A popular error exists with respect to the subject of chemical incompatibility, against which it may be here advisable to caution the inexperienced prescriber, viz. that _no important change is produced, on the admixture of solutions, unless precipitation is occasioned_. This however occurs only when the new compound produced is insoluble; thus _Sulphuric acid_ may be added to _Lime water_, by which a _Sulphate of Lime_ is formed, but as its proportion is not too large for the water to dissolve, no precipitate occurs; so again, a solution of _Nitrate of Silver_ is not apparently disturbed by the addition of _Ammonia_, because the resulting _Ammoniuret_ is a soluble compound. We should, nevertheless, commit a great error in supposing that, for such a reason, these bodies were not _incompatible_. On the other hand, the medicinal powers of a solution are not necessarily destroyed by the occurrence of a precipitate, although such a result should always be regarded with suspicion. III.—_The Methods directed for the preparation of the Ingredients are either inadequate to the accomplishment of the object, or they change and destroy the efficacy of the Substances._ The observations already offered upon _Formula 40_, will sufficiently explain the nature of the various errors comprehended under this head: so, again, if the virtues of a plant reside in _essential oils_, which are easily volatilized, or in _extractive matter_, which readily becomes oxidized, DECOCTION must necessarily destroy its efficacy; a striking example of this fact is presented us in the history of the _Laurel_ and _Bitter Almond:_ the poisonous influence of the essential oil and distilled water of these vegetable substances is well known, but their watery extracts are perfectly innocuous. A still more familiar example is to be found in the onion, or in garlic, which by simple coction is deprived of all its acrimonious qualities. On the other hand, an error equally injurious would be committed, by directing a simple infusion of a vegetable, whose medicinal properties depended upon resino-mucilaginous principles. Orfila found that an extract of Hemlock, prepared by boiling the dried powder in water and evaporating the decoction, was entirely destitute of activity. See _Decocta_, _Infusa_, _Extracta_. An instance of the baneful effects which may arise from an erroneous method of preparation happened some time ago to fall under my immediate notice and care; it was in preparing an infusion of the root of the _Veratrum_ with _Opium_, as directed by Mr. James Moore, when the dispenser ignorantly substituted a spirituous for a vinous menstruum.