Mineral Sulphides of Iron. 199 



line within the limits of the errors.* If we compare this line 

 with the dotted line joining the specific volumes of sulphur 

 and ferrous sulphide, we see that a considerable contraction has 

 taken place in the process of solution. f 



Equilibrium between solid pyrrhotite and the partial 

 pressure of sulphur in hydrogen sulphide. — The composition 

 of a variable phase of two components, iron and sulphur, as 

 pyrrhotite is shown to be, would of course be fixed when both 

 temperature and pressure are fixed. By heating in hydrogen 

 sulphide the pressure is fixed, though not independently of the 

 temperature. To obtain equilibrium, the products were heated 

 for about three hours, at the measured temperature, and then 

 by the device shown in iig. 6, the crucible and its contents were 

 quickly lowered to the bottom of the enclosing tube. The 

 process of heating and quick cooling in hydrogen sulphide was 

 repeated until the density of the product was constant. The 

 densities of the products thus successively prepared usually 

 agreed exactly in the third decimal place. The rate at which 

 the sulphur is absorbed by pyrrhotite in the cooling is too slow 

 to affect these results except possibly in the determinations 

 made at the highest temperatures (1100°-1300°), where a small 

 quantity of sulphur may perhaps be taken up. As this point 

 is important, some data on the rate at which the cooling 

 proceeded are here given. 



800° 600° 



Initial temperature 



Temperature after 1 min 



it a 9 u 



1300° 

 ~580° 



1100° 

 680° 



480° 



a 



it 



3 " 











a 



a 



3* " 







400° 



k 



u 



5 " 









a 



a 



6 " 



365° 







a 



u 



10 " 







165° 



420° 



305 c 



In Table V are collected the quantities of sulphur dissolved 

 in pyrrhotite at different temperatures in hydrogen sulphide 

 gas. The sulphur in Nos. 1 and 6 was determined by analysis ; 

 in the rest it was calculated from the specific gravity. The 

 results in Table VI, showing the dependence of composition 

 on temperature when the products are cooled in nitrogen, are 

 given by way of control. In fig. 8, the curve in space shows 

 how the composition of pyrrhotite varies with both tempera- 



* Three of the points are beyond the errors of the determination of 

 sulphur, and specific gravity, but if we allow a small error in homogeneity 

 in the process of preparation, probably occasioned by the splinters of por- 

 celain, the statement holds for these points also. 



f The density of rhombic sulphur is very nearly 2*075, and its specific 

 volume is therefore 0"48l9. The specific volume of the ferrous sulphide is 

 estimated by extrapolation to be 0'2098. 



