Lead cry@tAllloes in regular ortohedm, and sometimes in four-sided pyramids, but as usually obtained—by allowing a quantity of melted lead slowly to cool till the external part has solidified, then boring a hole throiigh the crust and pouring out the still fluid metal from the interior, on the sides of which the crystals will be found—they present a fern-like appearance, somewhat similar to the form in which chloride of ammonium crystallises. By repeated melting in a vessel to which air has access, head becomes more and more brittle, owing to its dis solving a portion of the oxide that forms on its surface ; this may be prevented by covering it with as layer of charcoal ; if already slightly oxidised it may be re-softened*V well agitating it with charcoal. head boils at a white heat, and even at a red heat begins to volatilise. Cold hydrochloric or sulphuric acids have no action upon it, and at a boiling temperature dissolve only as small portion. Nitric acid, espe cially when rather dilute, converts it. into nitrate of head with evolution of binoxide of nitrogen. Acetic acid in the presence of air rapidly attacks it. [Due MANUFACTURE elate kad.] Sulphate of lime and moisture also quickly corrode lead, hence in making cisterns and roofing houses, contact of the metal with plaster of Paris, &c., should be avoided. The alkalies appear to have little or no action upon lead.
Lead is soft, easily taking impressions, of a bluish-white colour, and not possessed of much ductility or tenacity. It melts at 620° Fahr., and at the moment of solidification contracts considerably, so that it is not well adapted for taking castings.
The equivalent of lead is 103'57, and its specific gravity 11'44.
A freshly cut surface of lead has a high metallic lustre, and this is retained if the metal be preserved in a hermetically scaled tube filled with perfectly dried air, or in a similar tube filled with water freed from all traces of air by long boiling. If the air, however, contain vapour of water, as is always the case under ordinary circum stances, then the bright surface rapidly tarnishes owing to the forma tion of a film of oxide. Pure water also in contact with air very rapidly and extensively acts upon lead, oxide i3 formed, carbonic acid is absorbed, and in a short time the metal becomes transformed into a white, flaky powder, composed of hydrated oxide and carbonate of lead (Pb0, HO + Pb0, CO,). A very moist atmosphere produces the mine result, though, of course, much more slowly ; moreover, the hydrated basic carbonate produced is ultimately converted into anhy drous protecarbonate, owing to the replacement of the water of hydration by the carbonic acid of the atmosphere. An old leaden coffin, examined by Trion Phil. Mag.; April, 1860), was found to
have been nearly all changed into this condition, 93 per cent. of it being pure anhydrous carbonate of lead (1'b0, CO,).
Compounds of Lead.
Lead and oxygen form four oxides, namely : 1. Dinexide of lend . . . . Pb,0 2. Protoxide of lead . . . . Pb0 3, Ited lead . x Pb0, Pb0, 4. Binoxide of lead . . . . l'bO, 1. Dinoxide of lead suboxide of lead, is a black powder ob tained by cautiously heating oxalate of lead in a retort from which air is excluded. It is an unimportant oxide.
2. Protoxide of lead (Pb0). Litharge. Massieot. The fused oxide that is produced in the process of cupehlation, already described in treating of the purification of lead, is the protoxide. On cooling, it solidifies into a mass of reddish-yellow scales, and is known in the arts under the name of litharge. When ground it is used as a pigment by painters and decorators, also in the manufacture of acetate of lead, and sometimes enters into the composition of glazes used at glass-works and potteries. Massie°, is protoxide of lead that has not undergone fusion. It is produced in making red lead.
The hydrated oxide (x 1'hO,H0) falls as a white precipitate en adding an alkali to an aqueous solution of a lead salt. It is soluble in solu tions of the fixed alkalis, compounds being thereby formed that some times are called plumbites. When heated it loses water, and yellow anhydrous protoxide remains I'rotoxido of lead is as powerful base, forming with acids salts of the protoxide. It has a great tendency to form basic salts.
3. 'Red-lead (Minium). Thia compound usually contains Ph,0,= 2Pb0 +1'b0„ but sometimes has the composition 1'h,0,, and occasion ally l'b,0,; in fact it may be looked upon as as compound of binoxide of lead with one, two, or three equivalents of protoxide. All three varieties are of is brilliant red colour. Red lead is largely used as as pigment by the painter and the paper-stainer, and by the glass-manu facturer. [Gum.] For the preparation of red lead, see LEAD MANUFACTURE.
4. Binoxide of lead (P1)0,) is usually prepared by digesting red-lead in boiling nitric acid, whereby the in dissolved out, and the Mixable remains as a pure-roloured powder. When fused with alkalies it tonne crystallisable salts, called plumbates. The potash salt contains Laud and Phosphorus form a somewhat, indefinite pito/dads. It is unimportant.
Lead and Sulphur form disulphide of lead, Pb,S, and protosulphide (PbS), obtained as a black precipitate on passing aulphitretted hydrogen through a solution of as salt of lead. The protomilphide occurs native under the amine of galena ; it has already been de scribed at the commencement of this article, and also in NAT. HIST. Div., under LEAD.