Mineral Sulphides of Iron. 



211 



the contents were found to possess a bright metallic luster, while 

 about 10 cm above the cover of the resistance furnace a ring of 

 sulphur was condensed on the inside of the tube. This exper- 

 iment was repeated with similar results. A sulphide contain- 

 ing 36*02 per cent of sulphur (FeS contains 36*45 per cent) 

 was heated as before. This time the manometer read 5*5 mm 

 at the melting temperature. When the apparatus was cold the 

 reading was 5*0 mm . This product was also perfectly bright and 

 a ring of sulphur was again visible* on the cool part of the 

 tube. The product was analyzed again and found to have 

 lost still more sulphur. The following table (Table VIII) 

 shows that the percentage of sulphur, the specific gravity, and 

 the last melting point of three different products prepared in 

 vacuo are in accord, i. e., the density increases with the per- 

 centage of free iron and the melting "point" falls. The melt- 

 ing temperature of No. 3 should of course be lower than No. 

 2, but these temperatures are not easy to locate exactly ; a part 

 of the difficulty is perhaps due to the fact that we are dealing 

 with a mixture, in which the heat absorption is not sharp. 



Table VIII. 

 Properties of pyrrhotite after melting in vacuo. 



No. 



Melting 

 temperature 



in 

 microvolts 



Melting 



temperature 



in 



degrees 



Sp. gr. 

 at 25° 



Density 



(4°) 



Sp. vol. 



Per cent 

 sulphur 



Free iron 

 calcu- 

 lated 



1 



2 

 3 



11508 

 11346 

 11416 



1165 

 1156 

 1157 



4*816 

 4*861 



4-888 



4*802 



4*847 

 4-869 



•2082 

 •2063 

 •2054 



36-02 

 35-71 

 35-41 



1-230 



2-08 

 2-91 



It is quite evident from the above data that the true melting 

 point of ferrous sulphide can only be determined by heating 

 in sulphur vapor, the pressure of which is equal to the disso- 

 ciation pressure of the sulphide. We may safely say that this 

 point will be found to lie between 1183°, the melting temper- 

 ature of a product formed in hydrogen sulphide of one atmos- 

 phere pressure, and 1165°, and that the point is within 5° of 

 1170°. Treitschke and Tammann,f working with impure mate- 

 rial, by extrapolation estimated the melting point of ferrous 

 sulphide at 1300°. Biltz,J heating precipitated sulphide of 



* The sulphur was identified by solution and crystallization from carbon 

 bisulphide. About 100 m s was thus recovered, 

 f Zs. anorg. Chem., xlix, 328, 1906. 

 % Zs. anorg. Chem., lix, 273, 1908. 



