Mercury sometimes occurs in the metallic state, sometimes as an amalgam with silver, and occasionally as chloride, bromide, and iodide of mercury. The chief source of the metal is the sulphide HgS, known as cinnabar.

Lead. This metal has a bluish-gray colour, and possesses considerable lustre; it is malleable, ductile, and tough, but has a feeble tenacity. The lustre of a freshly cut surface soon becomes dim when exposed to the air, owing to the formation of a film of suboxide of lead. Pure lead emits a dull sound when struck, but the presence of impurities renders it more sonorous; also when the pure metal is cast in the form of a hollow sphere it becomes somewhat sonorous. Its specific gravity is 11:45, and all base metals, when alloyed with it, lower its density. Its melting point is about 330° C., and it is not well adapted for castings, since it contracts considerably on solidifying. It is so soft that it can easily be cut with a knife, and squirted into the form of tubes or rods; two clean surfaces of lead can easily be welded together by pressure in the cold, and also when in a finely divided state, the metal can be pressed into a compact mass. Its specific heat between 0° and 100° C. is 0314, and its coefficient of expansion is 00003 for each degree between 0° and 100° C.

If lead is boiled with water containing oxygen it is partially dissolved, and the liquid affords an alkaline reaction. The metal is oxidised when exposed to moist air; it is somewhat volatile when heated in air, forming lead oxide PbO, and this oxide acts as an oxidising agent on many metals, such as copper, zinc, iron, etc. Lead and sulphur unite when heated together, forming lead sulphide PbS, which is a bluish-gray, brittle, and crystalline body.

Commercial lead is often nearly pure, but it generally contains some silver, copper, antimony, tin, and sulphur; and occasionally iron, arsenic, zinc, and manganese.

Bismuth is a hard, grayish-white metal with a reddish tint and bright metallic lustre. Its specific gravity is 99, which may be reduced by pressure; it melts at 270° C. and

volatilises at a high temperature, burning with a blue flame, forming flowers of bismuth BigOg; it expands in the act of solidifying. When the metal is melted, and allowed to cool until its surface begins to solidify, the crust broken, and the metal poured out, fine large crystals are obtained. They oxidise in air, and frequently become covered with an iridescent film of oxide. Bismuth unites with sulphur, forming a dark-gray, metallic-looking sulphide Bi,S.

Bismuth serves for the preparation of many pharmaceutical products and cosmetics. The chief use of the metal is in the preparation of fusible alloys, the melting points of which can be altered according to the proportions of their constituents. It occurs in nature in the metallic state, as bismuth-glance BiS, as bismuth-ochre Bi2O, and often in company with silver, lead, tin, copper, and cobalt

ores.

TIN GROUP. TIN, ANTIMONY, AND ARSENIC

§ 8. Tin is a white metal with a brilliant lustre; very malleable, as seen by the thinness to which tin-foil can be reduced. A bar of tin when bent produces a crackling sound, known as the "cry of tin," supposed to be due to the grinding action of its crystals over each other. Its specific gravity is 7.3, it melts at 230° C., and may be somewhat strongly heated without volatilising. When raised to a temperature near its melting point, and allowed to fall from a considerable height, the metal breaks up into the form of long grains, known as grain-tin. When tin is melted, and poured into a mould at a temperature little removed from the point at which it solidifies, the surface remains bright, if pure, but the presence of a little lead, iron, or other base metals imparts a more or less dull and frosted appearance, so that the brilliancy of the surface is a test of purity. Tin is easily crystallised superficially by treating its surface with a mixture of dilute sulphuric and nitric acids; the ornamental appearance, known as Moirée Metallique, is obtained in this way. Tin is an

inferior conductor of heat and electricity; it takes a high polish, and the radiation of heat from its surface is small. It forms a valuable metal for coating culinary vessels. It is little affected by air at ordinary temperatures, and is therefore used for coating iron to protect it from rust. It unites readily with sulphur on the application of heat, forming stannous sulphide SnS. Commercial tin often contains small portions of lead, iron, copper, arsenic, antimony, bismuth, tungsten, and sometimes manganese and zinc. The tin of

commerce is quoted as common, refined, and grain-tin. The refined-tin is made from the best ores, and is more perfectly refined than common tin. Grain-tin is obtained from the best pigs, which are heated and dropped from a height, as referred to on previous page.

The only important ore of tin is tin-stone, which contains tin dioxide SnO2.

Antimony. Ordinary commercial antimony is often very impure, containing iron, lead, arsenic, and sulphur, and is called "regulus of antimony." Antimony is a brilliant bluishwhite metal, highly crystalline, with fernlike markings on the surface, and very brittle, so that it may be easily powdered; its specific gravity is 6.72; it melts at 450° C., and volatilises at a higher temperature. It does not oxidise at ordinary temperatures, but when heated in air, antimonious oxide Sb2O is formed; and at a red heat antimony burns with a bluish-white flame producing dense white fumes of Sb2O3.

3

Antimony and sulphur readily unite when heated together, forming Sb,S,; the same compound is also formed by heating the oxide with sulphur, thus

2Sb2O3+98=3SO2 + 2Sb2S3.

Antimony unites with other metals to form valuable alloys, in consequence of its hardening properties, but it impairs the malleability and ductility of the malleable metals. The effect of even small quantities of antimony on the malleable metals, such as copper, gold, iron, etc., is most injurious, making them hard and brittle.

Antimony occurs native, and in combination with other ores, but the chief ore is "stibnite" SbS.

Arsenic. This metal has a brilliant, dark steel-gray colour, and metallic lustre; it is crystalline, exceedingly brittle, and may be readily reduced to powder. When heated to 180° C. in a closed vessel it begins to volatilise without fusing, and crystallises as it condenses in forms similar to those of antimony. Its vapour is colourless, and possesses a peculiar garlic-like odour. The metal may be exposed to dry air without undergoing change. If heated in air it absorbs oxygen, and burns with a bluish flame, forming arsenious acid As2O3, which is condensed as a white powder when in contact with a cool body. The specific gravity of arsenic is 4.71.

Arsenic occurs in nature as realgar As,S2, orpiment ASS, mispickel FeAs+ FeS, nickel pyrites NiAs, and kupfer-nickel or copper-nickel NiAs2.

The metal is obtained by heating nickel pyrites, mispickel, etc., in closed retorts, when the arsenic is expelled, and sublimes into condensing chambers.

Arsenic enters into the composition of some alloys, such as shot metal, its general effect being to harden, and render the alloys brittle, and more fusible. Its compounds are used in medicine, and in glass making.

IRON GROUP. IRON, CHROMIUM, MANGANESE, NICKEL, AND COBALT

§ 9. Iron.-Malleable iron is of a grayish-white colour, having a granular, crystalline, or fibrous fracture, according to the mode of treatment. When rolled or hammered hot, the iron becomes fibrous, but continued cold hammering induces a crystalline or granular structure, making it hard and brittle. The nature of the fractured surface varies also with the manner in which the iron has been broken, for specimens broken by progressively increasing stresses invariably show a fibrous structure, whilst the same specimen broken by a

sudden blow may be crystalline. The presence of impurities generally tends to impart a granular or crystalline fracture, and makes the iron less malleable. When impurities, such as sulphur and arsenic, render the metal unworkable at a red heat, it is said to be hot- or red-short. On the other hand, some substances, such as phosphorus, cause iron to crack when hammered cold; it is then termed cold-short. The specific gravity of iron is about 7.8. Its fusing point is said to be about 2000° C., but before melting it assumes a pasty form, at which point two pieces may be joined together by welding. To ensure a good weld the surfaces must be clean, and the metal at a white heat. In order to dissolve any scale the smith adds a little sand, which unites with the oxide (produced by union of iron with the oxygen of the air), and forms a fusible silicate. The presence of any foreign bodies, such as carbon, silicon, sulphur, phosphorus, copper, oxygen, etc., increases the difficulty of welding. Iron possesses considerable malleability, ductility, and tenacity. Its tensile strength ranges from 17 to 25 tons per square inch, but this, like all the other physical properties, is modified by the presence of impurities, which tend to make it harder, more fusible, and brittle. When iron is heated to dazzling whiteness in air it burns, forming the black oxide Fe3O4, the iron becoming friable and brittle, and is then termed "burnt iron." Iron may be magnetised by bringing it in contact with, or near to, a magnet, but soft iron loses its magnetism when the exciting magnet is withdrawn. Its specific heat is 1137; its conductivity about 120, silver being taken as 1000. Its electric resistance is 5.8 times that of pure copper. When iron is exposed to moist air it readily rusts or oxidises, so that it is often coated with some substance to prevent this action, such as tinning, galvanising, and painting. Professor Barff preserves iron from rusting by exposing it at a red heat to superheated steam, which imparts to it a coating of the black oxide Fe3O4.

Pure iron is not a commercial article, but it may be obtained in several ways. 1o. By reducing pure ferric oxide