Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/257

 was attended with some difficulty, and required a considerable quantity of water) there always remained a small proportion of earthy matter, which resisted all solvents, caustic potash excepted. This insoluble matter, I had thought from some of the first trials, amounted to about 1 gr. in 100 of the residue; but from some subsequent experiments in which the silica was separated by caustic potash, there appeared to be reason to suppose that this estimate was rather over-rated. I shall relate the process, to which, after various trials, I gave the preference.

2. 50 grains of residue being boiled with very dilute muriatic acid, a white flocculent substance remained undissolved, upon which neither acid nor water could make any impression. This substance being separated and boiled in a solution of caustic potash, readily redissolved with the exception of a few particles of highly oxydated iron which subsided. Muriat of ammonia being added to the clear alkaline solution in sufficient quantity to saturate the whole of the potash with muriatic acid, the white flocculent substance reappeared, which, after being well washed and heated to redness, weighed between 0,3 and 0,4 gr. This substance when heated with alkali ran into a vitreous globule, and muriatic acid being poured upon this, the alkali was dissolved, and the earthy matter remained untouched. It was therefore silica, the quantity of which may be estimated at 0,7 gr. in a pint of water.