388 Prof. Johnston on the Dimorphism of Chromate 



muriatic acids, giving with the latter a greenish solution, and 

 on evaporation chloride of lead mixed with a green substance 

 (chloride of chromium ?). 



From these characters there can be no doubt that the sub- 

 stance is chromate of lead. 



The specimens on which these minute red crystals occur is 

 from the Bannat. None of the crystals appear to exceed the 

 sixteenth of an inch in length, and they rest on a thin yellow 

 coating which resembles molybdate of lead. Besides the small 

 specimens he possesses Mr. Brooke informs me he has never 

 seen but one other. He has measured two of the crystals, 

 and satisfied himself that in form they are identical with the 

 molybdate. 



We are justified therefore in including the chromate of 

 lead among known dimorphous bodies, and in more confi- 

 dently anticipating that the analogous tungstates and molyb- 

 dates will prove so also. A strict examination of the speci- 

 mens already existing in cabinets may be expected to fill up 

 several of the gaps. In the following table: 



There is another member of this group, the tungstate of 

 iron and manganese (wolfram) which, though represented by 



the formula Fe Tu + Mn Tu, in which the bases are isomor- 

 phous, yet crystallizes in a form different from either of those 

 above mentioned, l^he two oblique rhombic prisms have 



in wolfram M, M = 101-5 P, M = 110^50' 



in chromate of lead = 93° 30' = 99° 10' 



We have here therefore a third form in which the members 

 of this group may possibly crystallize. As wolfram however 

 is a double salt, it is equally possible that this third form may 

 result from the union of the other two. A square prism of 

 tungstate of iron, with a less oblique prism of tungstate of 

 manganese, may be capable of producing the more oblique 

 prism of wolfram, or the union of the two salts in some other 

 way may produce the third form, without rendering it abso- 

 lutely necessary at present to have recourse to a trimorphism. 

 In a former paper on the dimorphism of baryto-calcitef, 1 



• I need not draw attention to the link which this new fact affords for 

 connecting the molybdic with the sulphuric and other analogous acids, 

 f Lond. and Edin. Phil. Mag., vol. vi. p. 1. 



