Lead is also found mineralized by the sulphuric and the phosphoric acids; this last is of a greenish colour, arising from a mixture of iron. The sulphate of lead is soluble in about eighteen times its weight of water. One hundred and forty-three grains of the dried salt represent 100 grains of lead. The phosphate of lead ore may be dissolved in nitric acid by means of heat, except a few particles of iron, which remain at the bottom. By the addition of sulphuric acid, the lead is thrown down in the form of white flakes of sulphate ; which, when washed and dried, discover the quantity of lead they contain, by the same allowance of 143 grains of the salt to 100 grains of metallic lead. The remaining solution being evaporated to dryness, affords phosphoric acid. Lead is abundantly found in com bination with sulphur, in the form of heavy, shining, black, or bluish, lead coloured cubical masses, whose corners are usually truncated ; its texture is lami nated, and its hardness variable. This is called galena, or potter's lead ore. Most lead ores contain more or less of silver. When antimony enters into its composi tion, the texture is radiated or filamen tous. There are also lead pyrites, which contain a considerable proportion of iron and sulphur ; and red lead spar, which consists of lead mineralized by sulphur and arsenic : this is very scarce.
If sulphuretted lead be boiled in nitric or muriatic acid of a moderate strength, the sulphur may be obtained pure, and collected on a filter. When iron or stony particles are contained among the undis solved part, the sulphur may be separated by digestion in a solution of pure fixed alkali, which converts it into sulphuret, and leaves the other insoluble matters behind. If the first solution be made with nitric acid, it may contain silver and lead, which, after precipitation by carbo nate of soda, may be separated by ammo nia, as mentioned in the humid analysis of the calciform ores ; when the muriatic acid is used for the solution of the ore, a large quantity of muriate of lead sepa rates, for want of a sufficient quantity of water to dissolve it. This requisite quan tity of water must be added to dissolve the Salt, before the precipitate is made by the fixed alkali.
All the ores of lead, except the phos phoric, are reducible to the metallic state, by dissipating their volatile contents by the blow-pipe on a piece of charcoal. In the large way, they are reduced by fusion with charcoal.
The ores of this metal are abundantly found in the mine countiesof England, and in Various other parts of the globe. Its uses are numerous, and scarcely need be mentioned. Its oxides are of great use as a pigment, and in the manufacture of glass. Lead is cast into thin sheets for covering building, making water-pipes, and various other uses ; and this is rolled between two cylinders of iron, to give it the requisite uniformity and thinness.
Lead is thought, and with some reason, to be not perfectly innocent, even for water pipes, and much less so for any other kind Of vessels. The workmen in any of the preparations of lead are gene rally subject to a peculiar colic, and para Jetic disorders, which most probably arise from the internal use of the metal ; for it is a fact, that these workmen are not suf ficiently cautious in washing their hands, or removing such particles of lead, or its preparations, as may casually intermix with their food.
Most of the acids attack lead. The sul phuric acid scarcely acts upon it, unless it be concentrated and boiling. Sul phurous acid escapes during the process, the acid being decomposed. When the distillation is carried on to dryness, a saline white mass remains, a small portion of which is soluble in water, and is the sulphate of lead ; it affords crystals. The residue of the white mass is an oxide of lead.
Nitric acid acts strongly on lead, and converts it into a white oxide, if the acid be concentrated; but if it be more di luted, the oxide is dissolved, and forms nitrate of lead, which is crystallizable, and does not afford a precipitate by cooling. It detonates on ignited coals. Lime and alkalies decompose the nitrous solution of lead. The sulphuric acid added to this solution combines with the metallic oxide, and falls down. The muriatic acid in the same manner carries down the lead, and fbrms a combination, Which is more soluble in water than the muriate of silver.
Muriatic acid acts directly, but sparing ly, on lead by heat, which it oxides, and dissolves in part. The muriate of lead is crystallizable.
The acetic acid dissolves lead and its oxides; though the access of air or oxy gen seems necessary for the Solution of the metal itself in this acid. White lead, or eeruse, is made by rolling leaden plates spirally up, so as to leave the space of about an inch between each coil, and placing them vertically in earthen pots, at the bottom of which is some good vine gar. The pots are to be covered, and exposed for a length of time to a gentle heat in a sand bath, or by bedding them in clung. The vapour of the vinegar, as sisted by the tendency of the lead to combine with the oxygen of the air which is present, corrodes the lead, and con verts the external portion into a white oxide, which comes off in flakes when the lead is uncoiled. The plates are thus treated repeatedly, until they are cor. roiled through. Ceruse is the only white substance used in oil paintings. It may be dissolved without difficulty, in the acetous acid, and affords a crystallizable salt, called sugar of lead, from its sweet taste. This, like all the preparations of lead, is poisonous.