11 
iVIr Brooke on Crystallisation. 
which chemical mixtures exert generally upon the modifications 
of crystals, has been stated by Mr Beudant and others to be 
very extensive. 
The faces on which the deposited crystals rest, and those 
which are parallel to them, are generally extended dispropor- 
tionately in relation to the other planes. But this is not inva- 
riably the case : Nor does the position of the deposited crystal, 
in reference to its axis, or its primary edges, always influence its 
form ; although Mr Leblanc appears to have produced changes 
in figures of certain crystals, by varying their position in the so- 
lution. If the crystalline particles were constantly more nu- 
merous towards the bottom of the solution than at its surface, 
we might suppose that crystallisation would first take place at 
the bottom, and that the lower planes of immersed crystals would 
be the most enlarged ; and this is frequently found to be the 
case. 
But we may observe that several salts begin to crystallise al- 
most invariably at the surface, and that, as the crystals thus 
formed sink and increase in size at the bottom, others are suc- 
cessively formed above. 
I have remarked also among some crystals of nitrate of 
lead, which were deposited at the same time, and in the same 
vessel, some which were unmodified octahedrons, and others 
which were hemitropes. Of the octahedrons, some had their 
axis perpendicular to the surface on which they rested, and 
their upper summits were regularly terminated ; others rested 
on one of their planes^ and others were attached hy an edge to 
the bottom of the vessel, consequently having one of the bases 
of the octahedron vertical, and the crystals being terminated by 
a horizontal edsre. 
These different crystals were deposited singly, as the crystals 
of nitrate of lead very frequently are ; and the fact manifests the 
inconstancy of the form of a crystal, relatively to its position in 
the solution from which it is deposited. 
The natural planes of crystals are subject to a considerable 
diversity of character, possessing sometimes the highest degree 
of brilliancy, and at others being so dull as to afford very imper- 
fect reflections. Sometimes they are striated, sometimes curved, 
and commonly exhibiting greater or less degrees of inequality 
