1826.] Mr. Levy on the TungstatedfLetai. S65 



molybdate of lead, present one of the preceding forms ; and as 

 far as I could ascertain, measure very nearly the same angles : 

 they are, therefore, most likely tungstate of lead. 



I was immediately struck with the almost identity of these 

 measurements with those offered by molybdate of lead, and the 

 only difference I could notice with respect to cleavage was, that 

 in the tungstate of lead, the cleavage perpendicular to the axis 

 appeared to me more easily obtained than in the molybdate. In 

 consequence of this great similarity of crystallographieal charac- 

 ters, 1 begged Mr. Children, about two years ago, to examine 

 chemically a small quantity of the substance, to ascertain if it 

 had not been wrongly named, and whether it was not simply 

 molybdate. The quantity he had to operate upon, however,, 

 was so small, that no decisive result could be obtained, and in 

 consequence of the following considerations, I have placed the 

 specimens in the collection apart from the molybdate, and under 

 the name of tungstate of lead. 



I had then noticed that the measurements of molybdate of 

 lead were very nearly the same as those of tungstate of lime y 

 it appeared besides from the great analogy of forms, macles, 

 and cleavages, as well as the near equality of angles of carbo- 

 nate of lead and arragonite, as well as of phosphate of lead and 

 phosphate of lime, that (to use the language of Prof. Mitscher- 

 lich, which now it is well known how to understand), lead and 

 lime were isomorphous bases. It was, therefore, to be expected 

 that tungstate of lead would measure nearly the same angles as 

 tungstate of lime ; and consequently nearly the same as molyb- 

 date of lead. Another inference to be drawn from what precedes 

 is the isomorphism of molybdic and tungstic acids ; that is, the 

 same analogy between them as has been proved to exist between 

 phosphoric and arsenic acids.^ With the desire to establish this 

 result upon more facts, I endeavoured last year to procure some 

 artificial molybdates and tungstates ; but Mr. Faraday, to whom 

 I applied, told me they were very difficult to obtain in a crys- 

 tallized state. I think, however, that the measurements of tung- 

 state of lead which Mr. Brooke was so good as to show me a few 

 days ago, and which he had taken upon small crystals of that 

 substance (a few specimens of which were lately received by 

 Mr. G. Sowerby), but without noticing their near equality to 

 those of molybdate of lead and tungstate of lime, confirm the 

 results I had previously obtained, and justify the inference I had 

 drawn from it. 



Besides this new example of isomorphism, I have been sometime 

 engaged in the examination of a class of substances which present 

 a remarkable analogy of forms, and near equality of measurements, 



* These two acids present a ca»e analogoixs to the one under consideration^ Their 

 combinations with two isomorphous bases producing isomorphous crystals,, those of arse- 

 niate of cobalt and of phosphate of iron. 



