352 Mr. William Phillips on the OxydofTm, 



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 Abbe Haiiy'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 octohedron, of which the 

 faces answer to o o, PI. Ixxx, fig. 179 and 180 of his former work, 

 or, which is the same thing, to those of P P, fig. 26, PI. 16 of the 

 series attached to this paper. He says further, that the joints which 

 gave this octohedron are extemely sensible on exposing fractures of 

 tin to a vivid light ; and again, that he has been led to the adoption 

 of this octohedron as the primitive form by the results of me- 

 chanical division. 



What the circumstances in the mechanical division of the crys- 

 tals 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 me- 

 chanical 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 Haiiy 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 



