90 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 170. 
phidc that may be formed — the sulphur of the sulphides becomes 
nearly, if not fully oxidized to sulphuric acid at the expense of the 
ferric iron in the rock, with the production of an equivalent amount 
of ferrous iron in addition to that resulting from the sulphide itself. 
Now, rocks with hardly an exception, and many minerals, carry pyrite 
or pyrrhotite, or both, often in considerable amount, often in traces 
only. My own experience has been that sulphur can almost always 
be detected in 2 grams of rock powder. 
Let us now see what the effect of these traces when fully oxidized 
amounts to. One atom of sulphur (32) requires for its complete con- 
version to trioxide the oxygen of three molecules of ferric oxide (480), 
which then becomes six molecules of ferrous oxide (432). In other 
words, 0.01 per cent of sulphur may cause the ferrous oxide to appear 
too high by 0.135 per cent, and 0.10 per cent of sulphur may bring 
about an error of 1.35 per cent in ferrous oxide. The case is still 
worse if the sulphur is set free as hydrogen sulphide from a soluble 
sulphide, for then the above percentages of sulphur produce errors of 
0.18 and 1.8 per cent, respectively, in the ferrous oxide determination. 
The error caused by sulphides tends to become greater the more 
there is present of either or both sulphide and ferric salt. Now, the 
highly ferruginous rocks usually carry more ferric iron than the less 
ferruginous ones, and they are often relatively high in pyrite and 
pyrrhotite; hence the increasing discrepancy between the results by 
the two methods as the iron contents of the rocks rise is fully in accord 
with the aboA^e explanation. 1 
Notwithstanding that the Mitscherlich method has thus been dis- 
credited in its general applicability to rocks and minerals, it is still 
capable of affording valuable assistance with those which are totally 
free from sulphides. Hence the conditions under which success can 
best be achieved by it are set forth in the following paragraphs. 
THE MODIFIED MITSCHERLICH METHOD. 
Strength of acid. — The method in its original and usual application 
calls for a mixture of three parts of sulphuric acid and one of water 
by weight, or about three to two by volume, though a still stronger 
acid is sometimes used. In some cases, however, perhaps in most, 
much better decomposition of the silicates is effected by reversing the 
proportions of water and acid, or at any rate by diluting considerably 
beyond the above proportion. Hereby the separation of salts diffi- 
cultly soluble in the stronger acid is avoided and the actual solvent 
effect on the minerals seems to be in no wise diminished. 
1 For details of experiments showing the worthlessness of the Mitscherlich method for rocks and 
minerals which contain even a trace of free sulphur or sulphides, see Hillebrand and Stokes, in an 
as yet unpublished paper in Jour. Am. Chem. Soc, Vol. XXII, and Zeitschr. fur anorg. Chem., Vol. 
XXV, 1900, entitled, Relative value of the Mitscherlich and hydrofluoric-acid methods for ferrous iron 
determinations. 
