FIRST GROUP OF MINERALS INVESTIGATED. 27 



late it quickly and with great exactness, or to hold it constant for long 

 intervals. An oxidizing or reducing atmosphere could also be easily 

 introduced whenever desired. It is, however, undesirable to expose 

 either coil or thermo-element too freely to oxygen at very high tem- 

 peratures on account of the considerable losses by sublimation to 

 which the platinum metals are subject. 



With the help of the standard metals mentioned, which are readily 

 obtainable and can be used repeatedly, thermo-elements or resistance 

 pyrometers can be calibrated in any laboratory, and used for all 

 measurements up to the limit of the Reichsanstalt scale ( 1 1 50 C. ) with 

 no greater error than that inherent in the scale itself. Above this 

 temperature up to 1600 the continuation of the thermo-electric scale 

 probably still furnishes the most convenient and trustworthy extra- 

 polation which has yet been perfected. 



The uniformity and certainty of this extrapolation will best be 

 illustrated by the measurements upon anorthite (the highest melting 

 point we measured). The melting temperature of a mineral of very 

 poor conductivity for heat and relatively low specific gravity is much 

 more difficult to measure than that of a metal, but the agreement of 

 the results tabulated below (see Anorthite, p. 37) is sufficiently good 

 to demonstrate the accuracy of the extrapolation. The thermo- 

 electric potential, therefore, appears to deserve entire confidence for 

 consistent extrapolation through the 450 immediately above the 

 present Reichsanstalt scale. 



FIRST GROUP OF MINERALS INVESTIGATED. 



The particular group of minerals chosen for the first investigation 

 was the soda-lime feldspar series, and orthoclase. The reasons for 

 this choice will be fairly obvious. Aside from its being altogether the 

 most important group of rock-forming minerals, unusual interest has 

 been attracted to it through Tschermak's theory that these feldspars 

 bear a very simple relation to one another, that they are (orthoclase 

 excepted, of course) in fact merely isomorphous mixtures of albite 

 and anorthite. This hypothesis has given occasion for serious and 

 extended study, both from the optical and thermal sides. 



A complete review of the literature of the feldspars will not be 

 attempted here. Although opinion is still somewhat divided,* it is 

 probably fair to say that the optical researches have not yet definitely 

 established or disestablished the isomorphism of the albite-anorthite 



* Fouque et Levy, Synthesedes Mineraux et des Roches, p. 145, 1882; C. Viola, 

 Tschermak Min. & Petr. Mitth., 20, p. 199, 1901; Lane, Journ. Geol., XII, 2, p. 

 83, 1904; J. H. L. Vogt, " Die Silikatschmelzlosungen," Christiania, 1903. 



