In the first appendix to his “ Traité de Minéralogie ” that celebrated mineralogist, who was then disposed to consider it as a variety of the mesotype, states that he was induced to regard its primitive crystal as a rectangular tetrahedral prism, having indications of subdivision in the direction of its two diagonals; but since in his subsequent work, entitled, “ Tableau comparatif des résultats de la Crystallographie et de l'Analyse chimique,” &c. p. 49. he has changed his opinion, and has declared its primitive crystal to be a rectangular octahedron, having its faces unequally inclined. The observations, which I have myself made on this substance, prevent my assenting to either of these two forms as its primitive crystal; and they have enabled me at the same time to present to the Geological Society a more complete examination and description of it. The possibility of my doing so I owe to the friendship of M. Gillet de Laumont, who lately sent me several specimens, among which was one of very considerable magnitude. On its way to me it was broken. Chance, which very frequently assists the observer when he is prepared to profit by it, produced by that accident what I should certainly never have attempted myself: it furnished me with an immense number of fragments, and of perfectly regular crystals, and at the same time by exposing to view the central part of the mass, which had never yet been acted upon by the atmosphere, and had therefore been preserved entirely unaltered, it enabled me to examine this substance before it had undergone any kind of change, a circumstance which must of necessity be but of very rare occurrence.
