HILLEBRAN1X] FERROUS IRON. S«.) 
position has been complete. The addition of hydrofluoric acid to the 
sulphuric in the tube, in order to insure this breaking up, is to he 
regarded as of very doubtful utility in most cases, since the glass may 
be SO strongly attacked as to add an appreciable amount of iron to the 
solution, and the hydrofluoric acid may have exhausted itself in attack- 
ing the glass before the more refractory minerals succumb. Never- 
theless, if decomposition can be effected by sulphuric acid alone (he 
results obtained are sharp and concordant, and what lias seemed 
especially remarkable, and up to almost the present moment without 
a satisfactory explanation, they are in rock analysis usually higher 
than when made by any of the modifications of the hydrofluoric-acid 
method now so extensively practiced. This difference is not very 
marked with rocks containing but 1 or 2 per cent of ferrous iron, hut 
it increases with rising percentage to such an extent that where the 
sealed-tube method will show 12 per cent FeO the other may indicate 
no more than 10 per cent. This is a fact of which the writer has long 
been cognizant, but it does not seem to be known to chemists or petr< >g- 
raphers at large, though Wiilfing 1 has noticed this difference in cer- 
tain analyses without appreciating its significance. Experiments with 
soluble iron salts of known composition, like ferrous sulphate and 
ferrous-ammonium sulphate, throw no light on the subject, for both 
methods give with them the same sharp and accurate results. 
In spite of several attempts to find a solution to the problem, none 
presented itself until very recently, when, as a result of observations 
made in this laboratory by Dr. H. N. Stokes in a pending investigation 
on the action of ferric salts on pyrite and other sulphides, the clue 
was given. Dr. Stokes found that ferric salts exercise a most marked 
oxidizing effect on pyrite and probably other sulphides. The reaction 
is not new (see J. H. L. Vogt in Zeitschr. fur prakt. Geol., 1899, pp. 
250-251), but the ease with which the change takes place and the com- 
pleteness of the oxidation of the pyrite, not only of its iron but of the 
greater part of the sulphur as well, were entirely unexpected. Pure 
pyrite itself is attacked with extreme slowness by boiling dilute sul- 
phuric and hydrofluoric acids, either alone or mixed, but the moment 
a ferric salt is introduced the case is altogether different. 
However, experiment has shown (p. 95) that with the amounts of 
sulphides usually found in igneous rocks their effect upon the estimat ion 
of ferrous iron by the hydrofluoric-acid method at atmospheric pressure 
and boiling heat is negligible, though by increasing the amount of sul 
phide the effect becomes more and more apparent, because of the greater 
surface of pyrite exposed to the action of the ferric iron of the rock. 
Under the conditions of the Mitscherlich method, on the other hand 
a temperature of 150 to 200° C, and even higher, high pressure, much 
longer time of action, and impossibility of escape of any hydrogen sul- 
i Bur. deutsch. chem. Gesell., Vol. XXXII, p. 2217, footnote, L899. 
