Relation to the Alkali Metal Triiodides. 465 



from alcohol. The measurements which were chosen as funda- 

 mental are d^d, 011*011=96° 34' and e^e, 102*102 = 

 78° 48' giving the axial ratio : 



a: b: c = 0-682S: 1 : 1-1217 



The dome g was determined by the measurement g^g, 

 012*012 = 58° 34', calculated 58° 34' and the pyramid p by 

 its position in the zones a—d and d—e. 



A description of this salt, including a figure, has been given 

 by Nickles. His salt, crystallized from ether, had the habit 

 shown in fig. 3, the letters in brackets being those used by 

 him and the position being changed to correspond with the 

 orientation of the alkali metal trihalides. He considered^ as 

 a prism, t as a macropinacoid and m and n as brackydomes. 

 No calculations are given and only the following four measure- 

 ments : 



Nickles. Measured. Calculated from author's measurement. 

 p^p = 100° 15', 101° 12' for e*e, 102^102 



p^ t = 39 22, 39 24 " e*c, 102^001 



p^m— 61 59 3 " e^d, 1.02^011 



n^t = 19 25, 20 30 " 013^001 



The agreement between the measured and calculated angles 

 is not very close, but tickles' measurements cannot be very 

 exact, for if we take p^t= 39° 22' and n^t= 19° 25' as 

 fundamental, we find by calculation p^p = 101° 16' and^>*m 

 = 57° 55', which vary considerably from his measurements. 

 Nickies crystals differ from ours not only in habit but in hav- 

 ing the one-third brachydome n, 013, which has not been ob- 

 served either in the T1I 3 prepared from alcohol or on any of 

 the alkali-metal trihalides prepared by us. 



The very close agreement between the forms of rubidium, 

 csesiuro and thallium triiodides is to be seen from the follow- 

 ing table of axial ratios : 



Rbl 3 d : b: 6 == 0-6858: 1 : 1-1234 



Csl 3 -. " " "=0-6824:1:1-1051 



T1I 3 " " "= 0-6828:1: 1-1217 



Our previous observation, that the exchange of one metal 

 for another in the trihalogen compound's usually has little or 

 no effect upon the crystalline form, is strongly confirmed by 

 these ratios, and the remarkable agreement between the ru- 

 bidium triiodide and the thallium compound is very striking 

 when the great difference between the atomic weights of the 

 two metals is considered. 



It was hoped that a pentaiodide of thallium could be pre- 

 pared in order that its form might be compared with that of 



