8 
Mr Brooke on Crystallisakon. 
sation is said to take place more rapidly in rough earthenware’ 
than in glass vessels ; and it is well known that threads, horse- 
hair, fine wire, sticks of glass, and other foreign bodies intro- 
duced into a solution, will be covered with crystals in preference 
to the surface of the containing vessel. 
These facts may be explained by the supposition, that a par- 
ticle deposited on a point, in which it is nearly surrounded with 
other particles, will be more quickly covered by them than 
where it rests on a plane, and when only a smaller portion of 
its surface is accessible to fresh particles. 
When a solution is ready to deposit crystals, they will be im- 
mediately produced, if a crystal, or even a fragment of the sub- 
stance dissolved, be introduced into it. If a fragment of any 
irregular shape be introduced, it will not retain this shape as it 
increases in bulk ; but the irregulai’ities will be first found to dis- 
appear, and then a regular form will be produced, which will 
be enlarged as the evaporation proceeds. 
The size: the crystals is generally influenced by the volume 
of the solution, and by its depth. When the volume and depth 
are considerable, and the evaporation slow, large crystals will 
generally be produced. 
But in operating upon small quantities of fluid, crystals of 
different sizes are frequently produced in the same vessel, and 
in apparently a capricious manner. I have seen the bottom of 
a small saucer covered with very minute crystals, when, on 
pouring the solution into another saucer, a crop of consider- 
ably larger ones has been suddenly deposited, and these have 
been succeeded by smaller and larger, without any degree of re- 
gularity, as the fluid has continued to evaporate. 
1 have not been able to connect these alternations, with any 
degree of certainty, with the changes of the atmosphere, for 
I have observed the deposits, both of large and small crystals, 
become more copious as the atmosphere has become more dry. 
But Mr Beudant has remarked, that solutions charged with 
electricity have deposited smaller crystals than when in their 
natural state ; and the changes just alluded to may possibly be 
connected with changes in the electrical state of the atmosphere. 
Magnetism is also supposed sometimes to influence the pro- 
cess of crystallisation. Thus, it has been stated, that if, into a 
