Elements of Pharmacy.

part J.

secondary forms make their appearance ; which the theory ofHaiiy explains, by supposing that, as the matter which envelopesthe primitive nucleus to form a secondary crystal is attractedin thin layers, each layer decreasing in size in consequence ofone or more rows of integrant particles being abstracted fromits primitive edges or angles. The decrements may be on theedges of the slices, which correspond with the edges of theprimitive nucleus; or on the angles, that is, parallel to thediagonals of the faces of the primitive, nucleus; or the decre-ments may be intermediate, parallel to lines situated obliquelybetween the diagonals and edges of the feces of the primitivenucleus. It would be impossible, however, to give a satis-factory view of this ingenius theory in the narrow compass ofthis epitome; and therefore I must refer the reader to Haiiy'sTraite de Mineralogic, tomes 1 and 2; to the Annates deChimie, torn. 17; and the 3d volume of the fifth edition ofThomson's System of Chemistry.

Such is the attraction of aggregation, and its general effects.It is frequently concerned in modifying pharmaceutical results;but it is a power of much less importance than the next varietyof contiguous attraction.

b. OF CHEMICAL ATTRACTION, OR AFFINITY. '

Chemical attraction, or affinity, is that power by which dis-similar substances placed under certain circumstances areenabled to unite, and form new aggregates, in which the pro-perties of the component particles are lost or changed. Itsaction is confined to the minute atoms or particles of bodies,and is exerted only at insensible distances: not indifferently,however, between the particles of all bodies, but electively.The result of its operation is a combination of the constituentparticles of the substances, so intimate that the componentscannot be recognised nor separated by any mechanical force.Thus, lime acts as a powerful caustic when applied to animalmatter, and is partially soluble in water; phosphoric acid hasan acid taste, and is very soluble in water; but phosphate oflime, the compound produced by the chemical combination ofthese substances, is inert when applied to animal matter, in-sipid, and insoluble in water; and cannot he again resolvedinto lime and phosphoric acid by any mechanical power.

Chemical combination, therefore, is the result of the affinityof two or more substances for each other. It differs frommixture, in which the substances are only blended withoutacquiring any new properties, and in which the dissimilarparts are easily discovered, and may be separated by mechani-

1 For many of the following observations on affinity, I am indebted to the re-marks of my late respected friend, Dr. Murray: see his System of Chemistry,vol. i.