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montlis. A very hard semi-translucent fragment contained, when first 

 removed from the jar, 2 3 "2 7 per cent, of water, which would correspond 

 to SiO^HO ; but after some months' exposure to dry air, it was reduced 

 to 9*59 per cent., or 38102,110. In both cases the fragment still con- 

 tained some carbonate of potash, so that no very accurate analysis of it 

 could be made. The gelatinous precipitate formed by passing carbonic 

 acid through soluble silicate of potash, even when exposed to the air in 

 considerable mass until it became dry, yielded only an amorphous white 

 anhydrous powder, or one containing only small and variable quantities 

 of water. A hydrate containing 16-5 per cent, of water, and which may 

 be represented by the formula 3Si02,2HO ( = Si03,H0), appears, how- 

 ever, to have been obtained by dropping slowly hydrochloric acid into 

 a solution of basic silicate of potash of moderate strength, and drying 

 the gelatinous precipitate in a vacuum or in dry air. This hydrate con- 

 sisted of a white powder ; but M. Doveri obtained a similar hydrate in 

 the crystalline state by precipitating a solution of silicate of copper dis- 

 solved in hydrochloric acid, by sulphide of hydrogen, and evaporating 

 the perfectly limpid solution of silica over quick-lime in a vacuum. When 

 the hydrate 3Si2,2HO in the form of a white powder was exposed 

 for some time to a temperature of 100° to 120° cent., it lost half its 

 water, and formed a definite compound, represented by the formula 

 3Si02,H0 (= 2Si03,HO), that is, the same compound as that which was 

 formed by the exposure of my hard semi-translucent silica for some 

 months to dry air. The latter, to which I have above assigned the 

 formula Si02,H0 (=2Si03,3HO), has the same composition as the re- 

 markable glassy hydrated silica obtained by Ebelman by exposing 

 silicic ether to the slow action of moist air. So far as I am aware, the 

 two hydrates which I have described are the only examples of definite 

 hydrated silica having been obtained in the form of opal. A strong so- 

 lution of silicate of potash put into a Eriet's apparatus, charged in the 

 ordinary way with bicarbonate of soda and tartaric acid, and left un- 

 disturbed for a few months, and then exposed to the air until it dried, 

 was horny here and there. The quantity of water in many varieties of 

 opal and hyalite is so small, that some mineralogists consider it not to 

 be chemically combined in those minerals. In what state, then, is it ? 

 Hydrated water may be held with so feeble a force as to appear attached 

 by cohesion. Mr. A. Gages, in a paper read before the British Asso- 

 ciation at Leeds, 'described an opaque siliceous skeleton which he obtained 

 by the long continued action of acids upon a mineral, and which became 

 transparent like hydrophane when plunged into water. The quantity 

 of water necessary to efiect this change appeared to be definite ; the 

 phenomenon was certainly an excellent example of mechanical cohesion 

 passing into chemical. Opal, hyalite, &c., as well as the semi-trans- 

 lucent gummy hydrated silica just described, probably belong to the 

 same category. The formation of some horny hydrated silica in the 

 Eriet's apparatus is interesting, as showing that time influences the 

 combining power of water and silica. A similar influence appears to 

 be exerted upon carbonic acid dissolvedin water under pressure, because, 



