6 
GENERAL PROPERTIES OF SILEX. 
mospheric influence, a large portion of this combined silica, when set free, retains its 
solubility. New or virgin soils are particularly rich in soluble silica and the elements of 
carbonic acid : hence the cereals find in them their best location, and yield abundant 
returns. When tillage has exhausted them of silica in its soluble state, the straw is weak, 
and the harvest fails, unless artificially supplied. All plants take up this earth, but it is 
in the cereals and grasses that it abounds. In some the quantity is so small, that it may 
be regarded as accidentally present. 
But I may yet remark, that many bodies which contain potash are easily decomposed 
by the ordinary atmospheric agents to which they are exposed ; thus, granite, which is in 
part felspar, is a remarkable instance where decomposition furnishes soluble silica. The 
potash with which it is combined in the hard rock, yields to the action of carbonic acid 
and water. The same may be said of hornblende and basalt, or of the pyrogenic rocks as 
a class. The clay slate, or the slates of the Taconic system, as well as those of the higher 
classes in the Nevv-York system, furnish both silica and potash by decomposition : hence 
a glass may be formed which is very soluble, and may be used as a manure. 
A remarkably striking instance of decomposing action is that of carbonic acid upon the 
hardest of substances, such as that which is constantly exhibited upon the tumblers used 
for dipping the carbonated waters of Saratoga. These, in the course of a few days’ use, 
lose their transparency, and look as if they really required washing before they would be 
fit for dipping. It is the alkaline matter of the glass which is attacked in the first instance. 
The silica, however, is all of it soluble, and is slowly washed away. This action will be 
increased in proportion to the amount of alkali used in the composition of the glass. 
The formation of silica, as described in the foregoing paragraphs, is an illustration of the 
mode by which soils have been produced. Only a small portion, however, of the debris of 
rocks is in a condition to become the food of plants at any given period. The process is 
slow, but, as must be seen, it is the one best adapted to the wants of vegetation. 
The most important and interesting fact in regard to silica, is the two distinct chemical 
characters which it possesses; that of being soluble in water and weak acids at one time, 
and almost insoluble at another. In one state, it is the sustaining and protecting agent 
in the tissues of a vegetable; in the other, it sustains the root and whole plant, and is the 
medium through which nutritive matters are introduced, that is to say, it forms the soil in 
which plants are destined to grow, and at the same time affords them an outward me¬ 
chanical support. It is in the most delicate and useful vegetables, the grasses and grains, 
that silica forms a large proportion of the tissues, or in those plants where it seems lime 
would be insufficient to perform the same office. 
ALUMINE, ALUMINA, SILICATE OF ALUMINA, OR CLAY. 
It will not be far from the truth, to assert that alumine or clay possesses characters op¬ 
posed to those of silica. Whether true or not in its full extent, it is certainly less soluble 
