are replaced by three triangular planes as shown in fig. 7. I have also others which have the edges replaced, and in which each of the angles is also replaced by both modifications (fig. 8.) The crystals above described vary in colour from light and almost transparent green, to dark green, sometimes having a brownish tinge: others are of a resinous appearance and are almost transparent. It may be remarked that the crystals of this substance are generally more or less, though not regularly striated on the surface, and that the striæ constantly take the directions described in fig. 2: they do not however admit of a fracture in that direction, nor have I satisfactorily obtained it in the direction of the faces of the cube. Some crystals of a dull green colour, on being broken, have been found to enclose other cubes of a darker colour, and iridescent on the surface. I have to regret the impossibility of giving the admeasurements of the various angles formed by the modifications and varieties of the crystal of the arseniate of iron, on account of their minuteness.
Description of the red Oxyd of Copper.
This substance, which during the last ten years, has been found in Cornwall in great abundance, was previously of very sparing occurrence in that county. I am not aware of its having been mentioned as a production of that district, by any foreign mineralogist, until very lately: even the celebrated Haüy has not quoted it in his Traité de Minéralogie as a Cornish mineral. Many cabinets however now abound with it. Previously to the labours of Mr. Chenevix on this substance its composition was by no means well understood. From the analysis by that able chemist it appears to contain copper 88,5, oxygen 11,5, and is therefore a sub-oxide of copper, and "exists in a state hitherto unknown in nature." In reference, doubt-
