Rubidium

a metallic element of the sodium group. It was detected in 1860 by Bunsen and Kirchoff by means of spectrum analysis applied to a residuum of mineral water. It gives two red lines in the blue part of the flame. It has a specific gravity of 1.52. At ordinary temperatures it is as soft as wax. In appearance and qualities, it resembles potassium, burning like that element explosively on water and slowly in air. See Potassium