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 effecting 

 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 on a 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, 1 

 invariably noticed a strong odor of sulphuretted hydrogen; the escaping gas 

 does, of course, not act on the sesquichloride of iron, and the amount of 

 zinc 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 Pre- 

 senilis 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 

 chamaeleon 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 Mn 3 4 are determined by 

 ignition. 



