Specific Gravity

(grav'e-te.) (Phys.) The weight of any kind of matter, considered with reference to that of an equal bulk of some other, which is a umed as a standard of comparison; and this standard, by universal consent, is distilled water generally at 62° Fahr. Comparison may be made with distilled water at any temperature, if the allowance required by its altered density is taken into account. It happens that a cubic foot of distilled water weighs 1000 ounces avoirdupois. Consequently, a uming this as the S. G. of rain-water, and comparing all other bodies with it, the numbers that expre the S. G. of bodies will at the same time denote the weight of a cubic foot of each in avoirdupois ounces; which is a great convenience in numerical computations. From the preceding definition we readily deduce the following laws:-1. In bodies of equal magnitude, the S. G. are directly as the weights, or as their densities. 2. In bodies of the same S. G., the weights will be as the magnitudes. 3. In bodies of equal weights, the S. G. are inversely as the magnitudes. 4. The weights of different bodies are to each other in the compound ratio of their magnitudes and S. G. Hence it is obvious that, of the magnitude, weight, and S. G. of a body, any two being given, the third maybe found; and we may thus ascertain the magnitude of bodies which are too irregular to admit of the application of the common rules of mensuration; or we may, by knowing the S. G. and magnitude, find the weight of bodies which are too ponderous to be submitted to the action of the balance or steel-yard; or lastly, the magnitude and weight being given, we may ascertain the specific gravities. The S. G. of a solid is found by weighing it first in air, and then while immersed in distilled water, or some fluid of known density which will not di olve it. The weight lost by immersion is the weight of a quantity of fluid equal in bulk to the body. The weight of the body in air, divided by the weight lost, will be its S. G. with reference to the fluid employed. If the body will not sink in the fluid, some substance that will make it sink must be attached to it. The effective part of the weight, added to the weight of the body, will be the weight of an equal bulk of the S fluid; and dividing the weight of the body in air by this, will give its S. G. with respect to the fluid. The most accurate and concise mode of ascertaining the density of liquids is to employ a small gla measure with a very short narrow neck, called a S. G. bottle, and adjusted to hold exactly 1,000 grains of distilled water. The ve el being counter poised, and then filled with any other liquid, its weight is observed, and the density of its contents, compared with that of water, may be found by merely cutting off three decimal places. After each operation, the gla must be carefully rinsed with pure water, and again dried, by heating it, and then sucking out the humid air by means ofa slender tube. The S. G. bottle enables us to take the S. G. of a body which is in powder -of a soil for instance. For this purpose, half the quantity of water the bottle would hold is to be placed in it; then just enough of the clay, ., to cause it to be filled with the mixture; and it is then to be weighed. The weight of the water being deducted, the remainder will be the weight of the clay, .; and this, divided by half the weight of the water which the bottle would hold, will be the S. G. of the clay, . The S. [s. 844]