stokes.] ACTION OF CUPRIC SALTS. 45 
turbed, and the reaction continues to proceed from Lefl to right, 
accompanied by formation of Cu' and Fe"' (and possibly (Hi) and their 
precipitation as insoluble products, until the accumulated acid is in 
equilibrium with both of these and further precipitation can not 
occur. The process is further complicated by a partial precipitation 
of copper as basic salt when the sulphate is used. A mixture of cupric 
and ferrous salts gives a strong reaction for ferric iron with sulpho- 
cyanate. This does not imply an appreciable oxidation of the ferrous 
iron in the mixture itself, bid a shifting of the equilibrium to the 
right in the above equation through the formation of the slightly 
dissociated ferric sulphocyanate. It is doubtless to the reduction of 
the cupric sulphate by sulphur and by ferrous sulphate that the forma- 
tion of chalcocite from pyrite is to be ascribed. 
GENERAL CONCLUSIONS FROM THE EXPERIMENTS. 
The conclusions which may be drawn from the above experimental 
data are: 
1. At 200 copper sulphate decomposes marcasite more rapidly than 
pyrite. 
2. Both ferrous and ferric salts are formed in both cases, the former 
being found wholly in the solution, the latter wholly in the precipi- 
tate. There is no marked difference in the relative amounts of these 
in either case. The precipitate also contains, besides cuprous sul- 
phide, cuprous oxide and perhaps metallic copper. 
3. The formation of ferric salt, is to be ascribed, in pari al Least, to 
the reaction of the ferrous salt upon the cupric salt (Experiment 7), 
while some oxidation of sulphur to sulphuric acid occurs (Experi- 
ment 6). Any ferric salt which may fail to be precipitated will be 
reduced by the sulphides. 
•4. The relative amounts of ferrous and ferric salts depend simply 
upon the establishment of equilibrium between the solution and the 
products of decomposition of the pyrite or marcasite, and not upon 
any fundamental chemical difference in the two minerals, and con- 
sequently the hypothesis of Brown is devoid of valid experimental 
basis. 
It would seem thai no method can be depended upon to give us an 
insight into the state of combination or valenc}^ of either iron or 
copper in their sulphides which does not take due account of the con- 
ditions of equilibrium between the reagents and the decomposition 
products. This applies not only to other experiments of Brown quoted 
in his paper, but to the attempt of Starke, Shock, and Smith 1 to prove 
the divalent condition of iron in arsenopyrite and of Morgan and 
Smith- to establish the same for chalcopyrite. That the product of 
heating chalcopyrite in dry hydrochloric gas consists of a mixture of 
cupric chloride and ferrous chloride is inconclusive on the question as 
1 Jour. Am. Chem. Soc, Vol. XIX, 1897, p. 948. 
2 Ibid., Vol. XXIII, 1901, p. 1U7. 
