568 PRINCIPLES OF CHEMISTRY 



flame. One which gives a red and violet band is named rubidium, 

 from rubidius (dark red), and the other is called caesium, because it 

 colours a pale flame sky blue, which depends on its containing bright 

 blue rays, which appear in the spectrum of caesium as two blue bands. 

 Both metals accompany sodium, potassium, and lithium, but in small 

 quantities ; rubidium occurs more frequently than caesium. The amount 

 of the oxides of caesium and rubidium in lepidolite does not generally 

 exceed one-half p.c. Rubidium has also been found in the ashes of many 

 plants, but it does not seem to accompany potassium in sea water. 

 Rubidium also occurs, although in very small quantities, in the majority 

 of mineral waters. In a very few cases caesium is not accompanied by 

 rubidium ; thus, in a certain granite on the Isle of Elba, caesium has 

 been discovered, but not rubidium. This granite contains a very rare 

 mineral called pollux, which contains as much as 34 p.c. of caesium 

 oxide. 41 Guided by the spectroscope, and aided by the fact that the 

 double salts of platinic chloride and rubidium and caesium chlorides 

 are still less soluble in water than the corresponding potassium salt, 

 K 2 PtCl 6 , Bunsen succeeded in separating both metals from each other 

 and from potassium, and demonstrated the great resemblance they 

 bear to each other. The isolated metals, 42 rubidium and caesium, have 



41 The salts of the majority of metals are precipitated as carbonates on the addition 

 of ammonium carbonate for instance, the salts of calcium, iron, etc. The alkalis 

 whose carbonates are soluble are not, however, precipitated in this case. On evaporat- 

 ing the resultant solution and igniting the residue (to remove the ammonium salts), we 

 obtain salts of the alkali metals. They may be separated by adding hydrochloric acid 

 with a solution of platinic chloride. The chlorides of lithium and sodium give easily- 

 soluble double salts with platinic chloride, whilst the chlorides of potassium, rubidium 

 and caesium form double salts which are sparingly soluble. A hundred parts of water 

 at dissolve 0'74 part of the potassium platinochloride ; the corresponding rubidium 

 platinochloride is only dissolved to the amount of 0'184 part, and the caesium salt, 

 0'024 part; at 100 5'13 parts of potassium platinochloride, K 2 PtCl 6 , is dissolved, 

 0'634 parts of rubidium platinochloride, and 0'177 parts of caesium platinochloride. 

 From this it is clear how the salts of rubidium and caesium may be isolated. The 

 separation of caesium from rubidium by this means is very lengthy. It is better 

 effected by taking advantage of the difference of the solubility of their carbonates in 

 alcohol ; caesium carbonate, Cs 2 CO 5 , is soluble in alcohol, whilst the corresponding salts 

 of rubidium and potassium are almost insoluble. Setterberg separated these metals ;is 

 alums, but the best method, that given by Scharples, is founded on the fact that from 

 a mixture of the chlorides of potassium, sodium, caesium, and rubidium in the pre- 

 sence of hydrochloric acid stannic chloride precipitates a double salt of caesium, which is 

 very slightly soluble. The salts of Rb and Cs are closely analogous to those of 

 potassium. 



42 Bunsen obtained rubidium by distilling a mixture of the tartrate with soot, and 

 Beketoff (1888) by heating the hydroxide with aluminium 2RbHO + Al = Rb A10 2 + 

 H 2 + Rb. By the action of 85 grams of rubidium on water, 94000 heat units are 

 evolved. Setterberg obtained caesium (1882) by the electrolysis of a fused mixture of 

 cyanide of caesium and of barium. 



