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Densities and Specific Gravities
DEFINITION 1.  The density^{(1)} of a substance at any temperature is the mass of a unit of volume of the substance at that temperature; thus the density of water at 4° C. is one gramme per cubic centimetre. DEFINITION 2.  The specific gravity^{(1)} of a substance at any temperature is the ratio of its density at that temperature to the density of some standard substance, generally the maximum density of water (i.e. the density of water at 4° C.). DEFINITION 3.  The specific gravity of a body is the ratio of the mass of the body to the mass of an equal volume of some standard substance, generally water at 4° C.
These relations are only true for the C.G.S. system, and any other systems in which the unit of mass is the mass of the unit of volume of water at 4° C.; but whatever be the system, the density of water at 4° C. is accurately known, although its numerical value may not be unity. Hence, in order to calculate the volume of a body whose mass is known, or vice versa, we require only to know its specific gravity, and hence the practical importance of determinations of specific gravity. It is generally an easy matter to determine experimentally the ratio of the mass of a body to the mass of an equal volume of water at the same temperature, but it would not be easy or convenient always to keep the water at its temperature of maximum density, throughout the experiment. The densities of bodies are therefore not usually experimentally compared directly with the maximum density of water in determining specific gravities, and the necessity for doing so is obviated by our knowing with great accuracy the density of water at different temperatures, (this is given in table 32); so that we are enabled, when we know the mass of a volume of water at any temperature, to calculate from the table the mass of the same volume at 4° C., and thus obtain the specific gravity required. We proceed to describe some of the practical methods in general use.


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