288 Mr. R. Phillips on the Crystalline Forms 



other memoir which will soon appear, I shall describe another 

 crystalline form of sulphate of nickel and of sulphate of zinc, 

 which is entirely differentfrom that which I have now described; 

 the production of these different forms depends upon the tem- 

 perature at which the crystals are formed. The seleniate of zinc, 

 which at a temperature of 50° Fahr. gives prismatic crystals, 

 changes its form when the prismatic crystals are exposed on 

 paper to the heat of the sun. This phenomenon may be also 

 extremely well observed in sulphate of nickel. A temperature 

 of 59° of Fahr., still produces prismatic crystals. If these 

 crystals, of a certain size, be exposed to the sun in a close 

 vessel, it frequently happens that they retain their external 

 form, so that the angles at which the planes meet may be mea- 

 sured ; but if they be broken, they are found to consist of 

 crystals frequently several lines in length, which are octahe- 

 drons with a square base, the angles of which I have been 

 able to measure. This change requires two or three days. 



"I have determined by a very complete analysis the quantity 

 of water contained in this compound. The octahedrons with 

 a square base, into which the prismatic crystals were con- 

 verted, by several days' exposure to the sun in an uncovered 

 [covered?] vessel, gave me 30*14 per cent, of sulphuric acid; 

 some other octahedrons with a square base derived from the 

 crystallization of a hot solution yielded 29*88. If we take the 

 mean of these two results, we must admit that the octahedron 

 of sulphate of nickel with a square base contains: 



Sulphuric acid 30*02 



Oxide of nickel 28*13 



Water . 41*85 



100*00 

 " It follows from this phenomenon," concludes M. Mitscher- 

 lich, "as well as from several others which I have before an- 

 nounced, that the isolated particles of matter in solid bodies 

 are moveable with respect to each other, and that they may 

 assume different relative positions to those which they origi- 

 nally had, without its being necessary to render the bodies 

 fluid. 



Now without asserting it to be the case, I do most certainly 

 think that Mons. M. has attributed the difference in the form of 

 these crystals to a wrong cause ; and at any rate I am quite 

 sure that the crystals of sulphate of nickel of both kinds may 

 be procured at pleasure, and totally independently of the tem- 

 perature at which the crystallization occurs. 



I have already stated that M. Mitscherlich considers as an 

 octahedron that which Mr. Brooke regards as a square prism ; 



either 



