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term relative density or specific gravity denotes the ratio of the mass of a certain volume of a substance to the mass of the same volume of some See also:

standard substance. Since the weights used in See also:

CONJUNCTION (from Lat. conjungere, to join together)

conjunction with a See also:

BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance are really standard masses, the word " See also:

WEIGHT

weight " may be substituted for the word " mass " in the preceding See also:

DEFINITIONS

definitions; and we may symbolically See also:

EXPRESS (through the French from the past participle of the Lat. exprimere, to press out, by transference used of representing objects in painting or sculpture, or of thoughts, &c. in words)

express the relations thus:—If M be the weight of substance occupying a volume V, then the absolute density A =MN; and if m, m, be the weights of the substance and of the standard substance which occupy the same volume, the relative density or specific gravity S = m/m,; or more generally if m, be the weight of a volume v of the substance, and ml the weight of a volume v, of the standard, then S=mv,/m,v. In the numerical expression of absolute densities it is necessary to specify the See also:

units of mass and volume employed; while in the See also:

case of relative densities, it is only necessary to specify the standard substance, since the result is a See also:

mere number. Absolute densities are generally stated in the C.G.S. See also:

SYSTEM

system, i.e. as grammes per cubic centimetre. In See also:

commerce, however, other expressions are met with, as, for example, " pounds per cubic See also:

FOOT

foot " (used for See also:

woods, metals, &c.), " pounds per See also:

GALLON

gallon," &c. The standard substances employed to determine relative densities are: See also:

WATER

water for liquids and solids, and See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen or atmospheric See also:

air for gases; See also:

OXYGEN (symbol 0, atomic weight 16)

oxygen (as 16) is sometimes used in this last case. Other See also:

STANDARDS

standards of reference may be used in See also:

SPECIAL

special connexions; for example, the See also:

Earth is the usual unit for expressing the relative density of the other members of the See also:

SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar system. Reference should be made to the See also:

ARTICLE (from Lat. articulus, a joint)

article See also:

GRAVITATION (from Lat. gravis, heavy)

GRAVITATION for an See also:

account of the methods employed to determine the "mean density of the earth." In expressing the absolute or relative density of any substance, it is necessary to specify the conditions for which the relation holds: in the case of gases, the temperature and pressure of the experimental See also:

gas (and of the standard, in the case of relative density) ; and in the case of solids and liquids, the temperature. The See also:

REASON (Lat. ratio, through French raison)

reason for this is readily seen; if a mass M of any gas occupies a volume V at a temperature T (on the absolute See also:

scale) and a pressure P, then its absolute density under these conditions is A=M/V; if now the temperature and pressure be changed to T, and P,, the volume VI under these conditions is VPT/See also:

PITT, and the absolute density is MP,T/VPTI. It is customary to re-duce gases to the so-called " normal temperature and pressure," abbreviated to N.T.P., which is o° C. and 76o mm. The relative densities of gases are usually expressed in terms of the standard gas under the same conditions. The density gives very important See also:

INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

information as to the molecular weight, since by the See also:

law of See also:

AVOGADRO, AMEDEO, CONTE

Avogadro it is seen that the relative density is the ratio of the molecular weights of the experimental and standard gases.

In the case of liquids and solids, comparison with water at 4° C., the temperature of the maximum density of water; at 0° C., the zero of the Centigrade scale and the freezing-point of water; at 15° and 18°, See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary See also:

ROOM

room-temperatures; and at 25°, the temperature at which a thermostat may be conveniently maintained, are See also:

COMMON

common in laboratory practice. The temperature of the experimental substance may or may not be the temperature of the standard. In such cases a bracketed fraction is appended to the specific gravity, of which the numerator and denominator are respectively the temperatures of thesubstance and of the standard; thus 1.093 (0°/4°) means that the ratio of the weight of a definite volume of a substance at 0° to the weight of the same volume of water 4 is I•o93. It may be noted that if comparison be made with water at 40, the relative density is the same as the absolute density, since the unit of mass in the C.G.S. system is the weight of a cubic centimetre of water at this temperature. In See also:

BRITISH

British units, especially in connexion with the statement of relative densities of alcoholic liquors for Inland See also:

Revenue purposes, comparison is made with water at 62° F. (16.6 C.); a reason for this is that the gallon of water is defined by See also:

STATUTE

statute as weighing to lb at 62° F., and hence the densities so expressed admit of the ready See also:

CONVERSION (Lat. conversio, from convertere, to turn or ,change)

conversion of volumes to weights. Thus if d be the relative density, then sod represents the weight of a gallon in lb. The See also:

BREWER, JOHN SHERREN (1810-1879)

brewer has gone a step further in simplifying his expressions by multiplying the density by s000, and speaking of the difference between the density so expressed and s000 as " degrees of gravity " (see See also:

BEER

BEER).

End of Article: DENSITY (Lat. densus, thick)

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