( 604 ) 



From this we see that a dassification exchisively according to 

 decreasing- SiO., percenhige, coijicides wilh an iiicreasing value of 

 tlie refractive index. 



Apparently the metal oxides present have oidy little iiitluence on 

 that vahie, at least tiiis influence falls within the limits of the 

 errors of observation. 



A chi'vsolite-norite and an augite syenite with about the same 

 SiOj percentage have also the same index, whereas the oxides, 

 especially MgO are sui-e to he there in quite another i-elation, for 

 in the chrysolite-norite the niijierals containing Mg come strongly 

 to the foreground. 



As regards the coloui- of the glass it will he almost wholly 

 dependent on the iron-percentage. 



With the exauiined glasses the colour changed from light green 

 to dark hrown. .lust as with isomoi-phous mineral series, as e. g. the 

 enstatite-hyperstenesei'ies, the dark colour most likely points to a 

 greater iron ])ercentage than the light one. 



The typical amorphous glassfracture can he easily distinguished 

 at the s|)linters under the microscope. 



The fusion of the rock-|)o\\ der took |)lac(* in a gasllame in \\hich 

 compressed o.xygen was blown. ,\s nil underlayer a cupel of chalk 

 or bone-ash \\as used. Ihit care has to be laken that the melted 

 magma of the i-iijiel remains isolated, because there is a chance that 

 oxides of alcalic earths will be absoi-bed by the cupel and in 

 consequence the conq)ositi()ji of the magnui does not answer any 

 more to that of the rock. This can be obtained by directing the 

 point of the tlame towards the middle; the upperlayer then fuses 

 quickly to a little ball, which remains isolated by the underlaying 

 rock-powder of the cui)el. To conti'ol the regularity found in the 

 independence of the refi-active index of the Si 0.^ percentage, two 

 niixtures of the following com[)Osition were made : 



I. 



SiC), 



AI.O3 

 CaO 

 MgO 

 K,0,Na, 



60 7„ 

 10 

 10 

 10 



5 



5 



II. 



60 7c 

 20 



5 



5 

 10 



Of both the mixtures the fused glass had the index 1.520; here 

 we see again the prevalent inlluence of SiOj. 



At last some slags and melted minerals were investigated. 



