j$fcM)^!WWVW*Vt 



Wells and Penfield — Thallium Triiodide, etc. 463 



by 8 swings, in 11 (a) by 7 swings. The same slit was con- 

 tinued through the back and the result was 13 swings. With 

 another box the side walls were slit horizontally, result 11 

 swings. These horizontal slits were then continued through 

 the back nearly meeting, result 18 swings. With the slit ex- 

 tended through and the upper half alone used the number of 

 swings was 34. The faces of the plane d of 13 had been 

 lacquered, and the effect of the horizontal slitting just de- 

 scribed would indicate that this separation of the halves of the 

 sphere has a very noticeable effect in reducing the damping 

 power. 



These observations do not furnish sufficient data for attempt- 

 ing to express in the differential equation of motion of a 

 vibrating magnet the functions representing the damping, 

 but they may possibly prove useful in the way of practical 

 suggestion, and it is with that hope that the paper is written. 



Jefferson Physical Laboratory, Cambridge, March, 1894. 



Akt. LYIII. — On Thallium Triiodide and its Relation to the 

 Alkali-Meted Triiodides ; by H. L. Wells and S. L. 

 Penfield. 



The well-known resemblance between the thallous salts and 

 many of the corresponding alkali-metal salts has led us to 

 prepare thallium triiodide and to compare its crystalline form 

 with that of the alkali-metal triiodides.* As a result, it has 

 been found that T1I 3 agrees, with remarkable closeness, in form 

 with Rbl 3 and Csl 3 , and thus a case of isomorphism is estab- 

 lished between the higher iodides of thallium and the alkali- 

 metals. 



This isomorphism is of special interest because our study of 

 the trihalogen compounds of caesium has led us to the conclu- 

 sion that these have the structure of double-salts. We con- 

 sider the evidence of this double-salt structure as very strong, 

 and since it seems necessary to infer that isomorphism indi- 

 cates the same arrangement of the atoms, we are obliged, in 

 spite of the apparent trivalence of thallium in thallic com- 

 pounds, to conclude that T1I 3 is also a double-salt, to which 

 the formula Til . I 2 should be given. It is not safe to assert 

 at present that all thallic salts must be similarly constituted, 

 for it is possible that thallium triiodide is not a true thallic 

 compound at all, and that thallic sulphate, nitrate, etc., have 

 an entirely different kind of structure. If it is granted that 



* This Journal, III, xliii, 17 and 475. 



