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We could not select a more interesting example of the very wide diffusion of some compound substances than this one of silicic acid. It is found in the mineral and vegetable kingdoms. The rough, sharp edge of a blade of grass sometimes makes an ugly cut on one's finger by means of the flint it contains.
By the simplest experiment, any one may satisfy himself that rain-water filtered through field or garden soil does not dissolve out a trace of potash, silicic acid, ammonia, or phosphoric acid. The soil does not give up to the water one particle of the food of plants which it contains.
The undissolved residue is dried, and after the filter has also been burned in due manner and the ash added, the weight is ascertained. It consists of clay, sand, silicic acid and gypsum.
If rain or other water holding in solution ammonia, potash, and phosphoric and silicic acids, be brought in contact with soil, these substances disappear almost immediately from the solution; the soil withdraws them from the water. Only such substances are completely withdrawn by the soil as are indispensable articles of food for plants; all others remain wholly or in part in solution."
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