Wells and Penfield — Series of Caesium Trihalides. 31 



The evidence which has just been given points strongly 

 towards considering the trihalides as double-salts. The con- 

 siderations based on the axial relations of the crystals, which 

 have been given in the crystallographic part of this article, 

 also indicate that these bodies cannot be viewed as compounds 

 of trivalent caesium, and, moreover, that a single halogen 

 atom of highest atomic weight plays an important part in their 

 structure. One view of the possible position of this peculiar 

 atom may be that it is a trivalent atom united to the others in 



X 



the manner indicated by the general formulae Cs — I<Cx an( ^ 



Cs— Br<y. This view may be objected to because the 

 strongest halogen atom is not directly united to the caesium. 



The structure Cs — X <" n is scarcely admissible in the com- 



X J 



pound CsI.AgI on account of the probable invariable univa- 

 lence of silver. 



Another view of the position of the peculiar iodine or 

 bromine atom may be based upon the idea, so ably advocated 

 by Kemsen,* that a bivalent group of two halogen atoms acts 

 the same part in double halides that oxygen takes in ordinary 

 oxygen salts. Supposing that a halogen atom of highest atomic 

 weight is linked to the caesium atom by means of the other 

 two halogen atoms, acting as a bivalent group, the trihalides 

 have the following formulae : 



Cs-(II)_I Cs— (BrBr)— Br 



Cs-(BrI)-I Cs— (ClBr)-Br 



Cs - (BrBr) —I Cs - (C1C1) -- Br 

 Cs— (ClBr)-I 

 Cs-(CICI)— I 



[Cs-(C1I)-I] [Cs-(C1C1)-Cl] 



These formulae allow the supposition that the most negative 

 halogen atom is in direct union with the caesium. They are 

 consistent with the view that there are two symmetrical series 

 of compounds, one with an iodine atom, the other with a 

 bromine atom in a special position. On these grounds they 

 seem quite plausible. 



Assuming that the structure of the trihalides is represented 

 by the above formulae, a comparison of their relative stability 

 leads to the view that the linking group of two halogen atoms 

 causes greater stability when composed of like atoms than 

 when these atoms differ. The groups (II) (BrBr) and (C1C1) 

 occur in the comparatively stable compounds, while (ClBr) is 

 in the most unstable body of the "bromine series," (BrI) is in 



* On the Nature and Structure of Double Halides, Am. Chem. Jour., xi, 291. 



