Density. Essentially the ratio of an object's or substance's weight (mass) to its volume; this is measured by specific gravity (its density to that of water). Density is expressed in grams per cubic centimeter and indicates how "heavy" an object is. It is determined by molecular arrangement. Gold, for instance, has a heavier molecular weight than, say, tin. While it might be thought that struck metal is more dense than cast metal of the same composition, their densities do not differ. (Temperature and pressure do, however, affect the density.) See specific gravity.