The degree of fineness to which substances should be reduced by pulverization, in order to obtain their utmost efficacy, is a very important question. The impalpable form appears to be extremely injurious to some bodies, as to _cinchona_, _rhubarb_, _guaiacum_, and to certain aromatics, in consequence, probably, of an essential part of their substance being dissipated, or chemically changed by the operation. Fabbroni, for instance, found by experiment that cinchona yielded a much larger proportion of soluble extractive, when only coarsely powdered. I think it may be laid down as a general rule, that _extreme pulverization assists the operation of all substances whose active principles are not easily soluble, and of compound powders whose ingredients require, for their activity, an intermixture; whilst it certainly injures, if it does not destroy, the virtues of such as contain as their active constituent, a volatile principle which is easily dissipated, or extractive matter which is readily oxidized_. 2. _Compound Powders._ The disintegration of a substance is much accelerated and extended by the addition of other materials; hence the pharmaceutical aphorism of Gaubius, “_Celerior atque facilior succedat composita, quam simplex pulverisatio._” Thus several refractory vegetable bodies, as _myrrh_, _gamboge_, &c. are easily reduced by triturating them with sugar or a hard gum; and some gum resins, as _assafœtida_ or _scammony_, by the addition of a few drops of almond oil. Upon the same principle the Pharmacopœia directs the trituration of aloes with clean white sand, in the process for preparing _Vinum Aloes_, to facilitate the pulverization and to prevent the particles of aloes, when moistened by the liquid, from running together into masses; some dispensers very judiciously adopt the same mechanical expedient in making a tincture of myrrh; so again, in ordering a watery infusion of opium, it will be judicious to advise the previous trituration of the opium with some hard and insoluble substance, as directed in the _Pulvis Cornu Usti cum Opio_, otherwise its particles will adhere with tenacity, and the water be accordingly unable to exert a solvent operation upon its substance.[295] It is equally evident that in the construction of compound medicinal powders, the addition of an inert ingredient, which the mere chemist might condemn and discard as useless, not unfrequently acts a very important part in the combination, owing to its effects in dividing and comminuting the mere active constituents: the _sulphate of potass_ in Dover’s powder acts merely in dividing and mixing more intimately the particles of opium and ipecacuan: the _phosphate of lime_ appears to act in the same mechanical manner in the Antimonial Powder; so again, in the _Pulvis Contrajervæ compositus_, the prepared oyster shells may be a necessary ingredient: in the _Pulvis Jalapæ compositus_ of the Edinburgh