Effect of Temperature, Acidity, etc. 411 



The numerical data are presented in Table V.* The first 

 point to be noted here is that the range of acidity within which 

 it is possible to work is very limited, for a comparatively low 

 concentration, about 1 per cent, inhibits precipitation entirely. 

 Such products as one can obtain within this range vary com- 

 paratively little in composition also. However, excepting in 

 the highest concentrations, the percentage of pyrite is seen to 

 diminish as the acid concentration increases. While the results 

 are too limited in range to determine accurately the slope of 

 the curve, it appears to be nearly the same as that for sul- 

 phuric acid solutions at 200°. The fact that any given quan- 

 tity of hydrochloric acid exercises a greater specific influence 

 on the crystalline form of iron disulphide than the chemically 

 equivalent quantity of sulphuric acid may be explained by the 

 greater hydrogen ion concentration in solutions of the former. 

 Still, we should logically conclude also that the slope of the 

 curve ought to be steeper from the same cause. This appears 

 not to be true, but perhaps the numerical values are, as sug- 

 gested above, too limited in range to make sure of it. 



Hydrochloric acid solutions unquestionably give results 

 which are analogous to sulphuric acid solutions in the follow- 

 ing particulars : they give products containing marcasite which 

 increases in quantity with increasing acid concentration ; the 

 highest acid concentrations give results abnormally high in 

 pyrite ; an acid concentration is found which inhibits precipita- 

 tion entirely, and finally, the product highest in marcasite is 

 of the same composition as that highest in marcasite obtained 

 from sulphuric acid solutions at 200° and 25°. 



G. Synthetic marcasite. — These facts lead to the suspicion 

 that the above product may be pure marcasite. This suspicion 

 is strengthened by the fact that acids of 0*6 per cent and 0*7 

 per cent average hydrochloric acid concentration give the same 

 product. Measured by the Stokes constant for natural marca- 

 site, the composition should be 95 per cent marcasite. We 

 know that this constant varies somewhat in different natural 

 specimens of marcasite, as well as pyrite, and that impurities 

 affect it. The purest natural marcasite from Joplin, Missouri, 

 contained no foreign metals, but it is possible it may have 

 contained a little oxide of iron, which would certainly have 

 raised the value of the constant. We have no positive evi- 

 dence of this, but it is strange that under three different sets 

 of conditions the same product should be obtained, unless that 

 product were a chemical compound. There is another explana- 



*It is interesting to note that with 0*05 per cent HC1 solutions a precipi- 

 tate of FeS 2 was obtained when no H 2 S was introduced, due to the formation 

 of H 2 S by the interaction of sulphur and water at 300°. When the initial 

 acid wfce greater the formation of H 3 S in this way was not observed. 



