Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/543

Rh LEAD.] to the air it oxidizes almost as readily as the more positive alkali metals, chiefly thallious oxide, T1 2 O, being formed ; and when heated to redness and plunged into oxygen it burns brilliantly with a pure green light, chiefly thallic oxide, T1 2 O 3, being produced. Four chlorides of thallium have been described, viz., T1C1 or T1 2 C1 2, T1 2 C1 4 , T1 4 C1 6 , and T1C1 3 or T1 2 C1 6 ; corre sponding bromides, and the two iodides, Til or T1 I 2, and T1I 3 or T1 2 I 6, have also been obtained. Thallious chloride, T1C1 or T1. 2 C1 2, separates as a white precipitate scarcely distinguishable at first sight from silver chloride, on the addition of hydrochloric acid to a solution of a thallious salt ; but in properties it more nearly resembles lead chlo ride, PbCl 2, being slightly soluble in boiling water, from which it separates again on cooling in crystals, and scarcely more soluble in aqueous ammonia than in water. Thallium sesquichloride, T1 4 C1 6, formed by melting thallious chloride in a stream of chlorine, crystallizes in brilliant orange-yellow plates, moderately soluble without decomposition in water slightly acidulated with hydrochloric acid. Thallic chlo ride, T1C1 3 or T1 2 C1 6, is formed on dissolving thallic oxide in hydrochloric acid; it crystallizes from hydrochloric acid in colourless prisms of the composition T1C1 3, H.,0 or T1 2 C1 , 2H 2 O. Little is known of the chloride T1 2 C1 4 , but the corresponding bromide is obtained on adding thallious bromide to a hot solution of thallic bromide, separating from the hot liquid in yellow shining needles. Thallious bromide is nearly insoluble in water, but thallic bromide is very soluble and deliquescent ; the bromide Tl.,Br 4 is de composed by water into thallious and thallic bromide and the bromide Tl 4 Br c, which crystallizes in orange-red laminae. This last compound is also decomposed by water, with separation of thallious bromide. Thallious iodide, Til or T1 2 I 2, has a brilliant yellow colour, and is almost insoluble in water, and scarcely more soluble in a solution of potas sium iodide; when heated it becomes scarlet, but its yellow colour is restored by friction. It is therefore opposite in cha racter to mercuric iodide, HgI 2, which is scarlet, but becomes yellow when heated, and is readily soluble in potassium iodide solution, although insoluble in water. Thallic iodide is an extremely unstable compound, and, like cupric iodide, readily decomposes into iodine and thallious iodide. Thallious oxide is almost black in colour ; like litharge, PbO, the corresponding lead oxide, it fuses readily, and the fused oxide is absorbed by bone-ash so that a silver thallium alloy may be cupelled like a silver-lead alloy. It dissolves readily in water, forming a colourless, powerfully alkaline solution, greasy to the touch, of thallious hydroxide, Tl(OH) or T1 2 (OH) 2. The latter compound may be ob tained in pale-yellow prismatic crystals; it is reconverted into the oxide by mere exposure over sulphuric acid in a vacuum at the ordinary temperature. The solution of thallious hydroxide has nearly all the properties characteristic of sodium hydroxide ; but it has a slight tendency to absorb oxygen. Thallic hydroxide, T1(OH) 3 or T1 2 (OH) 6, separ ates as a brown gelatinous precipitate on the addition of alkalies to a solution of a thallic salt ; by heating to about 260 C. it is converted into thallic oxide, T1 2 O 3. This oxide is a dark brown powder fusible with difficulty ; it is reduced to the lower oxide at a red heat. It has much less marked basic properties than thallious oxide, but dissolves readily in acids, forming thallic salts; it is insoluble in water. Thallious salts are not precipitated by hydrogen sulphide in presence of strong acids, but the thallium is completely precipitated as a brown sulphide from solutions of the salts of weak acids, such as thallious acetate, and by alkaline sulphides. Thallious hydroxide readily absorbs carbon dioxide, forming thallious carbonate, T1 2 C0 3. This salt crystallizes in prisms, and is soluble in about four times its weight of 531 boiling water, forming an alkaline solution; it is fusible, but decomposes when heated to dull redness. Thallious sulphate, T1 2 S0 4, is isomorphous with potassium sulphate ; it also forms an alum, T1 2 A1 2 (SO 4 ) 4, 24H 2 O, and double salts such as Tl 2 Mg(SO 4 ) 2, 6H 2 O, isomorphous with the corresponding potassium salts. A large number of thallious salts, in fact, are isomorphous with the corresponding sodium, potassium, and ammonium salts. Thallic sulphate, T1 2 (S0 4 ) 3, 7H 2 O, is decomposed even by cold water; it forms an anhydrous double sulphate with potassium sulphate quite different from the alums in composition, viz., T1 2 (S0 4 ) 3, 2K 2 S0 4. Lead is a bluish-white, extremely soft metal ; it fuses at 325 C. It is readily dissolved by nitric acid, and is slowly acted on when boiled with hydrochloric or dilute sulphuric acid, hydrogen being evolved. Although it tarnishes in the air, the oxidation is only superficial, but it readily absorbs oxygen when heated. Its oxides are very easily reduced. Only one stable chloride of lead, PbCl 2 is known ; when, however, the oxides Pb 2 O 3 and PbO 2 are dissolved in hydro chloric acid at a low temperature, solutions are formed which, apparently contain the corresponding chlorides PbgCl^ and PbCl 4, as the oxides may be reprecipitated by alkalies, but the slightest heat causes chlorine to be evolved. Lead chloride is difficultly soluble in water, from which it crystal lizes in delicate white needles. Lead iodide, PbI 2, crystal lizes in brilliant yellow plates sparingly soluble in water. Lead forms a subcxide, Pb.,0, a monoxide, PbO, a dioxide or peroxide, PbO 2, and a fourth oxide, Pb 3 O 4 , which may be regarded as a compound of the monoxide and dioxide. The suboxide is black ; like the corresponding oxides of mercury and silver, it furnishes the metal and a salt derived from the monoxide on treatment with acids. The monoxide, PbO, ordinarily known as litharge, is prepared on the large scale by the oxidation of lead in air ; the pure oxide has a lemon-yellow colour, and is insoluble in water ; it fuses at a heat below redness. The corresponding hydroxide, Pb(OH) 2, is obtained on adding ammonia to a solution of a lead salt such as lead nitrate, Pb(XO 3 ) 2 , as a white amorphous precipitate, which absorbs carbon dioxide on exposure to air ; it is converted into the oxide when heated above 100 C. The oxide and hydroxide are readily acted on by acids, and form stable salts ; but they are also soluble in alkalies. Lead nitrate is easily soluble, but lead sulphate and carbonate are insoluble in water ; they may be obtained in crystals isomorphous with those of the corresponding barium salts. The carbonate is readily decomposed when heated, and on heating the sulphate with lead sulphide metallic lead is obtained : PbS0 4 + PbS = 2Pb + 2SO 2. Lead sesquioxide, Pb 2 O 3, is little known; it has a brown colour, and is converted into the monoxide and oxygen when heated. Lead dioxide, PbO 2, also has a brown colour ; it may be obtained by passing chlorine into water in which lead oxide is suspended. It unites directly with sulphur dioxide, forming lead sulphate, sufficient heat being developed to cause the mass to glow ; it is converted into the monoxide when heated ; it is insoluble in most acids, but is dissolved by acetic acid, and apparently converted into a corresponding acetate ; by fusing it with potassium hydroxide, potassium plumbate is produced, which may be obtained in crystals of the composition K 2 PbO 3, 3H 2 O. By carefully heating lead monoxide to low redness in air it is converted into the red oxide or minium, the most usual composition of which is represented by the formula Pb 3 O 4, On treating this oxide with acids nitric acid, for example it furnishes lead nitrate and lead dioxide. Lead is precipitated by hydrogen sulphide, as a black sulphide, even from strongly acid solutions of its salts. Although no stable chloride of lead exists containing more than two atoms of chlorine, a stable tetraethyl derivative,.