Potassium- Trihalides, with their Crystallography. 479 



Series without iodine. 



CsBr.Br, 



0-6873: 1 : 1-0581 



1 



: 1-4550: 1-5395 



RbBr . Br 2 



0-6952 : 1 : 1 



•1139 



1 



: 1-4384: 1-6023 



CsCl . Br 2 



0-699 : 1 : . 





1 : 



1-430 : 



RbCl.Br, 



0-70 : 1 : 1 



•1269 



1: 



1-43 : 1-61 



CsCl • ClBr 



0-7186: 1 : 1-1237 



1 : 



: 1-3917 : 1-5638 



RbCl . ClBr 



0-7146:1 :1- 



1430 



1 : 



: 1-3994: 1-5995 





m/sm, 110 ^ llO 



d*d, 011^011 «a«, 102 a 102 



Rbl . I„ 



*68° 53' 





*96° 39' 



78° 38' 



KI.I„ 



*70 34 









RbBr. BrI 



70 58 





*98 40 



*78 27 



KBr . BrI 



71 12 





*98 55 



78 28 



RbCl . BrI 



72 2 





*99 10$ 



*77 51 



RbCl . C1I 



72 34 





*100 13 



*78 21 



KC1.C1I 



72 54 







*79 8 



RbBr . Br 2 



*69 37 





*96 10 



77 24 



RbCl . BrJ 



*70 appi 



•ox. 



*96 58 



76 approx, 



RbCl . ClBr 



71 6 





*97 38 



*77 18 



A comparison of the axial ratios of the trihalides shows that 

 the replacement of caesium by rubidium, and in one case by 

 potassium, has little or no effect on the form, while in two of 

 the compounds potassium causes a change in symmetry with- 

 out much change in the axes. It is evident that the rubidium 

 salts like those of caesium may be arranged in two symmetrical 

 series, one with and the other without iodine, in which the 

 ratio of two axes remains nearly constant throughout while 

 the third varies, and the conclusions which were arrived at in 

 our previous paper concerning the constitution of the caesium 

 trihalides, are confirmed by the rubidium compounds. 



The rubidium trihalides have a strong tendency to crystal- 

 lize and the solubility is such that, from solutions of not over 

 50 c.c. in volume, large and magnificent crystals, several centi- 

 meters in length, can readily be obtained. The size of the 

 crystals seems often dependent only upon the volume of the 

 solution and the size of the vessel containing it. Many of the 

 large crystals are complex, being built up of smaller ones in 

 parallel position. Some of the crystallizations were as beauti- 

 ful as any that we have ever seen. 



The rubidium trihalides containing iodine were measured at 

 ordinary temperatures ; those without iodine and the potassium 

 trihalides at about 0° C. It was found that the stability of the 

 compounds increased very rapidly with a diminution in tem- 

 perature and, by working in the cold, no difficulty was experi- 

 enced in making accurate measurements of the more unstable 

 salts. It is not considered necessary to give with each trihalide 



