158 



This colour is not, however, evenly distributed, but appears as 

 spots, between which there are uncoloured portions. When the 

 light vibrates perpendicularly to the с -axis, the section is 

 colourless. Sections parallel to the base show a regular, uni- 

 axial interference figure. The double refraction is positive. 

 The indices of refraction were determined by means of a pre- 

 pared prism with a refracting edge (parallel to the c-axis) of 

 44° 7' and were found as follows : 



Li Na Tl 



0) = 1,5492 1,5532 1,5576 



£ = 1,.5801 1,5842 1,5861 



Narsarsukite has been analyzed by Mr. Chr. Christen- 

 sen in Copenhagen. As there was a rich supply of pure and 

 fresh material for the analysis , the correctness of the different 

 determinations could be established by repeated trials. The 

 result of the analysis is as follows : 







Molecular 



ratios 



SiO^ 



61,63 



1,0271 



24 



TiO._ 



14,00 



0,1707 



4 



Fe,0, 



6,30 



0,0394 1 



1 



Al,0, 



0,28 



0,0027 1 



MnO 



0,47 



0,0067 





MgO 



0,24 



0,0060 



' 6 



Na^O 



16,12 



0,2600 . 





F, 



0,71 



0,0187 1 



1 



H^O 



0,29 

 100,04 



0,0161 j 



I 



-0 = F, 



0,30 







99,74 



If the sesquioxides are taken together with the fluorine 

 and //2 as a bivalent group FeF, and if this group is sub- 



