•4-58 Notices respecting Kew Booh. 



appearance of the primitive ones. The edges forrocd by 

 the union of the pyramids are very often replaced by narrow 

 planes, forming a short prism : hut the crystals more 

 usually become prismatic by the elongation of two opposite 

 faces on each pyramid, rendering the dodecahedron cunei- 

 Ibrni ; and the crystal then appears to be a long rhomboidal 

 tetrahedral prism of about 66^ and 114", wii^h itlrahedral 

 pyramids, the prism being formed by the four elongated 

 trapezoidal faces, and each of the |iyramid3 by four of the 

 faces of the dodecahedron which remain triangular. Some- 

 times two of the edges of the prism are replaced each by a 

 plane, and it then becomes liexahedral ; and when ihe 

 same occurrence takes place with the four edges, it is ren- 

 dered octohedral; frequently alio two edges of the pyramid 

 are replaced each by a plane, siivinc; them six instead of 

 four planes. These are not the -mlv forms which this salt 

 assumes ; but it would be difficult to give an intelligible 

 description of the rc>t without lia'ur'_'S. 



" The quantity of sulphat of barytes which 100 grains 

 of this salt yield omhe addition of muriat of barytes, is er- 

 roneously stated by Dr. Thomson to be 128 grains. I ob- 

 tained a preci|ntate which, dried by a red heat, weighed 

 136'7 grs. beinL^ only 1*45 grain more than the proportion 

 mentioned by Kirwan ; indicating about 3 2' 8 of sulphuric 

 acid, if with Dr. Thomson we allow 100 parts of sulphat 

 of barytes to contain 24 of sulphuric acid. This salt then 

 consists of 32*8 sulphuric acid with 67*2 potash and water, 

 instead of 31 sulphuric acid with G9 potash and water, as 

 stated by Dr. Thomson." 



The College has introduced the supcrsulphat of potash 

 into their materia medica; but their directions for preparing 

 it areas usual very defective, and Mr, ]\naiurally supposes 

 that *' neither the College nor its Committee have ever ex- 

 amined the true supcrsulphat of potash." The author re- 

 ceived " some extremely well defined crystals of this salt 

 from Messrs. Howard, of Stratford ; their form was di- 

 stincdy rhomboidal ; but as quartz sometimes occurs of this 

 figure, although, as well as sulphat of|)otash, it is more 

 usually dodecahedral, he supposed it possible that the rhom- 

 boid might in this case he derived from the dodecahedron, 

 and consequently that these crystals might be connnon sul- 

 phat of potash." Count de Bournon however examined 

 their crystalline form ; and on comparing it with that of the 

 sulphat above described, it appeared that " the rhomboid 

 differed so materially from that which would be derived 

 from the dodecahedron of sulphat of potash, that those 



salts 



