164 SEVENTH REPORT—1837. 
compound, assumed, on crystallizing, a form peculiar to itself, 
and that this form constituted an unfailing specific character,— 
(Haiiy.) Crystals belonging to the regular system presented 
the only apparent exceptions. 
2. After a time, however, the generality of this conclusion 
was further narrowed by the doctrine of isomorphism, which 
showed that form alone, even when not tessular, was insufficient 
in many cases to determine the chemical constitution of a body*. 
Still, in these new exceptions, the form indicated the nature and 
constitution of a substance within certain limits, that it was a 
member of this or that isomorphous group, elevating crystalline 
dimensions in such instances from the rank of a specific to that 
of a generic character. Even this place, however, they did not 
long retain undisturbed. 
3. Founded on the principle that the molecules of crystalline 
bodies have themselves a regular crystalline form, the doctrine 
of isomorphism hitherto recognised, that for each substance, 
simple and compound, this form was one and invariable ; though 
not necessarily a specific that it was a constant character. 
4, The earliest measurements of artificial crystals had been _ 
made on such as were formed in ordinary circumstances of tem- 
perature and by the most usual methods of manipulation, 
Occasionally, however, crystals formed at higher temperatures 
or under peculiar circumstances attracted attention; and in 
certain cases these new crystals were found to differ in form or 
dimensions from the ordinary form of the same substance, to — 
such an extent that they could not be derived from each other — 
by the ordinary laws of crystallization. Thus sulphur crystal- — 
lized from fusion differs in form from the natural crystals and — 
from those deposited from solutions of sulphurt. And as the — 
resources or results of analytical chemistry were multiplied so — 
as to place beyond doubt the chemical identity of different spe- 
cimens, the examples of such differences gradually increased in — 
number. Natural substances also were met with, crystallized — 
under circumstances not well understood and generally beyond — 
our imitation, which, though shown to agree in chemical con- 
stitution, yet differed wholly in form. Graphite and the dia- | 
mond, both forms of pure carbon ;—arragonite, and calc spar, 
both pure carbonate of lime, are groups of this kind. 
5. To mark the singular character possessed by these bodies, 
they have been distinguished by the term dimorphous, and the — 
abstract property by that of dimorphism. 
* Mitscherlich, dn. de Chim. et de Phys., XIV. p. 172. 
+ In bisulphuret of carbon, or in quadri (?) sulphuret of hydrogen, 
