ANORTHITE. 



37 



platinum and platinum alloyed with 10 per cent of rhodium. Now 

 the constants of such thermo-elements will usually differ among 

 themselves and require to be determined for each element by calibra- 

 tion with the gas thermometer or with the melting points of the 

 metals. It therefore offers an excellent test of the value of the extra- 

 polation if some sharp melting point can be found in the extrapolated 

 range to serve as a point of reference. The melting point of crystal- 

 line anorthite serves this purpose exceedingly well, and separate 

 determinations of it with three separate thermo-electric systems, 

 gave identical values within the limits of error of observation. Our 



Anorthite. 

 first preparation. 



Melting temperature, 1532 . 



confidence that the extrapolation for these 375 is reasonably correct 

 would, therefore, appear to be justified. Until the gas scale can be 

 extended over this range, the melting point of pure anorthite (1532 ) 

 determined in this way will serve as a useful point in thermometry. 



Ab,An 3 (PeatesII, III, IV, V). 



This feldspar decidedly resembles anorthite in its relatively low 

 viscosity, the readiness with which it crystallizes, the well-marked 

 break in the heating curve at the melting point, and in its tendency to 

 form comparatively large crystals. In general, we may say that all 

 these characteristics are somewhat less marked than in anorthite. 

 Our determinations of the melting temperature follow. 



