CHAPTER III. 



OF THE MINERAL ELEMENTS OF SOIL, THEIR PROPERTIES, AND 



CHEMICAL ACTION. 



62. The bases of the silicates have common properties, 

 which are : 



1st. Alkalhie. Whatever may be our idea of the effect 

 of an alkali, as exhibited by potash or soda, the same in 

 kind, but in degree less, is exhibited by lime, magnesia, and 

 alumina. Placing potash as the type of alkaline power, the 

 same power in a decreasing order is found in lime, magne- 

 sia, and alumina. 



2d. They are, most of them, soluble in water. Potash 

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

 magnesia, and totally disappears in alumina. This may 

 have some connection with the fact, that, widely diffused as 

 it is in all soil, it is very seldom found in plants. 



3d. They exhibit great affinity for carbonic acid. The 

 order of affinity is potash, soda, lime, magnesia ; alumina, 

 if it possesses it at all, exhibits it only feebly. The alkalies 

 form soluble, and the alkaline earths, and alumina insoluble 

 compounds with carbonic acid. 



4th. They have all great affinity for water, combining 

 with it, and forming what are called hydrates. Potash parts 

 not with this chemically combined water, by any heat which 

 has been produced ; lime and magnesia give up their water 

 readily, at a red heat : alumina requires, for this purpose, a 



