CHAPTER IV 



THE ELEMENTS NECESSARY FOR PLANT GROWTH- 

 OXYGEN HYDROGEN CARBON NITROGEN PHOS- 

 PHORUS SULPHUR POTASSIUM CALCIUM MAGNE- 

 SIUM IRON 



29. Oxygen. Oxygen is the most abundant of the ele- 

 ments. Upon it the life of plants as well as of animals 

 directly depends. The greater part of the energy mani- 

 festing itself in the motion of objects about us is the result 

 of the chemical activity of oxygen. Power for the pur- 

 poses of commerce, energy which drives electric and steam 

 cars, and the heat necessary for the sustenance of life 

 and the maintenance of a temperature which makes life 

 possible in other than tropical countries, all are derived 

 from heat produced when oxygen combines with com- 

 bustible substances. 



Occurrence. In a free condition oxygen exists in vast 

 quantities in the atmosphere, of which it is nearly 21 per 

 cent by volume. In combination with other elements it 

 is found in thousands of different compounds. Nearly 

 nine-tenths of water is oxygen. It forms nearly one-half 

 of the rocks composing the crust of the earth. It is rather 

 difficult to find, in any of the common objects about us, 

 a substance that is not combined with oxygen. Other 

 than those substances that have been artificially produced 

 by man and those that like carbon in coal are the product 

 of vegetable life, almost the entire earth is composed of 

 oxygen compounds. 



Properties. When free, oxygen is an odorless gas 

 without color or taste. It is a little heavier than air and 

 is soluble in water at ordinary temperature to the extent 



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