Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/396

 378 LEAD cements, and of lead plaster, and for other purposes. Massicot is important as being the raw material for the manufacture of &quot; red lead &quot; or &quot; minium.&quot; Finely divided massicot, freed from admixed metal by elutriation, is spread out on the flat sole of a kind of baker s oven, or (better) of a &quot; muffle &quot; heated from the outside, and therein exposed for twenty-four hours or more to air at a temperature of about 300 C. or 600 Fahr, The massicot, at a gradually decreasing rate, absorbs oxygen, and as the latter increases the colour becomes more and more intensely red, the point of saturation corresponding very nearly to the formula Pb 4 O. A more highly oxygenated kind of minium (&quot; orange lead &quot;) can be produced by substituting white lead for massicot as a raw material. The composition of orange lead approxi mates to Pb 3 O 4. It is very singular that this higher oxide cannot be obtained from massicot, although the first effect of heat on white lead is its conversion into the oxide PbO. Besides the two named there is another red oxide, of the composition Pb 2 O 3, but it is not much known. Red lead is largely used as a pigment and as an ingredient for flint glass, also for the making of certain cements. Any of these red oxides when treated with dilate nitric acid is converted into the binoxide PbO 2, protoxide passing into solution as nitrate : e.g., Pb 3 O 4 + 2H 2 ON 2 5 = 2 PbON 2 O 5 + Pb0 2 +2H.&amp;gt;0. The binoxide is a brown powder, in soluble in aqueous oxygenated acids, but converted by hot hydrochloric acid into chloride PbCl 2 with evolution of chlorine. To obtain the binoxide in the state of purity, the best method is to pass chlorine into a solution of acetate of lead mixed with excess of carbonate of soda. The hypochlorite formed oxidizes the PbO into PbO 2 , with formation of chloride of sodium and free acetic acid (Wohler). Action of Aqueous Reagents. &quot;Water when absolutely pure has no action on lead by itself. In the presence of free oxygen (air), however, the lead is quickly attacked, with formation of hydrated oxide (PbOH.,0), which is appreciably soluble in water forming an alkaline liquid. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbonate, so that there is room made, so to say, for fresh hydrated oxide, and the corrosion of the lead progresses. Now, all soluble lead compounds are strong cumula tive poisons, hence the danger involved in using lead cisterns or pipes in the distribution of pure waters. We emphasize the word &quot;pure&quot; because experience shows that the presence in a water of even small proportions of bicarbonate or sulphate of lime prevents its action on lead. All impurities do not act in a similar way. Nitrate and nitrite of ammonia, for instance, intensify the action of a water on lead. It is to be remarked, however, that even pure waters, such as that of Loch Katrine (which forms the Glasgow supply), act so slowly, at least on such lead pipes as have already been in use for some time, that there is no danger in using short lead service pipes even for them, if the taps, as in any house hold under normal circumstances, are being constantly used. Lead cisterns of course must be unhesitatingly condemned. G. Bischoff found that a water pipe made of a &quot; composition &quot; consisting of 17 per cent, of antimony and 98 3 of lead was rapidly corroded by a water which, in virtue of its composition, had no action on lead pipes. Action of Acids. The presence of carbonic acid in a water does not affect its action on lead (Pattison Muir). Aqueous non- oxidizing acids generally have little or no action on lead in the absence of air. Dilute sulphuric acid (say an acid of 20 per cent, of H 2 S0 4 or less) has no action on lead even when air is present, nor on boiling. Stronger acid (e. g., any acid strong enough to fairly fall within the meaning of &quot; vitriol &quot;) does act, slowly in general, but appreciably, the more so the greater its concentration and the higher its temperature. According to Hasenclever, whose experi ments were subsequently confirmed by A. Bauer and by James Mactear, pure lead, cxt. par., is far more readily corroded than a metal contaminate I with 1 per cent, or even less of antimony or copper. Hasenclever treated an almost pure lead with pure vitriol of 54 Beaume (Too sp. gr., or 64-65 per cent. H 2 S0 4 ) in a glass flask. At 40 C. an evolution of gas was observable, which at 80 C. became very distinct. The same lead, after having been alloyed with a little antimony, was not visibly attacked below 85 C. A decided gas-evolution commenced only at 140 C. Boiling concen trated vitriol converts lead into sulphate, with evolution of sul phurous acid. Dilute nitric acid readily dissolves the metal, with formation of nitrate Pb(N0 3 ) 2. Tin. Lead. Melts at Fine solder 2 1 340 F. Common do 1 1 370 F. Coarse do 1 2 441 3 F. Lead Alloys. Lead unites readily with almost all other metals ; hence, and on account of its being used for the extraction of (for instance) silver, its alchemistic name of saturnus. Of the alloys the following may be named : With Antimony. Lead contaminated with small proportions of antimony is more highly proof against vitriol than the pure metal. An alloy of 83 parts of lead and 17 of antimony is used as type metal ; other proportions are used, however, and other metals added besides antimony (e.g., tin, bismuth) to give the alloy certain pro perties. Arsenic renders lead harder. An alloy made by addition of about g^th of arsenic is used for making shot. Bismuth and Antimony. An alloy consisting of 9 parts of lead, 2 of antimony, and 2 of bismuth is used for stereotype plates. JBismuth and Tin. These triple alloys are noted for their low fusing points. An alloy of 5 of lead, 8 of bismuth, and 3 of tin fuses at 94 4 C., i.e., below the boiling-point of water (Rose s metal). An alloy of 15 parts of bismuth, 8 of lead. 4 of tin, and 3 of cadmium (Wood s alloy) melts below 70 C. Tin unites with lead in any proportion with slight expansion (Kuppfer), the alloy fusing at a lower temperature than either component. It is used largely for soldering. The following are the compositions and melting-points of frequently used compounds (Tomlinson): &quot; Pewter &quot; may be said to be substantially an alloy of the same two metals ; but small quantities of copper, antimony, and zinc are frequently added. Common pewter contains about 5 parts of tin for 1 of lead. In France a tin-lead alloy, containing not over 18 per cent, of lead, is recognized by law as being fit for measures for wine or vinegar. &quot;Best pewter&quot; is just tin alloyed with a mere trifle ( per cent, or less) of copper. Lead Salts. Of the oxides of lead the protoxide, PbO, is the only one which under ordinary conditions is capable of forming salts. Towards potash and soda it plays the part of a feeble acid, being readily soluble in solutions of either caustic alkali ; while with acids it behaves as a decided diacid base. By a &quot; diacid base&quot; is meant a base which can unite with two monovalent acids at the same time, and form a stable salt. Take, for instance, the case of chloride of lead, PbCl. 7, which is re lated to HC1 and Pb(OH) 2 exactly as KC1 is to HC1 and K(OH); but, while there is nothing between KC1 and K(OH), the two lead compounds readily unite into Cl Pb (OH), oxychloride of lead. This property, common to all diacid bases, is developed in lead oxide to a characteristically high degree. The nitrate, PbON 2 5 or Pb(N0 3 ) 2, easily obtained from the metal as explained above, or by dissolving the oxide in aqueous nitric acid, forms white cystals, difficultly soluble in cold, readily in hot water, almost insoluble in strong nitric acid. It is decomposed by heat into oxide, peroxide of nitrogen (N 2 4 ), and oxygen. It is used for the manufacture of fusees and other deflagrating com pounds. The numerous basic nitrates must here be passed over. The acetate, Pb(C 2 H 3 2 ) 2 .3H 2 (called &quot;sugar&quot; of lead, on account of its sweetish taste), is manufactured by dissolving massi cot in aqueous acetic acid. It forms colourless transparent crystals, soluble in one and a half parts of cold water and in eight parts of alcohol, which on exposure to ordinary air become opaque through absorption of carbonic acid, which forms a crust of basic carbonate. An aqueous solution readily dissolves oxide of lead, with formation of a strongly alkaline solution containing basic acetates (Acctum Plumbi or Saturni). When carbonic acid is passed into this solu tion the whole of the added oxide, and even part of the oxide of the normal salt, is precipitated as a basic carbonate chemically similar, but not quite equivalent as a pigment, to white lead. The carbonate, PbC0 3, exists in nature as ccrusite. It can be produced by addition of a solution of lead salt to an excess of carbonate of ammonia, as an almost insoluble white precipitate. Of greater practical importance is a basic carbonate, substantially 2PbC0 3 .Pb(OH) 2, which is largely used as a white pigment under the name of &quot;white lead.&quot; For the manufacture of this important substance two methods chiefly are used. In the Old Dutch method, pieces of sheet lead are suspended in stoneware pots so as to occupy the upper two-thirds of the vessels. A little vinegar is poured into each pot ; they are then covered with plates of sheet