen and forms nitric 

 acid. These chemical changes and combinations 

 take place only under certain cinumutances, which, 

 so far as they affect our subject, wd^ be considered 

 in the following pages. 



As the atmospheric elements of points are ob- 

 tained from matters existing in the atmospltore whic'\ 

 surrounds our globe, we will examine its co.*\rv^*vi.>vM\ 



CHAPTEE II. 



TUE ATMOSPHERE AJSfD ITS CARBOX. 



Atmospheric 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 



