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SECTION XIV.

OF LEAD.

Experiment 1. In order to obtain lead in the large way, the ore is picked from among the extraneous matter with which it was combined by nature. It is then pulverised and washed. It is next roasted in a reverberatory furnace in which it is to be agitated, in order to bring all its surfaces in contact with the air. When the external parts begin to soften, or assume the form of a paste, it is covered with charcoal, the mixture is stirred, and the heat increased gradually ; the lead then runs on all sides, and is collected at the bottom of the furnace, which is perforated so as to permit the metal to flow in a receptacle defended by a Jining of charcoal dust.

The scoria remaining above in the surnace still retain a considerable portion of lead; in order to extract it they are fused in a blast furnace. The lead is by that means separated, and cast into iron moulds, each of which contains a portion called a pig of lead. These pigs are sold under the name of ore lead.

Experiment 2. To disengage the silver from lead thus obtained, the metal is subjected to the action of the refining furnace. The continual application of a quantity of fresh air which is thrown by means of large bellows upon the fused lead, which is at the same time heated as intensely as possible, oxydizes the lead, and converts it into the yellow scaly oxyd, known by the name of litharge, or vitreous oxyd of lead.

This oxyd being driven off from the surface of the fused metal, as it is formed, leaves the silver alone unaltered at the bottom in a metallic state.

The litharge is then to be fused in contact with charcoal, in order to reduce it again to the state of. metallic lead.

Experiment 3. To obtain perfectly pure lead, the lead of commerce may be dissolved in nitric acid, and the solution be decomposed by adding to it, gradually, a solution of sulphate of soda, or sulphuric acid, so long as a precipitate ensues. This precipitate must be collected on a filter, washed repeatedly in distilled water, and then dried. In order to reduce it to the metallic state, let it be mixed with two or three times its weight of black flux, introduce the mixture into a crucible, and expose it briskly to a red heat.*

Lead ore may be analyzed by solution in nitric acid, diluted with an equal weight of water. The sulphur, if any, will remain undissolved. Let the solution be precipitated by muriate of soda. If any silver be present, it will be taken up by pure liquid ammonia. Wash off the excess of ammonia by distilled water; and add concentrated sulphuric acid, applying heat, so that the muriatic acid may be wholly expelled. Weigh the sulphate of lead, and, after deducting 70 per cent. the remainder shows the quantity of lead.

Muriate of lead may also be separated from muriate of silver by its greater solubility in warm water. From the solution, iron may be separated by prussiate of potash, and the solution decomposed by sulphuric acid.t Lead has been known at an early period. It is mentioned by Moses, but the ancients cofounded it very often with tin, or considered that it had a close resemblance to that metal.

Lead is of a bluish white colour; its specific gravity, according to Brisson, is 11.3523. It is very malleable, and may therefore be hammered into leaves of extreme thinness. Its tenacity is such, that a lead wire to inch in diameter is capable of supporting only 18.4 pounds without breaking. It melts at about 612o. At a considerable temperature it boils and evaporates. If it be cooled slowly, it crystallizes. The abbe Mongez obtained it in quadrangular pyramids, lying on one

[blocks in formation]

of their sides. It has been obtained, however, in the form of a polyhedron with 32 sides, formed by the concourse of six quadrangular pyramids. When exposed to the air, lead loses its lustre, and acquires a dirty gray colour, and at last becomes almost white. When melted and exposed in the same manner, its surface is converted into scoriæ, or oxyd. Lead is capable of combining with four doses of oxygen, and of forming four different oxyds. The yellow oxyd of lead may be prepared in the following manner.

Experiment 4. Dissolve lead in a sufficient quantity of nitric acid, so as to form a colourless solution; then supersaturate it with carbonate of potash; collect the precipitate, which is white, wash it, and heat it nearly to redness. It will assume a yellow colour.

Rationule. The lead is oxydized by the acid; the oxyd is taken up, and again separated by the addition of carbonate of potash; nitrate of potash is formed, and the carbonate of lead precipitated. On exposing this to heat the carbonic acid is disengaged, leaving the yellow oxyd of lead behind. According to Thomson, this oxyd contains 9.5 per cent. of oxygen.

Experiment 5. If lead be melted in an open vessel, its surface will become covered with a gray powder; if this be removed another will proceed; and, finally, the whole will be converted into an ash coloured powder; and,

Experiment 6. If the product of the last experiment be heated, and agitated for a short time in an open vessel, it will assume the appearance of a grayish yellow powder.

This powder is a mixture of the yellow oxyd and metallic lead; but,

Experiment 7. If this powder be exposed to heat for some time longer in an open vessel, it will assume a yellow colour, forming the massicct of commerce.

Experiment 8. If thin plates of lead be exposed to the vapour of vinegar, they will gradually be corroded, and converted into a heavy white powder, used as a paint, and called white lead. This preparation, though formerly considered a white oxyd, is considered a con

pound of the yellow oxyd and carbonic acid. See Salts of Lead.

Lead may be combined with a minimum of oxygen, forming the prot oxyd, in the following manner:

Experiment 9. Dissolve lead in nitric acid, and boil the crystals which that solution yields along with pieces of metallic lead. Small scaly crystals of a yellow coloar may be obtained from this solution. These crystals are composed of the prot oxyd of lead and nitric acid.

Expiriment 10. If the crystals of the last experiment be decomposed by potash, a yellow oxyd will be obtained, which is the prot oxyd of lead, and according to Proust, contains 81 per cent. of oxygen.

Experiment 11. If massicot, ground to powder, be put into a furuace, and constantly stirred while the fame of the burning coals plays against its surface, it is in about 48 hours converti d into a beautiful red powder, known by the name of minium or red lead.

This powder is considered the trit oxyd of lead, and contains about 12 per cent of oxygen.

Experiment 12. If nitric acid, of the specific gravi, ty 1.260, be poured upon red lead, 185 parts of the oxyd will be dissolved, leaving 15 parts in the state of a deep brown powder, being the per oxyd, or brown oxyd of lead of Scheele; or,

Experiment 13. Put a quantity of red oxyd of lead into a vessel partly filled with water, and make oxymuriatic acid gas pass into it. Into the solution, which will thus be formed, pour a solution of potash, and the brown oxyd of lead will precipitate.

This oxyd contains 20 per cent. of oxygen.

Experiment 14. If the red oxyd of lead be exposed to a heat sufficient to reduce it to a semi vitrified state, its parts become agglutinated into small thin scales, which always preserve their red colour, but have less brightness, This oxyd is now called litharge.

Experiment 15. If the product of the last experiment be exposed to a violent heat in a crucible, it will fuse and be converted into the glass of lead. This vitre. ous oxyd of lead constitutes the basis of the glazing for common pottery.

All the oxyds of lead are very easily converted into glass; and in that state they oxydize and combine with almost all the metals except gold, platinum, silver, and the metals recently discovered in crude platinum. Hence lead is used in cupellation. In the process of refining, litharge is also formed. The following experiment will give some idea of the process:

Experiment 16. Alloy a piece of silver with a portion of lead, place the alloy upon a piece of charcoal, attach a blow-pipe to a gasometer charged with oxy. gen gas. light the charcoal first with a bit of paper, and keep up the heat by pressing upon the machine. When the metals get into complete fusion, the lead will begin to burn, and very soon will be all dissipated in a white smoke, leaving the silver in a state of purity.

The oxyds of lead may be reduced in the following

manner:

Experiment 17. Take one ounce of red lead, and half a dram of charcoal in powder, incorporate them well in a mortar, and then fill the bowl of a tobacco pipe with the mixture. Place this in the middle of a common fire, and presently the lead will be reduced, when it may be poured out as metallic lead.

Experiment 18. if the red oxyd of lead be put into a receiver of hydrogen gas, and the oxyd be heated by means of a glass lens, the oxygen of the metal will combine with the hydrogen to form water, and the metal will be completely revived.

Experiment 19. Dip a piece of white calico in an aqueous solution of acetate of lead, and then drop a little solution of sulphuret of potass upon it. If this be now placed in the palm of the hand, the lead will be observed gradually to revive, and will soon be reduced to its metallic state.

Experimen: 20. Write with a solution of nitrate or acctate of lead.

When the writing is dry it will be invisible. Then having prepared a glass decanter with a little sulphuret of iron strewed over the bottom

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