356

OXIDES OF LEAD-LITHARGE.

oxide of lead, which combines with carbonic acid and water to produce the basic carbonate of lead (PbO. CO2, PbO. HO). The lead of old coffins is often found converted into a white earthy-looking brittle mass of basic carbonate, with a very thin film of metallic lead inside it.

When lead is exposed to the joint action of air and the acetic acid contained in beer, wine, cider, &c., it becomes converted into acetate of lead or sugar of lead, which is very poisonous. Hence the accidents arising from the reprehensible practice of sweetening cider by keeping it in contact with lead, and from the accidental presence, in beer and wine bottles, of shot which have been employed in cleaning them. The action of water upon leaden cisterns has been already noticed. Contact with air and seawater soon converts lead into oxide and chloride.

The bright surface of lead soon tarnishes when exposed to the air, becoming coated with a dark film, which is believed to consist of suboxide of lead. In a very finely divided state, lead takes fire when thrown into the air, and is converted into oxide of lead.

The lead pyrophorus, for exhibiting the spontaneous combustion of lead, is prepared by placing some tartrate of lead in a glass tube closed at one end (fig. 249), drawing the tube out to a narrow neck near the open end, and holding it nearly horizontally, whilst the tartrate of lead is heated with a gas or spirit flame as long as any fumes are evolved; the neck is then fused with a blowpipe flame and drawn off. The tartrate of lead (2PbO.C.H4010), when heated, leaves a mixture of metallic lead with charcoal, which prevents it from fusing into a compact mass. This mixture may be preserved unchanged in the tube for any length of time; but when the neck is broken off and the contents scattered into the air, they inflame at once, producing thick fumes of oxide of lead. Tartrate of lead is prepared by adding ammonia to solution of tartaric acid constantly stirred, until the precipitate of bitartrate of ammonia at first formed is just redissolved, and precipitating the liquid with solution of acetate of lead. The precipitated tartrate of lead is collected upon a filter, washed several times, and dried at a gentle

heat.

Fig. 249.

Oxide or protoxide of lead is prepared on a large scale by heating lead in air. When the metal is only moderately heated, the oxide forms a yellow powder, which is known in commerce as massicot, but at a higher temperature the oxide melts, and on cooling forms a brownish scaly mass which is called litharge (Xibos, stone; apyvpos, silver), probably because that obtained by the alchemists would always furnish a considerable proportion of silver, which was present in most samples of lead before the introduction of Pattinson's process. The litharge of commerce often has a red colour, caused by the presence of some red oxide of lead. When heated to dull redness, litharge assumes a dark brown colour, and becomes yellow on cooling. At a bright red heat it fuses, and readily attacks clay crucibles, forming a fusible silicate of lead, and soon perforating the sides.

MANUFACTURE OF RED LEAD.

357

When boiled with distilled water, litharge is dissolved in small quantity, yielding a solution which is decidedly alkaline, and becomes turbid when exposed to the air, absorbing carbonic acid, and depositing carbonate of lead. The presence of a small quantity of saline matter in the water hinders the solution of the oxide, but organic matter, and especially sugar, favours it. Two definite white hydrates of oxide of lead, HO. 2PbO and HO. 3PbO, may be obtained by precipitating solutions of lead with the alkalies. Oxide of lead is a powerful base, and has a strong tendency to form basic salts. Hot solutions of potash and soda dissolve it readily, and deposit it in pink crystals on cooling.

Litharge, from its easy combination with silicic acid at a high temperature, is much used in the manufacture of glass, and in glazing earthenware. The assayer also employs it as a flux. A mixture of litharge with lime is sometimes applied to the hair, which it dyes of a purplish-black colour, due to the formation of sulphide of lead from the sulphur existing in hair. Dhil mastic, used by builders in repairing stone, is a mixture of 1 part of massicot with 10 parts of brickdust, and enough linseed oil to form a paste; it sets into a very hard mass, which is probably due partly to the formation of silicate of lead, and partly to the drying of the linseed oil by oxidation favoured by the oxide of lead.

Red lead or minium is prepared by heating massicot in air to about 600° F., when it absorbs oxygen, and becomes converted into red lead. The massicot for this purpose is prepared by heating lead in a reverberatory furnace to a temperature insufficient to fuse the oxide which is formed, and rejecting the first portions, which contain iron and other metals more easily oxidisable than lead (as cobalt), as well as the last, which contain copper and silver, less easily oxidised than lead. The intermediate product is ground to a fine powder and suspended in water; the coarser particles are thus separated from the finer, which are dried, and heated on iron trays placed in a reverberatory furnace till the requisite colour has been obtained. Minium is largely used in the manufacture of glass, whence it is necessary that it should be free from the oxides of iron, copper, cobalt, &c., which would colour the glass. It is also employed as a common red mineral colour, and in the manufacture of lucifermatches.

When minium is treated with dilute nitric acid, nitrate of lead (PbO. NO) is obtained in solution, and peroxide of lead (PbO) is left as a brown powder, showing that minium is probably a compound of the oxide and peroxide of lead. The minium obtained by heating massicot in air till no further increase of weight is observed, has the composition 2PbO. PbO, or Pb,O,, which would appear to represent pure minium ; commercial minium, however, has more frequently a composition corresponding to 3PbO. PbO,, but when this is treated with potash, PbO is dissolved out, and 2PbO. PbO, remains. Minium evolves oxygen at a red heat, becoming PbO, hence the necessity for keeping the temperature below 600° F. during its preparation.

Peroxide, or binoxide, or puce oxide of lead, is found in the mineral kingdom as heavy lead ore, forming black, lustrous, six-sided prisms. It may be prepared from red lead by boiling it, in fine powder, with nitric acid, diluted with five measures of water, washing and drying. The binoxide of lead easily imparts oxygen to other substances; sulphur, mixed with it, may be ignited by friction, hence this oxide is a common ingredient in lucifer-match compositions. Its oxidising property is frequently

358

=

MANUFACTURE OF WHITE LEAD.

turned to account in the laboratory, for example, in absorbing sulphurous acid from gaseous mixtures by converting it into sulphate of lead; PbO2 + SO2 PbO. SO,. Binoxide of lead is not dissolved by dilute acids and has no basic properties; indeed, it is sometimes called plumbic acid, for it combines with potash and soda when fused with their hydrates. Plumbate of potash (KO. PbO,. 3HO) has been crystallised from an alkaline solution, but is decomposed by pure water.

256. White lead or ceruse is a carbonate of oxide of lead, or, strictly speaking, a basic carbonate, a combination of carbonate of oxide of lead (PbO.CO,) with variable proportions of hydrated oxide of lead (PbO.HO). This substance is a constant product of the corrosive action of air and water upon the metal. Its formation is, of course, very much encouraged by the presence of organic matters in a state of decay, which evolve carbonic acid.

White lead is manufactured on the large scale by two processes, which depend, however, upon the same principle; this may be stated as follows:when oxide of lead is brought in contact with acetic acid (HO. CH2O3), it displaces the water from the latter to form the acetate of lead (sugar of lead, PbO. C,H,O,). This salt is capable of combining with twice as much oxide of lead as it at present contains, forming the tribasic acetate of lead (3PbO. C1H,O), and if this be acted upon by carbonic acid, twothirds of the oxide of lead are converted into carbonate, whilst the neutral acetate of lead (PbO. C,H,O,) is left.

In the older of the two processes, commonly known as the Dutch process, metallic lead, in the form of square gratings cast from the purest lead, is placed over earthen pots containing a small quantity of common vinegar; a number of these pots being built up into heaps, together with alternate layers of dung or spent tan, the heaps are entirely covered up with the same material. The metal is thus exposed to conditions most favourable to its oxidation, viz., a very warm and moist atmosphere produced by the fermentation of the organic matters composing the heap, and the presence of a large quantity of acid vapour generated from the acetic acid of the vinegar. The lead is therefore soon converted into oxide, a portion of which unites with the acetic acid to form the tribasic acetate of lead, which is then decomposed by the carbonic acid evolved from the fermenting dung or tan, yielding carbonate of lead, which combines with another portion of the oxide of lead and of water to form the white lead. The neutral acetate of lead left after the removal of the oxide of lead from the tribasic acetate, is now ready to take up an additional quantity of the oxide, and the process is thus continued until, in the course of a few weeks, the lead has become coated with a very thick crust of white lead; the heaps are then destroyed, the crust detached, washed, to remove adhering acetate of lead, ground to a paste with water, and dried. Rolled lead is not so easily converted as cast lead.

The newer process is a more direct application of the same principle, for it consists in boiling acetic acid with an excess of litharge in order to produce the tribasic acetate of lead, which is afterwards decomposed by passing through it a current of carbonic acid obtained by combustion or fermentation, or even by exhalation from the earth. The solution of neutral acetate of lead is then again boiled with litharge, when tribasic acetate is produced, and is again precipitated by the carbonic acid. The precipitated carbonate of lead always carries down with it a variable pro

portion of the hydrated oxide of lead. This process is, of course, much more rapid than the old one, and dispenses with the grinding which is so injurious to the workmen ; but the white lead so produced, being crystalline, has less opacity or covering-power (body) than that obtained by the Dutch method.

The usual composition of white lead is expressed by the formula PbO. HO, 2(PbO. CO2), though other basic carbonates of lead are often mixed with it.

White lead being very poisonous, its use by painters and others is generally attended with symptoms of lead-poisoning, arising in many cases probably, from neglecting to wash the hands before eating, the effect of lead being cumulative, so that minute doses may show their combined action after many days. Diluted sulphuric acid and solutions of the sulphates of magnesia and the alkalies are sometimes taken internally to counteract its effect, since the sulphate of lead is not poisonous.

All paints containing lead, and cards glazed with white lead, are blackened even by minute quantities of sulphuretted hydrogen, from the production of black sulphide of lead. If the blackened surface remain exposed to the light and air, it is bleached again, the sulphide of lead (PbS) being oxidised and converted into white sulphate of lead (PbO. SO), but this does not take place in the dark. A little sulphide of lead or powdered charcoal is sometimes mixed with commercial white lead to give it a bluish tint.

The pure carbonate of lead is found in white crystals associated with galena.

Sulphate of lead is found in nature in prismatic and octahedral crystals of anglesite or lead-vitriol. It is nearly insoluble in diluted acids, and is one of the chief forms in which lead is precipitated from its solutions in analytical operations. The minerals lanarkite and leadhillite are compounds of sulphate and carbonate of lead. The chromates of lead have been already noticed.

Phosphate of lead (3PbO. PO,) is occasionally associated with the carbonate in the ores of lead.

257. Chloride of lead (PbCl) forms the mineral termed horn-lead. It is one of the few chlorides which are not readily soluble in water, and is precipitated when hydrochloric acid or a soluble chloride is added to a solution of lead. Boiling water dissolves about of its weight of chloride of lead, and deposits it in beautiful shining white needles on cooling. It fuses easily, and is converted into vapour at a high temperature.

The oxychloride of lead (PbC1. PbO) is formed when chloride of lead is heated in air. It is sometimes employed as a substitute for white lead in painting, being prepared for this purpose by decomposing finely powdered galena with concentrated hydrochloric acid (PbS+ HCl = PbCl + HS) washing the resulting chloride of lead with cold water, dissolving it in hot water, and adding lime-water, which precipitates the oxychloride

2РЬСІ + CaO = РЬСІ. РЬО + CaCl.

Turner's yellow (Paris yellow, patent yellow, mineral yellow) is another oxychloride of lead (PbCl. 7PbO), prepared by heating a mixture of litharge and sal-ammoniac. It has a fine golden yellow colour, is easily fused, and crystallises in octahedra on cooling. The mineral mendipite

is an oxychloride of lead (PbCl. 2PbO) which occurs in colourless prismatic crystals.

Iodide of lead (PbI) is obtained as a bright yellow precipitate on mixing solutions of nitrate or acetate of lead and iodide of potassium. If it be allowed to settle, the liquid poured off, and the precipitate dissolved in boiling water (with one or two drops of hydrochloric acid), it forms a colourless solution, depositing golden scales as it cools.

258. Sulphides of lead.-The subsulphide (Pb,S) has been mentioned as produced in smelting galena. Sulphide of lead, or galena, has been described among the ores of lead. It is always obtained as a black precipitate when hydrosulphuric acid or a soluble sulphide acts upon a solution containing lead, even in minute proportion.

A persulphide of lead, the composition of which has not been ascertained, is formed as a red precipitate when a solution of lead is mixed with a solution of an alkaline sulphide saturated with sulphur (or with solution of hydrosulphate of ammonia which has been kept till it has acquired a red colour).

Chlorosulphide of lead (3PbS. 2PbCl) is obtained as a bright red precipitate when hydrosulphuric acid is added in small quantity to a solution of chloride of lead in hydrochloric acid.

Selenide of lead (PbSe) occurs associated with the sulphide in some lead ores; it much resembles galena, and has the same crystalline form.

259. Equivalent and atomic weights of lead.-The analysis of the oxide and chloride of lead has shown that 8 parts by weight (1 eq.) of oxygen and 35 5 parts (1 eq.) of chlorine, are respectively combined with 103.5 parts of lead; the equivalent of lead, therefore, is 103.5. The specific heat of lead, and its isomorphism with other diatomic metals, show its atomic weight to be twice its equivalent weight, or 207. The atomic formula of some of its chief compounds would be-litharge Pb0, minium 2Pb0. PbO2, peroxide of lead PbO2, galena PbS, chloride of lead PbCl.

260. THALLIUM.-The discovery of this metal in 1861 was one of the first results of the application of the new method of testing by observation of coloured lines in the spectrum of a flame, described at p. 273. Crookes was examining the spectrum obtained by holding in the flame of a Bunsen burner the deposit formed in the flues of a sulphuric acid chamber, in which pyrites was employed as the source of sulphur. A green line made its appearance in the spectrum, which a less acute and practised observer might have mistaken for one of the lines caused by barium (see fig. 221), with which it nearly coincides in position; but the line was much brighter than that produced by barium, and on instituting a searching analysis of the deposit, a metal was obtained which did not agree in properties with any hitherto described, and was named thallium, from 8xxxós, a young shoot, in allusion to the vernal green colour of its spectrum line. It has since been detected in several mineral waters; but the pyrites obtained from Spain and Belgium appear to be its best source. From the flue-dust of the sulphuric acid chambers, the metal is extracted by a simple process, but large quantities must be operated on to obtain any considerable amount. The deposit is treated with boiling water, and the solution mixed with much strong hydrochloric acid, which precipitates the thallium as chloride; this is converted into acid sulphate of thallium by treatment with sulphuric acid, and this salt having been purified by recrystallisation, is decomposed by zinc, which precipitates metallic thallium in a spongy form, fusible into a compact mass in an atmosphere of coal-gas.

In external characters thallium is very similar to lead; but it tarnishes much more rapidly when exposed to air, and the streak which it makes on paper soon becomes yellowish, being converted into oxide of thallium. If a tarnished piece of the metal be allowed to touch the tongue, a strongly alkaline taste is perceived, for the oxide of thallium or thallous oxide (TIO) is very soluble in water, so that the tarnished