lis Mr. William Phillips on the Oxyd of Urantulif, 



crystals from Tol Carn mine are generally of great length and-. 

 remarkable transparency, those from Tin Croft are more generally 

 opake ; but the long and slender crystals from the former mine 

 i-arely suffer interruption from each other, being generally de- 

 posited at right angles, and thereby shewing a constant tendency 

 to assume a quadrilateral figure. 



There are in my possession about 55 specimens of the oxyd of 

 uranium from the various mines in Cornwall above cited, and up- 

 wards of 200 detached portions, each having one or more well 

 defined crystals on them and placed on pieces of wax ; from these 

 the accompanying drawings were made. PI. 5, 6, and 7. 



Varieties of the Fri?nitive Crystal. 

 It has been already noticed that the primitive crystal of the 

 oxyd of uranium is considered to be a tetraedral prism, with 

 square bases. I have never observed any crystal, exhibiting the 

 primitive planes only, of a greater elevation than that described 

 by fig. 1. Fig. 4 represents an elongated crystal : this elongation 

 i€ so considerable on several crystals from Tol Carn mine that 

 the length exceeds the breadth many times. 



First Modification* 

 The first modification is the result of a decrease along the lateral 

 edges of the primitive prism, by which each is replaced by a 

 quadrangular plane.* The crystals represented by figs. 6, 7, and 

 8, are the most common of the whole series. 



* The Count de Bouriion in his " Catalogue" has described a modification which I 

 have not been fortunate enough to discover on any crystal from Cornwall, whence every 

 crystal delineated in tlie annexed series was brought. The planes of this modification he 

 describes as being in eombluation with that above quoted as the first modification, aud aa 

 replacing the lateral edges of that plane by quadrangular planes. 



