208 ‘SEVENTH REPORT—1837. 
tirelyarragonite. Chloride of calcium precipitated by carbonate 
of ammonia in the cold gives calc spar, if both solutions be 
boiling the result is arragonite ; and yet at a low red heat arra- 
gonite is again changed into calc spar. Thus it would appear 
that the conditions as to temperature in which the molecules 
may unite to form calc spar are various and recurrent, and that 
so far as we yet know arragonite is not formed ata temperature 
below perhaps 80° or 100° F., and cannot exist above 700° or 
800° F. It may be necessary therefore to observe the forms 
assumed by bodies at many different temperatures, not perhaps 
very remote from each other, before we shall be able to pro- 
nounce as to their ability to assume more than one form. 
The application of the microscope to the examination of the 
phenomena of crystallization promises to add much to our know- 
ledge. In the hands of Ehrenberg, Frankenheim, Gustav Rose, 
and Talbot it has already given us much interesting information, 
but a rich harvest awaits the further use of this new instrument 
on a field hitherto almost untouched by it. 
62. But the clearest and most extended inference in regard 
to the number of individual substances which are likely to prove 
dimorphous (trimorphous perhaps or polymorphous), is to be 
drawn from the existence of a dimorphism in certain chemical 
groups, the individual members of which are only monomor- 
phous, or conversely from the known existence of dimorphous 
individuals in large strictly chemical and isomorphous groups. 
In a former section (section iv.) we have discussed the probabi- 
lity of a heteromorphism being observed in all the members of 
the groups of the first class, and of all the members of those of _ 
the second class proving isodi or isotrimorphous, and we have _ 
seen strong reason to believe that this expectation will not ulti- — 
mately be disappointed. How great a number of individuals — 
these observations when made will add to the substances in our | 
first table need not be pointed out; it is sufficient that in the — 
circumstance here alluded to we see another reason for believing — 
that in nature the assumption of two or more incompatible forms _ 
is very far from being a rare phenomenon. 
63. Theory and observation therefore unite in suggesting — 
that dimorphism, instead of being an exception, as it still in’ 
some measure appears, to the ordinary laws of crystallization, 
may prove to be a general, perhaps a universal consequence of — 
those laws. The utility of the present report consists mainly — 
in its bringing together the scattered fragments of our certain 
knowledge—in pointing out the inquiries they indicate, and the © 
conclusions to which they lead, and in its setting up a landmark 
to which it may be interesting, perhaps curious to refer in a fu- 
