Page:Proceedings of the Royal Society of London Vol 1.djvu/154

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The copious contents of this paper are arranged under the three following heads :—1. A description of the sulphuret of lead men- tioned in the title ; 2. Observations on the various modes of attrac- tion which inﬂuence the formation of mineral substances ; and 3. 0b- servations upon the different kinds of sulphuret of copper.

1. The cupro-antimonial sulphuret of lead, described in the ﬁrst part, has hitherto been found only in Cornwall; and though many specimens of it are to be met with in various collections in the king- dom, yet no writer has hitherto taken any particular notice of it, nor has it been classed by any of the late compilers of mineralogical systems. Mr. Hatchett is the ﬁrst who, on a careful analysis, has ascertained it to be a triple sulphuret, in which the sulphur is com- bined with lead, antimony, and copper; each of these ingredients exhibiting their characters in so striking a manner as to afford, in some measure, a new example of a natural compound in the mineral kingdom.

The following are its principal characters—It is of a dark gray colour; it has a brilliant lustre, and is very brittle; its hardness is such, that it very easily cuts calcareous spar, but is not sufﬁcient to scratch ﬂuor spar; it slightly marks white paper; when rubbed, it does not emit any smell; when powdered and thrown upon a hot iron, it emits a phosphorescent light; and its speciﬁc gravity is 5765. The form of its primitive crystal is a rectangular tetrahedral prism, with terminal faces perpendicular to its axis; but as no specimen has yet been discovered in which the above-mentioned form is totally destitute of secondary facets, the author enters into a minute investi- gation of the various modiﬁcations of this form, hoping by this means to promote essentially the knowledge of the crystalline character, so important in the study of mineralogy. These modiﬁcations are four in number. and can only be understood by inspecting the ﬁgures which are subjoined to the paper.

To these characters is added the more essential one, which is supplied by the proportions of the constituent parts of the substance. These, according to Mr. Hatchett’s analysis, consist of 42152 of lead, 24'23 of antimony, 1280 of copper, and 17 of sulphur: 1'20 of iron was likewise yielded in the process; but this is thought to have been a mere accidental mixture. It is next observed, that all the characters in tlu's substance indicate very plainly the mutual combination of the three sulphurets of which it is found to be composed; the whole of the external characters above described differing materially from those of either of the three sulphurets, and also from those of any metallic substance hitherto known ; and the pro-