Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/124

 to present the appearance of distinct tabular depositions progressively altering in size; and these are easily separated in the direction of the striæ, by that means discovering occasionally a thin ferruginous deposit between them, but not extending to the edges of the tables. Those crystals which are so small as to merit the term minute, though of a thickness nearly equal to their breadth, particularly those of a deep green colour, rarely exhibit the horizontal striæ on their lateral planes, which are perfectly brilliant: yet I have not been able to find one that could satisfactorily be submitted to the reflecting goniometer. On the measurement of the angles of the oxyd of uranium, it is therefore impossible for me to offer any thing.

The ease with which the crystals of this substance are separated parallel with their terminal faces, was long since noticed by the Abbé Haüy, who adds that its other cleavages can only be perceived by the assistance of a vivid light. The Comte de Bournon who acknowledges much attention to this substance, says (Cat. p. 340) that by the assistance of a strong light, he could perceive indications of cleavages in the directions of both the diagonals of a tetrahedral prism (Fig. 2 or 3), which by both these scientific mineralogists is adopted as the primitive form of the oxyd of uranium. I have very satisfactorily obtained cleavages parallel with the lateral planes of that solid.

The authors above cited do not however agree in regard to the height of the prism which both have assigned as the primitive form of this substance. The former of them has supposed that the height is to the breadth, as 16 to 5, while the Comte de Bournon considers that there is some, though not conclusive evidence for adopting a tetrahedral prism with square bases much less in height than that assigned to it by the Abbé Haüy. It is not in my power to offer any thing decisive of this question, for