7 ' 
Mr Brooke on Crystallisation. 
ing ; but if the air be admitted to it, and it be at the same time 
slightly agitated, the whole mass becomes almost instantaneously 
solid. If, however, gas of any kind be admitted to the solu- 
tion, M. Gay Lussac states, that a similar effect is produced. 
He also states that sub-carbonate of soda will crystallise in a va- 
cuum, but not when exposed to the air. Many other salts crys- 
tallise equally well in vacuo, and under exposure to atmospheric 
pressure. 
It happens not unfrequently that crystals are very slowly de- 
posited from solutions in perfectly closed bottles, which have re- 
mained a considerable time at rest, and from which little, if any, 
evaporation could have taken place. 
There is a considerable diversity in the manner in which the 
crystals belonging to different salts are deposited. Sometimes 
they stand singly, scattered, in a greater or less number, over 
the bottom of the vessel containing the solution. Nitrate of lead 
affords an example of this nature. 
In other instances, their tendency is to form into groups, the 
crystals of which appear sometimes to radiate from a common 
centre. This character is very conspicuous among natural crys- 
tals in wavellite, in some varieties of sulphate of lime, in arse- 
niate of cobalt, and others. 
The crystals of sulphate of magnesia, of rhombic sulphate of 
nickel, of nitrate of potash, and of many other substances, if 
rather rapidly produced, form slender prisms, which so inter- 
sect each other that the axes of the prisms lie in almost every 
direction. 
Some substances have a tendency to run up the sides of the 
vessel containing the solution, and to produce there very irregu- 
lar and imperfect crystals, long before any regular ones are de- 
posited. Muriate of ammonia possesses this character in a re- 
markable degree. 
Chromate of magnesia does not crystallise, if the quantity 
dissolved be small, until the bulk of the solution be nearly 
equal to the bulk of the crystals that are produced from it. 
And the solutions of other salts are known to require a high de- 
gree of concentration before they will deposit any crystals. 
The nature of the surface of the vessel containing the solu- 
tion, will influence the precipitation of crystals. This crystalli- 
