"All solid drugs have to be reduced to powder for percolation. If a coarse powder is required or allowed by the formula, bruising the substance, that is, breaking it merely into a coarse powder, in a deep mortar by the aid of a heavy pestle, might suffice. Mortars of iron or brass are best adapted for preparing powders by contusion. Some tough materials are conveniently reduced to powder by rasping.

" Most of the vegetable powders are, however, prepared on a large scale at the drug mills by grinding, the grinding surface being a bed of hard stone upon which either iron balls or millstones are made to revolve. A number of drug mills which may be worked by hand have been constructed, and serve a good purpose in preparing the coarser kind of powders, but none have been made which could be used with advantage for finely powdering moderate quantities.

"Whether contusion or grinding is resorted to, the'drugs to be powdered should be deprived of the moisture remaining in them by exposing them in a drying room to a temperature of about 50° C. (122° F.). Drugs, however, which owe their virtues to volatile oil, and which should be powdered in the air-dry condition or on a small scale, are conveniently dried over burnt lime. The different tissues of plants are not reduced to powder with equal facility; the fibro-vascular bundles and the bast-fibres are usually more refractory than parenchyma-tissue, and more or less of the former generally remains behind as a coarse powder. When the powder is intended to be exhausted by percolation or otherwise, the coarse particles are utilized with the fine powder. The fine powder which may be sifted off from time to time during the process of powdering should be carefully mixed to ensure uniformity in composition". - IV. D. Fineness of Powders. - The United States Pharmacopoeia designates five degrees of fineness - namely, very fine, when passed through a sieve of eighty or more meshes to the linear inch (No. 80 powder); fine, when passed through one of sixty meshes (No. 60 powder); moderately fine, through one of fifty meshes (No. 50 powder); moderately coarse, through one of forty meshes (No. 40 powder); and coarse, if passed through a sieve of twenty meshes to the linear inch (No. 20 powder). The fineness of powders is often conveniently described as being No. 20, etc., indicating that it has been passed through a sieve of that size". Compound powders are a mixture of several powders.

Fig. 388.   Mortar

Fig. 388. - Mortar.

Fig. 389.   Drug Mill

Fig. 389. - Drug Mill.