276 Mr. Brooke on some undescribed Minerals. 



other Vesuvian substances, from Dr. Somerville, from which cir- 

 cumstance I have named it Somervillitc. 



Its primary form is a right square prism, but the crystals are 

 modified on the solid angles and lateral edges, as in the annexed 

 figure. 



P on a . . 147° 5' 



P on M . . 90 



Mon e . . 161 33 



Mond . . 135 



M on M' . . 90 



Assuming the planes a to result from a decrement by one row of 

 molecules, the terminal edge of the primary form would be to the 

 lateral edge, as 16 to 25 nearly. The planes e result from a decre- 

 ment by three rows in breadth on the lateral edges. 



The crystals may be cleaved easily parallel to the terminal 

 planes, but imperfectly, if at all, parallel to the lateral planes or 

 to the diagonals of the prism. Their colour is a very pale dull 

 yellow. 



The substance for which this might at first view be mistaken is 

 the idocruse, although no plane corresponding in its inclination on 

 P with the plane a of the preceding figure, has yet been observed 

 on any crystal of that substance. But these crystals are much 

 softer than idocrase, the cleavage parallel to the terminal planes 

 much more distinct, and the cross fracture more glassy. 



They occur in cavities, with crystallized black mica, and with 

 'another substance which I have not yet examined ; and the mass 

 to which they adhere appears to be nearly all Somervillite, inter- 

 mingled with black mica. 



Mr. Children has taken the trouble to compare the characters 

 of this mineral under the blow-pipe, with those of idocrase. 



When exposed alone in the forceps it slightly decrepitates, which 

 idocrase does not, r.nd fuses, with greater difficulty than idocrase, 

 into a greyish glass, the globule from idbcrase being greenish. 

 With borax, in the reducing flame, idocrase produces a light 

 green, and this a colourless glass. 



