CRYSTALLIZATION. 
reduction of its temperature. At first long 
and slender spiculoe form in the fiuid, and 
from these otliers shoot out at a certain 
angle, and this continues till the interstices 
are filled with the crystals, and the whole 
becomes a solid transparent mass. We 
have also examples of it in the metals, 
which when melted and cooled slowly, as- 
sume symmetrical forms. Some infiamma- 
bles, as sulphur, crystallize in a similar way. 
Of the second kind of crystallization, 
the principal examples are derived from 
the order of salts,- and a few other solids, 
soluble in water; and with regard to this, 
several facts of importance require to be 
stated. 
The solution of a solid in a fluid, is in 
almost every case increased by heat, which 
weakens cohesion : hence a larger quantity 
of the solid is kept in solution at a high, 
than a low temperature. If, therefore, we 
prepare a solution of salt in hot water, the 
solution being saturated, or the fluid having 
dissolved as much of the salt as it can do, 
on allowing it to cool, the portion of the 
salt which the heat enabled the fluid to 
dissolve, will separate ; and unless the cool- 
ing of the solution has been very rapid, 
the particles of the solid, in approaching to 
each other, will pass into those regular ar- 
rangements which constitute crystals. 
The same result will be obtained by with- 
drawing parts of the fluid in which the solid 
is dissolved. If this be done slowly, or by 
spontaneous evaporation, the particles will 
obey the law of attraction which unites 
them in regular forms ; the crystals are in 
this way formed frequently more regular, 
and of a larger size than by the former 
miethod of reducing the temperature of the 
solution ; some can be crystallized only in 
this method. 
In both cases the fluid in which the crys- 
tals form, is still a saturated solution of the 
solid, and by a farther evaporation, joined 
sometimes with subsequent cooling, will 
again crystallize. 
In general it holds true, that the slower 
the formation of a crystal, the more perfect 
is its symmetrical arrangement; it is also 
larger, harder, and more transparent : 
whereas, when the process is too rapid, or 
is disturbed by agitation, or other causes, 
the arrangement is less regular and tlie 
form incomplete. Hence the crystals 
formed by nature are so much more perfect 
than those produced by artificial processes. 
Crystallization is promoted by affording 
a nucleus, or solid point, at which it may 
commence, and still more so if a crystal be 
iutroduced into the solution; crystalliza- 
tion immediately commences from it, if the 
solution be a saturated one, and it is even 
capable of causing part of the solid to be 
separated, if the temperature at which it 
takes place could have retained in solution. 
Even the regularity of the figure of this 
ci-ystal, seems to have an effect in render- 
ing the crystallization more or less regrdar ; 
and on this Le Blanc has founded a method 
of obtaining large and perfect crystals. It 
consists in selecting very regular crystals 
of a salt that have been newly formed, and 
putting them into a saturated solution of the 
same salt. They increase in size ; and as 
the side which is in contact with the vessel 
receives no increase, they are to be turned 
daily to preserve their regularity. After 
some time, the largest and most regular 
of these crystals are to be selected, and the 
same process repeated on them ; and thus 
crystals much larger and more regular than 
are usually formed in a solution may be 
obtained. 
The access of the air has an important 
influence on this process. If a saturated 
solution of salt when hot, be put into a 
vessel from which the air is excluded, it 
does not crystallize even when cold. But 
if the air be admitted, the crystallization 
immediately commences and proceeds witli 
rapidity. It has been shewn by Dr. Hig- 
gins, that any pressure, equivalent to that 
of the atmosphere, as the pressure of a 
column of mercury, has the same effect. 
During crystallization a quantity of heat 
is rendered sensible. In many cases the 
volume of the substance crystallizing is en- 
larged, as in the example of water, of iron, 
and of the greater number of salts ; but in 
others the volume is diminished. Quick- 
silver, in congealing, contracts about -J- of 
its whole bulk, yet it exhibits the crystal- 
line texture ; and when the congelation is 
partial, the crystalline figure can even be 
discovered. 
Crystals deposited from water always 
contain a part of it which is retained by the 
affinity of the solid, and has passed with it 
into the concrete form. It is termed water 
of crystallization. Its quantity is very va- 
rious ; sonretimes it equals or exceeds the 
weight of the solid, and sometimes it a- 
mounts only to a few parts in the hundred. 
Much of the cold produced during the so- 
lution of salts in water, is owing to this 
water of crystallization passing into the 
fluid state: hence crystallized salts gene- 
