ON DIMORPHOUS BODIES. 173 
Formula. Square Prism, Oblique Rh. Prism. 
Tungstate of Lead... PbCr | Common form ...| Not known. 
Molybdate ........+00. Pb Mo Do. aie BOTS 
Chromate .........06 Pb Cr | Not known... Common form. 
As an illustration of this point we might have taken the sulphate 
and chromate of lead, of which not only are the formulz every 
way analogous, but in which both the acid and the base are 
known to be isomorphous and capable of replacing each other, 
or we might have made one group of the sulphate, chromate, and 
molybdate, which all present themselves in different forms. I 
have however taken the case of the chromate and molybdate, 
because I think the probability of the two forms of these com- 
pounds being a real isodimorphism is very much strengthened 
by a specimen in the possession of my friend Mr. Brooke, of 
London, which he showed me as a molybdate of lead (a square 
prism the form of the molybdate) having the colour of the 
chromate. I am not without hopes of obtaining a fragment for 
the purpose of determining if it does not really contain chromic 
acid*. The case of substances represented by the general for- 
mule presenting themselves in more than two incompatible 
forms will be considered in a subsequent section of this reportf. 
22, But all the members of isodimorphous groups, much less 54, gin 
of groups simply isomorphous, are not necessarily represented with unlike 
by formule every way analogous. Of this the fourth member formule. 
of the first group in our table, the nitrate of potash, presents a 
striking example. In the formula for this salt (KO+NO,) 
neither the ratio between the positive and negative elements in 
the entire compounds, nor in the acid it contains is the same 
_with that which exists in the carbonates (RO +CO,) which form 
_the other members of the group. 
___ Among isomorphous bodies, known to assume only one form 
(monomorphous), it was early observed by Mitscherlich{ that 
potash (KO) might be replaced both in neutral and acid 
salts by ammonia with an atom of water (H;N+HO), without 
change of form, though neither the number of equivalents nor 
__ * Since this report went to press I have examined a fragment of this speci- 
men, and found it to be chromate, which has enabled me to insert this compound 
in Table I. among the other known cases of dimorphism. See Lond. and Edin. 
Phil. Mag. for May 1838. 
+ See p. 197. 
t Berz. Arsberiittelse, 1833, p. 136. 
