50 jaeger: melting temperatures of metasilicates 



The lithium silicate compound crystallizes beautifully in white 

 glistening needles with perfect cleavage in fine colorless lamellae, 

 like mica. The microscopic examination showed the preparation 

 to be quite homogeneous except for a few tiny air bubbles between 

 the lamellae which were probably crowded out in the crystalli- 

 zation of the mass. The crystallization is accompanied by a con- 

 siderable development of heat and increase of volume. The 

 needles are biaxial with rectangular extinction and very strong 

 birefringence. The direction of elongation of the needles is that 

 of the larger refractive index and the principal plane of cleavage 

 appears nearly parallel to the plane of the optic axis. The angle 

 of the optic axes is rather large. Microscopical examination 

 yielded the following additional data: Refractive index in the 

 direction of elongation of the needles, 1.609; perpendicular to the 

 direction of elongation, 1.589, giving a birefringence of about 

 0.020, corresponding nearly to augite; direct measurement of the 

 birefringence with the Babinet compensator gave 0.019, in good 

 agreement with the calculated value. 



The specific gravity of the finely powdered substance, deter- 

 mined by the picnometer method with xylene (0.8587) gave 2.5203 

 at 24.9° C. 



Three different samples of this preparation were heated suc- 

 cessively in the platinum furnace with all the precautions which 

 have been found desirable in the determination of standard melt- 

 ing points in the Geophysical Laboratory. The standard thermo- 

 element C, used by Day and Sosman in the work with the nitrogen 

 thermometer, was used. The results obtained are given on 

 page 51. Prior to these determinations, the thermoelement was 

 examined for homogeneity and found to be free from contamina- 

 tion. 



The melting temperature (11,953 microvolts = 1201.8° C.) is 

 as sharply defined as that of diopside and quite as reproducible, 

 as can be readily seen from the experiments themselves. 



While the melting temperature of pure lithium metasilicate is 

 sharply defined, the solidifying temperature obtained by cooling 

 down the molten mass is completely dependent upon the rate of 

 cooling, notwithstanding the tendency to crystallize and the 



