486 Wells, Wheeler and Penfield — Rubidium, etc. 



as the crystals could be measured. Much work has since been 

 done, without avail, in the hope of obtaining better crystals of 

 this compound. Unsuccessful efforts have been made to 

 obtain measurable crystals of all the corresponding silver 

 double halides (except the fluorides) with caesium, rubidium 

 and potassium. Two or three of these compounds had already 

 been described and it is probable that we could have proven 

 the existence of all the rest of them, but the poorly crystal- 

 lized products obtained had no interest in this connection and 

 were not analyzed. Repeated efforts also failed to produce 

 from potassium iodide and cuprous iodide a double salt that 

 could be measured. 



Theoretical. 



Arguments were given in the article on the caesium series 

 which have led us to regard the trihalides as belonging to the 

 class of bodies called double halides. We have indicated this 

 view in the present article by using the usual formulas for 

 such compounds. 



The well-known idea of a linking group of two halogen 

 atoms as an explanation of the structure of double halides was 

 advocated for the caesium trihalides, and, since the rubidium 

 and potassium compounds are entirely analogous, it is unneces- 

 sary to give their structural formulas here. We believe, how- 

 ever, that the trihalides throw some light upon the constitution 

 of the diatomic linking group. Remsen says,* " I cannot see 

 that at present we have any evidence which justifies us in the 

 use of the expression — C1=C1— rather than — CI — CI — ." 

 If, as we believe, the structure of rubidium tri-iodide is ex- 

 pressed by the formula Rb— (II) — I, the structure of the link- 

 ing group probably cannot be — I — I— ; for in that case a 

 single bivalent iodine atom could do the linking as well as a 

 group of two, and we should expect the existence of di-iodides, 

 no evidence of which, or of any other dihalide has been found 

 in the course of an elaborate investigation of the alkali-metal 

 polyhalides. Moreover, with the assumption of bivalent halo- 

 gen atoms, there would be no difficulty in supposing four 

 halogens to be linked together and the existence of tetra- 

 halides would be anticipated. Our investigations, however, 

 have shown the existence of only tri- and pentahalides.f The 

 double linking seems therefore the more probable of the two 

 forms mentioned by Remsen, but it may be added that any 

 union weaker or stronger than the others in the molecule", and 



*Atn. Chem. Jour., xi, 312. 



f The pentahalides will be described in a future article. 



