Mr. William Phillips on the Oxyd of Uranium. 117 



as it depends, in the first place on the precise admeasurements of 

 certain angles, which no crystal I have hitherto seen has enabled 

 me to make, and in the second place, on calculations founded 

 thereon, for which I possess not the requisite qualifications, I am 

 compelled simply to state the opinions of these crystallographers, 

 and shall be amply gratified if, in their estimation, what is now 

 offered in regard to cleavages not before obtained, and modifi- 

 cations not before noticed, shall tend to throw any light on this 

 important part of the subject. 



■ The occasional notice of what appear to be perfect cubes, added 

 to the certainty that cleavages are practicable parallel with every 

 plane of that solid, at first tempted me to presume that it ought to 

 be considered as the primitive form ; but the consideration of some 

 circumstances connected with the crystallization of this substance, 

 induced me wholly to abandon the idea. The inspection of the 

 annexed drawings will evince that, of the five modifications 

 described,. the Frst alone is compatible with the cube.* 



All the crystals represented in the accompanying series which 

 are so remarkably flat as to have the appearance of mere laminse, 

 are from Tol Carn mine, which yielded only such as are of that 

 description, while on the other hand, the greater part of those from 

 Tin Croft mine nearly approach in height the dimension of the 

 cube, or exceed it. A considerable proportion of such as are 

 rather less than that dimension exhibit no striae on the lateral 

 planes, which are very splendent : many of those which are deeply- 

 striated on these planes are iridescent on the surface. The 



* In reply to this observation it may be said that the plane which constitutes the third 

 modification of the annexed series, also occurs on cubes of fluate of lime and sulphuret of 

 iron ; but on these two substances, which are remarkable for the beauty and elegance of 

 their crystalline forms, that plane only appears in combination with two other similar 

 planes, placed on the edges, and replacing the solid angle of the cube. 



