Outlines of Geology. 8 1 



precipitation, under which it readily dissolves in potassa and 

 soda ; it is also easily combined with those alkalies by fusion, 

 such compound constituting the basis of glass, and where the 

 alkali greatly predominates, being soluble in water. This solu- 

 tion is called, sometimes, liquor of flints ; the acids decompose 

 it, and throw down a finely-divided hydrate of silica^ which is 

 soluble, to a small extent, in water ; a fact of some importance 

 in relation to our present subject, and accounting, in some 

 measure, for the existence of silica in certain mineral waters, 

 and more especially in those of the boiling fountains or geysers of 

 Iceland, which deposit it in abundant incrustations. Calcedony, 

 and some other minerals, are also occasionally referred to, aa 

 attesting the once fluid state of silica from a watery solvent : its 

 crystalline forms may have originated from igneous fusion, but 

 the existence of aquatic confervce, often of their native colour, or 

 discoloured by oxide of iron, and of mosses and lichens, in certain 

 agates, renders fire, in many instances, an inadmissible agent. 

 Upon these subjects, Dr. Mac CuUoch's inquiries are extremely 

 deserving notice. Even the chrysalis of a moth is, it is said, ex- 

 tant in agate. One other chemical character of silica must not 

 be overlooked, as bearing upon geological theory, which is, its 

 aflinity for other earths, either in igneous fusion, or aqueous 

 solution. In the former way, it combines with lime and alumina, 

 with magnesia, baryta, and strontia. Pottery and porcelain 

 are alumino-siliceous compounds, with an occasional proportion 

 of magnesia. Three parts of silica, 2 of magnesia, and 1 of 

 alumina fuse into a glass ; and we shall, by and by, find several 

 compound minerals of analogous constitution. Nor must the 

 humid attractions of silica be overlooked, for when its alcaline 

 solution is added to aqueous solutions of lime, baryta, or strontia, 

 or to an alcaline solution of alumina, compound precipitates of 

 the earths are the results, several of which have been particu- 

 larly examined by Mr. Dalton. (System, p. 841.) It may be 

 right to add, that silica is, probably, a binary compound of 

 oxygen with a peculiar inflammable base, which has been called 

 silicon^ and that it appears to contain about half its weight of 

 Vol. XIX. G 



