N. L. Bowen — The Binary System. 569 



Wallace found that a mixture of composition NaAlSi0 4 was 

 completely molten at 1350°. The writer finds that NaAlSi0 4 

 does not melt till a temperature of 1526° is attained, the actual 

 figures obtained in two determinations being 1527° and 1525°, 

 of which the figure given is the mean (Table I). Possibly 

 Wallace approached the unstable equilibrium nephelite-melt 

 on cooling. 



An indistinct heat effect was obtained by Wallace on the 

 cooling curve at 1260°. The writer found a small heat effect, 

 due to inversion, on the heating curve at 1305°. Allen found 

 a point at 1289°.* The diversity of results is due to the 

 indefinite nature of heating-curve breaks representing a slug- 

 gish transformation involving no great heat effect. The 

 determination by the method of quenching gave 1248° 

 (approx.) for the inversion point (Table II). 



Anorthite. — Anorthite, like neph elite, was first prepared in 

 1878 by Fouque and Levy 11 by simple fusion of its oxides. 

 No special interest attaches to the numerous other preparations 

 of this mineral. 



The melting point of artificial anorthite is sufficiently sharp 

 to be used by Day and Sosman 1 * as a reference point (1550°) 

 on the temperature scale. Brun 16 obtained by his very different 

 method (calorimetric) 1544°-1562 . 



Mixtures. — Schleimer 18 has made determinations of the melt- 

 ing 'points' of mixtures of anorthite and nephelite using natural 

 minerals. His mixtures are of very different composition from 

 those used in this investigation and the temperatures found 

 need have no relation to those found here. 



Application to Natural Minerals. 



Anorthite. — Several natural anorthites approximate closely 

 to the theoretical composition. The melting points of some 

 of these have been determined. Brun, 15 working with Seger 

 cones, and Douglas, 16 the latest worker with the meldometer, 

 have obtained figures that differ comparatively little from the 

 figure given by Day and Sosman for pure anorthite. 



Anorthite from Idsu, Japan, 1490-1520°— Brun. 

 Anorthite " Mte. Somma, 1505°— Douglas. 



Nephelite. — In a recent paper" 4 the writer has shown that 

 experimental results are in accord with Schaller's statement 

 that natural nephelites may be regarded as solid solutions of 

 the three molecules NaAlSi0 4 , KAlSi0 4 and NaAlSi 3 8 . 



Natural nephelites, then, differ considerably in composition 

 from the compound JSfaAlSi0 4 used in this work and the 



* Geophysical Laboratory, unpublished notes. 



