364 HAROLD L. ALLING 



modify such results in order to make them commensurate with the 

 values of natural minerals. 



To supplement the available data secured from the literature on 

 the subject, the writer has measured the indices of refraction of a 

 number of critical feldspars, by a development of the refractive 

 index liquid immersion method/ 



We find a serious difl&culty in that some feldspars are mixtures of 

 two different series. Foerstner has pointed out that such feldspars 

 actually occur. When this is the case the indices are intermediate 

 in value. The beta values of such feldspars when their indices are 

 plotted will not lie on a surface but in a zone or a "stratum" of 

 values. If a sufficient number of pegs has been available then the 

 maximum and minimum values would give the two surfaces — one 

 for "monoclinic" and the other for "triclinic" feldspars. Until 



^ The index of such oils is lowered in value with a rise in temperature while that 

 of the feldspars rises as they are heated. 



See C. H. Wright, Jour. Ind. Eng. Chem., 1919: 



Where n and Wi are the indices at the temperature / and /i, and it = modulus of 

 dilatation, which for fixed oUs generally = . 00076. 



The rate of decrease in value of the oUs, however, is far greater than the increase in 

 the index of the crystals when raised from room temperature to 70° C. The increase is 

 so slight that it may be ignored, as it involves an error of only one figure in the fourth 

 decimal place at 70° while most of the oils in the writer's collection have a coefficient 

 ran'^g from .000306 to .000609 P^r 1°) the average being between .0004 and .0005. 

 The indices of these oUs ha\'e been measured on a Spencer (Abbe) refractometer at 

 various temperatures from 10° to 70° C. The results were recorded in the form of 

 curves, which are periodically rechecked to insure accuracy. The instrument is accu- 

 rate to .0003, although it is said to be .0002 (H. S. Simms, Jour. Ind. and Eng. Chem., 

 June, 1921). The fragments (passed through loo-mesh and caught on 120-mesh screens, 

 approximately .3 mm. in diameter) are mounted in an oil whose index at normal room 

 temperatures is higher than any of the indices of the feldspar. The slide is then placed 

 in an electric hot stage (a remodeled biological stage, with an adjustable thermostat) 

 and the temperature raised slowly until the Becke test, using monochromatic light, 

 shows that the crystal and oU have the same index. The temperature of the stage is 

 noted (a Taylor Instrument Company's thermometer standardized against the 

 thermometer of the Spencer Refractometer) and the corresponding value in index read 

 from the curve for that particular oil. Furthermore these results have been checked 

 by using Merwin's dispersion diagram (E. Posnjak and H. E. Merwin, Jour. Amer. 

 Chem. Soc, September, 1922). In this manner the indices of both phases of a perthitic 

 feldspar may be conveniently measured from a single mount. 



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