JPotassitvtn Trihalides, with their Crystallography. 477 



that have been prepared the color becomes lighter as the sum 

 of the atomic weights of the three halogen atoms decreases. 



Stability. — It has been found by experiment that the potas- 

 sium trihalides are much less stable on exposure to the air 

 than the corresponding rubidium compounds, while these in 

 turn are less stable than the members of the caesium series. 

 The same relative stability of the three series is shown by the 

 temperatures at which they are completely decomposed by 

 rapid heating as given below : 



Approximate temperature of whitening. 



CsI.I 2 330° Rbl. I 2 270° KI.I„225° 



CsBr . BrI 320° RbBr . BrI 265° KBr. BrI 180° 



CsCl.Cll290° RbCl. 011265° KC1. 011215° 



CsCl . BrI 290° RbCl.Brl200° 



CsBr. Br, 160° RbBr . Br, 140° 



CsCl . ClBr 150° RbCl. ClBr 110° 



CsCl.Br, 150° RbCl. Br, 80° 



Fusibility. — The melting-points of the analogous compounds 

 become lower from csesium to potassium. In the open capil- 

 lary tube Rbl . I 2 melts at 194° and RbCl . C1I at 208°, while 

 all the other rubidium compounds whiten without melting. 

 The potassium compounds give practically the same melting- 

 points in open as in sealed tubes. The following table gives 

 the approximate melting-points in sealed tubes : 



CsI.I 2 .201°-208° Rbl. I 9 190° KI.I a 38°* 



CsBr . BrI 243°-248° RbBr . BrI 225° KBr . BrI 60° 



CsCl . C1I 225°-230° RbCl . C1I 180°-200° KC1 . C1I 60° 



CsCl. BrI 225°-235° RbCl . BrI 205° 



CsBr . Br 2 180° RbBr . Br 2 whitens 



CsCl . ClBr 205° RbCl . ClBr whitens 



CsCl. Br 2 191° RbCl . Br a 76° ? 



Behavior xoith Solvents. — The extreme solubility of the 

 rubidium and potassium trihalides in water has already been 

 referred to, and it has been pointed out that the members of 

 the potassium series are the most soluble. The rubidium 

 compounds which contain iodine can be recrystallized from 

 water without difficulty. These four bodies containing ru- 

 bidium and iodine are sufficiently stable to be soluble in alco- 

 hol, while the remaining rubidium compounds, as well as all 

 the potassium compounds, are more or less readily decomposed 

 by alcohol with the separation of normal halides. Ether 

 decomposes all the rubidium and potassium compounds, leav- 

 ing normal halides undissolved. 



* Johnson gives 45° for the melting-point of this compound (1. c.) 



