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LXIII. On the Dimorphisvi of the Chromate of Lead, By 

 James F. W. Johnston, AM., FM.SS. L, ^' E., RG.S., 

 Professor of Chemistry, University of Durham.* 

 "OESIDES those substances which, like the biniodide of 

 ^^ mercury and the carbonates of lime and of lead, are known 

 to be dimorphous as individuals, there are other bodies, simple 

 and compound, known to be dimorphous as groups, though 

 the individual members of these groups have not yet been 

 observed to assume more than one form. The analogous 

 compounds of the tungstic, molybdic, and chromic acids, pre- 

 sent groups of this kind. The tungstate and molybdate of 

 lead and the tungstate of lime occur in square prisms, while 

 the chromate of lead has hitherto been observed only in 

 oblique rhombic prisms ; the general formula for all of them 



being R R. 



In my report on the present state of our knowledge in re- 

 gard to dimorphous bodies presented to the last meeting of 

 the British Association, and which will appear in the ensuing 

 volume of their Transactions, I had already attributed this 

 difference of form exhibited by the several members of the 

 above, and many other analogous groups, to the existence of 

 a true dimorphism ; and while I considered that such groups, 

 being chemically analogous, might also be considered as cry- 

 stallographically dimorphous or heteromorphous, I expressed 

 my conviction that further observation would prove the se- 

 veral members of these groups to be also heteromorphous, 

 each individual assuming the forms already observed in any 

 of the others. I am now enabled to confirm these views by 

 a very interesting example. 



On a late visit to the cabinet of my friend Mr. Brooke, 

 whose skill as a crystallographer is so highly and so de- 

 servedly estimated, he showed me a small specimen of what 

 he called molybdate of lead (crystallographically so) with the 

 colour of the chromate. Of this specimen he has since kindly 

 favoured me with a few minute fragments, in all not exceeding 

 the fifth of a grain, but sufficient to enable me to determine 

 that the beautiful red crystals were not molybdate having the 

 colour of chromate of lead, hui chromate in the form of the mo- 

 lybdate. Fused with borax the mineral gave in both flames a 

 beautiful green bead ; with microcosmic salt a bead which at 

 a high temperature was nearly colourless, as it cooled became 

 reddish brown, and when solid was of a beautiful green, 

 A larger addition of the mineral rendered the glass opake, but 

 did not blacken it. It dissolved without residue in nitric and 



* Communicated by the Author. 

 2 M 2 



