Rubidium is a silver-like metal of lustrous appearance, but rapidly tarnishes through atmospheric oxidation. It is the softest metal, except caesium, its hardness on Rydberg's scale being 0.3. Its melting-point is given as 37.0° C., 37.8° C., 38° C., 38.5 C., and 39° C., and its boiling-point as 696° C. at 760 mm. pressure, the vapour being greenish-blue in colour. At 180° C. the vapour has a purple-red colour, which changes to orange above 350° C. Its density is given as 1.52 (Bunsen and Kirchhoff), 1.5248 at 0° C. (Hackspill), 1.522 at 15° C. (Erdmann and Kothner), and 1.532 at 20° C. (Richards and Brink). In melting-point and density rubidium and caesium are more closely related to one another than to the other alkali-metals, and a similar resemblance has been observed regarding the crystallography of the salts of the two metals. According to Rengade, the specific heat of the solid at its melting point is 0.019, and the heat of fusion per gram is 0.00615 Cal.
Several rubidium salts exhibit radioactivity,14 among them the sulphate, chloride, and chlorate. The action of the sulphate on the photographic plate is more powerful than that of potassium sulphate. |