A common metallic element of the alkali group, always found combined, as in common salt. When isolated it is a soft, waxy white metal, so readily oxidized that it combines with water and must be preserved under petro-leum. As a means of obtaining magnesium and aluminium, sodium is an important article of commerce. Its compounds are widely diffused in nature, and can be detected by the peculiar yellow color which they impart to a flame, or by the yellow line in the spectrum.