14 THE ELEMENTS OF PLANT FOOD 



change. It must not be supposed that a chemical 

 equation may be made up of any combination of ele- 

 ments that will make an equation. On the contrary, 

 the equation represents a chemical change and compar- 

 atively few chemical changes are possible. These must 

 be determined by experiment. 



OXYGEN (0) 



Description and Occurrence. Free oxygen is an 

 invisible gas. It makes up more than one fifth of the 



Ignition 

 Tube 



FIG. i. Preparing Oxygen. 



air, in which it remains uncombined with other ele- 

 ments. It exists in combination with many other ele- 

 ments. For instance, with hydrogen (H) it combines to 

 form water. By weight it is eight ninths of water. It 

 forms three fourths of all animal bodies and about one 

 half of the crust of the earth. Speaking generally, 

 oxygen forms about one half of all matter. 



Preparation. Although oxygen is so abundant in 



