Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/15

Rh acid to litmus paper. It is then dried on the water-bath, ignited, and weighed. Test at blowpipe as before. Sulphate of baryta and silicates, if both present, are not usually separated.

Weighing Precipitates. This general direction applies to almost all precipitates. If strong acids or acid and chlorate of potash are used, the liquid should invariably be diluted before filtering. In washing, allow the whole of the wash water to drop from the filter before adding more water. When thoroughly washed, fold the filter paper flat in the funnel, or, better, transfer the filter paper and its contents to a large watch-glass or flat basin, and dry on a water-bath. When dry, carefully unfold the filter over a quarto sheet of stiff glazed paper, with a feather brush off every particle of the precipitate adhering to the filter paper, collect the precipitate on the glazed paper, again using the feather, and cover over the precipitate with the funnel. Fold the filter paper till it assumes the appearance of a solid cylinder about one inch in length, and ignite with the Bunsen burner over a weighed platinum or porcelain capsule. After a while the filter paper becomes a charred mass of smaller dimensions, and drops from the wire into the capsule. The wire is cleaned into the capsule, by means of the feather, of any adhering particles, the charred paper is crushed with a glass rod, and ignited over a Bunsen until the ash is no longer black. The capsule is then stood on a porcelain slab, and the precipitate carefully transferred from a glazed paper into the capsule, the feather being employed to remove the last traces. The capsule is then ignited over an Argand burner until the weight is constant. The filter paper must always be ignited before the bulk of the precipitate is transferred to the capsule.

White Lead. The nitric acid solution obtained as before is boiled nearly to dryness, and if a precipitate forms a little water is added, then dilute sulphuric acid in small quantity, and the boiling continued for some minutes to expel the last