448 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tated, washed as before, and partly dried by suction, in a platinum 

 Gooch crucible. After further drying for some time the salt was ready 

 fur use. The portion thus prepared was used as Sample I. As will 

 be seen, the atomic weight of caesium found from this sample was 

 132.886. 



A second sample was prepared from the mother liquor of the first by 

 evaporating further in platinum vessels and precipitating, as before, with 

 hydrochloric acid gas. The product thus obtained was again dissolved 

 and again precipitated, and after being washed and dried gave Sample II, 

 which yielded the atomic weight 132.883. It was essentially similar to 

 Sample I, and is classed with it below. 



The temperature of the caesic chloride solution containing an excess 

 of hydrochloric acid in contact with platinum, must not be allowed to 

 rise above 60° or 70°, otherwise the platinum will be attacked, and 

 some caesium chlorplatinate precipitated, — a mishap which causes much 

 trouble. This difficulty was probably due to the presence of a residual 

 trace of undecomposed dichloriodide. The careful analyst well knows that 

 platinum, which he is forced to use, forms by no means the inert recep- 

 tacle which he desires. This fact was apparent more than once during 

 the present research, and it is not impossible that both Samples I and 

 II contained traces of platinum from this source, accounting for the 

 slightly high atomic weight. Subsequently this error was yet more 

 vigilantly guarded against, and the later specimens were as free as pos- 

 sible from platinum. 



A third sample of caesic chloride was prepared from a somewhat 

 impure caesium alum. In the first place the sulphuric acid was elimi- 

 nated by means of baric chloride ; the dichloriodide was obtained from 

 the filtrate by adding to it an excess of aqua regia, and subsequently 

 adding somewhat more than the calculated amount of iodine. The 

 trihaloid salt separated out, upon cooling, in beautiful rhombohedral 

 crystals. The iodine used in this preparation had been carefully re- 

 sublimed. The dichloriodide was recrystallized eight times, no trace of 

 rubidium being found by the spectroscope after the third recrystalliza- 

 tion, although the original alum had contained much of this impurity. 

 The normal caesic chloride was prepared from this product by heating 

 in the electric oven at 95° for a much longer time than before, until 

 long after all traces of iodiue had disappeared ; and great care was taken 

 not to allow the solution of caesic chloride to become at all hot when 

 precipitating this salt with hydrochloric acid gas in the platinum vessel. 

 The atomic weight obtained from this third sample was 132.873. A 



