Page:Proceedings of the Royal Society of London Vol 1.djvu/202

Rh above-mentioned genus. Dr. Babington, from its physical characters, and from some experiments made on its solution in acids, ascertained that it was a mineral substance not yet described, and that it con- tained a considerable portion of aluminous earth.

This mineral is generally found in small hemispherical groups of crystals, composed of ﬁlaments radiating from a common centre, and inserted on the surface of the schistus : sometimes, however, it forms small veins of irregularly disposed prisms. It is of a white colour, having sometimes atinge of gray, or of green; and, when beginning to be decomposed, of yellow. Its lustre is silky; it is generally al- most opake, but sometimes semi-transparent. It is fragile; but its small fragments are so hard, as to be capable of scratching agate. It has no smell when breathed upon; it has not any taste, nor does it adhere to the tongue till it has been strongly ignited. It does not become electrical, or phosphorescent, by heat or friction; nor does it decrepitate before the blowpipe, but loses its hardness, and be- comes quite opake. Its speciﬁc gravity, Mr. Davy thinks, does not exceed 270, water being considered as 100.

The white and semi-transparent specimens of this substance are soluble in the mineral acids, and also in ﬁxed alkaline lixivia, with- out eﬂ’ervescence; but when coloured or opake specimens are ex- posed to alkaline lixivia, a small part remains undissolved.

A small transparent piece, by being exposed to the greatest heat of a forge, had its crystalline texture destroyed, and was rendered opake, but was not fused. It now had lost more than one-fourth of its weight, and adhered strongly to the tongue; neither water nor alcohol had any eﬂ'ect on this mineral. When exposed, in a glass tube, to a heat of from 212° to 600°, it gave out an elastic vapour, which, when condensed, was a clear ﬂuid, having a slightly empy— reumatic smell, but not diﬂ'ering in taste from pure water.

The solution of this substance in sulphuric acid produced crystals in thin plates, which had the properties of sulphate of alumine, and from which, when re-dissolved and mixed with potash, octahedral crystals of alum were obtained.

The solid matter precipitated from the solution of this substance in muriatic acid, was not acted upon by carbonate of ammonia. con- sequently it did not contain glucine or yttria.

Several experiments were made on the opake and coloured varieties of this mineral, from which it appears that the substances which cause these varieties, are calcareous earth, manganese, and oxide of iron.

Mr. Davy then proceeded to the analysis of the mineral. For this purpose he made use of the whitest and most transparent pieces he could obtain. The particulars of this analysis we shall pass over; and shall merely state that, according to its general results, 100 parts of the mineral contain, of alumine 70, of lime 1-4, of ﬂuid 26'2, the loss amounting to 2'4 ; which loss Mr. Davy is inclined to attribute to some ﬂuid remaining in the stone after the process of distillation,