Specific gravity is the weight of a de terminate bulk of any body. For a stand ard of comparison distilled water has been assumed as unity. The specific gravity of solids is ascertained by comparing the weight of the body in the air with its weight when suspended in water. The quotient obtained by dividing its weight in air, by the difference between its weight in air and its weight in water, is its specific gravity. The specific gravity of fluids may be ascertained by compar ing the loss of weight of a solid body, such as a piece of crystal, when immersed in distilled water, with its loss when im mersed in the fluid we wish to examine; by dividing its loss of weight in the fluid by its loss of weight in the water, the quotient is the specific gravity of the fluid; or a small phial, containing a known weight of distilled water, may he filled with the fluid to be examined and weigh ed, and by dividing the weight of the fluid by the weight of the water, the specific gravity is ascertained.
Although these are the only general principles by which specific gravities are ascertained, yet as the result is always influenced by the state of the thermo meter and barometer at the time of the experiments, and as the manipulation is a work of great nicety, various ingenious instruments have been contrived to ren der the process and calculation easy. Of all these, the gravimeter of Morveau seems to deserve the preference.
It would be of material consequence to science and the arts, if specific gravities were always indicated by the numerical term expressing their relation to the specific gravity of distilled water. This, however, is unfortunately not the case, The excise officers in this country collect the duties paid by spirituous liquors, by estimating the proportion which they contain of' a standard spirit, about 0.933 in specific gravity, which they call hy drometer proof, and they express the re lation which spirits of a different strength have to the standard spirit, by saying that they are above or under hydrometer proof. Thus one in six, or one in seven, below hydrometer proof means that it is equal in strength to a mixture of six parts of proof spirit with one of water.
The only other mode of expressing specific gravities which it is necessary to notice is that of Beanie's areometer, as it is often used in the writings of the French chemists, and is little understood in this country. For substances heavier than water he assumes the specific gravity of distilled water as zero, and graduates the stem of his instrument downwards, each degree being supposed by him to ex press the number of parts of muriate of soda contained in a given solution, which however is not at all the case. • For sub
stances lighter than water the tube is graduated upwards, and this zero is af forded by a solution of 10 of salt in 90 water.
Mechanical Divitdon. By this process substances are reduced to a form better adapted for medical purposes; and by the increase of their surface their action is promoted, both as medical and chemical agents. It is per-firmed by cutting, bruis ing; grinding, grating, rasping, filing, pulverization, trituration, and granulation, by means of machinery or of proper in struments.
Pulverization is the first of these ope rations that is commonly employed in the apothecary's shop. It is performed by means of pestles and mortars. The bot tom of the mortars should he concave ; and their sides should neither he so in clined, as not to allow the substances ope rated on to fall to the bottom between each stroke of the pestle, nor so perpen dicular as to collect it too much toge ther, and to retard the operation. The materials of which the pestles and mor tars are formed should resist both the mechanical and chemical action of the substances for which they are used, Wood, iron, marble, siliceous stones. porcelain, and glass, are all employed : but copper, and metals containing cop per, are to he avoided, They should be provided with covers, to prevent the finest and lightest parts from escaping, and to defend the operator nom the effects of disagreeable or anxious substances. But these ends are more completely attained by tying a piece of pliable leather round the pestle and round the mouth of the mortar. It must be closely applied, and at the same time so large, as to permit the free motion of the pestle. In stone instances it will be even necessary for the operator to cover his mouth and nostrils with a wet cloth, and to stand with his back to a cnrr,ent of air, that the very acrid particles which arise may be carried from him. The addition of a little water or spirit of wine, or of a few almonds, to very light and dry substances, will pre vent their flying off. But almonds are apt to induce rancidity, and powders are always injured by the drying which is necessary when they have been moisten ed, 'Water must never be added to sub stances which absorb it, or are rendered cohesive by it.