184 AM en, Or ens/} aw, Johnston, and Larsen — 



forms, though some of the black precipitate is still unchanged 

 after it has been heated several days at 100° with excess of the 

 polysulphide. The oxidation coefficients of several such prod- 

 ucts formed at different temperatures, and carefully purified as 

 usual, are given in Table III. Evidently they are not pure 

 pyrite, a result somewhat surprising in view of the previous 

 work; for if we obtain marcasite from the more acid solutions, 

 marcasite with pyrite from those which contain less acid, and 

 pure pyrite from practically neutral solutions, we should natu- 

 rally expect pure pyrite from alkaline solutions. Further inves- 

 tigation has led us to believe that the products of the alkaline 

 solutions do not contain marcasite, but are mixed with amor- 

 phous disulphide. Stokes explained, very plausibly, that the 

 reason why marcasite gave more free sulphur than pyrite when 

 it reacted with ferric sulphate was because it was more soluble ; 

 a fortiori, amorphous disulphide would give, under similar 

 conditions, still more free sulphur because it is the most solu- 

 ble of the three. The evidence for the existence of amorphous 

 disulphide in the products of alkaline solutions is as follows : 

 While the products of acid solutions which contain the most 

 marcasite are the best crystallized, those from alkaline solu- 

 tions which, judging by their oxidation coefficients, contain 

 the most, are almost black, dull and lusterless at the lower 

 temperatures. The quantity of pyrite is increased by raising 

 the temperature or prolonging the time of reaction, — both con- 

 ditions which are favorable to the crystallization of an amorph- 

 ous substance. Moreover, marcasite is not changed by heating 

 in alkali polysulphide solutions, as we found by heating some 

 of the natural mineral in powdered form for several days at 

 300° with polysulphide of sodium. The oxidation coefficient 

 remained 14*5. The influence of temperature on the formation 

 of pyrite from ferrous salts and alkali polysulphides is shown 

 in Table III. The influence of time is proved by the two fol- 

 lowing experiments : 3 g. FeS0 4 .7H 2 0, 2*5 g. Na 2 S, and 0*75 g. 

 sulphur, and 100 cc water, were heated 2 days at 100°. The pro- 

 duct contained about 75 per cent pyrite.* A similar system 

 heated for 7 days-at the same temperature gave a product contain- 

 ing about 95 per cent pyrite. The results are calculated on the 

 supposition that they contain marcasite ; if they contain amor- 

 phous disulphide instead, the true percentage of pyrite should 

 of course be higher, since a given quantity of amorphous disul- 

 phide would be equivalent to a greater quantity of the less sol- 

 uble marcasite, but the order of the results would of course 

 remain the same. The products obtained at 300° were yellower, 



* There is an apparent discrepancy between this last result and the one 

 quoted in Table III under the caption " 100°." In the latter case the excess 

 of polysulphide was much smaller. 



