See also:matter contained in unit
See also:volume of any substance: this is the absolute
See also:density; the
See also: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 standard substance . Since the weights used in conjunction with a
See also:balance are really standard masses, the word "
See also:weight " may be substituted for the word " mass " in the preceding
See also:definitions; and we may symbolically
See also: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 units of mass and volume employed; while in the case of relative densities, it is only necessary to specify the standard substance, since the result is a mere number . Absolute densities are generally stated in the C.G.S.
See also:system, i.e. as grammes per cubic centimetre . In commerce, however, other expressions are met with, as, for example, " pounds per cubic
See also:foot " (used for woods, metals, &c.), " pounds per
See also:gallon," &c . The standard substances employed to determine relative densities are:
See also:water for liquids and solids, and hydrogen or atmospheric air for gases;
See also:oxygen (as 16) is sometimes used in this last case . Other
See also:standards of reference may be used in
See also:special connexions; for example, the
See also:Earth is the usual unit for expressing the relative density of the other members of the solar system . Reference should be made to the article GRAVITATION for an 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 reason for this is readily seen; if a mass M of any gas occupies a volume V at a temperature T (on the absolute 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/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 information as to the molecular weight, since by the
See also:law of
See also: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°,ordinary
See also:room-temperatures; and at 25°, the temperature at which a thermostat may be conveniently maintained, are
See also: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 units, especially in connexion with the statement of relative densities of alcoholic liquors for Inland 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 as weighing to lb at 62° F., and hence the densities so expressed admit of the ready 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 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) .
DENSATION OF GASES
MANIUS CURIUS DENTATUS
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