104 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
hours the solution is filtered, ammonium carbonate is expelled, and an 
ammoniacal solution of zinc oxide added, whereupon evaporation is 
carried on till the odor of ammonia is entirely gone. After filtering, 
add nitric acid in insufficient quantity to fully neutralize. 
The use of ammonium nitrate or chloride, instead of carbonate, for 
throwing out the silica and alumina is not to be recommended, because 
of loss of fluorine on evaporation (Rose). If the rocks are very basic 
it may happen that the amount of silica in the alkaline solution is so 
small that ammonium carbonate may be dispensed with and the ammo- 
niacal zinc oxide solution added at once. 
By whatever modification of the method the silica and alumina ma} T 
have been separated, the alkali carbonate must be converted into 
nitrate and not chloride if phosphorus or chromium, or both, are 
present. To remove the chromium and the last of the phosphorus, 
silver nitrate in excess is added to the solution containing still enough 
alkali carbonate to cause a copious precipitate of silver carbonate, which 
shall take up the acid set free and thus insure a neutral solution and 
consequent complete precipitation of phosphorus and chromium. 
After heating and filtering, the excess of silver is to be removed by 
sodium or potassium chloride, and sodium carbonate is to be added, in 
order to furnish by addition of calcium chloride in excess to the hot 
solution a sufficient admixture of calcium carbonate with the fluoride. 
At this stage there must be no ammoniacal salts in solution, otherwise 
calcium fluoride may be held up. 
The next operation is perhaps best conducted as recommended by 
Penfield and Minor. 1 To the gently ignited precipitate of carbonate 
and fluoride of calcium at most 1-2 cm. 3 of acetic acid are added in the 
crucible, which is then placed for a time on the water bath and after- 
wards its contents are evaporated to dryness. They are then taken 
up with hot water, the solution is decanted through a small filter, 
likewise the washings. The filter is burned in the same crucible, more 
acetic acid is added and the various operations are repeated until all 
calcium oxide and carbonate have been extracted. The above authors 
find that if a great excess of acetic acid is used at the start, the results 
are low. While their experiments related to the determination of 
fluorine in a mineral rich in that element — topaz — their precautions 
are probably not needless with the small amounts of fluorine met with 
in rocks. 
The well-washed and gently ignited calcium fluoride finally obtained 
in the course of this method should be converted to sulphate as a check 
upon its purity, and at the same time as a qualitative test to ascertain 
if it really is calcium fluoride b} r the characteristic odor of the gas 
given off. Should fluorine be found, and the weight of sulphate not 
correspond to that of the fluoride, the former should be dissolved in 
'Am. Jour. Sci., 3d scries Vol. XLVII, p. 389, 1894 
