[309] SCIENTIFIC MANIJAL. 23 



lime represent the different forms in which the lime and 

 phosphoric acid unite. In the first, the phosphoric acid 

 unites with only one equivalent of lime, the second only two 

 parts of lime, while the third, or tribasic, the usual form, 

 unites with three parts. The tribasic is the form from 

 which the superphosphate of commerce is manufactured. 



Since the "precipitated," or " reduced," phosphoric acid 

 is soluble in "soil water," it is considered of equal agri- 

 cultural value with the "soluble," the two being classed 

 together as AVAILABLE phosphoric acid. The insoluble 

 portion of a superphosphate, derived from animal bone, 

 (not charred,) is of greater value than that derived from 

 phosphate rock, since it more readily becomes available to 

 plants under the influence of acids and alkalies in the soil. 



Silicon, in combination with oxygen, forms silica, and 

 exists as such in a great variety of forms, such as quartz, flint, 

 sand and rock crystal. As sand, it is found in all soils, form- 

 ing the principal ingredient in some, and hardly perceptible 

 in others. 



Sand is formed by the disintegration of rocks containing 

 quartz, and is spread over the earth's surface by the action 

 of winds and water. 



Silica exists in soils in two conditions : 



1st. Insoluble silica, as pure grains of sand. 



2d. Soluble silica, in combination with some other min- 

 erals or elements as silicates. Its chief compound is that 

 of silicate of alumina, or clay. It enters largely into the 

 composition of plants, especially so in the grasses and the 

 common cane of our swamps, 



Pettified wood is a tree rock, formed by the entrance of 

 soluble silica into the cells of the wood, then hardening and 

 displacing, or producing decay of, the vegetable matter, 

 while retaining the woody form. 



Silica is abundant in all soils, and though but a small 

 per cent, of it is soluble, there is, usually, sufficient for the 

 use of vegetation. 



