OF AGRICULTURE. 199 



water. Sulphuret of silicon dissolves in water, and 

 gives silicon or sand in solution. 



GLASS ANALYSIS OP GRANITE. 



The general properties which silicic acid exhibits 

 in combination are these, viz. : 



1. All its compounds with excess of alkali are 

 caustic, and soluble in water. 



2. Those with an excess of silicon or sand are mild 

 and insoluble. G-lass is an example of the last, and 

 so are rocks. Green bottle glass is but a fused rock, 

 a mixture of the silicates of potash, soda, magnesia, 

 lime, and iron. These are the silicates which have 

 been already noticed as composing rocks, and the 

 amount and origin of those elements of the soil can 

 now be easily comprehended. This is practical 

 ground, and shows the value of the chemical analysis 

 of rocks. 



Let us take granite rock for an example. See of 

 what it is composed ? 



ANALYSIS OP GRANITE BOOK. 



Silex or Sand 74.84 



Alumina (or clay earth) 12.80 



Pofash 7.48 



Magnesia 99 



Lime 37 



Oxide of Iron 1.93 



Oxide of Manganese 12 



.100 



Thus, in every one hundred pounds of granite we 

 have seven and a half pounds of potash and three - 

 eighths of a pound of lime. 



These elements do not exist in the soil free; they 

 exist as silicates or salts, which are gradually acted 

 upon by the carbonic acid of the air, and by the grow 



