ON DIMORPHOUS BODIES. 185 
would compel us to reject, the suggestion we are now consider- 
ing. 
31. Before quitting this part of my subject I cannot refrain 
from laying before the reader a tabular comparison of the physical 
and chemical properties of some of the metallic oxides repre- 
sented by the general formula R,O;, though none of them is yet 
known to be dimorphous, as they present a beautiful example 
of the analogies which exist among isomorphous bodies, and as 
their densities exhibit a relation to their plesiomorphous differ- 
ences entirely the converse of that which Mitscherlich supposes 
| to exist among the earthy carbonates. 
Equivalent. Sule ot tbe Hardness.| Lustre. | Colour of Crystal. 
“Corun- | Ox. of 
dum. | chrom, 
of 
Corundum lj 321-167 | 86:6 Mohs. 9 | Vitreous | { Blue, vel.» me: aie 3°33 
'|Peroxide ‘ 85°58 Mohs. ; ' k ; : 
eer iran... ¢ {489213 |{86-10 Phat, | 5°5t063| Metalic | Steel grey ......] 5:9 | 4-88 
Oxide of ! ; - 
| Grereium |501°319 | 85°55 Rose ..... Be Thee | Blac ste a 6-09 
The same difficulty presents itself here as in the former ex- 
ample from the uncertainty of the determinations, but in these 
substances it is clear either that heat does not expand them so 
as to make them approach the cube, or that the difference of 
the chemical affinities considered as the cause of plesiomor- 
phism does not act in the same way as heat does. Peroxide 
of iron and oxide of chromium are much less dense than they 
ought to be, compared with corundum, and yet the acute 
angle of their rhombs is less; or, comparing the first two 
substances in the table with oxide of chromium their specific 
gravity is greater than calculation gives it, while their acute 
angles are less. Can it be that heat in expanding these acute 
thomboids makes them diverge from, while obtuse rhomboids 
it brings nearer to, the cubical form ? 
III. 
32. Of Analogous Chemical Groups, the members of which 
taken singly are Monomorphous, but which as Groups are 
Dimorphous.—In the remarks already made on the table of 
isodimorphous groups (21) I have adverted to the observation 
that like crystalline forms generally follow like chemical for- 
mule, and I have illustrated by one example in what way this 
observation leads us to infer and to look for dimorphism in sub- 
stances not hitherto observed in more than one form. Almost 
