370 O. D. Allen on Caesium and Rubidium. 



A second series of trials was made with tlie platinbromids of 

 potassium, rubidium and caesium. Tlie platinbroraid of potas- 

 sium is known to be readily soluble in water. The platinbro- 

 mids of caesium and rubidium readily separate from dilute solu- 

 tions of these three metals, but carry down potassium with them. 

 For the removal of the latter metai from the new alkalies the 

 platinbromids appear to have no advantage over the platinchlo- 

 rids, while they are equally inadequate to the separation of cae- 

 sium and rubidium from each other. In external characters the 

 three platinbromids closely resemble each other. 



Finally, recourse was had to the bitartrates, and with satisfac- 

 tory results. Carbonates of caesium and rubidium were first 

 prepared from the chlorids by converting them into sulphates, 

 separating the sulphuric acid with caustic baryta, and removing 

 the excess of baryta by carbonic acid. To the alkaline solution 

 thus obtained, twice as" much tartaric acid was added as was ne- 

 cessary to neutralize it. This solution was concentrated till it 

 was nearly saturated at 100° C. The crystals which deposited 

 on cooling when examined by the spectroscope, showed the rubi- 

 dium lines more intensely than did the origmai mixture, while 

 the caesium lines were much fainter. This product was dissol- 

 ved and recrystallized from hot saturated solutions three times. 

 The caesium reaction in these successive crops diminished until 

 in the fourth it disappeared, leaving the rubidium spectrum in 

 entire purity. 



In order to ascertain whether the more soluble bitartrate ot 

 caesium could be purified from rubidium by fractional crystalli- 

 zation, the solution from which the first crystals had been remo- 

 ved was concentrated to nearly one-half its original volume, 

 when by cooling, a very small quantity of salts of the two alka- 

 lies was deposited. This operation was repeated three timers 

 when a portion of the solution evaporated to dryness, and exam- 

 ined by the spectroscope gave only the lines belonging to cae- 

 sium. The several intermediate products containing both alka- 

 lies were then united and another portion of each salt separated 

 from them in the same manner. By repeating this process ot 

 fractional crystallization four times with about 40 grammes of the 

 mixed salts, 23-77 grammes of bitartrate of caesium, and 12-511 

 grammes of bitartrate of rubidium were obtained, while f'* 

 grammes remained unseparated. It was found that the caesium 

 salt thus obtained, although exhibiting no impurity when testea 

 by the spectroscope directly, i. e. after conversion by ignition 



converting it into chlorid, a faint line characteristic of the latter 

 metal was perceptible. The sciiaration of two or three more 

 small crops of crystals sufficed to render the residual solution 

 perfectly free from anv admixture that could be detected by a 



