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Sifting

This is a means employed pulverized by simply rubbing through a fine to obtain uniformity of fineness in a pulversieve; while many hard and gritty, and some ized substance; and is also of use in mixing soft substances, such as chalk, antimony, &c., different substances powdered to the same are pulverized on a large scale by elutriation. degree of fineness. The sieves used for this (See No. 14.) Others will only yield to a purpose are furnished with cloths of various rasp or file. Whichever method is adopted, materials and different degrees of fineness; the substance to be pulverized must be very consisting of brass wire, horse hair, buckram, dry, and may even require artificial drying book muslin, gauze, or raw silk; this last or desiccation. (See No. 12.) On the other constituting a bolting cloth for sifting ina bolting cloth for sifting imhand, a few substances, as rice, sago, nux palpable powders. These are stretched over vomica, &c., are often soaked in water, or a wooden cylinder in the same manner as the steamed, before being pulverized. In some head of a drum. During the process of pul- . During the process of pulcases, some other substance or intermedium is verizing, the use of the sieve is necessary from introduced to aid in the operation; thus, time to separate the finer powder from sugar is used in pulverizing civet, musk, nut- the coarser particles, which have to be re- , which have to be remeg, and vanilla; absorbing the moisture turned after each sifting, to the mortar for which could not otherwise be readily got rid further trituration. The powder is made to of. The addition of a very small quantity of pass through the meshes of the sieve by alcohol renders the powdering of camphor gently agitating it between the hands; a easy. Gold leaf is pulverized by mixing with rough jarring motion will force through some of the coarser particles, and destroy the uniformity of the powder. A sieve should be fitted with a drum head, top and bottom, the upper one to confine the dust of the substance being sifted, and the lower one to catch the sifted powder as it falls through the sieve. sulphate of potassa, and then removing the potassa by washing with water. (See also No. 2517.) Fusible metals are reduced by melting and rubbing in a mortar until cold, or by agitating when melted in a box covered inside with chalk or whiting. Glass, quartz and silicated stones require to be heated red An arrangement of this kind is called a drum hot and then thrown into cold water, to make or box sieve. them sufficiently friable for pulverization. 29. Solution. Under the head of solu- When powdering very dusty or costly articles in a mortar, it should be covered with a loose skin of leather, fastened firmly round the top of the mortar and the pestle, to prevent loss of the dust, and possible injury to the operator's lungs. When a substance is required to be reduced to an impalpable powder, a slab and muller are used; this process is termed porphyrization. tions, are properly included only those liquids which consist of water or an aqueous menstruum, in which has been dissolved an appropriate quantity of any soluble substance to impart to the liquor its peculiar properties.

When spirit is the dissolving medium, the liquid receives the name of alcoholic solution, spirit, or tincture, while substances dissolved in water form aqueous solutions. In cases where a substance is dissolved in an acid or alkaline solution, whose acid or alkali is afterwards neutralized by means of an alkali (to counteract the acid), or an acid (to destroy alkali), the solution is then termed a neutral solution. A saturated solution is a solution made according to No. 27.