362 Mr. William Phillips on the Qxyd of 'Tin, 



Sixth Modification. 



This modification, like the preceding, consists in a decrease on each 

 of the solid angles, caused by the meeting of the two pyramids of 

 the primitive form base to base ; by which each is also replaced by 

 two triangular planes placed on the edges formed by the meeting 

 of the two pyramids, but inclining on the axis passing through 

 the angles, more than those of the fifth modification, fig. 5S. 

 PI. 18. The planes of this modification are shewn in combination 

 with those of the primitive form by the dotted lines of fig. 59. and 

 with those of the secondary octohcdron by the lines within it. 



Seventh Modification. 



This modification consists in a decrease on each of the four 

 solid angles caused by the meeting of the two pyramids of the 

 primitive form, by which each is replaced by four triangular planes, 

 placed on the faces of the primitive form, but inclining on the axis 

 passing through the lateral solid angles, fig. Q^. PI. 18. 



By fig. 64. the planes of this modification are shewn in a more 

 advanced state, which renders the first figure in the series, that 

 of QiQ^ perfectly intelligible. Fig. QS. shews them in combination 

 with the secondary pyramid, and will, without pursuing the 

 series from fig. QQ. to fig. 70. cause the latter figure to be at once 

 understood. 



I have not satisfactorily obtained the incidence of the planes 

 of the primitive form with those of this modification. 



The principal part of the crystals delineated in the series of this 

 modification, in which the prism is long and the facet of the 7th 

 modification is small, are from Relistian tin mine, and are about a 

 line in thickness. I have not seen them from any other mine. 



