SPECIFIC GRAVITY (ML. specificns, spe cific, particular, from Lat. species, kind). The ratio between the weights of equal volumes of any substance and of some other, chosen as a standard. For solids and liquids the standard substance is water, for gases, air, or sometimes hydrogen. Since the volume of a given weight of a substance changes with temperature, it is necessary for exact determinations that the standard selected should be at a certain fixed temperature. Thus specific gravity is not the ratio of an equal volume of a given substance to water at any temperature, hut only at the stand ard temperature which is selected, as described be low. Likewise for a specific gravity determination to be of value the temperature of the substance at the time of making the measurement should be given. For gases not only the temperature, but the pressure must he mentioned. For many purposes the term density has replaced specific gravity, but the two are not synonymous. The density of a substance (at a given pressure and temperature) is the mass of that substance in a unit volume, provided it is homogeneous; other wise it is the mass of a volume so small as to be homogeneous, divided by the volume. Thus density is an absolute expressing the quan tity of a certain kind of matter in a given space, while specific gravity is merely relative, being in fact the ratio of the density of the substance to the density of the standard. The specific gravity of a gas or vapor is usually referred to air at the same pressure and temperature as that of the gas, but also frequently to hydrogen under like conditions. ]n terms of the latter it is
equal to half the molecular weight of the gas. See MOLECULES—NOLECULAR WEIGHTS.
The specific gravity of solids and liquids is variously referred to water at 4° C., 15° C., or 62° F., and sometimes still other temperatures. Specific gravities on these scales are respectively as 1.000045, 1.000913, 1.001180—the density of water at these temperatures being inversely as these numbers. The difference is obviously not of importance except for purposes of extreme accuracy. Density in the metric system is meas ured in grams per cubic centimeter (kilograms per liter). If a liter of water at 4° C. weighed exactly a kilogram, as was intended, the density of water at this temperature would be one and metric density and specific gravity referred to water at 4° C. would he identical. The density of water at this temperature is actually .999955, so that specific gravity referred to water at 4° is equal to density X 1.000045. Specific gravity referred to air at 0° C. and 76 centimeters pres sure is equal to metric density divided by the density of air = .001293.
On the English system density is variously ex pressed in pounds per cubic foot, pounds per cubic inch, grains per cubic inch. To reduce specific gravity referred to water at 62° F. to density on these scales multiply by 62.3546, .0360S4S, 252.593, respectively.