806 SILICA, SILICON, SILICATES OF POTASH A^TD jODA. 



is without colour, taste, or smell, and cannot be melted by the strongest 

 heat. As it occurs in the mineral kingdom — in the state of flint, of 

 quartz, or of sand — it is perfectly insoluble in pure water, either cold or 

 hot, does not dissolve in acid and very slowly in alkaline solutions. 

 "When mixed with potash, soda, or lime, and heated in a crucible to a 

 high temperature, it melts and forms a glass. Window and plate glass 

 consists chiefly of silica, lime, and soda, Jlint glass contains litharge 

 [oxide of lead] in place of the lime. But though the various forms of 

 more or less pure silica, which are met with in the mineral kingdom, 

 are absolutely insoluble in water, j'et it sometimes occurs in nature, and 

 can readily be prepared in a state in which pure water, and even acid 

 solutions, will take it up in considerable quantity. In tiiis stale it may 

 be obtained by reducing crown-glass to a fine powder, and digesting it 

 in strong muriatic acid, or by melting quartz sand in a large quantity of 

 potash or soda, and afterwards tre£'*^ng the glass that is formed with di- 

 luted muriatic acid. 



Silica is one of jtfae most abunda. i substances in nature, and in com- 

 bination with potash, soda, lime, magnesia, and alumina, it forms a 

 large portion of all the so-called crystalline (granitic, basaltic, &c.) 

 rocks. The compounds of silica, with these bases, are called silicates. 

 By the action of tiie air, and other causes, these silicates undergo decom- 

 position, as glass does when digested with muriatic acid, and the silica 

 is separated in the soluble state. Hence its presence in considerable 

 quantity in the waters of many mineral and especially hot mineral 

 springs, and in appreciable proportion in nearly all waters that rise from 

 any considerable depth beneath the surface, or have made their way 

 through any considerable extent of soil. 



In the substance of living vegetables it exists, for the most part, in 

 this state of combination — as well as in the form of an extrf mely deli- 

 cate tissue, of which the fibres are exceedingly minute, and therefore 

 expose a large surface to the action of any decomposing agent, or of any 

 liquid capable of dissolving it. In the compost heaps these silicates 

 undergo decomposition, — and the more readily the less they have been 

 previously dried, or the greener they are, — and the silica of the plant is 

 liberated in a soluble slate. Whether or not, when thus liberated, it 

 will be carried, uncombined, into the roots of the plants by the water 

 they absorb, will depend upon the quantity of potash or soda in the 

 compost or in the soil, and upon other circumstances hereafter to be 

 explained. 



2°. Silicon is known only in the state of a dark brown powder, which 

 has not as yet been met with in nature in an elementary form, and is 

 jirepared by the chemist with considerable difficulty. Whefi heated in 

 the air, or in oxygen gas, it burns, combines with oxygen, and is con- 

 verted into silica. Silica, therefore, in its various forms, is a compound 

 of silicon with oxygen. It consists of 48 per cent, of the former and 62 

 per cent, of the latter. 



3°. Silicates of Potash and Soda. — When finely powdered quartz, 

 flint, or sand, is mixed with from one-half to three limes its weight of 

 dry carbonate of potash or soda, and exposed to a strong heat in a cruci- 

 ble, it readily unites with the potash or soda, and forms a glass. This 

 glass is a silicaf.i or a mixture of two or more silicates of potash or soda. 



