166 .PROPERTIES AND FUNCTIONS OF THE SILICATES. 



than any of the salts here mentioned ; and in so far as their 

 use to plants is concerned, this increase of alkaline matter will 

 probably be an advantage. 



2. Properties. There are only two properties of these silicates 

 on which it is necessary here to insist. 



a The special valuable property of the silicates of potash and 

 soda is their solubility in water. Those which contain the 

 largest quantity of alkali are most readily soluble, and are, 

 therefore, most likely to exercise a beneficial action when 

 applied to the land. 



b These silicates are decomposed by the carbonic and by 

 most other acids. Hence, in the soil, where carbonic acid is 

 usually abundant, they are liable to be more or less completely 

 decomposed, forming carbonates of potash and soda, and libe- 

 rating the silica they contain. It is a property of silica so 

 liberated, however, that it is to a certain extent soluble in pure 

 water, and is thus able to enter directly into the roots of plants. 



3. Functions. The functions of these salts, in reference to 

 the soil, therefore, may be either to supply it with the soluble 

 silicates of potash or soda in which it may be deficient, or, if 

 they are decomposed in the soil, with soluble silica, and with 

 the carbonates of potash and soda. These carbonates, as I 

 have already explained in a previous section of this chapter, 

 are capable of performing important after functions in reference 

 both to the soil and to the plant. 



In the plant, the functions exercised by the silicates must be, 

 first, to convey into the sap and thence to the exterior part of 

 the stem, where it is usually deposited, the silica which it 

 requires in a soluble form. Second, having deposited the silica 

 in an insoluble state to supply alkaline matter to the sap, either 

 as an agent in producing chemical changes necessary to the 

 plant's growth, as a base to combine with the organic acids 

 formed in it, or as a building material required for the com- 

 pletion of its several parts. It is probably the carbonic acid 

 taken in by the leaf, or the other organic acids naturally formed 

 in the sap, which decompose the silicate, and enable it to 

 deposit the silica in a more or less insoluble form where the 

 structure of the plant requires it. 



