and Composition of the Sulphates of Nickel. 269 



either of which forms, from the relation which they bear to 

 each other, may be assumed as the fundamental crystal : 

 while he has not given at all the precise form of the prismatic 

 variety, arid which Mr. Brooke in a paper already quoted, has 

 described as a rhombic prism so nearly approaching that of 

 sulphate of zinc, that he is inclined to doubt whether there is 

 any real difference between them. 



In this memoir the difference between the crystalline forms 

 is clearly traced by Mr. Brooke ; and I have endeavoured to 

 prove that the difference is dependent not upon the propor- 

 tion of water, as Mons. M. appears to suppose, but owing to 

 one of the crystals containing more sulphuric acid than the 

 other, although not in atomic proportion. 



The analyses which I have given in the Annals of Philosophy, 

 N.S. vol. vi. p. 439, show that the quantity of acid in 100 of 

 the square, is to that in the rhombic prisms as 30 to 28*16, — 

 a difference of nearly 2 per cent. ; and the quantities of water 

 are respectively 45'54 and 43*8, a difference of almost If per 

 cent, instead of nearly 3, as stated by M. Mitscherlich. 



That excess of acid without any variation of temperature is 

 capable of producing variation of form, is proved by the fol- 

 lowing experiment : I dissolved 200 grs. of rhombic prisms in 

 water, and added to the solution about half its weight of sul- 

 phuric acid, and put the solution to crystallize in a room, the 

 temperature of which varied from about 60° to 64°. The cry- 

 stals first obtained were similar to those dissolved, viz. rhom- 

 bic prisms ; afterwards I procured a mixture of rhombic and 

 square prisms ; and lastly, square prisms only, and this with- 

 out any greater variation in the temperature than that which 

 I have already noticed. 



From this experiment it is evident, that owing to the for- 

 mation of rhombic crystals in the first instance without excess 

 of acid, the relative proportion of sulphuric acid to the oxide 

 of nickel was subsequently so much increased, that square 

 prisms were formed, which from the analysis already stated 

 contain a larger proportion. of acid. 



In corroboration of the inference that the difference of form 

 is dependent upon that of the quantity of acid, I shall merely 

 add that a solution of 200 grs. of rhombic prisms, to which 

 no sulphuric acid was added, and crystallized in the same 

 room already mentioned, yielded merely rhombic without any 

 admixture of the square prisms. 



To the foregoing statements I may add, that rhombic cry- 

 stals of sulphate of nickel, when exposed to the air, effloresce, 

 which is not the case with the octahedral variety; and it ap- 

 pears to me probable, that when rhombic prisms which have 

 'New Series. Vol.4. No. 22. Oct. 1828. 2P been 



