RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OF CAESIUM. 445 



For the purpose of finding the atomic weight of caesium, they made 

 four determinations of the amount of chlorine in the chloride prepared as 

 above. Silver nitrate was used to precipitate the chlorine, and the pre- 

 cipitated argentic chloride was collected on filter paper. Their highest 

 result was 133.15 and their lowest 132.89, the average being 133.04, if 

 oxygen is taken as 16.000 and the correction to vacuum is applied. 



A few months after the appearance of Johnson and Allen's paper, 

 .Bunsen published the results of his second investigation. His method 

 of purification was now not unlike that used by the other investigators, 

 the most important difference consisting in the fact that several recrys- 

 tallizations of chlorplatinate were made, and each successive product 

 was analyzed. The method of analysis also resembled that of Johnson 

 and Allen. The three final results were 132.949, 133.04, and 132.98, 

 averaging 132.99. 



In 1865 Redtenbacher * pointed out that there is a greater difference 

 in solubility between the alums of caesium and rubidium than between 

 the chlorplatinates ; that therefore these metals might be separated more 

 completely through the former salts than through the latter. Godeffroy 

 made use of this fact in preparing his pure caesium chloride. When he 

 had obtained caesium alum entirely free from rubidium, the alum was 

 dissolved in hot water and the aluminum precipitated with ammonia. 

 After filtering, the solution was evaporated to dryness in a platinum 

 dish, and heated to drive off ammouic sulphate. The residue was then 

 dissolved in water, and baric chloride was added to precipitate the re- 

 maining sulphuric acid. After filtering off the baric sulphate the solu- 

 tion was treated repeatedly with ammonia and ammonic carbonate in 

 order to eliminate the last traces of aluminium and barium. The 

 caesium chloride obtained by this treatment was fused in a platinum 

 dish to drive off any trace of ammonium salts that might have been 

 present. On dissolving this fused mass in water, the solution was found 

 to be alkaline, but the crystals deposited by evaporation were not hygro- 

 scopic. Godeffroy weighed the chlorine in this salt as argentic chloride, 

 as usual, obtaining four results, which ranged from 132.50 to 132.71, 

 averaging 132.57. 



This low result is probably due to the admixture of sodium salts 

 coming from the utensils employed in the complicated manipulation of 

 the material. The salt used by the earlier experimenters was undoubt- 

 edly much purer ; their errors probably lay in the analysis, and had the 



* Redtenbacher, Wiener. Acad. Anzerger f., 1805, 39. 



