RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OF CAESIUM. 449 



trace of thallium was found spectroscopically in the extreme mother 

 liquors. * 



In the search, which proved fruitless, for an unknown heavier metal 

 like caesium, a fourth sample of material was prepared. Since the in- 

 solubility of the dichloriodides seems to increase with the atomic weight 

 of the basic element, the unknown element would probably be found in 

 the extreme fraction of crystals. Hence, the most likely way of con- 

 centrating a trace of such an element would be to subject a large quan- 

 tity of the caesium dichloriodide to systematic recrystallization, until a 

 very small amount of substance remained. 



Such a process was carried out, starting with 150 grams of the di- 

 chloriodide belonging to the sample used in the first analysis. This was 

 systematically fractioned by crystallization twenty-five times, when only 

 about one gram of the substance remained. The normal chloride was pre- 

 pared from this, and its spectrum was carefully examined. No difference 

 could be distinguished between the spectrum given by this portion and 

 that given by any of the other preceding samples, or the extreme mother 

 liquor. The last four extreme crops of crystals, averaging twenty-three 

 crystallizations, were now combined, and enough caesium chloride was 

 obtained from these for several analyses. The value of the atomic 

 weight obtained from this fourth sample was 132.878, or about the mean 

 of the preceding values. 



This result agrees with the verdict given by the spectroscope, showing 

 that recrystallization has in no way affected the purity of the caesium 

 chloride. It must be concluded from this that the most efficient means 

 known to us at present are not capable of isolating any unknown element 

 from this material ; and further, that the occurrence of such an element 

 in the original mineral is unlikely. Wells has independently come to 

 the same conclusion. Having gained considerable experience in the 

 technique of all parts of the determination, it now seemed advisable to 

 make a series of determinations in which the highest possible degree of 

 accuracy would be attained at every step. 



To this end a fifth portion of caesium chloride was prepared from the 

 already pure chloriodide of Wells. The method of preparation was iden- 

 tical with that employed before, except that in every step greater care 

 was taken, and in driving off the iodine and extra chlorine the salt was 



* This experience agrees with that of Wells, kindly communicated to us in a 

 private communication. It is not impossible that thallium really represents the 

 recurrence of the alkaline characteristics in the periodic system, modified by con- 

 flict of overlapping tendencies belonging to other groups. 

 vol. xxxviii. — 29 



