CRYSTALLIZATION. 
rally produce more cold than when they 
are uncrystallized. If the water of crystal- 
lization be expelled from a crystal, it looses 
its transparency, and at length its form. 
Crystals which part with their water of 
crystallization when exposed to the atmos- 
phere, are said to eiHoresce, and to deli- 
quesce when they attract water and become 
humid. 
Some substances have so strong an affi- 
nity for the fluids in which they are dis- 
solved, or so little tendency for cohesion, 
that they do not crystallize. In some cases 
their crystallization may be effected by ad- 
ding to the solution a substance exerting 
an affinity to the fluid, and of course weak- 
ening its affinity for the solid it dissolved. 
As different bodies require very different 
quantities of water for their solution, it is 
possible when two such bodies are dissolved 
in one fluid, to obtain them separate by 
crystallization, the one which is least solu- 
ble, or most disposed to crystallize, first 
passing into the solid form ; and by farther 
evaporation the other is obtained. A fact 
on this subject, somewhat singular, is no- 
ticed by Mr. Kirwan. If into a saturated 
solution of two salts in water, a crystal of 
either be put, that salt crystallizes in pre- 
ference to the other. 
By crystallization, also, salts, the solu- 
bility of which is unequally promoted by 
heat, may be obtained separately from the 
same solution. Thus, if one salt be much 
more soluble in hot than in cold water, 
and another be equally soluble, or nearly 
so at any temperature, on evaporating the 
solution sufficiently, the latter salt will 
crystallize while the solution is hotj on 
cooling, tlie other will shoot into crystals; 
and by alternate evaporation and cooling, 
the two may be obtained uncombined, 
though generally with a little intermixture 
of each other. 
Sometimes, however, when two salts are 
in solution in the same fluid, and have even 
different tendencies to crystallization, their 
mutual affinity leads tliem to crystallize in 
one mass, and even to assume a form diffe- 
rent from that in which separately they 
would have crystallized. 
In other cases this mutual affinity, be- 
tween substances in solution, is sufficient 
to resist their crystallization, or to render 
it more difficult. 
Crystallization sometimes takes place, 
when bodies in the gaseous form become 
subject to the attraction of aggregation, as 
in sublimates ; and even solids separated 
from a liquid by chemical action, in some 
instances at the moment of their separation, 
assume a crystallized form. 
Every substance in crystallizing is dis- 
posed to assume a particular figure. Thus, 
sea-salt crystallizes in the form of a cube ; 
nitre in that of a hexaedral prism ; sugar in 
that of a four or six-sided prism, with trie- 
dral terminations. The crystalline figure 
in any substance, however, is not invariar 
ble, but may be altered by circumstances 
affecting the crystallization; and we find 
the same substance crystallized under a va- 
riety of forms. Sea-salt crystallizes, not 
only in cubes, but also in octaedrons ; and 
carbonate of lime is found in nature in the 
form of an hexaedral prism, an hexaedral, 
and a triedral pyramid. 
The effect of light upon the act of crys- 
tallization is very remarkable. It is found 
in general, that the crystals of salts are 
larger and better formed in the dark than 
when light falls upon the solution. But this 
relates only to such crystals as are formed 
in the fluid. In many, and indeed most 
salts, there are crystals formed, during the 
spontaneous evaporation of the solution, 
which rise above the surface into the air, 
either in contactwith the sides of the ves- 
sel, or supported by their own structure. 
This phenomenon is very striking and cu- 
rious, and it appears to have been well de- 
termined by experiments of Chaptal and 
others, that it does not take place without 
the presence of light. See Vegetation 
OF Salts. 
CRYSTALLOGRAPHY. Hauy has 
succeeded in developing the theory of 
crystals, so far as to shew, that in every 
crystallized snbstance, whatever may be 
the difference of figure which may arise 
from modifying circumstances, there is in 
all its crystals a primitive form, the nucleus, 
as it were, of the crystal ; invariable in each 
substance, gnd by various modifications, 
which he points out, giving rise to tire nu- 
merous secondary, or actually existing 
forms. 
The fact which led to these views, is that 
crystals can be mechanically divided only in 
certain directions, so as to afford smooth sur- 
faces, a fact long known by those who work on 
the gems. Suppose we have a crystal of calca- 
reous spar, a regular hexaedral prism, repre- 
sented in plate crystallography, fig. Sand 6, 
if we endeavour to divide it parallel to the 
edges which form the outlines of the basis 
of the prism, we shall find that three of these 
edges taken alternately, are the upper extrc 
