but not squares. From this table we perceive, that the angle of 129° 56′, arising from the retrogradation by a single row, belongs to the incidence of the plane produced on the narrowest sides of the prism ; while that of 128° 31′, which is next to it in the preceding table, belongs, on the contrary, to that of the incidence on the broadest side. We also see that the plane of substitution that makes an angle of 135° with the two adjacent sides of the prism, and which would be the product of a retrogradation by a single row, on the supposition of the bases being a square, may likewise exist, or at least with a very trifling difference, on the supposition of the Abbé Haüy. The fracture alone, and particularly that which the crystals themselves exhibit (for I could never obtain one sufficiently regular by cleavage), as well as its striking parallelism with the interior indications of natural joints, and the angle of 135° constantly formed by these fractures, appear to me most in favour of my opinion.
I ought however to add, that the peculiarity exhibited by the crystals of this substance, of undergoing a retrogradation only on one side of the edges of the primitive prism, would seem to support the view taken of it by the Abbé Haüy; though much less so than if a retrogradation took place on each side, and the retro gradations were of a different kind. On the whole, I think there is a majority of circumstances in favour of the opinion which I have formed, but on this point the crystallographical reader will decide.
