92 SOME PBINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
of mineral operated on. An average error equal to the above when 
employing 1 gram of ferrous minerals would make the percentage for 
FeO about 0.3 per cent too low. While the absolute error might be 
the sam 5 in all eases, the relative error would increase with minerals 
low in ferrous iron. 
THE HYDROFLUORIC-ACID METHOD. 
This method is the one which has been almost exclusively used since 
the earliest years of the Survey's existence. 
The specially ground powder, in a capacious crucible, is placed, 
alter stirring up with dilute sulphuric acid, on a small water bath of 
a single opening (fig. 13) and covered with a glass funnel, the stem of 
which has been cut off near the flare, resting in a depression of the 
specially made cover, into which water constantly drops from a tubu- 
lated bottle, thus securing a perfect water joint and serving to keep 
the bath full. Through a small metal pipe carbonic-acid gas flows into 
the bath above the surface of the water, and rising through orifices 
in the cover fills the funnel and crucible. 1 The lamp under the bath is 
lighted and hydrofluoric acid is poured into the crucible through a 
platinum funnel, which is left in place to serve as an occasional stirrer, 
for which a rod or wire may be substituted. After boiling commences 
the rapid gas current can be safely interrupted, to be restored when 
the lamp is extinguished after one-half to one or more hours. A full 
stream of cold water is then caused to flow from the tubulated bottle 
into the bath, the overflow from the outlet tube being caught in a 
receiver. As soon as cool the contents of the crucible are emptied 
into a platinum dish containing cold water, and titrated till the first 
permanent color appears, which usually will last for only a few sec- 
onds. Duplicate determinations are to be advised whenever possible, 
since even with the utmost care the results will occasionally differ 
more than is allowable. 
In absence of a suitable water bath an ordinary one can be used 
covered with a beaker, through a hole in the bottom of which a strong 
current of carbon dioxide is introduced, or the crucible may be set 
in a sand bath and covered in the same way with a broken beaker 
(Doelter). 
The cause of the rapid disappearance of the first pink blush when 
titrating in hydrofluoric-sulphuric solution appears to be the ready 
oxidizability of manganous fluoride by permanganate. The latter can 
be added by the cubic centimeter to solutions already containing man- 
ganous sulphate in presence of hydrofluoric acid without producing 
a more than passing pink blush. The solution, however, takes on in 
ever-increasing intensit}^ the red-brown color characteristic of man- 
i J. P. Cooke, Am. Jour. Sci., 2d series, Vol. XLIV, p. 347, 1867. 
