6 GENERAL PROPERTIES OF SILEX. 



mocpheric 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 haYvest fails, unless artificiall}' 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 arc 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 New-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 iji 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 



