JV. L. Bowen — The Binary System. 553 



A preliminary study was carried out by putting some of each 

 mixture in a separate platinum crystallizing dish and holding 

 all at an approximately constant temperature in an electric fur- 

 nace for a couple of days. The products were then examined 

 under the microscope. This procedure was repeated at mod- 

 erate temperature intervals over some range. The detail of 

 this work need not be given here because it is not in itself of 

 great importance, but is a very useful preliminary to the more 

 precise determinations. It was noted that all compositions 

 crystallize readily ; that, in the mixtures, no reactions pro- 

 ducing new components took place, leaving the system truly of 

 two components. The existence of a eutectic point in the 

 neighborhood of 1300° G. was indicated. Two different forms 

 of the soda compound were observed, the one appearing at low 

 and the other at higher temperatures. The low temperature 

 form is analogous to nephelite and will henceforth, for con- 

 venience, be referred to simply as nephelite. The high tem- 

 perature form had formerly been prepared at this laboratory, 

 and given the name carnegieite. At low temperatures mixtures 

 containing up to 30 per cent anorthite are perfectly homoge- 

 neous, showing the ability of the low temperature form to hold 

 over 30 per cent of the anorthite molecule in solid solution. 

 The lime compound was observed in only one form, anorthite. 



Heating Curves. 



With this preliminary information it was possible to proceed 

 with the exact determination more expeditiously. 



Small quantities of each mixture were crystallized at about 

 1200° C. (below the eutectic point indicated above), and heating 

 curves run on each. The method employed was that found at 

 the Geophysical laboratory to be the best in mineral work. ' * 

 The charge is of about 2 gms. ; the bare thermocouple of plati- 

 num-platinum-rhodium dips into the charge and is connected 

 with a potentiometer system which measures the E.M.F. set 

 up at the thermocouple contact. A curve has been prepared 

 giving the E.M.F. corresponding to temperatures between 0° 

 and 1550° C. for standard elements calibrated against the gas 

 thermometer. Such standard elements are used to calibrate 

 the elements employed in the course of the work. 



The furnace in which the charge is heated is an electrical 

 resistance furnace in which a coil of platinum wire is the con- 

 ducting material. 



In running a heating curve the temperature of the furnace 

 is caused to rise gradually and regularly. The temperature of 

 the thermocouple in the charge is read at regular time inter- 

 * For references see list at end of article. 



