32 THE PLANT. 



portion of sulphur, which is necessary to the forma- 

 tion of some of its parts. 



SILICIC ACID, OK SILICA. 



This is common sand. In its pure state it cannot 

 be dissolved and plants can make no use of it. It 

 unites with the alkalies and forms compounds, such 

 as silicate of potash, silicate of soda, etc., which are 

 soluble in water, and therefore available to plants. 

 If we roughen a corn stalk with sand-paper we may 

 sharpen a knife upon it. This is owing to the hard 

 particles of silica which its outer parts contain. 

 Window glass is silicate of potash, rendered insoluble 

 by additions of arsenic and litharge. 



Liebig tells us that there was discovered, between 

 Manheim and Heidelberg in Germany, a mass of 

 melted glass where a hay-stack had been struck by 

 lightning. They supposed it to be a meteor, but 

 chemical analysis showed that it was only the com- 

 pound of silicic acid and potash which served to 

 strengthen the grass. 



There is always enough silicic acid in the soil, but 

 it is often necessary to add an alkali to render it 

 soluble and available. When grain*, etc., lodge or 

 fall down from their own weight, it is probable that 

 they are unable to obtain from the soil a sufficient 

 supply of the soluble silicates to support their rapid 

 growth. 



