Specific Gravity
A term used to express the comparative weight of different substances. The specific gravity of a substance is the weight of a given bulk of it compared with the weight of an equal bulk of some other substance taken as a standard. The standard employed is a fixed one, being distilled water at a temperature of 60 degrees. The weight of a cubic inch of silver is 10 1/2 times as much as the same measure of water; accordingly, the specific gravity of water being 1, that of silver is 10 1/2. A cubic inch of cork weighs 24/100 as much as the same bulk of water; the specific gravity of cork, therefore, is 24/100 or .24. Mercury, water, and oil if thrown into a tumbler will arrange themselves in the order of their specific gravities: the mercury at the bottom, being the heaviest; then the water; on top of this the oil, being the lightest. Gases, like liquids, differ in their specific gravity. Smoke ascends, being lighter then air. Hydrogen is so much lighter than air that it will ascend with a loaded balloon. Contrary to this, because carbonic-acid gas is heavier than air, it remains at the bottom of wells, etc. A cubic inch of iron weighs 7 1/4 times as much as a like bulk of water, and will therefore sink in the latter; but if hammered out into a vessel containing more than 7 1/4 cubic inches, the same iron will float, simply because it is lighter than an equal bulk of water. A floating substance displaces its own weight of liquid; and a body immersed in water loses as much weight as the water it displaces weighs. The specific gravity of a liquid is easily obtained in the following manner: Fill a glass vessel, whose weight is known, with water to a certain mark, and weigh it; subtract the weight of the vessel and you have the weight of the water alone. Then fill the vessel to the same height with the liquid in question, weigh it again, and subtract the weight of the vessel as before. To find its specific gravity divide its weight by that of the water.
A simple way of finding the specific gravity of a solid would be to take a certain bulk, as a cubic inch or cubic foot, ascertain its weight, and divide it by a like bulk of water. There is difficulty, however, in obtaining any given bulk exactly, for which reason other methods are adopted. If the solid sinks in water, weigh it first in air and then in water by means of a balance provided for the purpose. Divide its weight in air by the weight it loses in water, and the quotient will be its specific gravity. This is exactly the same as dividing the weight of the solid by that of an equal bulk of water, for it has been shown that a solid weighed in a liquid loses as much weight as the liquid it displaces weighs. A piece of platinum weighs 22 grains in air and 21 in water. If we divide 22 (its weight in air) by 1 (the loss of weight in water), we obtain 22 for the platinum's specific gravity. The specific gravity of a solid that floats on water is found by attaching something heavy enough to sink it. These are then weighed in air and in water, and the loss of weight in water found by subtraction, as before. In the same manner find how much weight the heavy body alone loses in water, and subtract this from the loss sustained by the two, which gives the weight of a volume of water equal to the body under examination. Divide the body's weight in air by this remainder, and the specific gravity is obtained. The specific gravity of gases is found by a similar process to that for liquids. The standard is air. A glass flask with stop-cock is weighed when full of air, and again when the air has been exhausted; the weight of the flask full of air is the difference between these weights. The flask is now filled with the gas in question, and again weighed; this weight, less that of the exhausted flask, is the weight of a flask full of the gas. Divide the weight of the gas by that of the air, and the quotient is the specific gravity required.
If the specific gravity of a body is known, it is easy to discover how much any given bulk of it weighs. A cubic foot of water weighs 1000 ounces, or 62 1/2 pounds. The weight of a cubic foot of any given substance will therefore be equal to 62 1/2 pounds multiplied by its specific gravity; as follows: What is the weight of a cubic foot of silver? The specific gravity of silver is 10.474. This multiplied into 62.5 gives 653.478 pounds,--the weight required. See HYDROSTATIC BALANCE; WEIGHT OF METALS.