aS 
ON DIMORPHOUS BODIES. 167 
If these were crystals of sulphate they would indicate a dimor- 
phism in this salt also— Edin. Phil. Jour., vol. i. 
Other substances likely to prove dimorphous are inserted in 
Table III., and it is not impossible that some of those forms 
now considered pseudo-morphic, may hereafter appear to be true 
cases of dimorphism. 
Several observations suggest themselves on a glance at this 
table. 
12. The number of substances contained in it, and of which 
the dimorphism has been discovered in so short a time, renders 
it very doubtful whether the crystalline form assumed by any 
given substance is one only and invariable. 
13. The several forms of the same substance possess different 
physical properties,—different colour, hardness, density, or 
relations to heat and light. This is true of every pair of di- 
morphous bodies in the table, yet in all of them the chemical 
relations remain unchanged. ‘The only trace of an exception, 
yet observed, is in the different solubilities of the two forms of 
arsenious acid and in the different behaviour of garnet and vesu- 
vian before the blow-pipe. These chemical differences, how- 
ever, are too obscure to demand much attention in this place ; 
were they distinct and well-defined, the compounds which ex- 
hibit them, should be removed from the class of simply dimor- 
phous to that of isomeric bodies *. 
It appears, therefore, that dimorphous bodies exhibit in their 
several forms physical differences only, the chemical relations 
remaining unchanged. To this remarkable characteristic of 
such bodies we shall have occasion to advert when we come to 
_ inquire into the cause of dimorphism and its connection with 
_ isomerism. 
14. Inthe relation between the first and second forms of several In dimor- 
of the groups in the Table, a striking analogy presents itself, phous com 
In the carbonates of lime, of magnesia, of lead, and of iron, and in P97 ete. 
_ the nitrate of potash, the first form being a rhomboid of nearly ment is di- 
equal dimensions in all, the second form is aright rhombic prism ™orphous. 
similarly related in dimensions. In arsenious acid and oxide of 
antimony, the first form is the regular octohedron, the second a 
right rhombic prism. In each form these substances are iso- 
morphous, or they are isodimorphous. 
* Though alike in chemical constitution, the two forms of arsenious acid 
and garnet may be the result of isomerism. In minerals represented by so 
complicated a formula as garnet and vesuvian, it is impossible to say that the 
elements are not very differently arranged, that they are not, in fact, different 
substances. 
