188 GEOLOGICAL RECONNOISSANCE 
instead of boiling water, the process proceeds still slower, though the 
danger of loss consequent on oxidation becomes lessened, and perhaps 
entirely prevented. At any rate, the length of time required in eflecting 
a thorough washing is a serious inconvenience. I should mention here 
that I took the precaution of allowing the precipitate to settle before 
throwing it ona filter. The next step consists in placing the filter with 
the moist precipitate in a stoppered glass cylinder containing neutral 
sesquichloride of iron and dilute sulphuric acid [the addition of the latter 
is required in order to effect a complete decomposition]; in doing so, J 
invarcably noticed a strong odor of sulphuretted hydrogen; the escaping gas 
does, of course, not act on the sesquichloride of iron, and the amount of 
zine will be found, proportionally, too low. The method, therefore, is 
possessed of two sources of error, acting in a contrary sense; the two 
errors may, perchance, counterbalance each other, and the result may, 
consequently, be the correct one; but the method can hardly be relied 
upon—at least not as far as my experience goes. For this reason the 
results, thus obtained, have not been embodied in the Report. 
PSILOMELANE, 
Containing : water, free oxygen; silica; peroxide of iron, alumina; protox- 
ide of manganese, cobalt, baryta, and lime. 
I. To determine the amount of free oxygen, I used the method of Fre- 
senius and Will, with the modification of Mohr; it combines great sim- 
plicity with accuracy, and requires but little time. From 1.5 to 2.5, gram- 
mes of the dried mineral are introduced into a Florence flask, a measured 
volume of normal oxalic acid and some concentrated sulphuric acid added, 
and heated over a spirit lamp until the evolution of gas has ceased; if the 
ore is decomposable only with difficulty, the liquid is poured off from the 
dark-colored residue, some more normal oxalic acid and sulphuric acid 
added, and heated again until the residue appears white, or nearly so; the 
liquid thus obtained is diluted to 500 cubic centimeters; 100 cub. cent. are 
taken out with a pipette, largely diluted with water, sulphuric acid added, 
and the excess of normal oxalic acid determined volumetrically with 
chameleon mineral; the same process is repeated with another 100 cub. 
cent. of the solution; subtracting the undecomposed normal oxalic acid 
from the amount originally used, we obtain the quantity decomposed by 
the mineral. 1 cub. cent. of normal oxalic acid is equal to 0.008 gram- 
mes of free oxygen. . 
II. Water and oxygen above the composition Mn30* are determined by 
ignition. 
