Germanic oxide, as also the hydrate, appears to possess acid proper

ties.

On account of the unstable character of germanic sulphide it requires to be precipitated in the presence of considerable free hydrochloric acid.

When this compound is heated in a current of hydrogen, germanous sulphide, GeS, is produced. It may also be obtained as a red-brown precipitate by passing hydrogen sulphide into a solution of germanous chloride.

LEAD (Plumbum).

Symbol, Pb.

Atomic Weight, 206.4.

Valence, II and IV.

History. Lead has been known from the earliest times. was designated Saturn by the alchemists, who believed that some connection existed between the metals and the planets.

Occurrence.-Lead has been found in nature uncombined, but in small quantity only, as in certain volcanic tufa. The oxides of lead are found in a few rare minerals. By far the most abundant and important ore of lead is the sulphide or galena, PbS; this is very widely distributed, usually occurring with quartz, fluor-spar, calc-spar, and heavy spar. The most important deposits in England are those of Cornwall and Cumberland. In the United States there are two localities which furnish the chief supply of lead, the Rocky Mountain district, in which the lead is a byproduct in the preparation of silver, and the Mississippi Valley district, in which galena is worked for lead alone. The ores are known respectively as argentiferous and non-argentiferous, the former furnishing by far the greater proportion of lead. Cerussite or native carbonate, PbCO,, and anglesite or native sulphate, PbSO, are ores of minor importance when compared with galena.

The total production of lead in the United States during 1893 amounted to 193,928 tons, valued at 14,467,029 dollars. The total production of the world during that time amounted to something over half a million tons.

Extraction. Lead is almost exclusively prepared from galena. In case the ore is nearly free from other metallic sulphides and silica, it is roasted in a reverberatory furnace, whereby a part of the sulphide is converted into oxide and sulphate; air is then excluded, the temperature is increased, when the following reactions take place with formation of metallic lead:

This is known as the air reduction process, and is conducted in a furnace as shown in Fig. 82. The furnaces are constructed to

hold 20 tons of ore, which is introduced at T, and evenly spread over the hearth, which is hollowed out at B, so as to allow the metal to collect, and from which it can be drawn out by a taphole into an iron pot. When the ore is impure, charcoal is used to assist in the reduction. Small quantities of other metals are removed by melting the lead, and subjecting it to the oxidizing action of a current of air in a reverberatory furnace. The impurities oxidize first, rise to the surface, and are removed as slag.

Iron is sometimes used as a reducing agent. This is accomplished by placing alternate layers of the ore and an iron slag in a blast-furnace (Fig. 83). In this process a portion of the lead is reduced by the metallic iron, a considerable residue, however, remaining behind in the form of sulphide, together with the sulphides of iron and other metals contained in the ore. This slag or lead regulus is then worked up according to the character of the metals of which it is composed. Sometimes copper is a constituent, in which case it may be so rich in that metal as to pay for extracting it.

When argentiferous lead ores are worked the process becomes modified according to the richness of the ore in silver, or the lead is further treated. If silver predominates, the ore is oxidized and cupelled as described under that metal. It is much oftener

the case that the lead predominates, when Pattinson's process is employed. This consists in cooling the melted metal, when pure

FIG. 83.

lead first crystallizes out and is removed by perforated ladles. In this way a metal very rich in silver is obtained, which is cupelled.

Parke's process for desilvering lead is based on the fact that melted lead and zinc do not mix in all proportions. By adding, therefore, a certain proportion of zinc to the argentiferous lead, an alloy of zinc and silver is formed, which rises to the top and solidifies. It is then easily removed.

Properties.-Lead is a bluish-gray, lustrous metal; it is soft and malleable, but is not capable of being drawn into wire. It is easily cut with a knife, and when drawn over white paper leaves a gray streak. Lead melts at 327.7°, and quickly becomes covered with a layer of oxide; at a red heat it vaporizes slowly; and when heated to whiteness, distils. Melted lead crystallizes, when cooled slowly, in regular octohedrons. The specific gravity of lead is 11.37.

Lead furnace.

Pure water does not dissolve lead so long as air is excluded; when, however, air is present, lead hydrate is formed, which is slightly soluble in water. Rain water and distilled water also dissolve minute quantities of lead; hot water containing small proportions of various salts, notably chlorides and sulphates, does not dissolve the metal. With most natural waters, there

fore, lead pipes may be employed. Carbonated waters should not be passed through such pipes, unless the latter are well tinned on the inner surface.

Concentrated hydrochloric or sulphuric acid attacks lead but slowly, even when warmed, but nitric acid dissolves it readily. Weak acids like acetic will dissolve it in the presence of air.

Uses. On account of its power to resist the action of air, water, and acids, lead is employed for a great variety of purposes. Enormous quantities are used in the manufacture of lead pipe. It is also the best material for lining sulphuric acid chambers. Bullets and shot are made of lead; the latter contain one-half per cent. of arsenic in order to give them a spherical form.

Alloys of Lead.-Lead enters largely into the composition of a number of useful alloys. Type-metal is composed of 2 parts lead, 1 part each of antimony and tin. Solder consists of 1 part lead and 2 parts tin; a common variety is made of equal parts of these two metals.

LEAD AND OXYGEN.

Lead Suboxide, Pb,O.-This compound is formed on the surface of melted lead when the temperature is not too high. It may be prepared by heating lead oxalate in an atmosphere free from oxygen and at a temperature below 300°. It is a black, velvety powder, which, if heated without access of air, decomposes into the metal and monoxide.

Lead Monoxide, PbO. Plumbi Oxidum, U. S. P. Litharge. A rare mineral found near Vera Cruz is composed of this substance. There are two varieties of lead oxide in commerce,—namely, massicot, which is yellow in color and prepared by carefully heating lead carbonate or nitrate to low redness, and litharge, which has been prepared at a much higher temperature, and is a by-product in the desilverization of lead ores. Litharge is obtained at a temperature sufficiently high to fuse it, and in solidifying it forms shining, scaly masses, which are sometimes of a yellowish color, but oftener inclining to red.

Properties. The official oxide is "a heavy, yellowish, or reddish-yellow powder, or minute scales, without odor or taste. On exposure to the air it slowly absorbs moisture and carbon dioxide." It is almost insoluble in water, but when left in contact with it imparts to it an alkaline reaction. It is insoluble in alcohol, but soluble in acetic and dilute nitric acids, and in solutions of sodium and potassium hydrates.

On the application of heat lead oxide assumes a brownish-red

When heated before the blow

color, and at a red heat it fuses.

pipe on charcoal it yields metallic lead.

Uses.-Litharge is extensively used in the manufacture of flint glass, and is a constituent of the glaze used for earthenware. It is also useful as a starting-point for a number of other lead compounds.

Lead Sesquioxide,, Pb,Og, is formed as a reddish-yellow powder when cold solution of lead hydrate in sodium hydrate is mixed with a solution of sodium hypochlorite, or when a solution of red lead in acetic acid is precipitated by very dilute ammonia. It is decomposed by acids into the monoxide and dioxide, and is therefore considered to be a mixture of these two.

Red Lead or Minium, PbO4.—When litharge is carefully heated this oxide is formed. The commercial product is of variable composition. It is used as a pigment and in the preparation of the finer kinds of flint glass.

Lead Dioxide or Peroxide, PbO2.-This is also known as puce-colored lead oxide. It is obtained by boiling lead acetate solution with solution of bleaching powder, or in the dry way by heating 4 parts of litharge with 8 parts of potassium nitrate and 1 part of potassium chlorate. It is also readily obtained by heating red lead with nitric acid.

Lead peroxide is a dark-brown, amorphous powder, which is readily decomposed by heat into lead monoxide and oxygen. It occurs native in black, hexagonal prisms as plattnerite. Lead peroxide has a strong oxidizing action on many compounds, especially those of an organic character.

Lead Hydrate, Pb(OH)2, PbO.—This is more properly called oxyhydrate. It is obtained as a white precipitate when solution of lead acetate or nitrate is treated with so much solution of sodium or potassium hydrate as is necessary to produce an alkaline reaction. It is insoluble in water, but easily soluble in excess of sodium or potassium hydrate, forming the compounds Pb(ONa), and Pb(OK). When solutions of these are boiled the yellow or red lead oxide separates according to the concentration of the solution.

LEAD AND THE HALOGENS.

Lead Chloride, PbCl2, is found to some extent in nature as the mineral cotunite. It is most readily prepared by adding sodium chloride to a solution of lead nitrate or acetate, although it may be obtained by dissolving lead in hot aqua regia.

Lead chloride occurs in white, pearly crystals, soluble in 135 parts of cold water and in about 30 parts of that liquid at the boiling temperature. At 498° the crystals melt, and on cooling form a horny mass.

A number of oxychlorides are known which are employed as pigments ; Cassel's yellow is one of these; it is prepared by heating together lead oxide and sal-ammoniac. The oxychloride, PbO, PbCl, is found in nature as the mineral matlockite.