Wells and Penfield — Series of Caesium Trihalides. 29 



and also mixed trihalides, especially of the type RC1 2 I. These 

 organic bodies* are evidently analogous to the caesium com- 

 pounds under consideration, but since they have not been 

 sufficiently studied to throw any light on the structure or crys- 

 talline form of trihalides in general, they will not be described 

 in detail here. 



Theoretical Considerations. 



Thus far in this article the simplest possible formulEe have 

 been used. The probable structure of the compounds will 

 now be discussed. 



The trihalides previously known have been usually con- 

 sidered as weakly combined addition-products. Mendelejeff,f 

 for example, says that the instability with which I 2 unites with 

 KI and N(CH 3 ) 4 I is analogous to the instability of many 

 crystal-hydrates, e. g., HC1.2H 3 0. It must be noticed that 

 some of the caesium trihalides are very stable, but this fact is 

 evidently due to the strong positive character of the metal, and, 

 sinee others among them are comparatively unstable, it can 

 have no important bearing on their structure. 



Johnson;}: advances the formula K 2 I 6 for potassium triiodide 

 with no better reason than the existence of a higher iodide of 

 mercury, Hgl 6 .§ This formula may be dismissed at once, for 

 there is no more ground for it than for writing K 2 I 2 because 

 Hgl 2 exists, and moreover if the caesium salts were Cs 2 X 6 we 

 should expect to find in the series such compounds as Cs 2 Cl 5 I, 

 Cs 2 Cl 3 I 3 , etc., none of which were discovered. 



Since the members of the caesium series are crystallographi- 

 cally isomorphous they must all have the same structure, and, 

 as has just been shown, no multiple of the formula CsX 3 is 

 probable. 



A possible explanation of the caesium trihalides may be 

 made by supposing the metal to act trivalently, and the fol- 

 lowing arguments seem to favor this view : 



1st. Caesium has the highest atomic weight of the alkali- 

 metals, and it is a noticeable fact that, among the elements 

 in general, those with higher atomic-weights in a group have 

 the greater tendency to act with variable quantivalence. 2d. 

 Caesium is univalent in its ordinary compounds, and, following 



* See the following articles: Weltzien, Ann. Ch. Ph., xci, 33 ; xcix, 1. Muller, 

 ibid., cviii, 5. Hubner, ibid., ccx, 368. Ladenburg, ibid., ccxvii, 122. Zinke and 

 Lawson, ibid., ccxl, 123. Zinke and Artzberger, ibid., ccxlix, 366. Jorgensen, 

 J. pr. Ch., II, vols, ii, iii, xiv and xv. Dafert. Monatshefte, iv, 496. Dittniar, 

 Berichte, xviii, 1612. Ostermayer, ibid., xviii, 2298. Kaniensky, ibid., xi, 1600. 

 Tilden, J. Chem. Soc, xviii, 99. Hoogewerff and Dorp, Eec. Trav. Chim., iii, 361. 



f " G-rundlagen der Chemie," p. 563, foot-note 63. 



jj. Chem. Soc., 1878, 183. § Jorgensen, J. pr. Ch., II, ii, 357. 



