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1829.] impurities: but to prevent excessive solution of the nitrate, the same portions of water may be used for several basins of crystals washed at the same time, by making it pass from one to another in succession. Being thus cleansed, they are to be drained, put over the sand-bath, stirred and dried, and dually preserved in glass bottles. By this process much iron and sulphate of lead lure excluded; and the purified nitrate is found to yield a glass very far superior in colour to that prepared with the ordinary oxides of lead, and to exert not the slightest action on the platina: its use put an end to all the accidents and failures which resulted from the presence of metallic lead in the oxide. 166 parts by weight are to be considered as equivalent to one proportional or 112 parts of protoxide of lead.

18. Boracic acid.—The boracic acid for these experiments was obtained pure from the manufacturer, but before being used was carefully examined. It was rejected unless it was in white or bluish-white crystals, clean and entirely soluble in water. Its solution was tested for iron by the ferro-prussiate of potash and a drop of sulphuric acid, and also for other metallic impurities by a little solution of sulphuretted hydrogen. An ounce or two were heated and dissolved in a little water; and when cold, the soluble part separated and examined for sulphuric acid, by a few drops of nitrate of baryta and a little nitric acid. It was also examined for soda by dissolving three or four ounces in hot water, adding ten or fifteen drops of sulphuric acid, and allowing the whole to cool and crystalline; expressing the mother-water from the crystals; concentrating it; again crystallizing, and then acting upon the mother-liquor, obtained at the second time, by strong alcohol; continuing to wash with the latter fluid until all was dissolved or an insoluble part left. If the latter circumstance occurred, the insoluble substance was examined for sulphate of soda, which, if in any sensible quantity, occasioned the condemnation of the boracic acid. This care respecting alkali in boracic acid was taken in consequence of observing certain bad effects produced in glasses, which appeared referable to its presence.

19. When the boracic acid was acknowledged as pure, 36 parts by weight of the crystals were considered as equivalent to 24 parts or one proportional of the dry substance.

20. Silica.—This material is in its most convenient state