116 Mr. William Phillips on the Oxyd of Uranium, 



to ;p resent the appearance of distinct tabular depositions progressively 

 altering in size ; and these are easily separated in the direction of the 

 striae, 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 striae 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 

 Abbe Haiiy, 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. 1 have very satisfactorily obtained cleavage* 

 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 pri- 

 mitive 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 tetraedral prism wdth square bases much 

 less in height than that assigned to it by the Abbe Haiiy. It is 

 not in my power to offer any thing decisive of this question, for 



