Since the ratio between the weights of equal volumes of substances in the same place is the same as the ratio of their masses, we can use the term specific gravity in place of relative density. Since, also, pure water is taken as the standard in specific gravity, we may express it by the following formula:
In this expression W is the weight of the body in air, and W' its apparent weight in water.
Since there are different systems of weights and measures it is evident that the number representing the density of a substance will depend upon the system of units used, while its relative density or its specific gravity will be the
same in all systems. For example, the density of wrought iron in the C. G. S. system is 7.85, which means that it has a mass of 7.85 g. per c. c. In the F. P. S. system its density is 489, which means that its mass is 489 lb. per cubic foot. In both systems its specific gravity is 7.85, which means that its weight is 7.85 times that of an equal volume of water.
The density of a substance depends upon its physical condition. If the substance is a mineral, its density depends upon its purity. If the substance is a metal, its density is affected by the treatment received in the process of manufacture; for instance, whether it is cast, or drawn into. wire. The density of an alloy depends upon the proportional parts of the metals composing it.
The accompanying table of densities and specific gravities is made by taking the average of results found by different observers. With slight exception they are as reported in the Physical Tables published by the Smithsonian Institution.
