Very reflective white metal. Will spontaneous combust upon exposure to the atmosphere. m.p. 38.89 ºC; 102 ºF b.p. 688 ºC; 1270 ºF density (solid) 1.532 (20 ºC) (liquid) 1.475 (39 ºC) g/cc; (solid) 95.640 (68 ºF) (liquid) 92.081 (102.2 ºF) pound/cubic foot	memory peg
	1861 Robert Wilhelm Bunsen & Gustav Robert Kirchhoff, Germany
	rubidus = deep red (Latin)

Rubidium was discovered by Gustav Robert Kirchhoff (1824-1887) and Robert Wilhelm Bunsen (1811-1899) at the University of Heidelberg, in 1861, when they noticed the existence of new spectral lines in the mineral lepidolite from Saxony (note).

"Therefore we propose for this alkali metal, in respect to those two remarkable dark red lines, the name Rubidium with the symbol Rb from Rubidus which was used by the ancients to designate the deepest red."

Interesting is the note to Aulus Gellicus, a rather obscure Roman writer from the middle of the second century AD. Only one, incomplete book is left, the Noctes Atticae (Attic Nights). This work takes its name from having been begun during the long nights of a winter which he spent in Attica. In this work he had jotted down everything of unusual interest that he heard in conversation or read in books. Chapter 26 of the second book is about the words for the different colors in Latin and Greek,

"Sermones M. Frontonis et Favorini philosophi de generibus colorum vocabulisque eorum Graecis et Latinis; atque inibi color «spadix» cuiusmodi sit"
[= Philosophers Sermones M. Frontonis and Favorini on the origin of colors and their words in Greek and Latin, and how the color «spadix« [=brown] ranks among them],

"Non enim haec sunt sola vocabula rufum colorem demonstrantia, quae tu modo dixisti, «russus» et «ruber», sed alia quoque habemus plura, quam quae dicta abs te Graeca sunt: «fulvus» enim et «flavus» et «rubidus» et «poeniceus» et «rutilus» et «luteus» et «spadix» appellationes sunt rufi coloris aut acuentes eum quasi incendentes aut cum colore viridi miscentes aut nigro infuscantes aut virenti sensim albo illuminantes."
(«Russus» and «ruber» are not the only words that denominate the color red, as you've said recently; however, we have many others, which you said were Greek: «fulvus» [=gold yellow] and «flavus» [=blond or blushing red] and «rubidus» [=dark red] and «poenicus» [=purple] and «rutilus» [=yellowish red, maybe orange] and «luteus» [=gold yellow or pink] and «spadix» [=brown] are all names for the color red, either sharpening or lightening it, or mixing it with the color green, or darkening it, or gradually turning it fresh and white),

"«Rubidus» autem est rufus atrior et nigrore multo inustus, «luteus» contra rufus color est dilutior"
(However, «rubidus» is a darker and much more blackish red; «luteus» on the other hand is a lighter form of the color red).

The preparation of the metal was tried by Bunsen, but he never got samples with more than 18% of Rubidium. The separation of the metal was only accomplished by Hevesy, through the hydrolysis of melted Rubidium hydroxide. Later, Hevesy also obtained Rubidium through the reduction of that hydroxide Sodium, Potassium or hot Aluminum.

• Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 24 (1937).
• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 603-606.