Powders are usually mixtures of two or more powdered substances, and may be intended for either internal or external use. They are prepared by mixing the ingredient or ingredients ordered in the smallest quantities with a gradually increased quantity of the remaining material, either by gentle trituration in a mortar or mixing machine, or in the case of small quantities by mixing by means of a spatula on paper, the mixture being passed through a sieve of suitable mesh. Uniformity of the product is ensured by repeating the process of trituration. The degree of comminution obtained in the finished powder depends principally upon the degree of fineness pertaining to the powdered substances employed. When drugs are to be incorporated or administered in powders it is customary to employ the powdered drugs in general use, without reference to a special degree of comminution. Uniformity is sufficiently ensured by specifying, in certain cases, that substances shall be in fine powder before admixture with other ingredients. Substances passed through sieves containing from eighty to one hundred and twenty or more meshes to each linear inch are usually considered to be in fine powder. Substances passed through sieves containing from forty to sixty meshes to each linear inch are generally considered to be in powder. Substances passed through sieves containing from twenty to thirty meshes to each linear inch are usually considered to be in coarse powder. Deliquescent or hygroscopic substances, if ordered and administered in powder form, should be coarsely powdered, other ingredients (if any) being mixed in gently or added separately to each powder. Deliquescent or hygroscopic powders should be wrapped in waxed paper and finally in tinfoil or tinfoil paper.