GENERAL PROPERTIES OF SILEX. 5 



retains as long as it is moist; but when dried, and especially when ignited, it becomes 

 insoluble again : hence it is often spoken of as soluble and insoluble silica. In silicated 

 plants, as the cereals, it retains its solubility if they undergo in the ground a slow decay, 

 and we may dissolve it in our analysis along with the phosphates. If, however, these 

 vegetables are burned, their silica becomes insoluble mostly, although it is minutely divided. 



Silex is white when pure, and harsh and gritty to the feel, but fuses easily with soda 

 into a transparent bead. It is also dissolved in the hydrate'd vapor of fluoric acid, for 

 which substance it is regarded a test. 



In this connection, it is proper to speak of silica as a constituent of soils, and of its uses 

 and functions, as a part of the vegetable tissues. It lias its mechanical importance in both 

 relations ; and so abundant is it both in its separate and combined states, that it must be 

 regarded as one of the most essential bodies in nature. 



1. Silica as an element of soil. In quantity it forms more than 60 per centum, and 

 sometimes its percentage is as high as 95. Taking its average range as about 78 '79 

 per centum, we find it entering more largely into the composition of soil than any other 

 element. In this fact, we discern that its function as an earth in the midst of earths is 

 important : it must impart its own characters to the compound. In itself, silica is a dry, 

 white, harsh-feeling powder, destitute nearly of affinity for water, and admitting the free 

 passage of fluids through it without affecting them in the least. It is then an element 

 which is designed to give porosity to soil, in order that water and air may be admitted into 

 its texture. If soils contain too little of it, they are close and impervious; if too much, 

 water percolates too rapidly through them. Soil is not tempered with an excessive dose 

 of silex when it amounts to 85 per centum : above that proportion, the soil becomes rapidly 

 porous and loose, and can not be cultivated without annual additions of manure. 



2. Silica as an element of organized bodies. Silica or sand is not taken up by the roots 

 of plants as such, in consequence of its insolubility ; but it requires to be in combination 

 with other substances, in order to give it this property. What these substances are, has 

 been stated already, namely, potash and the alkalies. Silica then enters into the compo- 

 sition of vegetables, though not in equal proportions in different parts of the same plant. 

 Its presence can not be regarded as accidental ; for instance, in 100 parts of the ash of the 

 straw of the creeping wheat, it amounts to 69 "66 ; in the grain, to only 2 - 56. In the ash 

 of forest trees, it is never half as much. In the stalks of all the cereals, however, it exists 

 in large percentages. It gives strength to the straw, and may be regarded as the element 

 which supports and protects it. In order that the grains and grasses may take up silica, 

 it is necessary that the fluids of the soil should be able to dissolve it. 



Although an immense quantity of silica exists in a free and insoluble condition, and it 

 would seem necessarily so, yet, in the constitution of the globe, provision is made for its 

 solubility. This provision may be seen in its numerous combinations with the alkalies 

 and alkaline earths. With these it has been united by fusion ; and, as we have already 

 stated, this is one of the modes resorted to for giving it solubility. In the natural progress 

 of the decomposition of the silicated earths and alkalies in a moist soil, and by the at- 



