ON DIMORPHOUS BODIES. 207 
much less even the relations of these forms to temperature and 
other circumstances of an unusual character have been attended 
to, we shall see cause to believe that the number of bodies capable 
of assuming two or more forms must be vastly greater than we 
can as yet be aware of. 
59. In the great majority of cases we have yet to learn where 
and how to look for the second forms of bodies. This is stri- 
kingly illustrated by the beautiful observation of Frankenheim in 
regard to the crystallization of nitrate of potash from its solu- 
tion in water. As the evaporation proceeds crystals of two 
kinds are distinguished, prisms of the ordinary form and six- 
sided plates of the second form ; but as the prisms are prolonged 
they come in contact with the plates, give rise to an immediate 
movement among their particles, and incorporate them with 
their own mass, so that the ultimate result of the crystallization 
is an unmixed crop of crystals of the common form. 
In most cases of crystallization it is only the final result we 
can observe or have hitherto regarded—may there not be very 
many cases in which changes analogous to those observed in 
nitre may take place, a knowledge of which would enable us 
greatly to enlarge our list of dimorphous bodies ? 
60. An analogous observation of Ehrenberg* suggests the 
same question, and makes an affirmative reply still more pro- 
bable. In examining the crystallization of common salt under 
the microscope, he states that the first crystals formed were 
generally six-sided tables, in the centre of which a cubical point 
would suddenly appear and gradually increase in size, while the 
tabular crystal dissolved around it and at length disappeared. 
The hexagonal crystals had much resemblance to the hydrated 
tables observed by Mitscherlich at very low temperatures, so 
that the present does not appear to be a case of dimorphism. 
Still it points in the same direction as the observation of Frank- 
enheim, tells us to keep an eye on the same class of phenomena, 
instructs us not to rest satisfied with a knowledge of the final 
form of a crystallized body, but if possible to follow the march 
of the molecules, to note the successive stages at which they 
seem to rest for a time, and to mark the transformations they 
may undergo before they reach that form. 
_ 61. The circumstances also, the range of temperature for 
example, within which a certain form can exist, is sometimes 
very limited. Thus a solution of carbonate of lime in carbonic 
acid, if allowed to evaporate and crystallize in the cold gives 
only calc spar, if evaporated on the sand bath it is almost en- 
* Pog. An. Z. R, vi., p.» 240. 
