438 On the CrptaUine Form$ of Artificial Salts, [Dec. 



of the quantities he was about to examine, and he found that the 

 rhombic prisms had lost one atom of water, while the square 

 prisms experienced no loss. As the square prisms formed in Mr. 

 Phillips's solution were not deposited until that had been much 

 reduced by evaporation, it appeared probable that an excess of 

 acid might be necessary to their production. Mr. Cooper, 

 therefore, dissolved some of the rhombic prisms in dilute sulphu- 

 ric acid, and from this solution square prisms were obtained. 

 Thus it was ascertained that either the square or the rhombic 

 prisms might be produced at pleasure, by crystallizing the salt 

 Irom a solution in dilute sulphuric acid, or in water. 



It appears from the analyses of the two sets of crystals, that 

 between 1^ and 2 per cent, of the water of the rhombic prisms 

 has been replaced by sulphuric acid in the square ones. But as 

 this difference does not constitute any atomic disparity of com- 

 position in the two forms, we may probably ascribe their differ- 

 ence to some cause analogous to that which has impressed on 

 arragonite a crystalline form distinct from that of common car- 

 bonate of hme. 



Sulphate of Nickel in Rhombic Prisms, 



The form and measurements of this salt approach so very 

 nearly to those of sulphate oj' zinc, that I am inclined to doubt of 

 there being any real difference between them. If there be any, 

 it will not exceed 2' or 3' in the inclination of M on M', which, 

 in many of the crystals of this salt approaches nearer to 91° 10' 

 than to 91° 7'. We may, therefore, refer to the measurements 



fiven above for the angles of these crystals. But there is a 

 ifference in the cleavages of the two salts, for this may be 

 cleaved easily parallel to its lateral primary planes. 



Sulphate of Nickel in Square Pi-isms, 



This is the form of the crystals of this salt alluded to by Dr. 

 Wollaston in a paper which appeared in the Annals of Philoso- 

 phy, vol. 11, p. 286, but without any mea- 

 surements. The ciystals may be cleaved 

 parallel to the planes P, M, and M', of the /^c^i 



accompanying figure, which are its primary rf^Lj/az 



planes. 



P on M, or M'. 90° 0' 



fl, 126 24 'VvV//^4 



a^ no 40 • VA 



c 117 37 



MonM' 90 



Sulphate of Nickel and Potash, 

 This salt was first given to me by a friend as sulphate of nickel, 



