30 Wells mid Penfield — Series of Caesium Trihalides. 



the general rule, the next higher quantivalence should be 

 three. 3rd. Caesium is in the same group as gold, in Men- 

 delejefFs periodic system of the elements, and it is well known 

 that this element acts univalently and trivalently. 



On the other hand, the following arguments are in favor of 

 considering the bodies double salts : 1st. The compounds IBr, 

 IC1 and BrCl are definite bodies and all the trihalides may be 

 considered as molecular compounds of these, and also the 

 molecules I 2 and Br 2 , with normal halides. 2d. The fact that 

 CsBrJ is more stable than CsBi\T 2 and that CsCl 2 Br is more so 

 than CsClBr„, showing that the stability of these bodies does 

 not entirely depend upon the volatility of the halogens con- 

 tained in them, indicates that the halogen atoms have much 

 influence on each other, and that at least two of them are 

 probably bound together. A consideration of the fact that 

 CsCl 2 I is a very stable body while Cs01I 2 probably cannot be 

 prepared leads to the same conclusion. 3d. It has been pointed 

 out by Godeffroy* that the simple salts of caesium are as a rule 

 more soluble than the corresponding rubidium and potassium 

 salts, while with the double-salts the reverse is true. Work 

 now in progress in this laboratory shows that the rubidium and 

 potassium trihalides, as far as they have been investigated, 

 increase in solubility towards potassium, hence, if the rule 

 holds true, they must be double-salts. 4th. The new salt 

 CsI.Agl, f an undoubted double- salt, shows a close relation to 

 the trihalides in its system of crystallization, in the ratio of 

 the two axes, which alone were determined, and in its cleavage.;}: 

 The salt KI.AgI§ has been prepared, but it has not yet been 

 procured in crystals fit for measurement. Work will be con- 

 tinued on this class of compounds. 



*Berichte, ix, 1365. 



f This was made by dissolving Agl in a concentrated, hot solution of Csl and 

 cooling. It forms tufts of hair-like crystals, very seldom large enough to measure. 



Calculated for 

 Analysis gare CsI.Agl. 



Silver 19-03- 21-82 



Iodine . *. 50-91 51-32 



Caesium 26S6 



\ CsI.Agl crystallizes in the orthorhombic system. It was obtained in long 

 needles, not over \ mm. in diameter and too small to show distinct end faces. A 

 pinacoid and two other forms occur in the prismatic zone which correspond to 

 c(001), £(012) and d(Qll) of the trihalide series. Of these c and g are the best 

 developed while d is very small. Five independent measurements of c ~ g gave 

 values varying between 27° 29' and 27° 44', the average being 27° 36' which 

 gives the ratio ~b: c = 1 : 1-0456. The crystals show a imperfect cleavage parallel 

 to a (100) and probably a second parallel to c, but they were too small to allow of 

 this being determined with certainty. Under the polarizing microscope they show 

 parallel extinction. 



§Boullay, Ann. Chim. Phys., II, xxxiv, 377. 



