N. L. Bowen — The Binary System. 565 



good signals. The indices, determined by the immersion 

 method in sodium light, gave co = 1'537, e ■= 1*533. Bire- 

 fringence *004, slightly, but noticeably less than that in 

 carnegieite. 



About half a gram of these crystals was tested for tungstic 

 acid by boiling with hydrochloric acid. Only pure white 

 silica was left with no yellow tinge. The amount of tungstic 

 acid could not have been more than a trace. 



The density of these crystals is 2*619 at 21°. The degree of 

 correspondence of the artificial compound with the natural 

 mineral is shown in the following table : 



Opt. 



Cryst'n. Char. o e G habit 



NaAlSi0 4 (artif.) hex neg 1-537 1*533 2*619 prism and 



base 

 Nephelite (nat.) hex neg 1*541 1*537 255-265 prism and 



base 

 (predominant) 



It may be fortuitous that there is close correspondence with 

 Gladstone's law in the relation between the mean indices and 

 densities of the two forms of the soda compound, but the fact 

 is, perhaps, worthy of note. Gladstone's law states that the 



n i 



specific refractive energy is constant, whatever the state of 



molecular aggregation. 

 We have then 



for carnegieite 



1*512 — 1 



— *°037 



2*513 



for nephelite 



1*535 — 1 rt _ 



=-2042 



Q-A1 Q 



Nephelite Mix- Crystals. 



It was found possible to determine definitely the variation, 

 with composition, of the optical properties of the hexagonal 

 mix-crystals or solid solutions from Ne to Ne 65 An 35 . "With 

 increasing proportion of the lime molecule, the birefringence 

 of *004 (negative) grows less, passes through zero and finally 

 becomes *002 (positive). 



The indices were determined in sodium light at room tem- 

 perature by the immersion method. The results are shown in 

 Table III and in the plot Diagram III, in which the size of 

 the circles is a rough measure of the accuracy of the determi- 

 nations. 



Am. Jour. Sci. -Fourth Series, Vol. XXXIII, No. 198. -June, 1912. 

 37 



