62 
Proceedings of Royal Society of Edinburgh. [sess. 
Crystallisation. 
1 believe that the principle regulating the formation of crystals 
will have something to do with the symmetrical arrangement of the 
components of molecules, especially so with regard to complex 
molecules. This principle I believe to be the inequalities of 
'pressure in opposite directions which must exist within a solid body 
if the molecules are not symmetrically arranged. For instance, 
suppose the molecules of compound bodies, on solidifying, froze 
together anyhow ; it is easy to see that if this happened the spaces 
separating the molecules, owing to the peculiar shapes of the latter, 
would not be equal between contiguous molecules, so that the ether 
within those spaces would, owing to the vibrations existing within 
every body, be unequally compressed and unequally dilated after 
each vibration within the spaces. Now, this principle of the in- 
equality of pressure is, I take it, the very principle which arranges 
'x 
f,C. / 
the molecules of crystals in symmetrical patterns. For instance, if, 
when on the point of solidifying, a molecule be not acted on by 
the ethereal pressure equally in opposite directions , this very in- 
equality of pressures will turn the molecule about until it is in a 
position of equilibrium ; that is, until it is in a position where it 
would be acted on by equal pressures from opposite directions. 
In this position the molecules would freeze together; and the 
various shapes of the resulting crystals would be accounted for by 
the inequalities of size which existed between the constituents of 
the molecules of the various crystals. For instance, a molecule of 
ice would have the shape shown in plan in fig. 1 ; this shape de- 
always vary by two steps at a time. If this be so, there must be some, 
peculiarity in the circumstances attending the departure from the rule, or the 
departure, one would think, would have been discovered long ago. Possibly , 
though, this very departure, when the circumstances attending it are 
thoroughly inquired into, may serve to confirm my theory. 
