_Remark._--In the operations described above, as well as in those following, the difficulty in separating minute precipitates from the filter is often experienced. When the precipitate is to be dissolved in reagents that do not affect the paper, such as ammonia, tartrate of ammonia, and dilute acids, it can be brought in solution directly on the filter. In cases, however, where reagents which attack the paper are employed, the precipitate should be separated. This is accomplished by mixing a small quantity of pure silica, obtained by the decomposition of fluoride of silicium by water, with the solution, before filtering. The precipitate becomes intimately mixed with the silica, and can then be readily removed from the paper. The presence of silica does not interfere, it being insoluble in the reagents commonly made use of. TREATMENT OF FILTRATE B. A current of sulphuretted hydrogen is conducted for twelve hours through the solution, which is kept at a temperature of 70°. by means of a water-bath. The flask containing the liquid is then closed with a piece of paper, and allowed to remain in a moderately warm place until the odor of the gas is no longer perceptible. The solution is next filtered with the precaution mentioned in the preceding remark, and the precipitate (_a_) thoroughly washed. The water used in this operation is united to the filtrate, and the fluid (_b_) examined as directed further on. TREATMENT OF PRECIPITATE _a_. In order to free the precipitate from the organic substances possibly present, at the same time avoiding a loss of any metal, it is dried, moistened with nitric acid, and the mass heated on a water-bath. Some Swedish filtering paper is next added, the mixture well impregnated with sulphuric acid, and then maintained for several hours at a temperature of about 170°. until a small portion (afterwards returned) gives a colorless solution when treated with water. The residue is now heated with a mixture of one part of hydrochloric acid and eight parts of water, the liquid filtered, the matter remaining undissolved washed with dilute hydrochloric acid, and the washings united with the filtrate. The residue I. and the solution II. are separately examined as directed below. RESIDUE I. This may contain lead, mercury, tin, bismuth and antimony. It is heated for a considerable time with _aqua regia_, the solution filtered, and the second residue, should one remain, washed with dilute hydrochloric acid. If the second residue is fused with cyanide of potassium, the compounds present are reduced to the metallic state. The liberated metals are treated with nitric acid, which dissolves _lead_, but leaves _tin_ as insoluble metastannic acid. The nitrate of lead is then filtered from the metastannic acid, and both metals are identified as described in the treatment of residue A.