The American new dispensatory. Containing general principles of pharmaceutick chemistry, chemical analysis of the articles of materia medica, pharmaceutick operations ... / [James Thacher].

  • Thacher, James, 1754-1844.
Date:
1821
Catalogue details

Licence: Public Domain Mark

Credit: The American new dispensatory. Containing general principles of pharmaceutick chemistry, chemical analysis of the articles of materia medica, pharmaceutick operations ... / [James Thacher]. Source: Wellcome Collection.

    86/748 (page 80)
    When a substance, in passing to the solid state, assumes a regular geometrick form, the process is named crystal- lization, and these figured masses are denominated crys- tals. Their forms are various, though nearly constant with regard to each substance ; they are usually transpa-. rent, hard, and have a regular internal structure. The crystallization may happen in two ways, from a state of solution. Ifa saturated solution has been prepared with the aid of heat, the increased quantity of the solid which the heat has enabled the liquid to dissolve, separates as the temperature falls; and the attraction of cohesion be- ing thus slowly exerted between the particles, unites | them so as to form crystals. Or, if a portion of the sol- vent be withdrawn by evaporation, and especially by slow evaporation, the particles of the solid unite slowly, and with a similar result. In both these kinds of crystallization from a watery so- lution, the crystallized substance always retains a quantity of water, and frequently even a considerable proportion in its composition. It is essential to the constitution of the crystal, its transparency, structure and form, and is hence named the water of crystallization. Some crys- tals lose it from mere exposure to the air, when they are said to effloresce ; others attract water, and become hu- mid, or deliquesce. Crystallization is promoted by the mechanical action of the air; likewise by affording a nucleus, whence it may commence, and especially a crystal of the substance dissolved ; and with regard to a few substances, their affinity to the solvent requires to be diminished by the ad- dition of another substance to enable them to crystallize. In pharmacy, crystallization is of importance, by ena- bling us to obtain substances, especially those belonging to the class of salts, ina pure form ; different salts, even when present in the same solution, being thus separated by their different tendencies to crystallization, according as they are more or less soluble in the solvent, or have their solubility more or less promoted by heat; and each salt, when it does crystallize, being in general pure. These are the principal operations of pharmacy. Con- nected with this subject, there remain to be noticed the weights and measures which are usually employed. The division, according to what is named troy weight, is that