GENERAL PROPERTIES OF SILEX. 
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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 hydrated 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 has 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- 
