100 Allen and White — Polymorphic Forms of 



Melting Point of Pseudo- Wollastonite. 



This was determined in practically the same way as the 

 inversion, except that some of the readings were made every 

 half minute. The rates of heating varied between 2'1 and 



"7° per minute. 









First Sample. 



Four Meltings. 





Microvolts. 



Degrees.* 



El H 



16058 



1511-0 





16065 



1511-5 





16062 



1511-3 





16073 



1512-1 



Second Sample. One Melting. 

 Microvolts. Degrees. 



El G 16053 1512-0 (curve 3.) 



El L 16072 1512-1 



Third Sample. One Melting. 

 Microvolts. Degrees. 



El G 16060 1512-7 



Fourth Sample. Two Meltings. 

 Microvolts. Degrees. 



El F 16104 1513-5 



16104 1513-5 



Melting point, 1512. 



Summary and Conclusions. 



1. Wollastonite and pseudo-wollastonite are enantiotropic 

 forms of calcium metasilicate, showing- an inversion point at 

 about 1180°. The change W„ — >W/f is easily effected by 

 heating above this temperature, while the reverse change has 

 not been accomplished without the addition of other sub- 

 stances, owing to the sluggish nature of the silicate. Cases of 

 suspended transformation are common enough, but a reversion 

 which fails altogether even after heating for days in contact 

 with the stable form is remarkable. We may compare it to 

 the case of the glasses of the alkaline feldspars, which resist all 

 attempts to make them crystallize by heating and sowing with 

 nuclei. The fact very well illustrates one of the difficulties 

 which the experimenter constantly encounters in the study of 



* Fractions of degrees are given in order to show the agreement obtainable 

 in such determinations. The absolute value of the melting point is dependent 

 upon the thermoelectric extrapolation of the gas scale, and is subject to cor- 

 rection whenever the latter shall be extended to this point. 



f Let W, t represent the optically negative form, and W p the positive. 



