1819.] © . tn Reply to Dr. Wollaston. 129 
together sulphate of copper and sulphate of zinc, both quite 
free from iron ; and he says that he obtained crystals which had 
the form of sulphate of iron. 
The analogous experiments which I had made, and of which 
I have already given an idea in my memoir, proved to me that 
the crystals, similar to those of sulphate of iron, contain all of 
them traces of this last salt ; and [ am tempted to believe that 
those obtained by Dr. Wollaston contained it likewise.. But to 
discover it, we must analyze a considerable quantity of these 
crystals. When I employed sulphate of zinc and sulphate of 
copper, prepared with the greatest care, and which, when exa- 
mined in considerable quantities, gave no trace of sulphate of 
iron, I never obtained any thing else than crystals of sulphate of 
copper or sulphate of zinc. — 
After having thus stated the opinions which I think should be 
adopted, and after having given precision to those that I formerly 
advanced, I will add that I perfectly agree with Dr. Wollaston 
relative to the form of sulphate of nickel. It is certainly a sym- 
metrical octahedron with a rectangular base; or, if that is 
preferred, a right prism with a rectangular base. The crystals 
which M. Haiiy examined were called sulphate of nickel by 
Leblanc, who, in this point as well as in others, has not examined 
the results with sufficient care. His crystals undoubtedly belong 
to the double sulphate of potash and nickel : their primitive form 
is an oblique rhomboidal prism. “This is demonstrated by the 
particular symmetry which the different crystalline forms of this 
salt present, of which I have obtained several very beautiful. 
The observation which terminates -Dr. Wollaston’s letter, in 
which he describes crystals of sulphate of nickel in small octa- 
hedrons, cemented by the double sulphate of nickel and potash, 
appears to me of the greatest importance for the theory of the 
mixtures of different salts with the preservation of the form of 
one of them. It agrees perfectly with all the ideas suggested to 
me by the numerous experiments which I have made on the 
subject. 
1 finish by testifying to Dr. Wollaston how much I am flat- 
tered that my experiments drew his attention. I am anxious 
that the new experiments, which I have just presented to the 
Academy, may likewise merit it; and that he will have the 
ppecness to enlighten by his observations the new route which I 
ave endeavoured to traverse. J am, &c. 
F, F. BeuDanr. 
Vou, XIII, N° II. I 
