Sect; S. Pharmaceutical Operations,

cxv

cipitant, before performing the operation. When it is onlydifficultly soluble, we must content ourselves with washing theprecipitate, after it is separated by filtration. In some cases,the separation of the precipitate is much assisted by a gentleheat.

5i>2. Crystallization is a species of precipitation, in whichthe particles of the solvend, on separating from the solution,assume certain determinate forms.

552. The conditions necessary for crystallization are,

1. That the integrant particles have a tendency to ar-

range themselves in a determinate manner whenacted on by the attraction of aggregation ;

2. That they be disaggregated, at least so far as to pos-

sess sufficient mobility to assume their peculiar ar-rangement;

3. That the causes disaggregating them be slowly and

gradually removed.

553. Notwithstanding the immense variety in the forms ofcrystals, M. Hauy has rendered it probable, that there areonly three forms of the integrant particles :

1. The parallelopiped.

2. The triangular prism.

3. The tetrahedron.

55b. But as these particles may unite in different ways,either by their faces or edges, they will compose crystals ofvarious forms.

555. The primitive forms have been reduced to six.

1. The parallelopiped.

2. The regular tetrahedron.

3. The octahedron with triangular faces.

4. The six-sided prism.

5. The dodecahedron terminated by rhombs.

6. The dodecahedron with isosceles triangular faces.

556. Almost all substances, on crystallizing, retain a portion«f water combined with them, which is essential to their exist-ence as crystals, and is therefore denominated water of crystal-lization. Its quantity varies very much in different crystal-lized substances.

557. The means by which the particles of bodies are disag-gregated, so as to admit of crystallization, are solution, fusion,vaporization, or mechanical division and suspension in a fluidmedium.