3. _Hard Mineral Substances_ should be formed into an electuary with some conserve, as they are too ponderous to remain suspended in syrup. It deserves also to be noticed, that in consequence of the readiness with which metallic preparations undergo change, it will be generally adviseable to keep the active ingredients in the form of powder, and to add them to the syrup only just before they are required; the Electuary of the French Pharmacopœia, which is commonly called “_Opiata Mesenterica_,” will furnish a good example, “_quantumvis molle fuerit recens, progressu temporis, ob ferrum quod ipsi inest, mirè indurescit_.” LIQUID FORMS. MISTURÆ. _Mixtures._ These preparations are generally _extemporaneous_, in which different ingredients are mingled together in the liquid form, or, in which solid substances are diffused through liquid, by the medium of mucilage or syrup: for prescribing mixtures the following general rules may be laid down. I. Substances which are capable of entering into chemical combination, or of decomposing each other, ought not to be mixed together, unless it be with a view of obtaining the new products as a remedy. II. Transparency is not a necessary condition,[302] and hence insoluble powders may be advantageously introduced into mixtures, if the following precautions be observed. 1. They must be divisible and mechanically miscible in the liquid. 2. They must not possess too great a specific gravity. 3. They must not render the liquid too mucilaginous or thick; _thus_, f℥j _should seldom contain more than_ ʒss _of a vegetable powder_, ℈ij _of an electuary_, _and conserve_; _or_ grs. xv, _or_ ℈j _of an extract_. III. The taste, the smell, and the general aspect of the mixture should be rendered as pleasant as possible; _thus milk covers the taste of bark, of the tinctures of guaiacum and valerian, and that of lime water_; _and a light decoction of the liquorice root disguises a bitter taste more effectually than sugar_. The Physician may also produce occasional changes in the appearance of his mixture, in order to reconcile a delicate taste to its continuance; he never ought however to alter the essential part of plans which he finds advantageous. A DRAUGHT differs merely from a mixture in quantity; it is usually taken at once, and should not exceed f℥iss; it should be always preferred when, 1. _The remedy is to be taken in a precise dose._ 2. _Whenever it is liable to spontaneous decomposition._ 3. _Whenever the action of the atmosphere occasions change._