INTRODUCTION 23 



crust, an(1, though it forms a very important ingredient in soil 

 and occurs in many plant ashes, it does not appear to be 

 essential as a plant food. Recently it has been shown that 

 soluble silica in a soil enables plants to subsist in the presence 

 of a smaller quantity of phosphoric acid than would be neces- 

 sary without the silica. 



The element itself is of little importance. It is prepared 

 from the oxide, silica, SIO^, by the action of some substance 

 having a very strong affinity for oxygen, the most suitable re- 

 ducing agents being the alkali metals, potassium or sodium, or 

 magnesium powder. It is usually a brown solid, but, like 

 carbon or sulphur, can be obtained in several allotropic 

 forms. 



The oxide, silica, SiOj, is a very abundant substance, occur- 

 ring free as quartz, flint and sand, and, in combination, as the 

 very numerous and important class of minerals, the silicates. 

 It has been estimated that nearly half the solid mass of the 

 outer crust of the earth consists of silica. 



