CRYSTALLOGRAPHICAL RELATIONSHIP 243 



an indium salt which corresponded in formula to (NH 4 ) 2 InCl 5 

 H 2 0, but which he did not examine crystallographically. A 

 review of previous work seemed then to indicate the possibility 

 of obtaining two fairly long isomorphous series, of the types 

 R 3 MX 6 ?H 2 and R 2 MX 5 H 2 O respectively, where R=K, 

 NH 4 , Rb, Cs, and possibly Tl ; M=T1, In ; and X=C1, Br. 

 Still another series of the type RMX 4 ?H 2 O might prove of 

 interest in this connection, more especially with X=Br or I. 

 Nickles (12) obtained the salts KTlBr 4 2H 2 and NH 4 TlBr 4 

 2H 2 0, and considered them to be rhombic, but gave no 

 crystallographical details ; while Pratt (8) found that the salts 

 RbTlBr 4 H 2 O and CsTlBr 4 crystallised in the cubic system. 

 As part of the proposed investigation it was considered 

 necessary to redetermine the crystallography of such members 

 of isomorphous series as had already been investigated. A 

 certain amount of the published data was too indefinite in 

 detail to be made use of in an investigation on iosmorphism, 

 where it is imperative that the data be the most accurate that 

 can be obtained. It was also necessary to analyse the com- 

 pounds of the type R 3 MX 6 ?H 2 0, with reference especially to 

 the amount of water which they contain. If the formulae 

 given for these salts is correct, it would seem that the water 

 of crystallisation is in solid solution in the crystals, and does 

 not play any fundamental part in determining the crystal 

 structure. 



METHODS OF PREPARATION AND ANALYSIS 



The general method adopted for the preparation of the 

 crystals was as follows. Thallic or indie oxide was dissolved 

 in rather more than the requisite amount of halogen acid, and 

 the alkali added in the proportion required by the formula. 

 The salt was then allowed to crystallise on evaporating the 

 solution either at room temperature or by gradual cooling in 

 a crystallisation apparatus. As the salt sought for might be 



