RUBIDIUM 723 



ROTTEN-STONE. A polish ing-powder which is much used for giving lustre 

 to brass, silver, and even to glass surfaces. According to the analysis of Richard 

 Philips, the rotten-stone of Ashford in Derbyshire consists of, carbon, 10; alumina, 

 86'0 ; silica, 4'0. Rotten-stone is nearly peculiar to this country, being found princi- 

 pally in Derbyshire, near Bakewell, and in Carmarthenshire and Breconshire, South 

 Wales. 



It is thought by geologists to be derived in Derbyshire from the siliceous limestone, 

 ' the lime being decomposed, and the silex remaining as a light earthy mass.' This 

 does not, however, agree with the above analysis, in which alumina occupies so large 

 a proportion. The total annual produce of the country is under 300 tons. 



ROUGE. (Fard, Fr.) A cosmetic employed to brighten a lady's complexion. 

 See CAKMINE. 



ROUGE, JEWEIiIiERS'. An oxide of iron prepared with much care. See 

 OXIDES FOB POLISHING. 



ROUND ORE. In Lead mining, the largest pieces of lead ore selected free from 

 the carbonate of lime or quartz, with which it may be associated in the lode. 



ROWLEY RAG. A peculiar augitic trap, forming the chief portion of the 

 Kowley Hills, near Dudley. Attempts have been made to fuse it, and then casting it 

 into ornamental shapes in moulds, to use it for architectural purposes; but the experi- 

 ments have not been successful. 



ROT AXi BIiUE. (Bleu de Soi, Fr.) A fine deep blue prepared from cobalt, and 

 used for enamel and porcelain painting. 



The term Bleu de Boi, or Boyal Blue, has of late been applied to one of the finest 

 of the aniline blues. 



RUBBLE. A local term used by quarrymen and miners for loose angular gravel, 

 or a slightly-compacted brecciated sandstone. H.W.B. 



RUBXACnr. Madder-orange ; Krapp-orange. A yellow colouring-matter dis- 

 covered by Runge. See MADDER. 



RUBIANT. A deep-yellow matter discovered by Schunk in madder. For the 

 analogous substances to, and derivatives of.rubian, consult ' Dictionary of Chemistry,' 

 by Watts. See MADDEE. 



RUBICEXiXiE. The yellow or orange-red varieties of spinel. See RUBY. 



RUBIDIUM. (See also C.ESIUM and THALLIUM.) Prof. Bunsen, in examining, 

 by means of the spectrum, the alkalis of a mineral spring at Diirkheim, in the 

 Palatinate, noticed the appearance of some bright lines which he had not observed 

 in any previous investigation of like kind ; and, as he had, by well-established 

 chemical methods, separated all the non-alkaline metals, he concluded that these 

 lines must be caused by the presence of some new alkaline metal. Although he 

 had obtained only 15 * 00 part of a gramme of the substance, he did not doubt the 

 accuracy of his conclusion ; so delicate and so reliable are the indications of the spec- 

 troscope. He resolved, therefore, to obtain for examination a larger quantity of the 

 presumed new body ; and, with this view, he proceeded at once to evaporate 40 

 tons of the mineral water. It soon became evident that tivo new alkalis were pre- 

 sent; and from the 40 tons of water Bunsen siicceeded in preparing about 7 

 grammes of the chloride of the one metal, and 9 grammes of the chloride of the 

 other. To the first of these substances he gave the name of Ceesium, from ccesius, 

 bluish-grey, on account of its spectrum being characterised by two bright blue lines. 

 For the second he proposed the name Rubidium, from rubidis, dark red, because of 

 the existence in its spectrum of two red bands. Caesium and rubidium, in their chief 

 chemical properties, closely resemble potassium ; so closely indeed that their existence 

 would probably have escaped notice, had it not been for the peculiarities which 

 their spectra exhibit. For the purpose of separating the two new metals from sodium- 

 and potassium-compounds, Bunsen took advantage of the fact that the chlorides of 

 rubidium and caesium form, with bichloride of platinum, double salts, much more 

 sparingly soluble in water than the corresponding double salt. By washing the pre- 

 cipitate containing the platinum salts of rubidium, caesium and potassium, the whole 

 of the latter salt is easily removed. The absence of the well-known potassium line in 

 the spectrum of the salt serves as a test to indicate the absolute purity of the new 

 metals. It was, however, much more difficult to separate caesium from rubidium, so 

 closely do they resemble one another in their properties. But carbonate of caesium 

 was found to be soluble in alcohol, in which carbonate of rubidium, like the other 

 alkaline carbonates, is insoluble. The metals thus isolated have been carefully studied 

 by Bunsen ; and small as were the quantities at his disposal, he soon succeeded in 

 determining the composition, crystalline form, and general properties of many of 

 their salts, besides establishing their numerical equivalents. Both metals form salts 

 strictly isomorphous with the salts of potassium. The equivalent of rubidium, 

 Rb, is 85'36; that of caesium, Cs, 133. After the publication of the memoir on 



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