METHODS OF MEASUREMENT. I. DENSITY. Two courses may be pursued to determine the density of a substance: (1) Weigh a portion of it and measure its volume; (2) determine its specific gravity referred to a substance of known density. For approximate purposes the former is best, particularly in the case of solids of regular shape. To attain a high degree of accuracy, however, is difficult, since the direct determina tion of volume is subject to many errors. Hence the density of most substances is found by deter mining, by one of the methods below, its specific gravity referred to water or hydrogen, the density of each of these having been carefully investi gated. The density of water is found by observing the loss in weight of a solid of regular shape when immersed in water, some of the best results being obtained with a glass cube whose exact dimensions were found by a method based on the interference of light.
Ti. SPECIFIC GRAVITY OF SOLIDS AND LIQUIDS. Most of the methods given below are based on Archimedes's principle, which states that a solid floating or immersed in a liquid loses weight equal to that of the liquid it displaces. Hence if it is totally immersed the loss of weight is the weight of a volume of liquid equal to the volume of the solid. Hence: Weight of solid = specific gravity of solid.
Loss of weight = specific gravity of liquid. Thus if either is known the other may be found. For exact purposes the temperature of the liquid must be known, weighings in air must be cor rected for the buoyancy of the latter, and the solid must be supported in the liquid so that only a single fine wire cuts the surface. Air bubbles are also a serious source of error, and for this reason water when used should be freely dis tilled or boiled.