Although the external diameter of the tube is only half an inch, 0·29 gram of efflorescence was obtained per foot by gentle rubbing. This dried at 100° C. contained 1·02 per cent. of sulphuric acid, corresponding with 4·1 per cent. of sulphate of lead. The remainder, except 1·13 per cent. of a residue insoluble in nitric acid, is carbonate of lead.
The alternate exposure to air and water appears not essential to the corrosion, as I have observed a similar effect when the same tubing remained constantly under water. The interior of the tube has also been corroded, although of course no permanent efflorescence could be formed, owing to the rapid flow of the water.
The frequent practice of plumbers of using composition tubing in connection with water supplies is therefore highly reprehensible, being fraught with considerable danger to the health of those using the water for drinking or cooking.
Mr Louis Siebold detected lead in eight out of ten samples of concentrated solution of acetate of ammonia as well as in a sample of the ordinary solution of the British Pharmacopœia. In pursuing his investigations Mr Siebold found that solutions of acetate of ammonium are capable of dissolving lead from glass. He therefore advises that all forms of the solution, more particularly the concentrated liquor employed by many chemists for making the weaker solutions, should be kept in bottles free from lead.
With the acids lead or its oxides form salts, usually white in colour, and in the majority nearly insoluble in water, but readily soluble in acids.
Tests. The oxides and salts of lead, mixed
with a little carbonate of soda, and exposed on a charcoal support to the reducing flame of the blowpipe, readily yield a soft and ductile globule of metallic lead, and the charcoal, at the same time, becomes covered with a yellowish incrustation of oxide of lead. Both metallic lead and its oxides are soluble in nitric acid, furnishing a solution which may be examined with ease.
Solution of lead salts may be recognised by the following reactions:—Sulphuretted hydrogen, sulphydrate of ammonium, and the alkaline sulphides, give black precipitates, insoluble in the cold dilute acids, alkalies, alkaline sulphides, and cyanide of potassium. Potassium and sodium hydrates give a white precipitate, soluble in excess. Ammonia (except with the acetate) gives a white precipitate, insoluble in excess. The carbonates of potassium, sodium, and ammonium, give a white precipitate, insoluble in excess. Dilute sulphuric acid (in excess), and solutions of the sulphates give a white precipitate, sparingly soluble in dilute acids, but soluble in a hot boiling solution of potassium carbonate. Chromate and bichromate of potassium give yellow precipitates insoluble in dilute nitric acid, and soluble in solution of potassium hydrate. Iodide of potassium gives a yellow precipitate, soluble in great excess by heat, and separating in small, brilliant, golden-yellow scales, as the liquid cools. A piece of polished zinc precipitates metallic lead in an arborescent form, hence called the lead tree. To prepare for these tests, a solid supposed to contain lead should be digested in nitric acid, when the solution, evaporated to dryness and redissolved in water, may be tested as above.
Estim. This has been already referred to under previous heads. The ores of lead (galena) may be digested in nitric acid, when the solution may be treated with sulphuric acid, and the lead estimated from the weight of the precipitated sulphate. This is called an assay in the wet way. The method adopted by practical mineralogists is an assay in the dry way, and is conducted as follows:—A small but powerful air-furnace, charged with coke, is brought to as high a temperature as possible, and a conical wrought-iron crucible plunged into the midst of it; as soon as the crucible has attained a dull-red heat, 1000 gr. of the galena, reduced to powder, are thrown into it, and stirred gently with a long piece of stiff iron wire flattened at the one end, in order to expose as large a surface of the powdered ore to the air as possible, observing now and then to withdraw the wire, to prevent it becoming red hot, in which case some of the ore would permanently adhere to it, and be reduced before the intended time; the roasting is completed in 3 or 4 minutes, and any portion of the ore adhering to the stirrer being detached by a knife, and returned into the crucible, the latter is covered up, and allowed to attain a full cherry-red
heat, when about 2 or 3 spoonfuls of reducing flux are added, and the whole brought to a full white heat; in 12 to 15 minutes, the portion of metal and scoria adhering to the sides of the crucible are scraped down into the melted mass with a small stick of moist green wood, after which the crucible is again covered, and the heat urged for 2 or 3 minutes longer, so as to keep the mass in a perfectly liquid state during the whole time; the crucible is then removed from the fire with the crucible-tongs, and adroitly tilted so as to discharge its contents into a small, ingot-mould of brass, observing to rake the scoria from the surface to the sides of the crucible, so as to allow the molten lead to be poured out without it; the scoria is then reheated in the crucible with about 1⁄2 spoonful of flux, and after being cleansed with a piece of green wood, as before, is at once poured into a second mould, which is instantly inverted; the little button of lead thus obtained is added to the lead in the other mould, and the whole is accurately weighed. The weight, divided by 10, gives the per-centage of lead (including silver, if present) in the ore examined.