RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OF CAESIUM. 447 



The recry Stall ization may be carried out very easily without much 

 •decomposition by cooling from moderately dilute hydrochloric acid 

 (sp. gr. 1.06). The crystals should then be dried at room tempera- 

 ture ; but after they have become almost dry they can be heated to 75° 

 or 80° without much decomposition. In order to test the efficiency of 

 this method, we experimented upon a mixture of 150 grams of lithium, 

 sodium, potassium, rubidium, and caesium chlorides. From this mix- 

 ture were obtained, after six recrystallizations as dichloriodide, about 

 eight grams of caesic chloride, spectroscopically free from all the other 

 metals, while only traces of caesium remained in the last few residual 

 mother liquors. The greater part of the caesium used in this research 

 had been prepared by Professor Wells in this way.* To test its purity 

 as regards rubidium, the mother liquor from one recrystallization had 

 been evaporated down to an extremely small bulk by systematic recrys- 

 tallization, and was finally tested in the spectroscope. Not the slightest 

 trace of rubidium could be detected. In all four successive recrystalliza- 

 tions as dichloriodide were made from about a kilogram of the salt, the 

 crystals being well washed each time, and nothing was used over from 

 any mother liquor. The product, weighing about 280 grams, was re- 

 ceived from Professor Wells in this state, and our results show that it 

 had reached a remarkable degree of purity. 



To obtain the normal chloride from this pure caesium dichlorio- 

 dide, the latter was placed in a porcelain crucible, and heated at 

 from 90° to 100° in an electric oven, — a temperature far below the 

 melting point of this salt (about 240° C). The iodine and extra chlo- 

 rine are slowly driven off, leaving the chloride beautifully white and 

 very porous. 



To eliminate any possible trace of iodine which might remain, one 

 might dissolve the salt in a very small amount of warm water, and pre- 

 cipitate with pure alcohol. A better method, however, seemed to be to 

 precipitate it by saturating the solution with hydrochloric acid. The 

 caesic chloride was therefore transferred to a platinum dish, and dis- 

 solved in the least possible amount of warm water ; hydrochloric acid gas 

 from pure boiling concentrated aqueous hydrochloric acid was passed into 

 the solution by means of a platinum tube ; the mixture was allowed 

 to cool, and the mother liquor was poured off into another platinum 

 basin, the pi'ccipitate being washed with a little pure concentrated 

 hydrochloric acid. Tliis salt was again dissolved in water, precipi- 



* Tlie source of tliis maturial was poUucitc, louiul at ilt. Mica, I'aris, Maine. 



