It has been observed that the practice of combining certain vegetable tonics with lime-water, although very common, is in cases where we are desirous to obtain their _astringent_ effects, of very doubtful propriety; for the fact is, that _Tannin_ forms with the alkalies and alkaline earths, compounds that are not soluble in water,[280] and which are therefore probably inefficacious. It may perhaps be said that such an argument cannot avail, because if the astringent matter be even introduced into the stomach in its purest form, it will immediately form an insoluble compound on its contact with _gelatine_.[281] We know so little of the laws of _gastric chemistry_, that it is difficult to learn what changes take place in the animal laboratory; but it would seem probable that the powers of the stomach rather consist in decomposing the ingesta into simple forms, than in complicating them by favoring new combinations; besides which, if such a compound were formed, it would be subsequently decomposed _in transitu_; for the experiments of Sir H. Davy shew that vegetable astringent matter passes through the body unchanged. (_page 82._)
It is impossible to furnish any general rule that may enable the practitioner to avoid mixing together substances which are incompatible with each other; a knowledge of their chemical habitudes must in every case direct him, and these are enumerated in the second part of this work, under the history of each medicinal substance. The physician however will find it useful to retain in his remembrance the simple and beautiful law which has been so ably developed by the eminent author of the “STATIQUE CHIMIQUE,” that _whenever two salts in a state of solution are brought together, which contain, within themselves, elements capable of producing a soluble and insoluble salt, a decomposition must necessarily arise_;[282] he illustrates this law by the example of _Nitrate of Silver_ and _Muriate of Potass_, whose elements are capable of forming within themselves a soluble salt, _Nitrate of Potass_, and an insoluble salt, _Muriate of Silver_. It deserves also to be remembered, that a table of chemical affinity will not upon all occasions prove to the medical practitioner an unerring pilot; in those cases for instance, where a _super or sub Salt is readily formed, a substance less weakly attracted by another than a third, will sometimes precipitate this third from its combination with the second_, thus in the production of _Nitric acid_, we decompose the _Nitrate of Potass_ by virtue of the superior affinity of the _Sulphuric acid_ for its base, the nitric acid is accordingly disengaged, and a _Sulphate of Potass_ remains in the retort; now, paradoxical as it may appear, if nitric acid be poured upon the _Sulphate of Potass_, a quantity of nitre will be reproduced, in consequence of the saturation of a portion of the base, in such a proportion as to enable the remaining atoms to form a _Bi-Sulphate of Potass_. In the same manner the _Tartrate of Potass_ (Soluble Tartar) is, contrary to the usual affinities, decomposed by all sub-acid vegetables, which neutralize a portion of the base, and convert the salt into the _Bi-tartrate of Potass_ (_Cream of Tartar_). The same effect is even produced by _Carbonic acid_.[283]