186 Allen, Crenshaw, Johnston, and Larsen — 



Cal. for the whole, 80*830 g. iodine solution. The iodine solu- 

 tion contained '005445 g. iodine per g. 



80*675 and 80*830 g. iodine solution are respectively equiva- 

 lent to 0*858 and 0*860 g. Na 2 S 2 3 .5H 2 0. Therefore 7*142 g. 

 and 7*140 g. Na 2 S 2 3 .5H were consumed in the reaction. 

 The equation demands 7*137 g. for 5 g.FeS0 4 .7H 2 0. 



Found Cal. 



FeS 2 + S 1-547 1*556 



FeS 2 0-872 0-863 



Sodium thiosulphate when heated with water in sealed tubes 

 forms hydrogen sulphide and sodium sulphate. JSa 2 S a 8 +H 2 = 

 H 2 S+Na 3 S0 4 . The reaction at 200° is quite incomplete, 

 though no thiosulphate was obtained when a solution of sodium 

 sulphate saturated with hydrogen sulphide was heated under 

 the same conditions. When 1 g. Na 2 S 2 3 .5H 2 and 20 cc water 

 were heated 4 days at 200°, the thiosulphate undecomposed, as 

 determined by standard iodine solution, was 0*753 g. 0*247 g. 

 decomposed is equivalent to 0*141 g. Na 2 S0 4 . The solution 

 after titrating with iodine was precipitated with barium 

 chloride. BaS0 4 found 0*223 g. equivalent to 0*136 Na 2 S0 4 . 



At first it was thought that the reaction between ferrous sul- 

 phate and sodium thiosulphate was to be explained as follows : 

 (1) Na 2 S 2 3 + H 2 = Na 2 S0 4 +H 2 S, (2) FeS0 4 +4H 2 S = FeS 2 + 

 3S+4H 2 0. Later it was found that from ferrous chloride the 

 same mixture of sulphur and FeS 2 was precipitated. Of 

 course, the reaction represented by equation (2) could not go 

 on with ferrous chloride. Therefore, the following is probably 

 the true explanation of the reaction. 



(1.) FeS0 4 + Na 2 S 2 3 = NaJ30 4 + FeS.O, 



(2.) FeS 2 3 + 3Na 2 S 2 3 = FeS, + 3S + 3Na„S0 4 



Form of FeS. 2 obtained by heating ferrous salts with sodium 

 thiosulphate. — Neither marcasite nor pyrite is obtained pure in 

 this reaction. The product shows in crusts the color of pyrite, 

 but it is poorly crystallized and may contain amorphous disul- 

 phide. A product prepared at 90°, tested by Stokes' method, 

 behaved like a mixture of 70 per cent pyrite and 30 per cent 

 marcasite. Another product formed at 300°* tested in the 

 same way acted like a mixture of 72 per cent pyrite and 28 

 per cent marcasite. Though this reaction — the reduction of 

 ferrous thiosulphate by sodium thiosulphate — doubtless has no 

 significance as applied to geology, it is probable that the fer- 

 rous thiosulphate might be reduced by other reagents, and it is 

 possible that ferrous thiosulphate may be formed in nature 

 under some conditions, but of this we have no evidence. 



* This was the maximum temperature. The reaction may have been com- 

 plete before this temperature was reached. 



