476 Wells, Wheeler and Penfield — Ridndium and 



Csl . I a Rbl . I a KI . I„ 



CsBr.X ------ ----- 



CsBr . BrI RbBr . BrI KBr . BrI 



CsCl.Brl RbCl.Bi-I 



CsCl . C1I RbCl . CII KC1 . C1I 



CsBr. Br, RbBr. Br„ 



CsCl.Br, RbCl. Br, 



CsCl.ClBr RbCl.ClBr 



It is to be noticed that there is but one member lacking in 

 the rubidium series to make it as complete as that of caesium. 

 We have repeatedly tried to prepare this compound, RbBr.I,, 

 using alcoholic solutions of varying strength and great con- 

 centration at low temperatures, but with no success. The 

 failure to make this body doubtless depends upon the com- 

 parative instability of the rubidium series. We have even 

 attempted to prepare RbCl . I 2 and RbCl . Cl„, corresponding 

 to which no caesium compounds could be made, but, as was 

 anticipated, these efforts were entirely without success. 



In the potassium series only those bodies could be prepared 

 which correspond to the more stable caesium and rubidium 

 compounds. They show a great decrease in stability in 

 comparison with the rubidium compounds. A product was 

 obtained at a very low temperature, which was probably 

 KBr . Br 2 , but we did not make a satisfactory analysis of it. 



We have attempted to j)repare a number of sodium and 

 lithium trihalides. There is no doubt that some of them exist, 

 but they are so extremely soluble and unstable that we have 

 abandoned work in this direction. 



Method of Preparation. — The rubidium and potassium 

 compounds are made, like the caesium series, by dissolving a 

 normal halide with the proper halogen or halogens in water 

 with the aid of heat and cooling to crystallization. The 

 members of the rubidium series, being very soluble, require 

 very concentrated solutions for their preparation. The potas- 

 sium compounds, being still more soluble, require the greatest 

 possible degree of concentration and are usually best obtained 

 by exposing the solutions for a considerable time to a winter 

 temperature, evaporation in the desiccator being sometimes 

 also necessary. 



Color. — The colors of the rubidium and potassium com- 

 pounds are very similar to those of the corresponding mem" 

 bers of the caesium series, but since they usually form larger 

 crystals their apparent color is generally somewhat darker. 

 They vary in color from brilliant black in Rbl . I 2 and KI . I 2 , 

 through various shades of yellowish-red and orange to bright 

 yellow in the compound RbCl . ClBr. In all the compounds 



