396 Allen, Crenshaw, and Merwin — 



1*05 to 1*1 £. is ground for 1 li. with water in a McKenna ore- 

 grinder and further purified as described in the preceding 

 paper.* 



Disulphide of iron was formed by the above method at 25°, 

 100°, 200° and 300°. All the products with the possible excep- 

 tion of those obtained at 100° were evidently crystalline to the 

 naked eye, and those obtained at the higher temperatures, 

 especially the products formed from solutions which contained 

 2 per cent to 3 per cent of sulphuric acid consisted of crystals 

 of relatively large size. The finest were crystallized from 

 hydrochloric acid solutions ; these formed beautiful aggregates 

 of splendid lustre the individuals of which were not in- 

 frequently 0-7 mm in length X 0'2-0-3 ram in width. 



C. Chemistry of the formation of iron disulphide. — The 

 chemical reaction which is involved in the formation of iron 

 disulphide is expressed by the equation : 



H a S + S + FeS0 4 = H 2 S0 4 + FeS 2 . 



Table I may be put in evidence on this point. The figures 

 there given show that the increase in acidity and the weight of 

 the sulphide formed are in practically the ratio which the 

 equation demands. This is strictly true within the limits of 

 error at the lower temperatures, 25° and 100°. To be sure the 

 errors are greater than they are in ordinary gravimetric opera- 

 tions because we are under the necessity of recovering the 

 sulphide in pure condition ; what clings to the filter can not 

 be saved by burning and this fact entails an appreciable loss. 



At the higher temperature and the higher acid concentra- 

 tions a side reaction becomes manifest, viz : the reduction of 

 the sulphuric acid to sulphur by the hydrogen sulphide, fol- 

 lowed in turn by the interaction of the sulphur with the sul- 

 phuric acid and the appearance of sulphur dioxide. This 

 reaction is pretty slow at 200°. The first series of results at 

 this temperature (see Table I) show very little reduction ; the 

 second series more. The cause of this difference is not clear. 

 At first it was ascribed to differences in the glass tubes, which 

 were taken from two different lots. The glass of the second 

 lot was believed to have neutralized more acid than the other. 

 Blank experiments made by heating dilute sulphuric acid in 

 sealed tubes at 200° show that the suspicion was incorrect, 

 but the results are given because the behavior of " Jena com- 

 bustion " glass with aqueous solutions at 200° is a matter of 

 some interest (see Table II). At 300° the reduction of sul- 

 phuric acid by hydrogen sulphide and sulphur is considerably 

 greater in the same time than it is at 200° (see Table I). 



* This number, p. 384. 



