on the crystals of the oxyd of tin, joined base to base, being those of 2, 2. fig. 27. Pl. 16.
In 1809, a new work of the Abbé Haüy's made its appearance, entitled “ Tableau Comparatif, &c.” in which he says (p. 285) that a revision of his researches on the subject of the oxyd of tin, in consequence of his having obtained some crystals from Cornwall, proved to him that the true primitive form is, not as he formerly supposed, the cube, but a rectangular octahedron, of which the faces answer to o o, Pl. lxxx, fig. 179 and 180 of his former work, or, which is the same thing, to those of P P, fig. 26. Pl. 16 of the series attached to this paper. He says further, that the joints which gave this octahedron are extremely sensible on exposing fractures of tin to a vivid light; and again, that he has been led to the adoption of this octahedron as the primitive form by the results of mechanical division.
What the circumstances in the mechanical division of the crystals of this substance leading to this result were, have not been explained, but having been unexpectedly led to the same conclusion by the cleavages I have obtained, I shall proceed to describe them.
While preparing this paper, with a view of presenting it to the notice of the Geological Society, and while an attempt at the mechanical division of the crystals of the oxyd of tin was on my list of agenda, Dr. Wollaston informed me that he had succeeded in obtaining it, in a direction parallel with the faces of the prism, and I have since had the same success in numerous instances, so as to procure on the planes of the fracture an incidence of 90° by the reflecting goniometer.
Thus is the conjecture of Haüy before cited, that he perceived the natural joints parallel with the planes of the prisms verified. I have also obtained numerous cleavages parallel with the diagonal of the