becomes malleable and ductile if heated to a temperature of 100° to 150° C., but beyond that point it again becomes brittle. Its specific gravity in the cast state is 6.9, which may be increased to 7.1 by rolling or forging; it contracts but slightly on cooling from the liquid state, and is thus well adapted for castings. The castings made at a high temperature are brittle and crystalline; but when cast near the solidifying point are more malleable. Zinc melts at 412° C. and boils at 1040° C. At a red heat in air it rapidly oxidises, and burns with a greenish-white flame, forming zinc oxide ZnO; if raised to a bright-red heat in a closed vessel, it may be readily distilled. When rolled zinc is exposed to air and moisture a gray film of suboxide is formed, which preserves the metal from further oxidation. Ordinary zinc readily dissolves in dilute hydrochloric and sulphuric acids, while the pure metal is unaffected; both kinds dissolve in nitric acid and in alkalies. Zinc displaces silver, gold, platinum, bismuth, antimony, tin, mercury, and lead from their solutions. The chief impurities of the commercial metal are iron, lead, and arsenic.

Zinc and sulphur do not readily unite, but when a mixture of finely divided zinc and sulphur is projected into a red-hot crucible, some zinc sulphide ZnS is formed. It is also formed by heating zinc with cinnabar HgS.

Zinc forms compounds with phosphorus and arsenic, when these bodies are heated with it, having a metallic lustre and somewhat vitreous fracture.

The chief ores of zinc are :-The oxide ZnO called zincite or red oxide of zinc, which is white when pure, but generally red from the presence of oxide of manganese. The sulphide ZnS, known as "blende" and "black jack," is the principal source of the metal, and generally black or yellowishblack in colour, but sometimes it has a reddish tint from the presence of galena; when pure it is white, and contains 67.03 The carbonate ZnCO, called calamine. And the silicate (2ZnO,SiO2,OH2) called electric calamine. Zinc forms with other metals a most important class of

per cent of zinc.

alloys, such as brass, German silver, etc. It is used in the form of sheets, worked into a variety of shapes; it protects iron from rusting, as in galvanised-iron; it forms the electropositive element in many batteries; and in the form of fine dust it is obtained in large quantities mixed with zinc oxide, and forms a valuable reducing agent.

Cadmium.—In the process of zinc extraction it was observed that a volatile vapour, in some cases, was distilled off with the first portions of zinc; this was found to be the metal cadmium. It possesses a tin-white colour, has a fibrous structure, and takes a high polish. It is harder than tin, malleable, ductile, and readily volatile. It has a density of 8·6; it melts at about 320° C., and boils at 860° C. Its vapour is of a dark-yellow colour, with a disagreeable odour. Like tin it emits a crackling sound when bent. It is used in alloys to produce a fusible metal, which melts below 100° C.; and an amalgam of cadmium is employed as a stopping for teeth, such amalgam being soft when first prepared, but soon becomes hard.

Magnesium.—This metal possesses a brilliant-white colour, but soon tarnishes when exposed to moist air, due to the formation of magnesium oxide. It is stated to possess great tensile strength, being nearly equal to that of aluminium bronze. Its specific gravity is 1.74. At a temperature of 450° C. it can be rolled and worked into a variety of forms. Screws and threads made of this metal are sharper and more exact than those made of aluminium. When ignited in a flame it burns with a dazzling white light, which is said to have been seen at sea from a distance of twenty-eight miles. This light is used for purposes of photography. Magnesium occurs abundantly in nature in combination with other elements forming compounds, such as magnesite MgCO3, dolomite MgCa(CO3)2, etc.

ALUMINIUM

§ 11. Aluminium. With the exception of oxygen and

silicon, this is the most widely distributed of the elements, and contained in the largest quantity in the solid crust of the earth. It occurs in a variety of forms as oxide, but more generally in combination with other metals, such as zinc, iron, magnesium, etc., forming aluminates; as silicate in all clays, and as fluoride in cryolite (6NaF,Al2F6).

Aluminium is a white metal which takes a fine polish. It has no taste or odour. It is soft, very malleable, and ductile, with an elasticity and tenacity about equal to that of silver. Its specific gravity is 2.5, which is increased by hammering; it melts at a temperature a little above that of zinc, and is not volatile when strongly heated out of contact with air. Its conductivity for heat and electricity is said to be very high; but, according to Mr. Roby, it is very low, and diminishes the conductivity of copper considerably when alloyed with it. It does not oxidise in air or combine with sulphur; it is insoluble in cold nitric acid; sulphuric acid has no action on it; but hydrochloric acid and alkalies dissolve it readily.

Aluminium is valuable for making articles where lightness is an important feature; this, combined with its lustre, unalterability in air and sulphuretted hydrogen, non-poisonous properties, and ease of working, gives it a widespread interest. It is, however, in its alloys that its greatest value appears. In some cases it imparts strength, in others it modifies the colour, while in others it promotes soundness in castings.

ALKALINE-EARTHY METALS

§ 12. The term "earth was formerly used to denote those bodies which are insoluble or but slightly soluble in water, and unaltered by exposure to a high temperature. Some of these were found to have an alkaline reaction, and to easily neutralise acids; hence the term "alkaline earth." These oxides-viz. baryta, strontia, lime, and magnesia—were found to be composed of metals in combination with oxygen.

Barium is a pale yellow metal, malleable, and fusible at a red heat. It rapidly tarnishes in air, and burns brilliantly at a red heat forming barium oxide. Its melting point, according to Frey, is above that of cast-iron. It decomposes water rapidly at the ordinary temperature. Its specific gravity is 4.

Strontium is similar to barium in colour; it is malleable, fusible at a red heat, quickly oxidises on exposure to air, burns brilliantly in air when heated, and violently decomposes water. Its specific gravity is 2.58.

Calcium is a yellow metal, tenacious and malleable; it melts at a red heat, oxidises in air, and burns when heated ; it decomposes water rapidly. Its specific gravity is 1.58.

The alkaline-earthy metals, although their compounds are widely distributed, do not occur in nature in the metallic state, and the isolated metals have little application in the arts, on account of their easy oxidation. They may be useful in removing oxygen from other metals and their alloys.

ALKALI METALS. SODIUM, POTASSIUM, LITHIUM, ETC.

§ 13. The word "alkali was originally used as the name of a soluble salt obtained from the ashes of sea-plants, and is now applied to a well-defined class of bodies having the following properties: They turn red litmus blue, completely neutralise acids, are soluble in water, and their solutions exert a caustic action upon animal matter. The alkalies proper are the oxides of sodium, potassium, lithium, rubidium, and cæsium. To these is added the hypothetical metal ammonium NH4, which is called the volatile alkali in contradistinction to potash and soda. The metals of the alkalies are soft, readily fusible, volatile bodies, easily oxidised on exposure to air, and they rapidly decompose water at ordinary temperatures.

Sodium. This metal melts at 96° C. and volatilises forming a dark blue vapour. It rapidly oxidises in air, and when strongly heated burns with a yellow light. It decomposes

water rapidly at ordinary temperatures. It is a silverwhite metal, with a specific gravity of 98. Sodium is used for the preparation of aluminium, magnesium, boron, and silicon. As an amalgam it is used in the extraction of gold, and in the laboratory as a reducing agent. It occurs very abundantly in nature in a state of combination; in the forms of chloride, nitrate, borate, carbonate, and silicate.

Potassium. This element is very similar to sodium in appearance and properties. It is a silver-white lustrous metal, having a specific gravity of 86; it is brittle at 0° C., but at 15° C. it becomes soft, malleable, and weldable; it melts at 62.5° C., forming a liquid like mercury in appearance; at a red heat it boils, emitting a green-coloured vapour. has a strong affinity for oxygen, and decomposes water, with evolution of great heat. It is used for similar purposes to those of sodium, and occurs abundantly in nature in analogous forms.

It

Lithium. This is a widely diffused element, being found in many micas and felspars, in the ashes of plants, and in sea-water. It has the colour and lustre of silver, is soft and weldable, melts at 180° C., is volatile at a high temperature, burning with a white flame, and rapidly oxidises in contact with air at ordinary temperatures; its specific gravity is 58, and it is therefore the lightest of all solid and liquid bodies.

Rubidium and Casium.-These rare metals so closely resemble potassium that they cannot be distinguished from that metal by many of the ordinary tests. Their presence is detected by means of spectrum analysis.

NATURE OF ALLOYS

§ 14. When two or more metals are caused permanently to unite the resulting mixture is termed an alloy. When mercury is an essential constituent the mixture is then termed an amalgam. The general method of effecting combination is by the agency of heat, but with certain soft metals true