Specific Gravity, the ratio of the weight of one body to that of an equal volume of another, adopted as a standard of reference. For solids and liquids the standard is pure water, at a temperature of 60° F., the barometer being at 30 inches. Air is the standard for aeriform bodies. A cubic foot of water weighing 1,000 oz., if the same bulk of another substance, as for instance cast iron, is found to weigh 7,200 oz., its proportional weight or specific gravity is 7.2. It is convenient to know the figures representing this proportion for every substance in common use, that the weight of any given bulk may be readily determined ; and for all substances the specific gravity is used among other tests for the purpose of distinguishing bodies from each other, the same substance being found, under the same circumstances, to retain its peculiar proportional weight or density. Hence tables of specific gravity are prepared for reference, and in every scientific description of substances the specific gravity is mentioned. In practical use, the weight of a cubic foot is obtained from the figures representing the density by moving the decimal point three figures to the right, which obviously from the example above gives the ounces, and these divided by 16 the pounds avoirdupois, in the cubic foot.

Different methods may be employed to ascertain the specific gravity of solids. That by measuring the bulk and weighing is rarely practicable, nor is it desirable. As a body immersed in water must displace its own bulk of the fluid, the specific gravity may be ascertained by introducing a body, after weighing it, into a suitable vessel exactly filled with water, and then weighing the fluid which is expelled. The proportional weight is then at once obtained. Wax will cause its own weight of water to overflow; its specific gravity is then 1. Platinum, according to the condition it is in, will cause only from 1/21 to 1/21.5 of its weight of water to escape, showing its specific gravity to be from 21 to 21.5. But a more exact method than this is commonly employed. The difference of weight of the same substance, weighed in air and when immersed in water, is exactly that of the water it displaces, and may consequently be taken as the weight of its own bulk of water. The specific gravity then is obtained by weighing the body first in air, and then, suspended by a fibre of silk or a hair, in water, and dividing the weight in air by the difference.

If the body is lighter than water, it is to be attached to one heavier, to make it sink; then find the loss of the two by immersion, and also the loss of the heavier body; the difference will express the weight of water displaced by the lighter body, whose weight divided by this difference will give its specific gravity. It is hardly necessary to say that the substance examined must be free from mixture of foreign matters, and especially from cavities that may contain air. Minerals, if suspected to contain such, should be coarsely pulverized, and then the second method above may be conveniently applied to determine their density. The specific gravity of fine powders may be determined by one of the methods employed for ascertaining the specific gravity of fluids, viz. : by comparing the weight of a measured quantity with that of the same quantity of water. A glass vessel called a specific gravity bottle is commonly employed, which is furnished with a slender neck, upon which is a mark indicating the height reached by 1,000 grains of water.

The substance to be examined is introduced till it reaches the same mark, and, the weight of the empty bottle being known, only one weighing is required to obtain the result. - A common method for finding the specific gravity of fluids is by the instrument called a hydrometer or areometer, of which several kinds are in use, all dependent on the principle that the weights required to immerse a light body, as a bulb of glass, in different fluids, are proportional to the densities of these fluids. Such instruments are used for ascertaining the specific gravity of liquors, as an indication of their strength. (See Hydrometer.) Gaseous bodies are weighed in a thin glass flask or other vessel made for the purpose, and provided with a stopcock. The vessel is exhausted of air before the introduction of the gas. The experiment requires particular care, as the result will be found to vary under different conditions of pressure, temperature, and the hygrometric state of the atmosphere. The temperature of the air should be 60° and barometric pressure 30 inches.

The specific gravities may also be calculated from the atomic weights of the gases : when the atomic volume is equal to that of hydrogen, it is obtained by multiplying the specific gravity of hydrogen by the atomic weight of the gas; when the atomic volume is half that of hydrogen, the specific gravity of the gas is equal to the specific gravity of hydrogen multiplied by twice the atomic weight of the gas; and when the atomic volume is twice that of hydrogen, the specific gravity of the gas is equal to the specific gravity of hydrogen multiplied by half the atomic weight of the gas. - The proportions of two ingredients in a compound, as in an alloy of gold and silver, may be found by multiplying the specific gravity of each ingredient by the difference between it and the specific gravity of the compound. As the sum of the products is to the respective products, so is the specific gravity of the body to the proportions of the ingredients; then as the specific gravity of the compound is to the weight of the compound, so are each of the proportions to the weight of its material. - The following table presents the specific gravities of substances most likely to be referred to, collected from various sources.

The weight of a cubic foot in ounces avoirdupois is seen by moving the decimal point three figures to the right.

Table Of Specific Gravities

Specific Gravity #1

See Gravity, Specific.