446 PROCEEDINGS OF THE AMERICAN ACADEMY. 



opposite tendency. Either the loss of chlorine from the caesic chloride 

 during its ignition, or the solution of argentic chloride by the water used 

 in washing would tend to increase the apparent atomic weight. Hence 

 one might have expected the atomic weight of caesium to lie between 

 133.0 and 132.6; probably nearer the former than the latter value. The 

 value usually selected by Clarke and others, 132.9, has been in accord 

 with this conclusion, and the present investigation shows it to have 

 been surprisingly accurate. 



III. The Analysts of Caesic Chloride. 

 (a) llie Preparation of Materials. 



Caesic chloride is a colorless salt, crystallizing in anhydrous cubes 

 which are very soluble in water. The aqueous vapor tension ot" the 

 saturated solution is somewhat greater than that of the average air of 

 American steam-heated laboratories, hence the crystals are deliquescent 

 only on damp days. The German ascription of deliquescence to the 

 salt is due evidently to the moister conditions prevalent in that country. 

 The addition of alcohol to its solution precipitates much of the dissolved 

 salt ; and since it has no great tendency to form an acid chloride in solu- 

 tion, hydrochloric acid also precipitates it. Caesic chloride fuses into 

 a limpid, colorless liquid at about 600°, losing a trace of its chlorine 

 thereby if moisture is present and hydrochloric acid absent. On the 

 other hand, the salt shows no trace of alkalinity after fusion in perfectly 

 dry air, especially if it has been recrystallized from a hydrochloric acid 

 solution. In the latter case, acid must always be present in the micro- 

 scopic inclusions wherever water is, and the trace of iiydrolysis on fusion 

 is effectually prevented. Its properties thus fit it admirably for accurate 

 work. 



Wells * has shown that caesium may be separated from rubidium and 

 other alkali metals by making use of the difference in solubilities of their 

 trihalide salts. Of these the dichloriodide (CsCLI) offers peculiar advan- 

 tages ; for this salt not only is from eight to ten times less soluble than 

 the corresponding rubidium salt, but also will crystallize below 70° in a 

 rhombohedral form, while the other metals give the corresponding salts 

 in the orthorhombic form only. By keeping the temperature below this 

 point, crystals of the caesium salt may be obtained which are not likely 

 to carry down rubidium, because of this helpful heteromorphy. 



* Wells, Am. J. Sci. (3), 43, 17 (1901) ; Chem. News, 84, 2184, Oct 4 (1901). 



