Wells and Pen-field — Series of Caesium Trihalides. 21 



three compounds containing no iodine, but in this series the 

 ratio between the two constant axes varies slightly from the 

 corresponding ratio in the iodine compounds. 



If an arrangement according to molecular weights is made 

 with all the compounds containing bromine, or in like manner 

 with all those containing chlorine, a symmetrical series of 

 axial ratios is not formed. This leads to the conclusion that 

 the two series given in the table have a special significance, 

 and that iodine, with the highest atomic weight, plays an im- 

 portant part in the constitution of the first, while bromine acts 

 in the same way in the second. Since several of the com- 

 pounds contain only a single halogen atom of highest atomic 

 weight, it follows that a single atom throughout exerts an 

 influence on the symmetry of the series. This peculiar part 

 played by an iodine or bromine atom may be explained by 

 supposing it to be closely united either with the caesium or 

 with one of the other halogen atoms. It seems probable from 

 these considerations that the three halogen atoms in these 

 compounds do not have similar positions in the molecule, and 

 consequently that the trihalides are not compounds of trivalent 

 caesium but have some other structure. 



Csl % . — Of this salt, crystals from both aqueous and alco- 

 holic solutions were examined. On L 2 . 

 the former the forms a, c, m, d 

 and f were observed. The habit 

 was different from anything else in 

 the series, being needle-like, with a 

 and m in the prismatic zone, termin- 

 ated by d, fig. I, or by c, d, andy, fig. 

 2. The crystals did not give very 

 satisfactory reflections, but the best 

 measurements, from a number of 

 selected crystals, agreed closely with 

 those given in the table. The crys- 

 tals examined were 20-30 mm in length 

 and seldom over 2 mm in diameter. 

 On the crystals from alcohol, the 

 forms a, o, c, m, g, d, f and e were observed, 

 shown in fig. 3. There was a tendency in the 

 crystals to arrange themselves in parallel posi- 

 tion forming plates, showing large a faces, but 

 the separate individuals were small, scarcely 

 over 3 mm in greatest diameter. The faces gave 

 excellent reflections. The crystals are black, 

 transmit a brownish red light only on the 

 thinnest edges, and are too opaque for optical 

 examination. 



CsBrI r — On two separate samples of this 

 salt the forms a, h, c, m, d, f and e were ob- 



m 



m 



m 



m 



The habit is 



m 



