( 358 ) 



is triclinic, the Rb-salt monoclinic, the {XH^ysalt. rlionibic, altliough 

 so analogous to the /^/>salt, that it should be regarded as pseudo- 

 symmetric. Finally in the series of the alkaU-platiauni-jodo-nitrites, 

 the /v-salt is tetragonal the Rb-%^\{ trielinic-pseudorhombododecaedric, 

 the CVsalt monoclinic, with a pseudorhombododecaedric habit. 



8. The K- and /^6-chromates are rhombic-bipyramidal ; the CV-salt 

 is dimorphous, namely rhombic and ditrigonal-scalenoedric ; and finally 

 the (AWj-salt is monoclino-prismatic, but certainly dimorphous, — as 

 it can form witli {N'H,)„_ SO, mixed crystals of great concentration. 

 The K- and Cs-dlchromates are isomorphous : the (A^/Zj-salt is 

 monoclinic and totally different from the two other salts ; the /i6-salt 

 is dimorphous, namely monoclinic and triclinic, whereas the mono- 

 clinic form is probably isomorphous wiih the (iV^J-salt. 



Still more complicated is the matter with the tric/womntes. The 

 /v-salt is monoclino-prismatic; the AY>-salt is hexagonal and rhombic; 

 the Cv-salt is trigonal and quite different from the hexagonal /^6-salt; 

 finally the (iVi/J-salt is rhombic but quite difTerent from the second 

 form of the Rh-ssdi. We must therefore assume here an isotri- or 

 isotetra-morphous replacement of the alkali-metals. 



9. But by far the most complicated of all is the behaviour of the 

 alkali-metals in their nitrates! 



The /v-salt is dimorphous: rhombic-pseudo-hexagonal and trigonal; 

 the 7i6-salt is rhombic but not isomorphous with the /v-salt; at 161° 

 it becomes cubic and at 219° trigonal, while all the conversions are 

 reversible; the CV-salt is rhombic and cubic; the 7V-salt is trimor- 

 phous, namely rhombic, cubic and hexagonal. Finally, the (A'//J-salt 

 is tetramorphous : rhombic, monoclinic, tetragonal and cubic, wliereas 

 according to Wallerant the tetragonal form again returns at — 16°C. 

 Here we have a rich field for the study of the binary systems 

 concerned, which is still very incomplete. 



§ 2. The above summary, which could be easily extended by 

 quoting other examples, for instance, from the nitrides, polyhalogenides, 

 hexafluosilicates, tetrachloro- and tetrabromoaurates may prove our 

 contention, that the mutual morphotrjpic relations of the alkali-iuetals 

 form an exceedlnyhj complicated problem, and that it is unwarrantable 

 to lay down rules, which are not supported by a sufficient number 



of facts. 



A peculiar contribution to the above question is given here. 

 During an investigation concerning trichloroacetic acid, I obtained 

 the acid potassium-salt of this acid ; it forms extraordinarily beautiful 

 crystals, which on investigation showed a few peculiarities of 



