Effect of Temperature, Acidity, etc. 405 



Below the concentration 0*015 per cent H 2 S0 4 we have 

 made no effort to experiment ; we were deterred by the fear 

 of getting amorphous products except possibly from solutions 

 of very low iron concentration, and these on account of the 

 small yield would be impractical to work with. Alkali 

 sulphide solutions at this temperature unquestionably give 

 pyrite, but what may happen in the narrow Held between 

 these two classes of solutions, i. e., in neutral or barely acid 

 solutions, has not yet been experimentally proven. (For 

 further considerations on the significance of these experiments 

 for the formation of marcasite in nature, see pp. 427-429.) 



2. The products at 100°. — The results obtained on the 

 sulphides precipitated at 100° were unsatisfactory. Not only 

 was no regular relation between the composition and conditions 

 of formation of the products found, but the results were not 

 reproducible. The products at all other temperatures gave 

 results consistent with one another, and the peculiar behavior 

 of these has not been fully explained. In general it was found 

 that the products contained more marcasite for a given acidity 

 than at higher temperatures, and the products all seemed to be 

 preponderantly marcasite ; apparently none contained more 

 than 25 per cent pyrite. This was as it should be, but none of 

 the products seemed to be pure synthetic marcasite, as we 

 should expect from the results at other temperatures. Their 

 anomalous behavior is possibly due to the presence of some amor- 

 phous disulphide. The products are for the most part crystal- 

 line ; but most if not all contained a variable quantity of a fine, 

 black powder which did not show any crystalline form or 

 luster under the microscope. The occurrence of amorphous 

 zinc sulphide of low refractive index was common in the zinc 

 sulphides and if any amorphous material was present in the 

 iron sulphides it would doubtless affect the results. One would 

 expect, however, that the Stokes constant (x — a) would be 

 higher for the amorphous substance than for marcasite, since it 

 is higher for marcasite than pyrite, and the amorphous sulphide 

 is less stable and more soluble than marcasite, as marcasite is 

 less stable and therefore more soluble than pyrite. As a matter 

 of fact, the constant (x — a) was always lower for these 

 products than for marcasite. It may also be stated that the 

 color of the films floated on water (p. 339) were somewhat dif- 

 ferent from those of marcasite-pyrite mixtures. Judging from 

 the Stokes method, the products ranged from 15 per cent to 25 

 per cent pyrite. 



3. Products at W0° and 300°.— -Most of the investigation of 

 the iron disulphides centers on the products formed at 200° 

 and 300°. Probably because these were better crystallized, 

 the results were always reproducible and the relations between 



