PART I.

Elements or Pharmacy.

shape of the crystals which are obtained. In both cases,however, the affinity of the saline particles at length ceases toact, while the fluid still retains as much saline matter as it canhold dissolved at the temperature of the atmosphere, or is asaturated solution ; but, by a greater reduction of temperaturein the one case, and a further evaporation in the other, it willagain yield crystals.

~ByJiisio?i, bodies which are not soluble in water, as glass,sulphur, &c. are enabled to assume the crystalline form. Inthis case the body is, as it were, dissolved in caloric ; and theparticles being separated from each other, these, when thecooling is gradual, assume, in aggregating again, the regulararrangements which take place in crystallization. This modeof crystallizing substances is never used for pharmaceuticalpurposes.

Crystallization is promoted or retarded by various circum-stances, to be afterwards noticed. (See Section iii.) Its theoryis still obscure j but some light has been thrown upon it bythe experiments of Haiiy. He found that crystals may bemechanically divided, and reduced to certain primitive forms,which are always the same in the same kind of substances, anddepend upon the figure and the mode of combination of theintegrant particles composing the crystals. The varieties offigure of these particles, notwithstanding the great diversityof crystalline forms, are reducible to three: namely, 1. theparallelopiped, the faces of which are six, parallel two andtwo; 2. the triangular prism; and 3. the tetrahedron, or four-sided pyramid: and these particles, therefore, according tothe mode in which they unite, which may be either by theirfaces or their edges, form primitive crystals, which are thenuclei of the secondary crystals. The forms of primitive crys-tals may be reduced to the following six: 1. the parallelopiped,including the cube, the rhomboid, and all solids terminated bysix faces, parallel two and two; 2. the regular tetrahedron;3. the octahedron with triangular faces; 4. the six-sided prism;5. the dodecahedron, terminated by rhombs; and 6. the dode-cahedron, with isosceles triangular faces. The variations ofthe forms of secondary crystals are considerable in the samesalt, and depend, in general, either on variations in the pro-portions of the ingredients which compose the integrant par-ticles, or on the properties of the solvent in which the crystalsare formed: thus, alum crystallizes in octahedrons, but theaddition of a h'ttle alumina produces cubes : and an excess ofthis earth prevents crystallization altogether: thus also, muriateof soda, which crystallizes in cubes, when dissolved in water,assumes the regular octahedral form when it is crystallized inurine. Independent, however, of these causes, a variety of

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