Rubidium (Lat. rubidus, dark red), a metal of the alkalies, discovered by Bunsen and Kirchhoff in 1860 by means of the spectroscope. The lines characteristic of the new metal are two remarkable bands of dark red lying beyond Fraunhofer's A, and consequently in a part of the spectrum visible only by unusual methods. Two blue and some yellow and green lines on the spectrum have since been observed. Rubidium occurs* in a considerable variety of potash minerals, among which may be mentioned the deposits of the Stassfurt salt mines; lepidolite from Rozna in Moravia, and from Goshen and Paris, Me.; orthoclase, triphylline, carnallite, and saltpetre; in mineral waters; in beet root, tobacco, ashes of tea and coffee, crude tartar, ashes of oak and of a great variety of plants, being very widely though sparingly distributed. The metal was extracted by Bunsen from the acid tartrate, 1,100 grains of which when distilled furnished about 80 grains of a brilliant metallic mass. It is silver white, with a slightly yellow lustre, oxidizes rapidly in the air, and takes fire spontaneously. It is soft like wax at 14° F., melts at 101°, and at red heat furnishes a blue vapor.

It is more electro-positive than potassium, and when thrown upon water takes fire and burns with a violet flame resembling that of potassium. It burns readily in chlorine, bromine, iodine, sulphur, and arsenic vapors. Its symbol is Rb; specific gravity, 1.52; atomic weight given by Bunsen at 85.36, by Piccard at 85.41. The salts of rubidium are with difficulty distinguished from those of potassium, and the only certain test is the appearance of the flame in the spectroscope. The sparing solubility of the chloride of rubidium and platinum in boiling water is employed as one of the means of obtaining pure salts of rubidium.