242 INDIUM AND THALLIUM IN 



1 JH 2 O, which was first prepared by Meyer (6), and re-examined 

 by Fock (7), was found to be isomorphous with K 3 T1C1 6 2H 2 O 

 and with (NH 4 ) 3 T1C1 6 2H 2 O, and mixed crystals were obtained 

 by Fock of K 3 InCl 6 IJH 2 O and K 3 T1C1 6 2H 2 O, the crystal 

 constants of which closely resembled those of its components. 

 Pratt (8) found that K 3 TlBr 6 H 2 O was isomorphous with the 

 above three salts, so that the unusual case is here presented 

 of salts of the same chemical type, but varying in the number 

 of molecules of water of crystallisation, showing isomorphous 

 relationships. In this connection Groth (9) remarks : ' The 

 explanation of these remarkable relationships can naturally 

 only be obtained by again completely and systematically 

 examining these and all analogous compounds.' 



So far then as the crystallography of the salts of indium 

 has been determined, it is evident that the results show only 

 indefinite isomorphous relationships between indium and a 

 wide class of trivalent metals, including Fe, Cr, Al, Ga, and Tl, 

 and do not point to any specially close relationship within the 

 sub-group Ga, In, Tl. It seemed advisable to compare the 

 crystallographical character of corresponding salts of thallium 

 and indium in particular, especially as the work on the double 

 sulphates had brought to light no isomorphous relationships 

 between indium and thallium compounds. As the complex 

 alkali halides gave promise of the best results, and as the 

 work on these salts might at the same time lead to the elucida- 

 tion of the unusual case of isomorphism referred to above, 

 attention was confined to the chlorides and bromides of 

 indium and thallium with K, NH 4 , Rb, Cs, and Tl. 



As already mentioned, the work of Rammelsberg (10) and 

 Fock (7) on K 3 T1C1 6 2H 2 O, of Meyer and Groth (6), and of 

 Fock(7),onK 3 InCl 6 liH 2 0,ofRammelsberg(10)on(NH 4 ) 3 TlCl 6 

 2H 2 O, and of Pratt (8) on Rb 3 TlBr 6 H 2 O, seemed to indicate iso- 

 morphism in this rather remarkable group. Pratt also found 

 that the salts Rb 2 TlCl 5 H 2 O and Cs 2 TlCl 5 H 2 O were isomorphous 

 in the rhombic bipyramidal system, and Meyer (11) prepared 



