Where found.

In the com. pact state.

when it is recent. The crystals are difficult to determine," both on account of the great number of faces they fréquently exhibit, and of the irregularity of these faces. They consequently require a great deal of attention on the part of the observer, to be perfectly ascertained.

The endellion in a state of completely determined crystallization was first observed in Cornwall, in the mine of Huel-boys, in the parish of Endellion; and from this mine have been obtained the finest groupes of this substance that are seen in collections, where it is in general very rare. The endellion exists likewise in Siberia, where too it appears to be very scarce, as I know of but a single specimen, which is in my possession. I have seen in the shop of Mr. Maw several fragments of this substance, sent with other minerals from Brazil, the biggest of which was a very large single crystal, of the variety represented pl. vii,* fig. 8. Lastly I am indebted to Dr. Wollaston for some small fragments of the same substance, in which the endellion is in the compact state, mixed irregularly and very visibly to the eye with the double yellow sulphuret of copper and iron; and in which are observable little cavities, including very small but well defined crystals of the same substance, intermixed with minute specks of carbonate of lime and sulphate of barytes. These fragments came from Peru. The endelJion, which I mentioned above as coming from Brasil, is in like manner mixed very perceptibly to the eye with yellow sulphuret of copper and iron.

The groupes of crystals of endellion from Cornwall are frequently accompanied with crystals of brown sulphuret of inc, and in several sulphuret of antimony is likewise observed, commonly in fine needles, and frequently even capillary.

In the compact state.

The same pieces, that include crystals of this substance in Cornwall, include also parts more or less large, in which it is in the compact state. When this occurs, its fracture is

The plates to this article are unavoidably deferred to our next number.

thus

thus rendered irregular and granular, and its lustre is much inferior to that of the fracture of the crystals.

This compact variety in Cornwall is frequently mingled with sulphuret of zinc, which may easily lead to mistakes; and which Mr. Hatchett has very judiciously noticed in the analysis he made of this substance.

The compact endellion of Brasil and Peru also is intimately mixed with yellow sulphuret of copper and iron.

Description of the crystalline forms of endellion, and observations respecting them.

The primitive crystal of this substance, as I have already Primitive crys said, is a rectangular tetraedral prism, pl. vii, fig. 1, the tal height or side of which is to the sides of its terminal faces in the ratio of three to five*. I have not yet seen this crystal in its perfect state, that is to say, without the planes of any of the modifications belonging to it; and it cannot be obtained by splitting, the attraction of cohesion, that unites the integrant molecules of this substance, being too strong to be overcome. By means of some of the accidental fractures however, that occasionally exist, I have been able to discover the direction of its lamina, and perceive that this direction, as well as that of the secondary faces, agree perfectly with the results of calculation.

I do not think however, that this prism is at the same Integrant partime the form of the integrant molecule of this substance: ticle. but hitherto nothing has led me to form any particular

opinion with respect to the form of this molecule.

primitive crys

As the crystals of endellion are frequently loaded with Various alterfacets, which the mineralogist may find embarrassing, ations of the have thought it necessary, for the ease of the reader, to give tal. separately, in fig. 4, the various retrogradations† experienced by the lamina of the crystallization, which I have observed

The method I have pursued for the determination of the primitive crystal will be seen hereafter.

+ I give the name of retrogradation [reculement] to that act of crystallization, which has hitherto been known by the name of decrement, an expression that is totally false in many cases, as I have shown in the second volume of my Treatise on Mineralogy, p. 206, in the part-relating to the theory of erystallization.

1

on

Pst modifica. ton.

on the longitudinal edges of the primitive prism; in fig. 5, those I have observed along the edges of the terminal faces; and in fig. 6, those that I have observed at the angles of these faces. These three figures are intended for the same purpose of convenience, as those which, in my former paper on this substance*, were given solely with this view, and the exact models of which had not yet been observed in nature. My experience in crystallography has frequently led me to remark, that, when the crystals of a substance are liable to any considerable number of modifications, and at the same time actually undergo several of them, this method is extremely useful, and frees the mineralogist, who is desirous of ascertaining one of these crystals, from a task not unfre quently very troublesome. There are even substances, in which this method is very advantageous to the most expert crystallographer, and the present is one of them,

If modification. The planes arising from this modifica tion substitute for the longitudinal edges of the primitive prism a plane equally inclined to those contiguous to it, They are produced by the retrogradation of one row of the particles of the laminae along these edges. These new planes are frequently striated, as is shown in fig. 2. Sometimes they cause the complete disappearance of the faces of the primitive crystal, giving rise to another prism, which is Jikewise a rectangular tetraedron with square bases, but secondary to the primitive prism; and in the crystals of this variety that I have seen the faces were constantly striated, as in fig. 3. This variety even led me into a mistake, whea I wrote the first paper on this substance I presented to the Royal Society, by inducing me to consider the planes of the prism of the varieties represented at figs. 10, 11, 15, 16, and 17, as belonging to them: but a more attentive examination, elucidated by observations since made on a great number of other crystals, has taught me, that these striæ were the simple effect of aggregation, and that these same planes belonged to their primitivest.

I shall

*Additional note, and for which I was so unhandsomely reproved by Mr. Smithson, in his critique printed in the Philosophical Transactions.

The striæ, that occur so frequently on the planes of crystals, are

often

I shall not enter into any similar details concerning the other modifications, the number of each of these being affixed in each crystal to the planes belonging to it, and the table of these modifications, which will be annexed to this paper, pointing out in this respect at a single view particu-Jars, which could scarcely be expressed by long circumlocution. In consequence I shall confine myself to the following observations.

All the varieties, from fig. 2 to fig. 20 inclusively, are Different vari met with among the fragments of this substance, that are eties. brought from Cornwall. I have a very fine groupe of those represented at figs. 7 and 8, and a separate crystal of 8 and of 9. The variety fig. 8 is a regular aggregation, in the form of a cross, of two of the crystals fig. 7 elongated parallel to the planes of the 6th modification. It might also, and perhaps more justly, be considered as resulting from five crystals, similar to fig. 7, united by one of their planes. I have likewise crystals of the varieties 11, 12, 14, 15, and 19. With those at figs. 10, 13, 16, 17, and 18, I was furnished by two very fine groupes, and a superb single crystal, in the possession of Mr. R. Phillips. Fig. 16 answers to that numbered 17 in my former paper, which was not quite accurate. In 15 and 16 of the same paper the prism was much too thick, and they are replaced at present by 17 and

18.

fine

I have likewise the varieties from 21 to 26, in a very group, which, with the fragments from Peru and Brasil al

often of very great use to indicate the direction of the laminæ of crystallization, and not seldom are they the only means, that the crystals of a Substance afford. Thus the lenticular rhomboidal carbonate of lime pretty constantly indicates by its stræ the direction of the laminæ, and consequently that of the plants of the primitive crystal. In the hexae dral prism of the same substance, the same striæ point out the direction to be given to the fractures, on which Mr. Haily has established the dimensions of its primitive rhomboid. But we must beware of the illusion, that may sometimes arise from striæ, which are indebted for their existence only to an aggregation of crystals, such frequent instances of which are exhibited by the tourmalin, thallite, sulphuret of antimony, &c.: an illusion by which I have shown I was at first misled myself with respect to the endellion, after having observed, that among its varieties there existed a rectangular tetraedral prism, the planes of which are frequently striated.

ready

ready mentioned, constitutes the only specimens I have yet seen from any place except Cornwall. The groupe, which assuredly is not English, was given to me as coming from Siberia. Its gangue is an irregularly crystallized quartz, part of which is of a dark blackish gray, in consequence of a mixture of very minute particles of sulphuret of lead, imperceptible to the naked eye. The crystals of endellion are covered by a slight stratum of green carbonate of copper; and some small crystals of common dodecaedral pyramidal carbonate of lime (métastatique of Haüy) are disseminated among them as well as in the quartz. Several small parcels of sulphuret of lead and blue copper are likewise observable on it.

Such is the result, which the most careful examination, and continual attention to every thing, that could render me better acquainted with this scarce and interesting substance, enables me at present to lay before the Royal Society. I am far however from imagining, that I have seen every thing Probably other pertaining to its crystallization. Undoubtedly other varie

varieties.

ties, and other modifications, may exist; and it is probable, that, among the small number of specimens of it in different collections, such may be found. Froin the numerous varieties, that exist in a single groupe of this substance, its primitive crystal appears to have a great tendency to be modified but the modifications of this crystal, which I have given, are unquestionably sufficient, to render it easy to ascertain any new ones, if they should occur. These reflexions are not introduced here without reason. Among the different specimens of this substance examined by me, I have seen several crystals belonging to some of the varieties I have given, on which there existed likewise slight traces of planes belonging to other modifications, but which it was altogether impossible for me to determine. As an example of this I shall mention the crystal represented at Pl. VIII, fig. 27, not only because it is one of the most strik ing for elegance of form, but because it is in my own possession. The faces indicated by the letters x, y, and z, are certainly owing to an intermediate retrogradation at the angles of the terminal faces: but the impossibility of measuring with precision the augle of inclination between these planes

and