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Encyclopedia Britannica



DENSITY (Lat. densus, thick)

This article appears in Volume V08, Page 46 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: DEM-DIO
DENSITY (Lat. densus, thick) , in physics, the
mass
  or quantity of matter contained in unit volume of any substance: this is the
absolute
  density; the
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
balance
  are really standard masses, the word "
weight
  " may be substituted for the word " mass " in the preceding
definitions
 ; and we may symbolically 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.
system
 , i.e. as grammes per cubic centimetre. In commerce, however, other expressions are met with, as, for example, " pounds per cubic foot " (used for woods, metals, &c.), " pounds per gallon," &c. The standard substances employed to determine relative densities are: water for liquids and solids, and hydrogen or atmospheric air for gases; oxygen (as 16) is sometimes used in this last case. Other standards of reference may be used in
special
  connexions; for example, the 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 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 law of 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
  room-temperatures; and at 25, the temperature at which a thermostat may be conveniently maintained, are 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 Io93. 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 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
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 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 BEER).


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