OF AGRICULTURE. 197 



1. The silicate of alumina (clay, earth), with potash, 

 forms feldspar. 



2. Silicate alumina and lime, with magnesia and 

 iron oxyde, forms hornblende. 



3. The silicate of magnesia, forms serpentine and 

 talc, and silica almost pure in quartz. 



This brings us to the third branch of our subject. 

 The mineral ingredients of the soil, their properties 

 and chemical action. 



CHAP TEE III. 



THZ PROPERTIES AND CHEMICAL ACTION OP THE MINERAL 

 INGREDIENTS OP THE SOIL. 



The bases of the silicates have common properties, 

 which are: 



1. Alkaline, as exhibited in potash and soda, and 

 in a less degree in lime and magnesia, etc. 



2. They are almost all soluble in water. Potash 

 stands first here, also; the solubility decreases in lime, 

 and totally disappears in alumina and clay. 



3. They have a great affinity for carbonic acid. 

 The order of affinity is potash, soda, lime, magnesia. 



4. They all have a great affinity for water. 



The metalloids or unmctallic substances, also, have 

 common properties : 



1. They combine with the pure base of silicates, 

 sftid form silicinrets, phosphurets, carburets and sul- 

 phurets. Thus are formed carburet of iron or plum- 



