Mr. William Phillips ojt the OxydofTin, 361 



base to base, by which each is replaced by a plane, perpendicular to 

 the axis passing through those edges, fig. 34. PI. 17. 



The planes of the primitive crystal are rarely found in combination 

 with those of this modification, except when they seem only to be 

 the result of a decrease on the edges of the secondary pyramid as in 

 fig. 39. Fig. 35. shews the combination of the planes of this modi- 

 fication with those of the secondary pyramid, which is thus given, 

 because, as the secondary pyramid is that commonly observed on the 

 crystals of this substance, it seems to facilitate the tracing of the 

 various combinations in the succeeding figures. 



Fifth Modificatiotu 



The fifth modification arises from a decrease on each of the solid 

 angles caused by the meeting of the two pyramids base to base, by 

 which each is 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 lateral solid angles, fig. 45. PI. 17. 



The planes of the primitive crystal are also shewn by the dotted 

 lines of fig. 46. together with the planes of this modiHcation in a 

 more advanced state. The latter are also exhibited in combination 

 with those of the secondary pyramid, by the lines within that 

 figure. 



It will be noticed how nearly the crystals given by fig. 55, 

 approach the cube, and that of fig. 47. the secondary octohedron. 

 The crystal represented by the latter figure does not exceed in size 

 the head of a small pin, but all its planes are remarkably brilliant 

 and well defined. 



Vol. II. 2 z 



