1818.] la Determination des Esptces Minerahs. 285 



quantities, the crystals obtained have also the same form of an 

 oblique rhombic prism, similar to that of mere sulphate of iron ; 

 but the colour is a pale greenish white, as observed by M. 

 Beudant. 



Since the compounds of each of the foregoing salts with 

 sulphate of iron assume apparently the same form of crystal, it 

 is not surprising that the more compound salt containing all 

 these sulphates together should also present the same rhombic 

 prism ; and it must be admitted that the most natural interpret- 

 ation is that of M. Beudant, namely, that the sulphates of 

 copper and zinc are to be regarded as an intermixture of foreign 

 matter grouped together by sulphate of iron, to which alone the 

 crystalline form may be ascribed. 



The transparency of the crystals, however, militates against 

 such an explanation of the forms of the preceding salts, and the 

 doubt thence arising is confirmed by observation of a fourth 

 compound having, to all appearance, the same form, although 

 entirely free from sulphate of iron. 



If equal quantities of sulphate of copper and sulphate of zinc 

 (both perfectly free from iron) be dissolved together, and suffered 

 to crystallize, the first crystals formed are those of sulphate of 

 copper, of their usual form and customary intense blue colour 

 (but of course impure). The after crystals are of a paler blue 

 colour, and consist of the two sulphates of copper and zinc 

 combined, presenting, to all appearance, the form of the same 

 rhombic prism before observed, with the same obliquity of its 

 terminal face. 



Judging from the apparent quantities of the two salts formed 

 in the preceding experiment, that the sulphate of zinc in the 

 compound salt might exceed the sulphate of copper in the pro- 

 portion of about four to one, I formed a solution of the two salts 

 in this ratio, which afforded no crystals like sulphate of copper, 

 but presented solely the oblique rhombic prism as before, and so 

 nearly agreeing in the measures with those of sulphate of iron, 

 that 1 cannot at present undertake to say wherein any difference 

 consists. 



But since no geometric reason can be assigned for the preva- 

 lence of this peculiar form among several bodies consisting of 

 different chemical ingredients, ii maybe presumed that some 

 difference will hereafter be found either in the angles or linear 

 measures, and may enable us to distinguish them without having 

 recourse in all instances to chemical analysis. 



It is not merely with regard to the triple sulphate of copper 

 and zinc that this hope may be entertained ; but even those salts 

 which do contain sulphate of iron, if they be really chemical 

 compounds, as their transparency would lead us to presume, may 

 be expected to exhibit some difference from the simple salt 

 when crystals can be formed with sufficient regularity for accu- 

 rate measurement. 



