206 SEVENTH REPORT—1837. 
In regard to the difference of physical properties exhibited by 
the unlike forms of the same substance, M. Voltz considers that 
the axes as well as the forces resident in each being independent 
in magnitude, the physical properties in the direction of the 
three axes must always differ in a greater or less degree. The 
density, hardness, refraction, reflection, dilatability by heat 
and compressibility along the unlike axes being unequal in the 
molecule, must vary also in the crystalline mass with the way in 
which the molecules are grouped together to form it, and hence 
the physical properties of the mass will depend in some mea- 
sure on the system of crystallization to which its form belongs. 
These views of M. Voltz may not be correct, yet they are 
deserving of much consideration. They may embody only a 
part of the truth, or they may hereafter prove to be wholly in 
error ; yet they have more the air of a vera causa than any of the 
other hypotheses we have considered, and they may be instru- 
mental in pointing the way to something still more satisfactory. 
X. 
56. Extent of Dimorphism.—Is dimorphism or heteromor- 
phism universal; may all substances assume two or more in- 
compatible forms? ‘To this question we cannot at present give 
a direct reply ; there are considerations, however, partly theo- 
retical and partly drawn from observation, which seem to render 
it probable, that if not all, at least a very great number of cry- 
stallizable substances are heteromorphous. 
57. According to any of the suppositions (53, 54, 55) by © 
which dimorphism has been accounted for, as above stated, the 
power of assuming more than one form ought not to be restricted 
to any number or to any class of bodies whether simple or 
compound. If it be caused by change of circumstances, all _ 
substances may be placed in new conditions; if to a change 
in the relative intensity of the axial forces, all ought to be more or 
less liable to such a change ; while the theory of Voltz implies, 
that all being made up of molecules with three unlike axes, may ~ 
assume one or other of a much more numerous suite of forms — 
than observation has hitherto givenus reason to suspect inany one 
known substance. Still these explanations are all hypothetical ; 
and though we ought not altogether to lose sight of the conclu- — 
sion to which they would direct us, we are not justified in allowing — 
such theoretical views to do more than awaken in our minds a 
suspicion that all substances may ultimately prove to be dimor- 
phous. 
58. Again, if we turn to the department of observation, and 
consider how little the forms of bodies have been studied, how 
