S Mr Brooke on Crystallisafmi. 
distances, introducing attraction or repulsion among the particles 
subject to their influence, and possibly bearing some close ana- 
logy to electricity. We may suppose these latter forces acting 
upon molecules held in solution at considerable distances, and 
predisposing them to approach each other, and to arrange them- 
selves at the same time in some regular order; and we may imagine 
the same forces continuing to exert themselves, until the particles 
are brought within that distance from each other at which the 
cohesive force begins to operate. We may then imagine this 
cohesive force to fix and retain the molecules in those positions 
in which, when their numbers are sufficient, they will constitute, 
by their aggregation, visible and regular solids. 
It is evident that we may frame other theories to account for 
the formation of visible solids ; but there does not appear to be 
any hypothesis capable of accounting for the variety of forms 
under which crystals present themselves. 
I shall, however, proceed to consider shortly the general phe- 
nomena of the production of crystals, and the circumstances un- 
der which the crystallisation of minerals probably takes place. 
The different hypotheses which assign an aqueous or an ig- 
neous origin, to what has been termed the crust of our earth, a 
small portion only of the depth or thickness of which has been 
yet penetrated by the industry of man, suppose the minerals 
which have been discovered upon or beneath its surface to have 
been produced by corresponding causes. 
It is, however, certain, that many of the natural crystals 
with which we are acquainted, were not formed contempora- 
neously with the bed in which they have been discovered, but 
that they have been produced at different periods, and possibly 
under very different circumstances. Sometimes their origin may 
be ascribed to the action of heat, sometimes to the solvent power 
of some fluid, and in other instances to the united influence of 
both these causes. 
The observations of Sir Humphrey Davy “ upon the state of 
water and aeriform matter in cavities found in certain crystals,” 
printed in the PM. Trans, for 1822 , render it not improbable 
that natural crystals are formed under very different states both 
of pressure and temperature. 
There arc several appearances not uncommon among crystals. 
