and 12, and several others which are not delineated, would have induced the suspicion that the primary form is not a perfect geometrical solid, if some of the rhomboidal dodecahedrons had not been sufficiently brilliant to shew, by the reflecting goniometer, that the adjoining planes meet at an angle of 120 degrees.

It may be said that these crystals are accidental, and ought not, therefore, to be figured in the series of the forms which this substance assumes. To this it may be replied, that some specimens consist only of crystals in these forms, and that if they were omitted, and if only such varieties were given as are perfectly symmetrical, it might be supposed to exist only in the latter; while such specimens as consist only of irregular crystals, might be supposed to belong to some non-descript species. I possess a specimen, of which the crystals belong to the variety described by figure 9, and which are not less in size than the figure itself. In general, however, the crystals vary in size, from the extremely minute to one-fourth of an inch in diameter.

The irregular crystals of this substance, and indeed of many other substances, of which the primary forms are regular geometrical solids, induce the conclusion that the giving to every crystal a descriptive designation, a plan adopted by the celebrated Abbé Haüy, would prove an endless and almost impossible, not to say, an useless, task. It cannot be denied that now and then these designations are intelligible in description; but if the plan be pursued to its extent, about six hundred names will be required for the crystals of the carbonate of lime alone. The most familiar, and upon the whole, the most intelligible, mode, would be to describe a crystal as possessing the planes of the first, third and fifth modifications, &c.; but even for this more simple method, crystallography is not yet far enough advanced in that department which consists in the delineation of all the known forms; until this be done, we cannot hope for a system built upon a generalization of the whole.

This substance usually occurs deposited upon other ores of copper, chiefly the iridescent and botryoidal varicties of copperpyrites; it is sometimes accompanied by black copper, vitreous copper, and buntkupfererz.

It has been found in the veins of Dolcoath, Cook's Kitchen and Tin Croft copper mines near Redruth, and in Huel Virgin, Huel Unity and Huel Jewel, near St. Die. The specimen which has already been alluded to, as occurring in very large crystals, was raised about forty years ago, and is in the catalogue of my collection 'supposed to be from Treledra near Padstow;' the crystals are very thinly coated by copper-pyrites.

If, however, only one character of this substance were consulted, namely, its crystalline form, it might be assumed to be a variety of fahlerz; for the three planes which are found replacing each solid angle of the tetrahedral crystals of the latter, (Haüy, Pl. 70, o on o, Fig. 81.) meet under an angle of 120°, being precisely the same as that afforded by any two adjoining planes of the rhomboidal dodecahedron, in which form this mineral is found; but the superior hardness of this substance, as well as its fracture and specific gravity, and the effects resulting from the action of the blow-pipe, induce the conclusion that it ought not to be considered as a variety of Fahlerz; nor have any of the mines in which the mineral now under consideration have been found, yielded any tretrahedral crystals. It is to analysis that we must look to decide upon the difference between these minerals.

It is, however, extremely difficult to determine what the essential elements of the fahlerz are. A specimen analyzed by Chenevix, afforded 52 of copper, 23 of iron, and 14 of sulphur; and this being the most simple of the numerous analyses that have been made, (see Haüy, Tableau, p. 254), is almost sufficient to decide its essential elements to be copper, iron, and sulphur. A specimen from Airthrie in Scotland, analyzed by Thomson, afforded 19.2 of copper, 51 of iron, 15.1 of arsenic, and 14.1 of sulphur; while the several analyses of Klaproth shew, that it is sometimes composed in part of lead, antimony, or silver, occasionally even in considerable proportion, and in one instance of five per cent. of zinc. By all the analyses (ten in number) of this celebrated chemist, copper, iron, and sulphur appear to be the only invariable constituents; which tends to strengthen the conclusion that they are its essential elements; and that the arsenic, lead, antimony, silver, and zinc, although the

three first have been found to exceed in quantity the copper and iron, are only accidental ingredients; and therefore, since they do not affect the crystalline forms of the fahlerz, which are invariably tetrahedral, it may be inferred, that they are only mechanically mixed, not chemically combined with the copper, iron, and sulphur.

ART. XII. Analysis of the Copper Ore, described in the preceding Paper. By Richard Phillips, F.R.S.E. F.L.S., &c.

I. HAVING ascertained that the constituents of this ore are copper, iron, arsenic, and sulphur, I boiled 100 grains of it reduced to powder in nitric acid, until the whole of the metallic matter appeared to be dissolved. Fourteen grains remained unacted upon by the acid; of these a large portion was evidently pure sulphur; by heat nine grains were volatilized, and five remained, which were merely silica, that had been mechanically mixed with the ore.

II. The nitric solution was decomposed by potash, and being heated with excess of it, peroxide of copper and iron were precipitated together. This mixed precipitate was washed until it ceased to be alcaline, and was then dissolved in nitric acid. To the solution ammonia in excess was added; by this, peroxide of iron was precipitated, and the peroxide of copper held in solution; the former being separated, washed and ignited, weighed 13.3 grains, equivalent to 9.26 of iron.

III. The ammoniacal solution of copper was heated, and when the greater part of the ammonia was expelled, potash was added to the solution; and, by continuing the heat, peroxide of copper was precipitated, which being washed and ignited, weighed 56.6 grains, equivalent to 45.32 of copper.

IV. The alcaline solution obtained in III, and the water employed to wash the mixed precipitate of oxide of copper and iron, were evaporated together, and then saturated with nitric acid. This solution contained the sulphur and arsenic converted into acids, and combined with potash. Nitrate of barytes being added, sulphate 「was precipitated, which, being washed and ignited, weighed 126 grains, equal, according to Dr. Wollaston's scale, to 17.14 of sulphur, which, added to 9, before obtained, 26.14. After this an accident happened to the solution, which prevented the separation of the arsenic acid; therefore,

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V. One hundred grains of the ore were again treated with nitric acid; with the silica 11 grains of sulphur were obtained, and the nitric solution was decomposed by excess of potash as before, in order to separate the oxide of copper and iron.

VI. The alcaline solution being saturated with nitric acid nitrate of barytes was added to it, as long as precipitation took place. The precipitated sulphate of barytes being washed and ignited, weighed 150 grains, 20.4 sulphur, which, added to 11, separated without acidifying, 31.4; the mean quantity of this and the first experiment being 28.74.

VII. To the solution from which the sulphuric acid had been separated by nitrate of barytes, nitrate of lead was added as long as arseniate of lead was thrown down; and this, when washed and ignited, weighed 53 grains.

According to Dr. Thomson, 21.25 of arseniate of lead contain 7.5 of arsenic acid, equivalent to 4.75 of arsenic; if then, 21.25 give 4.75; 53 of arseniate of lead will indicate 11.84 of arsenic. It appears from these experiments, that this ore consists of nearly

In performing this analysis, some circumstances occurred which I think worthy of notice. In a preliminary experiment, I endeavoured to separate the copper from the iron by means of ammonia, without previously separating the arsenic acid; this I found impracticable, for it appeared that the arseniate of iron, at first precipitated, was eventually dissolved by the ammonia. In some treatises on chemistry, the arseniate of barytes is described as an insoluble salt: this, as may be deduced from