94 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
oxide. But this action is not so immediate as to affect the ferrous 
oxide determination if the end point in the latter has been properly 
observed. 
pkatt's modification of the hydrofluoric-acid method. 
J. H. Pratt 1 has shown that very satisfactory ferrous iron determi- 
nations can be secured by simple boiling of the rock powder with hydro- 
fluoric and sulphuric acids in a large crucible fitted with a cover and 
platinum tube for introduction of carbon dioxide. His test experiments 
on ferrous sulphate show that there need be practically no oxidation, 
even if the heating lasts several hours. The directions given on page 
150 of his paper, with reference to the treatment of very refractory 
minerals which are not fully decomposed by this treatment, must be 
understood as referring only to homogeneous minerals and not to rocks, 
where the relations of ferrous and ferric iron in the undecomposed 
portion are certainly different from those in the part dissolved. 
INFLUENCE OF SULPHIDES, VANADIUM, AND CARBONACEOUS MATTER ON 
THE DETERMINATION OF FERROUS IRON BY THE HYDROFLUORIC-ACID 
METHOD. 
A dark color of the undissolved residue may be due to pyrite, 
graphite, or carbonaceous matter. The first of these affects the result 
but little, the second probably not at all, and they can be distinguished 
by their behavior toward nitric acid. Organic matter of course ren- 
ders impossible the estimation of ferrous iron. 
Sulphides. — Pyrite, in the quantities usually met with in igneous 
rocks, is probably without serious effect on the ferrous-iron determina- 
tion by any of the hydrofluoric-acid methods. This sulphide is very 
resistant toward attack in the absence of oxygen, as is shown by the 
fact that if present in any quantity it can be readily recognized in the 
residue after titration. In any case it is impossible to allow for an 
error introduced by its possible decomposition, and the result of titra- 
tion must count as ferrous iron. In the case of soluble sulphides two 
sources of error are introduced — that of reduction of ferric iron by 
hydrogen sulphide evolved, and that due to the ferrous iron which the 
sulphides themselves may contain, especially if pyrrhotite is present. 
The first of these is perhaps negligible, since most of the hydrogen sul- 
phide would probably be expelled without reducing iron. The second 
is approximately measurable if it is known that pyrrhotite is the only 
soluble sulphide present, and its amount has been ascertained by deter- 
mining the hydrogen sulphide set free on boiling with hydrochloric 
acid in a current of carbon dioxide. In this case a correction is to be 
i Am. Jour. Sci., 3d series, Vol. XLVIII, p. 149, 1894. 
