286 Dr. Wollaston on M. Beudant's Memoir. [April* 



The existence, however, of mixed crystals, such as these are 

 conceived to be by M. Beudant, cannot be questioned, and must 

 continue to mislead those who think it possible to rely on crys- 

 talline forms alone. 



In mineralogy the most prominent instance is the crystal of 

 Fontainebleau, without need of referring to other cases of more 

 intimate mixture, but equally indisputable. 



Among the salts formed by M. Beudant, it can hardly be 

 questioned that various instances of real mixture occurred such 

 as he represents, though possibly owing their forms in some 

 cases to a more compound base than he has supposed. 



There is also one instance observed by myself some years 

 since which I will take the present opportunity of noticing, not 

 merely as adding one to the numerous list that might be quoted, 

 but because it is in itself instructive with regard to the real form 

 of sulphate of nickel. 



Having offered to reduce for the late Mr. Tennant some pure 

 nickel, if he would be at the pains of preparing some crystallized 

 sulphate of nickel for that purpose, I received, in the first 

 instance, a quantity of square prisms with pyramidal summits as 

 the first set of crystals formed in his solution. 



The next set consisted principally of octohedrons, formed by 

 junction of the summits without any intermediate prism. 



A third set was afterwards produced, having the form of an 

 oblique rhombic prism, which I contended could not. be real 

 sulphate of nickel, but was probably some triple salt of the same 

 metal. 



To this Mr. Tennant replied, that he had re crystallized a part 

 of this salt, and had obtained octohedrons from them as before. 

 We had the satisfaction of being both right ; for upon careful 

 examination I found that each crystal contained within it visible 

 octohedrons, of true sulphate of nickel, cemented together by a 

 triple sulphate of nickel with potash, which gave the outward 

 form to the mass.* 



It must be owned that the foregoing remarks leave the subject 

 involved in difficulty ; but it is to be hoped that they may at 

 least serve to excite the industry of others, and answer the pur- 

 pose for which they are designed, by suggesting to chemical and 

 crystallographical inquirers a train of curious and useful investi- 

 gation. I remain, Gentlemen, 



Your obliged and very obedient servant, 



Wm. H. Wollaston. 



* A friend has suggested to me that the Abbe Haiiy was probably misled by 

 some such deceptive appearance, when he assigned the form of an oblique rhombic 

 prism to sulphate of nickel. Traile, torn. iii. 509, fig. 115. 





