66 THE ELEMENTS OF PLANT FOOD 



flowing water. The various colors are due to the 

 presence of iron oxides. 



SILICON (Si) 



Silicon does not occur free in nature, but its com- 

 pounds are very abundant; alone it makes up about 

 one fourth of the crust of the earth. Silicon is not 

 an important plant food, the popular notion that it 

 serves to give stiffness to the straw of some of the 

 cereal plants being an error. 



Silica (SiO 2 ) is the most common compound of silicon. 

 It occurs in the form of sandstone, quartz, and quartz 

 sand. Quartz is crystallized SiO 2 . It often has some 

 coloring matter in it, which gives us the rose quartz, 

 the smoky quartz, and the amethyst. The opal is 

 uncrystallized silica combined with variable amounts of 

 water; flint, agate, and jasper are imperfectly crys- 

 tallized silica. 



Glass is made by melting together quartz sand, car- 

 bonate or sulphate of soda or potash, and limestone 

 or lead. CO 2 is driven off in the process, arising through 

 the molten mass in bubbles. The mass is kept in liquid 

 form while the bubbles are being formed. A very clear 

 glass is made by keeping the liquid hot for some time, 

 freeing it from bubbles and refining it. 



Water glass is made by melting together fine sand, 

 silica, and sodium carbonate, forming sodium silicate 

 (Na 2 SiO 3 ). It is soluble in water, and when it dries it 

 leaves a transparent coating on substances over which 

 it is spread. It is used extensively as a filler in the 

 manufacture of artificial stone. A solution of one 

 part water glass and nine parts of water makes one 



