284 Dr. Wollaston on M. BcudanCs Memoir sur [Ape it,, 



tions it assumes in its less simple state, I have remarked a 

 manifest difference in one direction of the crystal, proving that 

 even if the angular measures were really equal, still the solid 

 could not be regarded as a rhomboid ; but must be viewed as a 

 rhombic prism on account of some difference in its linear dimen- 

 sions. The angles, however, when carefully examined, are not 

 equal ; for though two of them have the same measure of 80-1- , 

 the third is 82°, or more ; and accordingly this angle is to be 

 considered as the measure of a rhombic prism, the terminal face 

 of which is equally inclined to the sides, the direction of its ob- 

 liquity being from one of the acute edges of the prism. 



But though I thus differ from M. Beudant with regard to the 

 primitive form of the common. sulphate of iron, 1 must admit the 

 justness of his remark, that the forms assumed by mixed sul- 

 phates of copper and iron, of zinc and iron, or of copper, zinc, 

 and iron, appear the same as that of simple sulphate of iron 

 alone. 



I am, however, inclined to think that M. Beudant is not cor- 

 rect in considering all the salts so obtained to be mixed crystals, 

 owing their form to the sulphate of iron alone, by which the other 

 metallic sulphates are grouped in the same manner as sand is 

 agglutinated by carbonate of lime in the crystals from Fontaine- 

 bleau. Some more intimate chemical union may be presumed 

 to occur in those instances at least in which a transparent crystal 

 is obtained ; for it is obvious that no mixture of bodies which 

 differ so much in refractive power * as the sulphates of copper 

 and of iron can suffer light to pass directly through the mass 

 with the transparency that is observable in some of these salts ; 

 and, though I am not aware that any compound salts of the 

 older metals have been hitherto observed, there may, neverthe- 

 less, be numberless instances of such union yet to be discovered 

 by inquiry properly directed to that object. 



I believe, indeed, that various triple salts of this kind will be 

 found among those sulphates that formed the subjects of M. 

 Beudant's experiments. 



If equal parts of sulphate of copper and sulphate of iron be 

 dissolved together and suffered to crystallize, the crystals pro- 

 duced are transparent rhombic prisms, similar to those of mere 

 sulphate of iron. 



If two parts of sulphate of copper be dissolved with one of 

 sulphate of iron, the first crystals formed are those of mere 

 sulphate of copper (but of course impure), and then the above 

 triple sulphate of copper and iron, the prismatic form of which is 

 in general so elongated as to show obviously that it is not a 

 rhomboid. 



If the sulphates of zinc and iron be dissolved together in equal 



* The index of refraction for sulphate of copper is 1*52 ; while that for sul- 

 phate of iron is only W5. 



