182 Allen, Crenshaw, Johnston, and Larsen — 



and washing out the excess of reagent, the black residue dis- 

 solved without leaving any sulphur behind. The product 

 therefore must have contained free sulphur and could not have 

 been ferric sulphide, Fe 2 S 3 , though the latter, according to 

 G-edel,* decomposes with dilute acid, giving the same products 

 as the above mixture. A product made "by the action of 

 hydrogen sulphide on ferric hydroxide was washed into a glass 

 tube with about 100 cc water, saturated w T ith hydrogen sulphide 

 at room temperature, sealed and heated at 14.0° for seven days. 

 The solution when cooled and opened still smelled strongly of 

 hydrogen sulphide. The product had become quite dense and 

 had a yellowish grey color. It was boiled in hydrochloric acid 

 for some time to dissolve any unchanged ferric hydroxide, or 

 ferrous sulphide, and further purified as usual. The oxidation 

 number was 49, corresponding to 87 per cent of pyrite. The 

 work was repeated with ferric hydroxidef which had been 

 dried at 100° to make it easier to handle It proved, however, 

 much less susceptible to hydrogen sulphide. It had to be heated 

 repeatedly at 140° with saturated hydrogen sulphide water 

 before the color of the oxide of iron had disappeared entirely. 

 After purification, the product gave the oxidation number 50*4, 

 corresponding to 90 per cent pyrite. 



Action of sulphur on pyrrhotite in the presence of a sol- 

 vent. — The formation of pyrite, just described, is evidently a 

 result of the direct union of sulphur and ferrous sulphide, the 

 first product of the reaction. The hydrogen sulphide water 

 probably acts as a weak solvent. Similarly, the marcasite may 

 be regarded as a product of the addition of sulphur to ferrous 

 sulphide, which forms gradually from solution. The formation 

 of pyrite by the action of sulphur on crystalline pyrrhotite, 

 the relation of which to ferrous sulphide will be shown farther 

 on, proves conclusively that, at a given temperature, it is not 

 the exact nature of the solid phase which reacts with the sul- 

 phur, but the composition of the solution in which it forms, 

 that determines whether the product shall be pyrite or marca- 

 site. 2*2 g. pyrrhotite prepared in the laboratory, and 0*8 g. 

 of sulphur were put into a glass tube, to which was added a 

 solution of 0*1 g. of sodium bicarbonate in 100 cc water. Before 

 sealing the tube, the solution was partially saturated with hydro- 

 gen sulphide. In composition this solution was similar to that 

 of a warm " sulphur" spring, and it served as a solvent for the 

 sulphur, which was gradually absorbed by the pyrrhotite. The 

 tube and its contents were heated for two months at 70°. 

 The product at the end of that time still contained sulphur and 

 undecomposed pyrrhotite. To remove the latter it was boiled 



* Jour, fiir Gasbel., xlviii., pp. 400 and 428, 1905. 



f According to Gedel (loc. cit.), Fe 2 3 .H 2 is thus obtained. 



