stokes.] ACTION OF FERRIC SOLUTIONS. 19 
From the variation in the relative oxidation of the sulphur with 
concentration and temperature it appears certain that no single equa- 
tion expresses the oxidation of pyrite and marcasite by ferric salts to 
ferrous sulphate, sulphur, and sulphuric acid, but that it is the 
result of two, or possibly three, Independent reactions of different 
orders, which produce results varying according to circumstances. 
It is therefore extremely probable that other oxidizing agents, as 
atmospheric or dissolved oxygen, behave in a similar manner. The 
oxidation with separation of free sulphur presents no difficulties; as 
is well known, finely powdered pyrite and marcasite on exposure to 
the air for a short time give free sulphur, which can be detected by 
extraction with ether, and the formation of sulphur dioxide has been 
noted by various observers; finely divided moist marcasite evolves it 
strongly when heated on the water bath. The difficulty is rather to 
account for the complete oxidation of the sulphur usually occurring 
in nature. Whether with stronger solutions and continued reoxida- 
tion by air, or the presence of retarding agents, 100 per cent can be 
reached can not be stated at present, and experiments bearing on 
this point are desirable. 
REASON FOR THE DIFFERENT BEHAVIOR OF PYRITE AND MARCASITE. 
The different proportion of sulphur oxidized under identical condi- 
tions in the two cases is doubt less due to the different solubilities of 
pyrite and marcasite, or at least the different rates at which they are 
attacked. That the sulphur is not completely oxidized is due to its 
smaller oxidation rate under the given conditions. The iron is at- 
tacked so fast that the oxidation of the sulphur falls behind and a 
portion is liberated as free sulphur, which, as my experiments show, 
is scarcely oxidized by the boiling ferric solution when collected to 
aggregates of sensible size. This phenomenon is a familiar one when 
precipitated sulphides are treated with nitric acid; a portion of the 
sulphur is oxidized and the remainder is liberated. When a ferric 
salt is reduced by hydrogen sulphide the sulphur of the latter is part ly 
liberated, but a very considerable part is oxidized to sulphuric acid. 1 
Whatever retards the solution of the sulphide without affecting the 
concentration of the oxidizer gives more time for the complete oxida- 
tion of the sulphur. This may be brought about by certain salts in 
solution or by a difference of physical structure in the crystals. 
Pyrite dissolves more slowly than marcasite, hence its sulphur is more 
completely oxidized. Under conditions where the solution is very 
slow, it may perhaps approximate completeness in both cases, a sub- 
ject well wwthy of investigation from its bearing on the oxidation of 
pyrites in nature. 
1 Gmelin Kraut, Vol. I, pt. 2, p. 219. I have found that when a 1 per cent boiling solution of 
ferric chloride is reduced by hydrogen sulphide very nearly one-third of the reduction is 
effected by the sulphur. 
