243 



i^i 6. The (/lass, obtained by snddenlj chilHiig tlie molten mass 

 ill mercury or cold water, is colourless, perfectly clear and exceed- 

 ingl}' hard. It can be removed from the platinum-crucibles in an 

 easier way than the crystallized mass, which fact is connected with 

 the volume-change in crystallizing. 



The refractive index of the glass a|)peared to be: ?z,> = 1,541. 

 We have prepared sevei'al glasses of varying chemical composition, 

 all in the vicinity of the composition of the pure compound, with 

 the purpose to measure accurately the refractive indices and the 

 dispersion, in order to get information about the intluence of the 

 chemical composition on the ojitical behaviour of these glasses. They 

 were ground in a flat cylindrical form, and in all directions care- 

 fully polished ; then they were investigated by means of an Abbe- 

 crystalrefractometer in light of ditferent wave-lengths. 



For pure pseudo-eucvijptite-glass of the composition LiAlSiO^, 

 we found : 



Li'. 



Na: 



Wavelength in A.U. Angle of Total- Refractive 



reflection : Index : 



6708 62°11' 1,5450 



5893 



61°54' 



1,5410 



A : 



0,0040 

 0,0056 



Tl: 5350 6r31' 1,5354 



In the following are summarized the measurements with the glasses : 

 //. Compos: 47,57„,S/a; 40,07„ .4/, O3 ; 12,5"/, Li,0 

 ƒƒ/. Compos: 42,57„6Va,; m,i\' U Al, 0, ; 18,97„ Li,0. 

 /F. Compos: 48,47„,S/(>,; 39,37„ .4/, O, ; 12,37„ Li,0. 



II 



