Mineral Sulphides of Iron. 



207 



Rinne and Boeke to facilitate the transformation, first by 

 raising the temperature at which it takes place, and second by 

 loosening the crystal structure. The transformation point in 

 the presence of 7 per cent dissolved iron is found to be 188°, 

 and further addition of iron does not change it. At lower 

 temperatures a temperature interval is found, and the inver- 

 sion is lowered, but the temperature cannot be followed in 

 mixtures containing less than 5 per cent dissolved iron. Our 

 results confirm the above in so far as the experiments were 

 carried. The change could not be detected in pyrrhotite or 

 pure troilite. It is a natural inference that the change does 

 take place in these also, but that presumably the heat absorp- 

 tion in them is very gradual and is therefore overlooked. 



Melting temperature of pyrrhotite. — Pyrrhotite melts in 

 hydrogen sulphide at 1183°. (Table VII.) Of course, a solid 

 solution melts through an interval and not at a point, but we 

 have as yet no quantitative methods which enable us to deter- 

 mine the length of such an interval at high temperatures. The 

 maximum of the heat absorption falls at the above point. The 

 melts seem to be quite thin,** and there is usually little or no 

 undercooling, if the temperature is not too rapidly lowered. 



Table VII. 



Prepara- 



Thermoele- 

 ment 



Melting temperatures 



Freezing temperatures 



tion 



mv. 



degrees 



mv. 



degrees 



1 



V 



11589 



1181 



11590 



1181 



2 



u 



11585 



1181 







3 



a 







11585 



1181 



4 



A 



11683 



1183 







5 



a 



11687 



1183 







6 



a 



11675 



1182 







6 



a 



11700 



1184 







1 



a 



11705 



1184 







8 









11690 



1183 



Examples of freezing and cooling curves of pyrrhotite in 

 hydrogen sulphide are plotted in Hg. 13. By reference to fig. 

 9 we notice that the liquid pyrrhotite contains still a consid- 

 erable excess of sulphur above the ferrous sulphide ratio, and 

 that the quantity is less than it is in the solid. From Beck- 

 mann's formulaf for the change of melting temperature with 



* See Friedrich, Metalhirgie, vii, 257, 1910. 



f Ostwald's Lehrbuch der Chemie, vol. ii, pt. 3, p. 38. 



