244 



77 5350 61°9' 1,5301 



It is diffictilt to deduce a simple relation of cheiiHcal composition 

 and oi)tical pi-operties from these data. Generally speaking, an 

 increase of the amount of Li J) seems to cause an increase 

 of the refractixe power (except in IV), while a larger amount 

 of SiO^ appears just to diminish the refractive index, the exceptional 

 case IV could be explained by the superposition of these two causes. 

 This dependence of the quantities of the oxides present, appears to 

 bear some connection with the relatively higher refractive power of 

 the lithiumoxide, and the smaller one of the silica ')• 



The pseudoeucryptite-glass is, with respect to the opaque, crystal- 

 lized compound, a typical metastahle phase : already on heating the 

 glass during a very short time in a BuNSEN-gasburner, the pieces of 

 glass become primarily yellowish, then thej become opaque, 

 and finally they appear under the microscope wholly changed into 

 the mentioned crjptocrystalline aggregation of birefringent grains. 

 If heated only for ten minutes at 900° C, they are completely 

 changed, and the same occurs, on heating the finely powdered glass 

 during some time with molten LiCl or LiF in a platinum crucible. 



The specific gravity of the pure pseudo-eucryptite-glass was deter- 

 mined by means of . the method of swimming, in a mixture of 

 bromoform and benzene, at 13'' C; it was found to be : c/40 = 2,429. 

 Thus both the refractive index and the density of the glass are 

 somewhat higher than for the crystallized compound. 



§ 7. P'inally we ha\'e compared the artificial product with a 

 natural eucryptite of Brancheville (Conn.). The mineral, of which 

 a thin section was prepared, looked as an aggregation of crypto- 

 crystalline, homogeneously extinguishing fields: however, although 

 they had superficially some analogy with the artificial product, they 

 must be considered as composed of much larger crystals, showing 

 apparently the kind of structure, somewhat similar to the so-called 

 "schrifV'-granite. Locally it is intermixed with a much more strongly 



1) F. M. Jaegeu and H. S. van Klooster, these Proceedings, loco cit. (1914). 



