Mr Haidinger on Dimorphism. 301 



Art. XXII.— Notice regarding Professor MitscherlicKs 

 Observations on the Dimorphism of Hydrous Sulphate of 

 Zinc, and Hydrous Sulphate of Magnesia. By "W illi am 

 Haidinger, Esq. F. R. S. E. Communicated by the Au- 

 thor. 



Dcjring my stay at Freiberg, I had prepared solutions of 

 sulphate of zinc, and of sulphate of magnesia, to examine and 

 measure crystals newly obtained of the hydrous salts. When 

 the solution of the sulphate of zinc was quite concentrated, 

 and the temperature of the stove rather high, on which I had 

 placed the solutions for producing a slow diminution of heat, 

 I likewise obtained crystals. These crystals, however, bore 

 no resemblance to the ordinary ones produced at lower tem- 

 peratures, but their forms belonged to the hemi-prismatic sys- 

 tem, somewhat like borax, and their degrees of transparency 

 were very low. The sulphate of magnesia, which, on ac- 

 count of the isomorphism of magnium and zinc, I examined 

 under the same circumstances, gave the same result. These 

 facts, isolated as they were, I mentioned to Professor Mits- 

 cherlich while at Edinburgh, in 1824. He found that sul- 

 phate of nickel, whose second form, a pyramidal one, had been 

 described before, did not yield a hemi-prismatic species, when 

 exposed in the same manner to a higher temperature. Some 

 time afterwards, when he was examining the changes produ- 

 ced by heat in the double refraction of crystallized bodies, he 

 observed that it remained unaltered in the hydrous sulphate 

 of magnesia, till at once the whole crystal, which was heated 

 in oil, became opaque. On being broken, it showed the struc- 

 ture of a pseudomorphous crystal, consisting of a number of 

 individuals, beginning at the surface, and meeting in the in- 

 side of the original crystal. Professor Mitscherlich repeated 

 the experiment under various modifications, from which it 

 appeared, that this change always ensued at a temperature of 

 about 42° It. (126°Fahr.)both in the sulphate of magnesia, and 

 the sulphate of zinc. When the crystal is exposed in a glass 

 tube to the heat of the spirit-lamp, its decomposition takes place 

 without the loss of water, except, perhaps, what has been me- 



