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] There are besides certain cases wherein _Triple Salts_ are produced, which afford apparent exceptions to the usual affinities of the bodies involved in the combination; we have a very good illustration of this truth in the decomposition of the _Liquor Ammoniæ Acetatis_ by _Magnesia_; if the practitioner refers to a table of affinities, he will perceive that _Acetic acid_ has a greater attraction for _Ammonia_ than for _Magnesia_; but if upon this assurance he were to administer these bodies together, he, or his patient, would soon discover that ammonia is developed with considerable pungency; now in this case the _Magnesia_ forms a _triple Acetate_ with one part of the ammonia, and consequently sets the remainder at liberty.