A manual of materia medica and therapeutics : including the preparations of the pharmacopoeias of London, Edinburgh, and Dublin, with other approved medicines / by J. Forbes Royle.

Royle, J. Forbes (John Forbes), 1798-1858.

Date:: MDCCCLVI [1856]

Credit: A manual of materia medica and therapeutics : including the preparations of the pharmacopoeias of London, Edinburgh, and Dublin, with other approved medicines / by J. Forbes Royle. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

27/818 (page 13)

$of foreign bodies, &c. and by agitation ; but the crystals then pro- duced are small, as also when the process is quickly performed. The majority of crystals, when deposited from their watery solu- tions, carry with them a certain portion of water, which they render solid, but much of which they also part with by the mere application of heat or by exposure to the air. This is called water of crystalli- sation, and it exists in crystals in its equivalent proportion, or some multiple of it. Professor Graham considers a portion of this water, when it replaces a base, as essential to the constitution of the salt. This is then sometimes described as water chemically combined. A salt or body combined with water is called a hydrate, one without any, anhydrous. Salts which contain much water of crystallisation, when heated, undergo aqueous fusion, and, the water being dissi- pated, they are left as dry salts : e.g. burnt Alum. Some, when exposed to the air, lose this water, and are said to effloresce ; others absorb water, and are said to deliquesce ; while those which undergo no change in the air are called permanent. Crystals are also studied with respect to their forms and the nature of their formation. Crystallography is now an extensive and inde- pendent science, the principles only of which we need notice, as they are treated of in separate works, as well as in systems of Chemistry. As many substances have forms peculiar to themselves, it is neces- sary to be acquainted with those which characterise different phar- maceutical salts, as they may thus be distinguished even when very minute. Some forms, however, are common to several distinct minerals, and these are therefore sometimes grouped together accord- ing to their external characters. The forms of these crystals have been distinguished into those which are Primary, and others which, from being cons dered as modifications of them, are called Secondary. That the external forms are connected with a certain regularity of internal structure, is evident from the well-known facts of the cleavage of crystals, also their refraction of light and different degrees of ex- pansion by heat. The primitive forms are,—1. The Cube. 2. The Tetrahedron, contained under four equilateral triangles. 3. The Oc- tohedron, contained under eight equilateral triangles. 4. The Hex- angular Prism. 5. The Rhombic Dodecahedron, limited by twelve rhombic faces ; and 6. The Dodecahedron with isosceles triangular faces. The secondary forms may be produced by modifications of the above, as by decrements of particles taking place on their edges and angles, which would produce a great variety of forms. Thus the Octohedron, Tetrahedron, and others, may be produced from a Cube, and many secondary forms from each. Though there can be no doubt that the internal structure of crys- tals must be as regular as their external figures, because it seems evident from the unequal expansion and contraction of certain crystals by changes of temperature, and also from their different modes of refracting light, yet, to use Mr. Daniell’s words, crystallographers now confine themselves to the experimental determinations, and the$