RUBID'IU (sym. Rb. eq. 85.4) and CiltS1U3I (sym. Cs. eq. 133) are two alkaline metals, discovered in 1860-61, by Bunsen and Kirchhoff, by means of spectrum analysis. They resemble potassium more nearly than any other substance, and their names are derived from rubSdas, dark red, and ea8i118, sky-colored, in consequence of two red lines of remarkably low refrangibility being present in the spectrum of the former, and two characteristic blue lines in that of the latter. They are widely diffused in nature, but always occur in very small quantities. They have been detected in many mineral waters, and in certain minerals; as, for example, lithia-mica, petolite, and feldspar: and they have been found in the alkaline ashes of the beet-root. The best material for the preparation of rubidium is lepidolite, which will sometimes yield as much as 0.2 per cent of the metal, while the principal source of efesium is-the brine of Darkheim, in which both these metals were originally found; every ton of the water containing about three grains of chloride of caesium, and rather less than four grains of chloride of rubidium. It has, however, been recently asserted that the mineral pollux contains no less than 34.07 per cent of oxide of carsium. Both metals are so analogous to potassium, that they cannot be distinguished either from it or from one another by reagents, or before the blow-pipe. Like potassium, they form double salts with bichloride of platinum,
which are much more insoluble than the corresponding potassium salt; and it is on this property that the separation of these metals from potassium is based. It is unnecessary to enter into any details regarding the compounds of these metals. It is worthy of remark that rubidium is electropositive toward potassium, and that emshun is electro positive toward rubidium and potassium, and is thus the most electro-positive of the known elements.
The notice of cwsium is included in this article, because its recent discovery pre vented it from being considered in its proper alphabetical place in the original edition of this work. For the same reason, we may here briefly notice another metal, Ikomm, discovered by Reich and Richter in the Freiburg arsenical ores. Its most striking property, and that which lcd to its discovery, is the indigo-blue line which all its com pounds (in so far as they have been investigated) show in the spectroscope. Its eq. is 37—new system, 113; its sp. gr. varies from 7.1 to 7.31 its color is between those of tin and silver; it is exceedingly soft and very ductile; and its fusing-point is about that of lead.