Like forms 
generally 
follow like 
formule. 
172 SEVENTH REPORT—1837. 
mula RO+RO,+7HO the same ratio prevails among the several 
members in both the substances as yet known to belong to the 
isodimorphous group it represents. 
Now as we know that there are several carbonates isomorphous 
with the first form of the first group in our table, and several 
with the second, all of which are represented by the same for- 
mul, there is reason to believe that they also are dimorphous, 
and that our knowledge of them might be represented as fol- 
lows: 
| Rhomboid. Rt. Rh. Prism. 
Carbonate of Manganese ...| Found native....| Not known. 
ING sdoses estes Do. seof!, Db. 
Baryta...s.ss Not known...... Found native. 
————— Strontia ...... Do, sacl 1G. 
and so with the rest of the isomorphous carbonates. 
In like manner we are justified in looking forward to the en- 
largement of the fourth group by the addition of the other iso- 
morphous neutral sulphates and seleniates of protoxides with 
seven atoms of water. It was supposed that the sulphates of 
zinc and magnesia had been met with in two forms, but later 
observations of Mitscherlich have shown that the second form 
contains only six atoms of water. 
21. It is generally true, so far as observations have gone, that 
isomorphous substances are analogous in constitution ; the ratio 
of the positive and negative equivalents in the whole formule, 
and in their several members, being the same. The converse 
of this, however probable it may be, is by no means so generally — 
established. A knowledge of the principle of dimorphism how- 
ever, and especially of that of isodimorphism, enables us to un- 
derstand how bodies may be isomorphous and yet not present 
themselves to us in ordinary circumstances under the same 
forms. Thus the chromate and molybdate of lead are represented 
by formule, which are analogous in every respect, and contain — 
the common base oxide of lead, and yet they occur in nature in © 
different forms. If we suppose them to be dimorphous, then — 
the ordinary form of each may be considered as representing — 
the second or rarer form of the other, and including tungstate — 
of lead, which is isomorphous, with the molybdate, we have the 
following isodimorphous groups :— ; 
