Page:Proceedings of the Royal Society of London Vol 1.djvu/229

Rh

The mineral substance treated of in this paper, is similar to that of which Mr. Davy, some months ago, gave an account, under the title of Hydrargyllite or Wavellite. That which is now described by Mr. Gregor is produced from a mine called Stenra Gwyn, in the county of Cornwall. -

Two species of this substance, the author says, are found in the above-mentioned mine; the ﬁrst, and most common, consists of an assemblage of minute and delicate crystals, in radiated tufts, attached to quartz crystals. These crystals are in general white and trans- parent; sometimes, however, they have a yellowish hue. They vary considerably in their size, but seldom exceed a quarter of an inch in length.

Among these crystals are frequently seen two kinds of crystalline laminae; one of them being in the form of parallelopipedons, with truncated angles, and of a green colour; the other forming an as- semblage of square plates, varying in thickness, and the angles of which are not always coincident; these are of a bright wax yellow. This last kind is also found adhering to the sides of quartz crystals, in the cavities of granite.

The other species of the substance here treated of, consista of crystals closely compacted together in the form of mammillary pro- tuberances, generally of the size of small peas, and forming a stratum about one eighth of an inch thick, upon quartz, in the cavities or ﬁssures of compact granite. The striae of these mammillae diverge from a centre, like zeolite.

The detached crystals of the ﬁrst speciesare easilyreduced topowder. Their speciﬁc gravity, at 56° Fahr., was found to be 2'22. The se- cond, or more compact species, is suﬁiciently hard to scratch calca- reous spar : its speciﬁc gravity, at the temperature of 55°, was 2'253.

The crystals of the ﬁrst species, when suddenly exposed to the action of the blowpipe, decrepitate; if gradually exposed to its ac- tion, they grow opake, but show no signs of fusion, even under the strongest heat. Both speci, when exposed for some time to a red heat, experience a diminution in weight of about 30 per cent.

Some other experiments upon these substances are related, and a very minute account of the mode in which they were analysed is given; of this we must necessarily conﬁne ourselves to give merely the results.

Fifty grains of the crystals of the ﬁlst species yielded alumina 29H, grs.; silica, 3;, grs.; oxide of iron, 1%,- grs.; lime, 3‘,— grs.; volatilized matter, 14+; grs.