230 



The forms observed are: o = |iri|, great and very lustrous; 

 c =1 |001j, small, but well developed and yielding good reflections; 

 711 = {llOj, broad, but commonly with curved and rudimentary facets, 

 and thus practically not well measurable. Probably also a form 

 g' = |021| occurs, and in the case of the crystals of fig. 2a doubtless- 

 a=i|100|, as a broad pinacoidal face, and r = \h o k\. 



Angular values-. 



C :(?:=(001):(111) = 

 0:0 = [\n) :(JI1) = 



0:0 = {i\l)'.{\l\) = 



No distinct cleavability was found. 



There cannot be the least doubt about the complete isomorphism 

 with the corresponding Co- and RIw-^'ó\{\ 



6V-salt a:h:c = 0,8632 : 1 : 0,8652. 



Co-salt a: h:c = 0,8700 : 1 : 0,8699. 



/Mo-salt a:b:c = 0,8541 : 1 : 0,8632. 



Up till now we have had no opportunity to prove this iso- 

 morphism also in the case of the optical antipodes, because no 

 suitable crystals could be obtained. There can be however no doubt, 

 that the said relation also exists in this case. 



Laboratory for Physical and Inoryardc 

 Chemistry of the University. 

 Groningen, June 1918. 



