134 - J. B. Ferguson and A. F. Buddington — 



ence should be made, therefore, to their paper. 6 The 

 melting points used in the calibration of the thermoele- 

 ment were: gehlenite 1590°C. ; anorthite 1550°C. ; aker- 

 manite 1458° C, and diopside 1391.5° C. The presence of 

 considerable iridium in the platinum foil, used to hold the 

 charges, gave rise to an extremely annoying contamina- 

 tion of the thermoelement, necessitating repeated calibra- 

 tions and rendering almost impossible the obtaining of 

 thermal results which could be considered to have a rela- 

 tive accuracy of better than 4°C. 7 



For the study of the optical properties of the crystals 

 of intermediate composition, the method of crystallizing 

 undercooled glasses gave the best results. Zoned crys- 

 tals usually were obtained from charges which had been 

 heated at temperatures at which both liquid and solid 

 phases could co-exist. For the first method, a treatment 

 overnight at a temperature just below the temperature 

 at which the beginning of the melting took place was 

 sufficient. 



Thermal Results and Their Discussion. — The results 

 of the quenching experiments are given in Table I and 

 interpreted graphically in figure 1. 







Table I 







The quenches 



which yielded significant thermal results. * 



Composition 





Temperature 



Time 





wt. percent. 





in degrees 



in 





Gehlenite. Akermanite. 



Centigrade. 



minutes. 



Phases present. 



100 





1592 



15 



Glass 







1588 



15 



Crystals 







1590 



15 



Crystals + trace 

 glass 



100 





1459 



15 



Glass 







1457 



15 



Crystals 



10 90 





1431 



15 



Glass 







1429 



15 



Glass 







1427 



15 



Glass 







1424 



15 



Glass -f- crystals 







1420 



15 



Glass -j- crystals 







1416 



15 



All crystals 



8 This Journal, 48, 88, 1919. 



7 Under the best conditions results which have a relative accuracy of 

 1°C may easily be obtained. 



