PHARMACY. 
mixture to settle after we think that enough 
of tlie precipitant has beenadded, and try a 
little of the clear solution, by adding to it 
some of the precipitant ; if any precipitation 
takes place, we have not added enough of 
tlie precipitant. This is necessary, not only 
to avoid loss, but in many instances the pre- 
cipitant, if added in excess, re- dissolves or 
combines with the precipitate. 
After the precipitation is completed, the 
precipitate is to be separated from the su- 
pernatant fluid by some of the means already 
noticed. 
When the precipitate is the chief object 
of our process, and when it is not soluble in 
water, it is often advisable to dilute to a 
considerable degree both the solution and 
precipitant before performing the operation. 
AVhen it is only difficultly soluble, we must 
content ourselves with washing the precipi- 
tate after it is separated by filtration. In 
some cases the separation of the precipitate 
is much assisted by a gentle heat. 
Crystallization is a species of precipita. 
tion, in which the particles of the solvend, 
on separating from the solution, assume 
certain determinate forms. The conditions 
necessary for crystallization are, that the 
integrant particles have a tendency to ar- 
range themselves in a determinate manner, 
when acted on by the atrraction of aggre- 
gation ; that they be disaggregated, af 
least, so far as to possess sufficient mobility 
t^ assume their peculiar arrangement ; and 
that the causes disaggregating them be slow- 
ly and gradually removed. 
Notwithstanding thp immense variety in 
the forms of crystals, M. Hauy has rendered 
it probable that there are only three forms 
of the integrant particles, the parallelopi- 
ped, the triangular prism, and the tetrahe- 
dron. But as these particles may unite in 
ditferent ways, either by their faces or edges, 
they will compose crystals of various forms. 
The primitive forms have been reduced 
to six, the parallelepiped, the regular tetra- 
hedron, the octahedron with triangular 
faces, the six-sided prism, the dodecahedron 
tei-minated by rhombs, the dodecahedron 
with isoscles triangular faces. 
Almost all sustances on crystallizing retain 
a portion of water combined with them, 
which is essential to their existence as crys- 
tals, and is therefore denominated water of 
crystallization. Its quantity varies very 
much in different crystallized substances. 
The means by which the particles of 
bodies are disaggregated, so as to admit of 
ci ystallization, are solution, fusion, vapori- 
zation, or mechanical division and siispen. 
sion in a fluid medium. The means by 
which the disaggregating causes are remov* 
ed are evaporation, reduction of tempera, 
ture, and rest. 
When bodies are merely suspended in a 
state of extreme mechanical division, no- 
thing but rest is necessary for their crystal- 
lization. When they are disaggregated by 
fusion or vaporization, the regularity of 
their crystals depends on the slowness with 
which their temperature is reduced ; for if 
cooled too quickly, their particles have not 
time to arrange themselves, and are con- 
verted at once into a confused or unvaried 
solid mass. Thus glass, which when cooled 
quickly is so perfectly uniform in its appear- 
ance, when cooled slowly has a crystalline 
texture. But in order to obtain crystals by 
means of fusion, it is often necessary, after 
the substance has begdn to crystallize, to 
remove' the part which remains fluid ; for 
otherwise it would fill up the interstices 
among the crystals first formed, and give 
the whole the appearance of one solid mass. 
Thus, after a crust has formed on the top of 
melted sulphur by pouring off the still fluid 
part, we obtain regular crystals. 
Tlie means by w'hich bodies which have 
been disaggregated by solution are made 
to crystallize most regularly, vary according 
to the habitudes of the bodies with their 
solvents and caloric. 
Some saline substances are much more 
soluble in hot than in cold water. Therefore 
a boiling saturated solution of any of these 
will deposit, on cooling, the excess of salt, 
which it is unable to dissolve when cold. 
These salts commonly contain much water 
of crystallization. O ther salts are scarcely, 
if at all, more soluble in hot than in cold 
water; and, therefore, their solutions must 
be evaporated either by heat or sponta- 
neously. These salts commonly^ contain 
little water of crystallization. The beauty 
and size of tlie crystals depend upon the 
purity of the solution, its quantity, and the 
mode of conducting the evaporation and 
cooling. 
When the salt is not more soluble in hot 
than in cold water, by means of gentle eva- 
poration, a succession of pellicles are formed 
on the top of the solution, which either are 
removed or permitted to sink to the bottom 
by their own weight ; and the evaporation 
is continued until the crystallization be 
completed. But when the salt is capable 
of crystallizing on cooling, the evapora- 
tion is only continued until a drop of the 
