stokes.] ACTION OF FERRIC SOLUTIONS. 15 
basis. Only the results obtained by the perfected method arc pre- 
sented in this paper. 
ACTION OF FERRIC CHLORIDE. 
It has long been known that ferric salts attack pyrite. L. L. 
de Koninck 1 found that pyrite is oxidized at 170°-200° C. by ferric chlo- 
ride or ferric ammonium alum, with formation of a ferrous salt and sul- 
phuric acid. J. H. L. Yogt 2 tnent ions that pyrite is very slowly attacked 
in the cold by 30 per cent ferric chloride solution, giving a trace of sul- 
phuric acid. He regards the formation of sulphuric acid as subordi- 
nate, and docs not ascribe an important role to ferric salts in the 
decomposition of pyrite. Julien 3 observed a reduction of ferric chlo- 
ride by marcasite. No quantitative study oi I hese reactions has been 
made, so Car as I can ascertain. 
PERCENTAGE OF SULPHUR OXIDIZED. 
Preliminary experiments showed thai very dilute solutions of ferric 
salts attack pyrite rapidly at the boiling temperature, and much more 
slowly, but quite appreciably, in the cold. It is upon the determination 
of the percentage of sulphur oxidized in the total mineral decomposed 
that the new method is based. If ferric chloride be used, the amount 
of mineral decomposed can l><> found from the increment of iron in the 
solution and the SO, determined as barium sulphate. An experiment 
with a ferric chloride solution containing 1.11 grams Fe'" per liter, 
with a Little hydrochloric acid, which was boiled to complete reduction 
with an excess of powdered pyrite \'vcc from sulphate, in a current 
of carbon dioxide, showed that 65 per cent of the sulphur was oxi- 
dized, a result agreeing well with the results presented below. 
FERRIC SULPHATE OXIDATION METHOD. 
Owing to the tediousness of the gravimel ric met hods and of the S0 4 
determination in the presence of iron, 1 have adopted a much more 
expeditious indirect method, which involves the use of ferric ammo- 
nium alum and titration with potassium permanganate. 
Principlt of tin method. — The oxidation of FeS 2 by Fe 2 (S0 4 ) 3 may 
be regarded as taking place by two independent reactions: 
(1) FeS 2 + Fe 2 (S0 4 ) 3 = 3FeS0 4 + 2S. 
(2) 2S + 6Fe 2 (S0 4 ) s + 8H 2 = 12FeS0 4 + 8H 2 S0 4 . 
If the reaction be carried out in the presence of an excess of FeS 2 
to complete reduction of the ferric salt, it is easily seen that equation 
(1) holds even if the theory of Brown be true, that the iron in pyrite 
is partly ferric, for in this case the ferric salt formed will in turn be 
1 Annates Soc. geol. Belgique, Vol. X. 1883, p. 101. Zeitscl.r. anorg. < 'heniie. Vol. XXVI, 1901, p. 123. 
2 Trans. Am. Inst. Min. Eng., Richmond meeting, February, 1901. 
3 Annals New York Acad. Sci., Vol. IV., 1887, pp. 138-139. 
