Mr. William Phillips on the Oxyd of Tin, 359 



considerable size, almost covered with well defined crystals repre- 

 sented by figs. 21. and 25. 



Neither the planes of this, nor of any other modification have 

 I believe been found in simple combination with those of the 

 primitive crystal. They are thus given preceding the series of each 

 modification, in the hope of thereby rendering each the more 

 intelligible ; to this I have generally added a figure representing 

 their combination with the planes of the secondary octohedrons, 

 being those of the second modification, because the planes of that 

 modification form the pyramid most commonly found on the 

 crystals of this substance. 



Second IS/LodiJicat'ion. 



Each of the four solid angles formed by the meeting of the two 

 pyramids of the primitive form base to base, is by this modification 

 replaced by two triangular planes ; each plane being placed on an 

 edge of the pyramid, but inclining on the axis passing through the 

 solid angles, fig. 22. PI. 16. 



This modification is represented in an advanced state, by the 

 dotted lines of fig. 23. shewing by the lines within it, that when 

 complete, it produces a secondary pyramid considerably more 

 acute than that of the primitive form. The secondary pyramid 

 produced by this modification is that commonly observed on the 

 crystals of this substance, and by fig. 24. is represented within the 

 dotted lines of the primitive form. 



Fig. 25. shews the first and second modifications in combination 

 with the planes of the primicive crystal. The lines on the faces of 

 this figure denote the direction in which the strise are sometimes 

 to be observed on the crystals. 



