SOURCE OF SILICA. 175 



Some plants contain sulphate of lime or gypsum, as 

 clover, while in others, the lime is found as a tartrate or 

 malate, that is, in combination with organic acids, which lat- 

 ter must have been formed before the lime could be assimi- 

 lated. 



Phosphate of lime is a powerful manure, and may, in 

 small quantities, enter the organs of plants. But in this 

 case, as in that of potash, the great point is to furnish it in 

 any form. 



5. Alumina is sometimes found in plants, but in very 

 small quantities. It may enter in combination with phos- 

 phoric acid, or with crenic and apocrenic acids. 



6. Silica or silicic acid. The epidermis, or outer bark 

 of trees, the vessels in which the sap circulates, and the sur- 

 face of the grains and grasses, are composed in part of this 

 acid. As it is not soluble in water, nor in cold alkalies, the 

 most common solvents in the soil, it has been a point of 

 some difficulty, to determine the form in which it can be in- 

 troduced. 



It is supposed by some, that it is combined with crenic 

 acid, a compound which is found in river water, and in the 

 soil. This substance must enter the roots of plants with 

 water, and may then be assimilated. 



It is possible, however, that the silica, found in plants, is 

 introduced by means of galvanic action, or the catalytic force 

 of life. 



There is still another mode of introducing this substance. 

 It forms soluble salts with alkali, as potash, and when first 

 liberated from its combinations, according to a well known 

 law, the silica becomes soluble, and capable of entering the 

 organs of plants. 



7. The metallic oxides of iron and manganese are found 

 in some plants, and are derived from the soil. Iron is the 



One grain of lime, in a hundred of the soil, will produce fertility, 

 Avhere all v/as barren before. 



