bright, metallic iron-a knife-blade, or a knittingneedle becomes coated with metallic copper.

If a quantitative determination of the copper is desired, the solution is weighed, and a small weighed portion of it used for the qualitative tests; the remaining portion is heated to ebullition, and the oxide of copper precipitated by caustic soda; the precipitate is collected on a filter, washed, dried, and ignited in a porcelain crucible until the filter is completely consumed. It is then weighed and, from the weight obtained, the whole amount of oxide found by calculation. Or, the whole of the liquid is used for this precipitation, and the weighed oxide afterwards dissolved in nitric acid and sulphuric acid; the resulting liquid is then used for the qualitative tests. After having precipitated with caustic soda, the liquid usually retains a blue color, even if the original precipitate of sulphide of copper was not collected on a filter, and if chlorate of potassa was used for the destruction of the organic matters. this case, a little grape-sugar (honey) is added, when the last trace of copper is precipitated as red suboxide; by ignition it passes into protoxide.

§ 63. Lead. If the precipitate, caused by sulphureted hydrogen, consisted of sulphide of lead, it becomes decolorized by the action of nitric acid. Some sulphuric acid (and, if necessary, some chlorate of potassa) is added, and the whole evaporated to dryness. On adding water to the residue, a white powder, sulphate of lead, remains undissolved.

The contents of the dish are next washed into a conical glass; the powder is allowed to settle, and

repeatedly washed with water. The decanted liquids are united in a large beaker, to recover any washedoff particles. With the necessary patience and circumspection, the whole of the sulphate of lead may thus be collected, without the use of a filter. The pulverulent salt tends to adhere to the sides of the vessel; but, by carefully stirring the liquid above the precipitate, the greater part is readily brought down, and the remaining portion usually adheres so fast to the glass, that it does not pass out with the liquid.

The decomposition of the sulphate may now be effected in the same vessel. A solution of carbonate of soda is poured on the precipitate, and left in contact with it for several hours, the precipitate, during this time, being frequently stirred up. The limpid liquid is then poured off, and the operation repeated with a fresh solution of carbonate of soda. Carbonate of lead is formed, which is several times washed with water. The decanted liquids are, as before, united in a large beaker.

According to H. Rose, solutions of neutral alkaline carbonates exercise a slight solvent action on carbonate of lead; for this reason, it might be better to substitute the carbonate of soda by the bicarbonate of soda or ammonia. A liquid which is turbid from finely suspended particles of sulphate of lead, appears much less so after addition of carbonate of soda.

A little water is then poured on the carbonate of lead, and some nitric acid added, drop by drop. It dissolves completely to a perfectly limpid liquid, in which the presence of the metal may be shown by means of the usual reagents. Chromate of potassa

produces a yellow precipitate of chromate of lead. Iodide of potassium throws down yellow iodide of lead; the precipitate dissolves on boiling the liquid, and separates, on cooling, in gold-like scales. Sulphuric acid precipitates white pulverulent sulphate of lead, etc.

If the lead is to be quantitatively determined, the sulphate of lead, collected in the conical glass, may be washed into a weighed porcelain dish, dried, and weighed. It is, then, again softened with water, and decomposed by a solution of carbonate of soda, as above explained.

Although filtration may very well be avoided in operating after the method just described, there is, of course, no objection to collect the sulphate as well as the carbonate of lead on a filter. If the sulphate is collected on a weighed filter, it may, after washing and drying, be weighed with the filter, and thus quantitatively determined. It is, then, together with the filter, if this cannot be conveniently removed, digested with a solution of carbonate of soda, washed, and the carbonate dissolved in dilute nitric acid.

§ 64. Mercury.-If the precipitate, produced by sulphureted hydrogen, consisted of sulphide of mercury, it suffers no change by the action of nitric acid; it remains black. (If the precipitate was not thoroughly washed and, thereby, freed from any chloride, solution may take place.) Hydrochloric acid is then added, heat applied to the mixture, and the resulting liquid evaporated till nearly dry. The residue is dissolved in water (if, on addition of water,

some yellow protosulphate of mercury should remain undissolved, a few drops of hydrochloric acid must be added), and the liquid tested with the proper reagents. A drop of a freshly prepared solution of protochloride of tin (by dissolving tin in hydrochloric acid), produces a white turbidness, or a white precipitate of subchloride of mercury; on addition of a larger quantity of the reagent, the white color passes into grey, owing to the reduced mercury. A drop of the liquid is placed on a piece of bright copper-foil, or copper coin (cleaned with nitric acid), and the copper touched, through the liquid, with a zinc rod; metallic mercury is precipitated; the liquid is carefully removed by means of blotting-paper, and the spot rubbed with the finger, or with a piece of soft wood, when it assumes a white appearance. These reactions are conclusive; to obtain the wellknown indications with potassa, ammonia, and iodide of potassium, it requires the presence of a considerable quantity of mercury.

The presence of mercury being thus demonstrated, the whole of the liquid is transferred into a small Florence flask, and, aided by a gentle heat, precipitated with protochloride of tin. The contents of the test-tube, which served for the qualitative test with protochloride of tin, are added to this liquid. The reduction being completed, the liquid is carefully poured off, and the metallic mercury washed into a small, weighed porcelain dish. It is washed with water, the last drops of adhering liquid removed by blotting-paper, and dried by exposure to air, or by application of a very gentle heat. By gently heating the finely divided mercury with a little hydro

chloric acid, it usually unites to larger globules. The dried mercury is weighed with the dish.

To prevent any loss of the metal, the washings are united in a glass of conical shape. The weight of the mercury having been ascertained, a few additional experiments may be performed. By rubbing the finely divided metal with the finger, it unites to larger globules; by rubbing a little of it on a piece of copper, the copper becomes amalgamated; by placing a few globules in a glass tube and applying heat, the characteristic coating of metallic mercury is produced, which, by friction with a piece of wood, forms bright metallic globules.

§ 65. Antimony-Tin.-If the precipitate, produced by sulphureted hydrogen, is of a light color, it may consist of sulphide of antimony, or sulphide of tin (or sulphide of arsenic). It is collected on a filter, washed with water, and treated with sulphide of ammonium, in order to dissolve it. The filter, for this operation, may either remain in the funnel-and the solution is promoted by stirring the precipitate with a feather, or by using the washing-bottle-or it is placed in a porcelain dish, unfolded, sulphide of ammonium poured on it, and the solution filtered.

The solution is evaporated to dryness in a porcelain dish, the residue moistened with concentrated nitric acid, and the acid removed by gentle heat. The residue is mixed with pure carbonate of soda and some nitrate of soda, and the mixture heated to fusion in a porcelain crucible (v. § 13). The quantities of the carbonate and nitrate of soda should be proportional to the amount of residue, and the cru