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CHAPTER IV
DENSITY AND SPECIFIC GRAVITY
Density.—The density of a given substance is the mass of
unit volume. When the metric system of weights and measures
is used, as is customary in most scientific work, the density equals
the weight, in grams, of 1 cc of substance.
Specific Gravity.—Specific gravity is the ratio of the density
of a given substance to that of some other arbitrarily chosen
substance. For liquids and solids the substance which is gen-
erally chosen for reference is water. Since the weight of a
milliliter (which, for all practical purposes, may be taken as a
cubic centimeter) of water at 4° is, by definition, 1 gm, the
density of any substance becomes its specific gravity if the latter
is referred to watei^ at 4°. This is the preferred method for
expressing specific gravity and the figure so determined is
stated as
t°
specific gravity^0
for the specific gravity when the substance is at temperature f,
Most of the laboratory methods for determining specific
gravity involve measuring either (a) the buoyant effect of the
(liquid) substance upon an immersed "float/' (6) the com-
parative weights of equal, but unmeasured, volumes of the
(liquid) substance and water, or (c) the weight of water displaced
by a weighed, but not measured, quantity of the (solid) sub-
stance. To carry out such experiments at 4° is a problem
offering great experimental difficulties and the work is usually
performed at some more convenient temperature, higher than 4°.
On this account it is customary to express the results as
f
specific gravity*0
this quantity being the ratio of the density of the substance
at t° to that of water at the same temperature. This quantity
t°
can be converted into the specific gravity at 75 by multiplying
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