Page:Proceedings of the Royal Society of London Vol 1.djvu/64

Rh proceeds to describe the external appearance of the specimens he has had opportunities to examine; and from these characters, to- gether with an accurate analysis of the different sorts made by Mr. Chenevix, he deduces the following classification of this new mineral

The principal distinction is in arseniates of copper and of iron. Of the former there are four species:—

1. Arseniates of copper, in the form of an obtuse octaedron. This is said to be the most simple, and appears to be the original form of an ore. The minute descriptions of the crystals are in this, as in all other species, illustrated by delineations. This sort is very light, its average specific gravity being 28-819. Its hardness is inconsiderable, and its transparency seldoni perfect. Its colour is either a beautiful deep blue or a fine grass-green, and sometimes white with a slight blue cast. According to the analysis, this variety of colours depends chiefly on the quantity of water which enters into the crystallization

2. Arseniate of copper in hexaedral laminae with inclined edges. The colour of this ore is a fine deep emerald green, and sometimes a light green. often assume the appearance of metallic foils. Its specific gravity is 25-488. It is less hard than the preceding species. Its thin crystals are transparent, and it frequently decrepitates in fire.

3. Arseniate of cópper in the form of acute octaedrons. Its colour is a brown or bottle-green, the surface often reflecting a golden tint. Its specific gravity is 42-809. It is sufficiently hard to scratch fluor spar, but not glass. Its transparency is generally very great. These general characters are by no means permanent. Its crystals are not always determined, nor does it at all times exhibit a similarity of colours. These differences of form and aspect have induced the Count to distinguish five varieties, which are denominated as follows: -1. Capillary of a determinate form; 2. Capillary of an indetermi- nate form; 3. In crystals perfectly regular in one part of their Iengths, and fibrous at their extremity; 4. Amianthiform, consisting of very delicate flexible fibres; and 5. Hematitiform, not unlike the tin ore known by the name Wood-tin,

4. Arseniate of copper in the form of a triedral prism.-As the crystals of this species are seldom sufficiently insulated to be distinctly perceivable, and are in general so small as to escape the naked eye, all their various forms are here more particularly described, and also their progressive change of figure as they deviate from thcir primitive configuration. Its specific gravity is 42-809 Its hardness is not so great as that of the preceding species, as it does not easily scratch calcareous spar. Its crystals are often transparent, and of a very beautiful blueish-green colour; but their surface easily decomposes, and the crystals then become black and perfectly opaque

The second class, being the arseniates of iron, is distinguished into two species:—

1. The arseniates of iron, properly so called.-This ore crystallizes in perfect cubcs, sometimes, though rarely, a little flattened. Its

Two of the six sides have so great a lustre that they