The Eclectic and general dispensatory : comprehending a system of pharmacy, materia medica, the formulae of the London, Edinburgh, and Dublin pharmacopoeias, prescriptions of many eminent physicians, and receipts for the most common empirical medicines / collated from the best authorities, by an American physician.

Date:
1827
Catalogue details

Licence: Public Domain Mark

Credit: The Eclectic and general dispensatory : comprehending a system of pharmacy, materia medica, the formulae of the London, Edinburgh, and Dublin pharmacopoeias, prescriptions of many eminent physicians, and receipts for the most common empirical medicines / collated from the best authorities, by an American physician. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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    portance than the next variety of contigu- ous attraction. b. OF CHEMICAL ATTHACTIOJT, On AFFI- HITT.* Chemical attraction, or affinity, is that power by which dissimilar substances, pla- ced under certain circumstances, are ena- bled to unite, and form new aggregates, in which the properties of the component particles are lost or changed. Its action is confined to the minute atoms or particles of bodies, and is exerted only at insensible distances : not indifferently, however, be- tween the particles of all bodies, but elec- tively. The result of its operation is a combination of the constituent particles of the substances, so intimate (that the compo- nents cannot be recognized nor separated by any mechanical force. Thus, lime acts as a powerful caustic when applied to ani- mal matter, and is partially soluble in wa- ter; phosphoric acid has an acid taste, and is very soluble in water; but phosphate of lime, the compound, produced by the che- mical combination of these substances, is inert when applied to animal matter, insi- pid, and insoluble in water ; and cannot be again resolved into lime and phosphoric acid by any mechanical power. Chemical combination, therefore, is the result of the affinity of two or more sub- stances for each other. It differs from mix- ttire, in which the substances are only blended without acquiring any new pro- perties, and in which the dissimilar parts are easily discovered, and may be separa- ted by mechanical powers. Chemical com- pounds can, however, be decomposed, either by exposure to a high temperature, which weakens the force of attraction ex- isting between their principles; or, by mixture, under favourable circumstances, with some other chemical agent, which has a more powerful affinity for one of the components than these have for each oth- er : and by such means, which constitute chemical analysis, the principles of a com- pound may be ascertained. As analysis separates compounds into their constituent principles, so synthesis may reproduce them by recombining these principles ; and when this can be effected, it is the surest proof of the accuracy of any analysis. In many instances, however, this is impossible: and the evidence of the truth of an analysis is to be drawn from oth- er sources. It is an acknowledged law of chemical affinity, that a compound does not possess properties merely intermediate between those of its component parts, but has ac- • For many of the following observations on affi- nity, I am indebted to the remarks of my late res- pected friend, Dr. Murray : see his System tf Chemis- try, vol. i. quired others more or less new. One of the most general changes is that of form. The combination of two gases, for exam- ple, may produce a fluid or a solid: that of two fluids may form a solid ; and the com- mon process of solution presents to us the fact, that by the combination of a solid with a fluid, the solid assumes the fluid form. In the last-mentioned instance the fluid is generally regarded as the active substance ; but, nevertheless, the attraction of affinity is reciprocal; and hence, the general mode of expressing the fact, that the fluid dis- solves the solid, or is the solvent or men- struum, is, in strict language, erroneous'. These terms, however, are more correctly applied, when the properties of the solid, except form, are scarcely sensibly altered ; as, for example, when common salt, muri- ate of soda, is dissolved in water. Chemical combination produces an al- teration of density—that of the compound not being the mean of the components, but often different. In the greater number of cases the density is increased ; and there is- a diminution of volume, owing probably to the compound atom being of a form admit- ting a more compact aggregation than the component atoms in their separate state; but the specific gravity of a compound can- not be determined by calculation from the specific gravity of its ingredients. There are- cases of combination, however, in which the density is diminished: and there is an in- crease of volume in the resulting com- pound ; for instance, when a solid is dis- solved by a liquid, the increase of volume acquired by the solid in passing into the fluid state may be greater than the con- densation resulting from its union in that state with the liquid; and this happens from the solution of a considerable number of the salts in water. The exertion of chemical attraction i& accompanied by a change of temperature. Thus, if four parts of sulphuric acid and one part of water, both at the temperature of 32°, be mixed together, the temperature of the mixture rises to 300°; and the den- sity of the compound is much greater than the mean of the densities of the components. The heat also which is evolved by combus- tion, and in fermentation, is the direct con- sequence of chemical combination. In all case6 the increase of temperature is accom- panied with an increase of density, to which, and the change of form suffered by one or both of the components, its produc- tion may be ascribed. Thus water which is solidified by being mixed with quicklime, parts with a large portion of caloric. The contrary effect, however, or an absorption of caloric, is also produced by chemical combination, when the density of the com- pound is less than the mean; as, for in- stance, ^ hen, by solutions of salts in water,