10 Of Simple Earths. expofed to the open air, more obftinately than the foregoing earths: however, in a freezing cold, clay contradls more than any other earth, and, in contradting, fqueezes out its water, and thus parts with more of it in that circumftance than other earths do; a fadt of fome importance in agriculture, as well as mineralogy. Siliceous Earth. This earth is generally found in a ftony ftate, it abounds in flint, and is thence calledßUceaus; but it is ftill purer in mountain cryftal, and in quartz: the pureft is perfeflly white or colour- lefs; its fpecific gravity is 2,66. ln its ufual ftate of concretion it appears info- luble in water; but, in that ftate of divilion in which it exifts, when precipitated frorn its folu- tion in fixed alkalis, it is perfedtly foluble in 1000 parts of water. In general it combines with no acid, except that' extradted from the ftones called fluors, a variety of which is generally known under the appella- tion of Derbylhire fpar ; but, in the moment of precipitation from its lolution in fixed alkalis, many eminent chemifts think it capable of uni- ting with moft acids * ; at leaft, it is certain that an alkaline lolution of filex, mueh di- luted with water, may be faturated and fuper- faturated with an acid without any precipitation; * Mr. Dolomieu, 40 Roz. 379, thinks it is in its ufual ftate United to inflammable air, but frced front that air when united to alkalis, and in that cafe combinable tvith acids, in feparating from which it again retakes this air from water. and