402 Allen, Crenshaw, and Me?nvin — 



from any other source than the moist sulphide itself; it could not 

 have come from the soda, since pyrrhotite crystallized from a 

 fusion and ground dry, gave off no hydrogen sulphide even 

 when heated for an hour or more at 700° under similar con- 

 ditions. 



A careful study of the table (Table III) proves that the 

 quantity of hydrogen sulphide given off by the products bears 

 no relation whatever to the quantity of pyrite indicated by the 

 Stokes test. Thus a preparation which, according to the 

 Stokes test, contained 28 per cent pyrite, gave off less hydro- 

 gen sulphide than the natural marcasite ; and the preparation 

 which contained the least pyrite (3%-5f ) gave off more hydro- 

 gen sulphide than the one which contained the most pyrite (45 

 per cent). True, the synthetic preparations usually gave off 

 more gas than the natural mineral, but we attributed this differ- 

 ence to the greater fineness and hence larger surface of the 

 former, a condition which is well known to favor the reten- 

 tion of water. That the synthetic products are finer-grained 

 appears from the fact that they are more readily transformed 

 into pyrite. Thus products which contained originally 5 per 

 cent and 45 per cent pyrite, after heating in vacuo for about 

 1 h. at 300° to 350° showed 21 per cent and 62 per cent respec- 

 tively, while pure natural marcasite after the same treatment 

 had changed only to the extent of 6 per cent. Previous 

 experience in this laboratory shows that fine powders are trans- 

 formed into polymorphic modifications more readily than 

 coarse materials. 



Though no direct experiments for the detection of any 

 possible sulphur dioxide or ferrous sulphate in the synthetic 

 sulphides were made, there was no indication of the presence 

 of either. Only the products formed at a maximum tempera- 

 ture of 300° could have contained sulphur dioxide, because 

 none was formed under other conditions. If we accept the 

 conclusion that at 200° the products occluded no reducing 

 impurities, there is no reason to believe that occlusion occurred 

 at 300°. The composition of the three-hundred-degree prod- 

 ucts as determined by the Stokes reaction shows a similar 

 dependence on the acid concentration as that of the two- 

 hundred-degree products ; both curves are smooth and the rela- 

 tion between them is rational (see fig. 1). 



In regard to the occlusion of ferrous sulphate in the prod- 

 ucts, we can simply say that its presence would increase the 

 quantity of iron dissolved in the Stokes reaction, which would 

 then indicate too much marcasite, while, on the contrary, in 

 the products obtained at three different temperatures (25°, 

 200° and 300°) the maximum percentage of marcasite indi- 

 cated was always the same — not far from 100 per cent as com- 



