ON DIMORPHOUS BODIES. 203 
mechanical cause, the difference of the effects produced in each 
case being due to the specific properties of the substance itself. 
IX. 
51. Cause of Dimorphism.—From what has been stated in 
the previous part of this report in regard to the infancy of our 
knowledge in this department, it will be evident that we are 
not yet in a condition todo much more than merely hazard 
conjectures as to the cause of dimorphism. Our observations, 
however, are already so multiplied that some of the earliest con- 
jectures may now be safely laid aside. I shall briefly notice the 
several explanations which have hitherto been given. 
52. Presence of a foreign hody.—When the phenomenon 
of dimorphism was first recognised in carbonate of lime, it ap- 
peared most easy to account for the difference between cale spar 
_and arragonite by supposing that the latter actually contained 
some other ingredient besides carbonic acid and lime. And 
though the experiments of Thenard and Biot failed in showing 
the presence of any other constituents, yet the detection of 
strontia by Stromeyer seemed to set the matter at rest, and the 
failure of the French chemists was attributed to their deficiency 
in analytical skill. Now, however, that we can change arra- 
gonite into calc spar, and by a proper regulation of the tempera- 
ture can cause one and the same portions of several other sub- 
stances to assume either of two known forms, the influence of 
foreign bodies in these cases can no longer be admitted. It is pos- 
sible that the presence of such bodies might produce a change 
of form, but they cannot be considered necessary to the pro- 
duction of a dimorphism, or to afford any insight into the pro- 
bable cause of the pheenomenon. 
53. Influence of circumstances.—In the preceding section 
we have seen that the assumptior of one or other form by di- 
morphous bodies is very much influenced by circumstances. 
Hence dimorphism has been said to be dwe to the different cir- 
cumstances under which a substance crystallizes. But this is 
only to look on the surface of the change, and would imply that 
you have only to vary the circumstances in order to produce 
another and another form, and that thus the number of forms 
in which a substance may exhibit itself is limited only by the 
number of changes that can be effected in the circumstances. 
Tt implies also that similar circumstances, or a similar change of 
circumstances, should produce a similar effect on all substances : 
but neither of these things is the case, so far as observation has 
gone ; there must, therefore, be something in the internal struc- 
ture of the mass, in the form, the mechanical arrangement or 
