82 ON PYRITE AND MAKCASITE. [bull. 186. 
The equal ion 
c— a 
applies, in its original sense of giving the per cent of sulphur oxidized, 
only to FeS 2 . We may, however, use it in a broader way as simply 
indicating a certain relation between the permanganate values of the 
ferrous, ferric, and total iron, and in this sense it may be applied to 
any sulphide or other compound capable of reducing ferric salts, and 
is useful in instituting comparisons between these and FeS 2 either 
alone or in a mixture. 
HEMATITE AND MAGNETITE. 
The presence of these is indicated during the extraction of the 
powder with 20 per cent hydrochloric acid if repeated portions of acid 
continue to be colored yellow after heating. Difficultly soluble fer- 
ric silicates, of course, show the same result. As these iron oxides 
are scarcely soluble in the weakly acid ferric solution, their presence 
in small amounts is without appreciable effect. 
GALENA. 
Galena is rather slowly attacked by the ferric solution, and the 
value of j> is infinity. A mixture of galena and pyrite (_p=60.8) con- 
taining 3 per cent lead gave 
p=59.5. 
From this it appears that small amounts of galena exert a slight 
depressing influence and that the curve for galena-pyrite varies from 
00.4 to infinity, with a minimum near the pyrite end. 
NICKEL AND COBALT. 
These show a very strongly elevating influence on the oxidation 
coefficient, as shown by specimens Nos. 27 and 32. 
ARSENOPYRITE. 
The action of the ferric solution on arsenopyrite, or on pyrites or 
marcasites containing arsenic in notable amounts, is very character- 
istic, in that the value of b is always greater than that of c. This is 
due to the fact that a portion of the permanganate represented by b 
is consumed in oxidizing arsenious to arsenic acid. It maybe further 
observed that in such cases a permanent end-reaction for b can not 
be obtained, as the last portions of arsenious acid are oxidized but 
very slowly in the cold by permanganate. In this event the titration 
must be stopped at the first change of color, and since arsenic acid is 
precipitated very slowly by hydrogen sulphide, it is necessary to con- 
duct the gas through for several hours at a temperature of about 80° 
