204: SEVENTH REPORT—1837. 
physical relations of its molecules, which incline it to assume 
one or other of a certain number of forms, and to assume each 
only under certain fixed conditions. Were these conditions 
fully understood, some light would be thrown on the internal 
cause ; or were the form and relations of the molecules known, 
we might be able to specify what crystalline forms they are fitted 
to build up, and under what conditions. It is stopping short 
however to attribute the phanomena to the circumstances under 
which they are displayed; for though we may not be able at 
present to see far beyond them, yet we should be ready to 
perceive and to avail ourselves of the first glimpse of light. 
54. Change in the intensity of the axial forces.—The optical 
phznomena exhibited by certain crystallized bodies, as the topaz, 
when raised to a high temperature, and of others when submitted 
to mechanical pressure, have suggested to Sir D. Brewster the 
idea that under the new conditions, a change takes place in the 
relative intensity of the axial forces resident in the molecule, 
and that of this change the new phenomena are a consequence. 
And as his beautiful researches have shown that the optical 
phenomena are almost universally true indices of the crystal- 
line form, he attributes the phenomena of dimorphism to a more 
or less permanent change in this relative intensity of the forces, 
caused by the circumstances in which the bodies happen to be 
placed during crystallization. If the attractive forces in the di- 
rection of two axes, A and B, be respectively + and —, and if by 
an alteration of temperature the intensity of the one be elevated 
and the other depressed, so that they change signs and become 
respectively — and +, itis easy to understand how, if at liberty 
to move, the molecules in which this change takes place should 
make a partial revolution, and build up a crystalline mass of a 
new form. But this only removes the difficulty a step further 
back ; it merely explains how heat and other circumstances may 
produce the phenomena, it does not affect to explain why. The 
true question still remains behind, What specific relations, me- 
se 
chanical or physical, exist among the molecules of each sub- ~ 
stance, that the same circumstances do not affect all alike? 
55. Union of the Molecules in the direction of different axes. : 
—This difficulty is in some measure got over by the supposition 
of Voltz*. He supposes the crystalline molecules of all bodies 
to be possessed of three unequal axes, in which reside polar — 
* Transactions of the Nat. Hist. Soc. of Strasburg, 1833. The only know- — 
ledge I have of M. Voltz’s views is derived from L’Jnstitut, 29th March and — 
8th Aug., 1834, and from a paper by Mr. Dana, in Silliman’s Journal, xxx. 
p- 294; it is not impossible therefore that in endeavouring to give a clear state- _ 
ment of his views I may have unintentionally misrepresented them. 
