108 



Day and Allen — Isomorphism and Thermal 



The close agreement of these determinations is of very 

 considerable significance with reference to the method of 

 temperature measurement employed. It will be remembered 

 that the established temperature scale ends at 1150° and that 

 temperatures beyond that point are extrapolated with the help 

 of some trustworthy phenomenon which varies with the tem- 

 perature. We chose for this purpose the thermoelectric force 

 developed between pure 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 calibration with the gas 

 thermometer or with the melting points of the metals.* It 

 therefore offers an excellent test of the value of the extrapo- 

 lation if some sharp melting point can be found in the extra- 

 polated range to serve as a point of reference. The melting 

 point of crystalline anorthite serves this purpose exceedingly 

 well, and separate determinations of it with three separate 

 thermoelectric systems gave identical values within the limits 

 of error of observation. Our 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 x An 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 some- 

 what less marked than in anorthite. Our determinations of 

 the melting temperature follow : 









ABxANs 















First Preparation. 















Electromotive 



Temper- 







Date 





Element 



Force in MV. 



ature. 



Eemarks. 



Dec. 9, 1903 



A 



15501 



1504° 



slow 



heating 



" 11, 



a 



A 



15363 



1493 



rapid 



u 



i—i 



a 



No. 3 



15507 



1498 



a 



u 



" 12, 



a 



3 



15599 



1505 



a 



u 



a a 



a 



3 



15594 



1505 



a 



a 



a a 



u 



3 



15604 



1506 



slow 



i( 



a a 



a 



A 



15518 



Meai 



1505 



a 



cc 





i 1502° 





Second Preparation, 

 Apr. 9, 1904 No. 3 15520 1499 slow heating. 



2 15637 1497 



u a (( 



a a 



Mean 1498°. 



Melting temperature 1500°. 



* Day and Allen, loc. cit. 



