208 



the peculiar brown or black parts nearest the surface; at the 

 outmost part there is often a coherent dark stratum, from which 

 brown tongues shoot inwards along side fissures in the crystal; 

 in a lateral direction these tongues may either by degrees 

 assimilate with the lighter surroundings or they may send out 

 fine brown threads in different directions, forming a more or 

 less regular network; this is especially prettily developed on 

 the crystals of type П. Of other minerals aegyrite appears 

 rather frequently, either as scattered crystals which are often 

 pressed and partly bent or as grained aggregates, fiUing out 

 fissures in the steenstrupite. Sometimes more isodiametric 

 spaces are found, partly filled with a variated mixture of 

 aegyrite and other minerals of several different colors. 



The refraction was determined with Thoulet's fluid. The 

 fluid was of such a degree of concentration, that small 

 grains of the mineral, distributed in it, did not show total 

 reflection under the microscope ; the refraction of the fluid 

 used is then stated by its specific gravity. In this way the 

 refraction of the pure steenstrupite (type III) is fixed to be 

 1-662; that of the more alterated (type I) 1-656. The double 

 refraction is very different with the difi'erent types. The 

 crystals of type III are perfectly double refracting and almost 

 similar in this respect; byBabinet's compensator the double 

 refraction is calculated to be 0-00818 being an average of 3 

 measurements, varying from 0-00732 to 0-00870. The crystals 

 of type II show without exception single or at all events 

 excessively little double refraction in the middle. However they 

 are distinctly double refracting in the outer parts, though 

 hardly to such a high extent as type III. Sometimes the limit 

 between the two parts of the crystal lies very regularly parallel 

 with the crystal faces, in other cases the single refracting parts 

 lie quite irregularly. There are often smooth transitions between 

 the different parts, however a sharp limit is also often found 

 and in such cases the small difference in refraction, which has 



