CRYSTALLOGRAPHICAL RELATIONSHIP 267 



crystals are markedly poorer in faces, the general similarity of 

 chemical composition, and of angular values, justifies the 

 inclusion of this salt in the isomorphous series. In comparing 

 this salt with the others of the series, the values already quoted 

 will be used. 



Isomorphous with the second series are two salts, K 2 FeCl 5 

 H 2 O and (NH 4 ) 3 FeCl 5 H 2 O. The former is the mineral 

 Erythrosiderite (20), for which the crystal constants are (adopt- 

 ing the same placing of the crystal as in the isomorphous series) 

 a : b : c :=*9628 : 1 : 1*3931, and in which the combination 

 jlOlj, jOllj, jlOOj, and J120J occurs : on crystals formed in the 

 laboratory the octehedral-like combination of JlOlj and jOllj, 

 similar to that already described for Cs 2 InCl 5 H 2 O, is charac- 

 teristic. The salt (NH 4 ) 2 FeCl 5 H 2 O has been described by 

 Johnson (15). He found that the forms JlOlj and JOllj pre- 

 dominate, |100j is sometimes large, and J120J small. Two 

 twinning laws were observed (1) twinning axis perpendicular 

 tojlllj; (2) twinning plane jllOj. 



System : Rhombic bipyramidal. 



Axial constants : a : b : c='9749 : 1 : 1*4239. 



S. G.=l*99. 



These two compounds of iron are the only salts hitherto 

 described which are undoubtedly isomorphous with the series 

 of indium and thallium salts under discussion. The crystallo- 

 graphy, as quoted above, will be used when comparison is made 

 between the various members of the series. 



