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[1] 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.[2]
- ↑ This precipitant, which was, I believe, first proposed by Mr. Chenevix, is much more appropriate than acids, because if an excess of acid be incautiously added, the precipitate is re-dissolvsd; whilst with muriat of ammonia, an excess of the test is attended with no inconveniences.
- ↑ The presence of silica was also shewn, and its quantity attempted to be ascertained by the following process. A portion of residue was boiled in caustic potash: this dissolved not only the silica, but also the alumine; both these earths were precipitated from the alkaline solutions by muriat of ammonia, and separated; muriatic acid being now added, both the silica and alumine were re-dissolved (for silica, just precipitated from its solution, and not desiccated, is soluble in acid); and this solution being evaporated to dryness in a water-bath, by which means the silica parts with its acid and becomes insoluble, the muriat of alumine was washed of by distilled water, and the silica remained undissolved. This method, though affording a very useful means of discrimination, must obviously be liable to inaccuracy as to proportions, when very minute portions of silica are to be separated from considerable quantities of alumine. This however was the process to which I trusted on a previous occasion (IX. 2), to free the alumina from the silica which was mixed with it.
