Rubidium
a metal much resembling cæsium, with which it was discovered in 1860, by Bunsen and Kirchoff, during the analysis of a spring of water which contained these metals in minute quantities. Rubidium has since been found in small quantities in other mineral waters, in lepidolite, and in the ashes of many plants.
This metal is closely related, in properties, to potassium, but is more easily fusible and convertible into vapor, and actually surpasses that metal in its attraction for oxygen, rubidium taking fire spontaneously in air. It burns on water with exactly the same flame as potassium.
Its oxide, rubidia (RbO), is a powerful alkali, like potash, and its salts are isomorphous with those of potash.