wherry: crystal form ^ and optical properties 325 



by R in the figure, but speculation as to the arrangement of the 

 atoms within this group seems profitless. 



Exceptions 



It now seemed desirable to inquire into the relations in cases 

 where inverse proportionality of the axes and the refractions fails 

 to hold, and five illustrations of this were found among the 50 

 representatives of the tetragonal system listed by Groth,^ 

 namely: calcium-cupric acetate, ammonium-uranyl acetate, i- 

 erythrite, /3-methyl-glucoside, and guanidine carbonate. 



In calcium-cupric acetate the inverse refraction ratio is 1.088 

 and the standard axial ratio 1.032. These are apparently too 

 far apart for observational errors to cause their divergence, so 

 it is concluded that in this substance either the atoms are irreg- 

 ularly arranged in one direction, or that a marked anisotropism 

 of the copper atoms is associated with the coloring and pleo- 

 chroism of the substance. 



In ammonium-uranyl acetate the inverse refraction ratio is 

 0.979, and the standard axial ratio 1.412. If the form usually, 

 called (102) is made (111), the latter ratio becomes 0.998, 

 approaching the former one. Probably some layers of atoms have 

 no effect in fixing the position of the prominent form (111) on 

 which the standard axial ratio is based, and yet their existence 

 finds expression in the appearance of the subordinate form known 

 as (102). The crystallographic measurements of this substance 

 made by different observers vary nearly one degree, which 

 may explain the lack of more exact agreement; and moreover 

 some anisotropism of the uranium atoms is probably present as 

 well. 



In i-erythrite the inverse refraction ratio is 1.037, the standard 

 axial ratio 0.376; but if the form taken as (111) is really (102) the 

 latter ratio would be 1.068, which is nearer, yet still not identical 

 with, the refraction ratio. However, the substance must possess 

 a rather complicated structure, containing two asymmetric 



'Chemische Krystallographie III (also a few in I and II). 



