GEOPHYSICAL LABORATORY. 



135 



simultaneous rotation of the nicols; (2) an electric resistance furnace com- 

 pletely inclosed within a cylindrical water-jacket and mounted upon the 

 microscope table. The furnace differs from those which have been used 

 hitherto in having an opening at the side through which passes a hollow 

 shaft perpendicular to the axis of the furnace and optical system of the 

 microscope. The shaft supports the crystal in the axis of the furnace and 

 permits it to be rotated upon the horizontal axis as desired. A graduated 

 circle on the shaft measures the rotation. The hollow core of the shaft 

 carries the thermo-element with the junction in contact with the crystal and 

 provides for temperature measurements when desired. 



N. L. Bowen. Am. Joiir. 



(12) The melting phenomena of the plagioclase feldspars. 

 Sci. (4), 35, 577-599. 1913. 



The method of quenching was applied to the determination of the melting 

 intervals of pure, artificial plagioclase feldspars. It was found possible to 

 determine accurately the temperatures of beginning of melting (solidus) for 

 compositions ranging from pure An to AbsAui, and of completion of melting 

 (liquidus) for the range An to AbgAui. Very pure natural material, Bakers- 

 ville oligoclase, was used to determine the point on the solidus corresponding 

 to its composition. Similar material, Amelia County albite, served to fix 

 the melting-point of albite. The results of this work are summarized below. 



It was also found possible in several instances to determine the composi- 

 tion of the liquid phase present at temperatures within the melting interval, 

 by measuring the refractive index of the quenched glass. Thus points on 

 the liquidus were determined by an independent methodand were found to 

 be in excellent agreement witb the results of the temperature method. 



In the theoretical discussion, equations are developed which express the 

 concentrations on the liquidus and solidus at any temperature in terms of the 

 melting temperatures and latent heats of melting of the two components. 

 The experimental results, therefore, make possible the calculation of the 

 latent heat of melting of anorthite and of albite. The calculated values are 

 104.2 calories per gram for anorthite and 48.5 calories per gram for albite, 

 and these values remain practically constant for all ranges of composition. 

 The calculated latent heat of anorthite is in excellent agreement with the 

 figure found by direct measurement, 105 calories per gram. No direct 

 determinations of the latent heat of albite have been made. 



It is shown that if these values of the latent heats are taken and liquidus 

 and solidus curves calculated, the resulting curves pass very close to the 

 experimentally determined temperatures (within the limits of error of the 



