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 form

ing 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. It 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.

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 Cæsium.—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

alloys may be formed by subjecting the constituents to considerable pressure, even at the ordinary temperature.

Alloys, such as those briefly referred to in the historical sketch, were doubtless first discovered by the metallurgical treatment of mixed ores, from the simultaneous reduction of which, alloys would be formed; or in some cases, as in ores of gold and silver, naturally formed alloys would be obtained by a simple melting process. The direct preparation of alloys by the simple melting together of the constituent metals has been enormously developed in modern times, and the attention which mixed metals are now receiving by chemists is far greater than in any period of history.

Comparatively few of the metals possess properties such as render them suitable to be employed alone by the manufacturer; but most of them have important applications in the form of alloys. Even among the metals which can be used independently, it is often found expedient to add portions of other metals, to improve or otherwise modify their physical properties. Thus gold is hardened, and made to resist wear and tear, as well as to lower its cost, by the addition of copper; silver is likewise hardened by alloying it with copper; and the bronze coinage is formed of an alloy of copper, zinc, and tin for similar

reasons.

The purposes for which metals are alloyed are as various as the uses of the metals themselves, but, as a rule, the combination is employed to harden, render more fusible, alter the colour, or to reduce the cost of production. Thus the class of alloys known as solders, which are used for joining the several parts of a body or bodies together, are formed so as to possess melting-points below that of the articles to be soldered. The well-known class of alloys termed "brass " furnishes a good illustration of the effect of alloying in producing different shades of colour. These bodies are composed of the metals copper and zinc in varying proportions, the colour depending to a great extent on the quantity of copper present. When the copper predominates the colour is yellow,

or reddish, when the two metals exist in equal proportions the colour is still yellow; beyond this, when the zinc is in excess, the colour gets white, or bluish-white, resembling impure zinc. Nickel is added to brass to whiten it, forming German silver.

Again, some metals, such as copper, can only with difficulty be made to produce sound castings; and the metal is too tough to be conveniently wrought in the lathe or with the file, but when alloyed with zinc or tin, good castings can be readily obtained, and rolled, turned, or filed with considerable facility. In some cases the tensile strength of a metal is enormously increased by the addition of another metal, sometimes in very small proportions; the various bronzes may be cited as examples. The addition of a second metal is often a source of weakness, as in the case of adding antimony to lead. One might be led to consider that the alloying of two malleable metals would produce a malleable alloy, and while in many cases this undoubtedly is so, there are others in which the opposite is the fact. Thus lead added to gold in very small quantity makes the gold exceedingly brittle and weak.

The specific gravity of an alloy nearly always differs from the mean specific gravities of the constituents, sometimes being greater and sometimes less. When the density is increased it shows that contraction has occurred, and chemical combination has probably taken place between the components. This is the case with bronze rich in copper, while with similar alloys rich in tin expansion occurs, the specific gravity being less than the mean of the specific gravities of the two metals. One of the greatest difficulties connected with the subject of alloying is the tendency of the constituents to separate on cooling according to their specific gravities. As a rule, it is more difficult to alloy three or four metals than two metals, especially when the components differ widely in fusibility, unless the combination forms a true chemical compound. The mixture is promoted by constant agitation when the body is in the liquid condition, and by pouring the

metal into the mould at the lowest possible temperature consistent with the proper degree of liquidity.

Most metals are capable, to some extent, of existing in a state of chemical combination with each other, but, as a general rule, they are united by feeble affinities, for it is necessary, in order to produce energetic union, that the constituents should exhibit great dissimilarity in properties. It is probable that the metals do unite in definite proportions, but it is difficult to obtain these compounds in a separate condition, since they dissolve in all proportions in the melted metals, and do not generally differ so widely in their melting points from the metals they may be mixed with, as to be separated by crystallisation in a definite condition. For these reasons it has been questioned whether alloys are true chemical compounds. Definite compounds do, however, exist in definite proportions by weight in the native as well as the artificial state. Such is the case with mercury and silver, which are found crystallised together in the proportion of one atom of silver to two or three atoms of mercury. A good illustration of chemical combination between two metals is seen in the alloy of copper and tin, which may be represented by the formula SnCu,, containing 38.4 parts of tin and 616 parts of copper. It is distinguished by its peculiar colour, its homogeneity after repeated fusions, its brittleness, and by having a greater density than any other alloy of these metals.

As a general rule, it may be stated that all metals which unite with oxygen to form only bases, have a strong tendency to unite with metals some of whose oxides possess an acid character; and those metals which are allied in regard to basicity or acidity, have little affinity for each other. Thus sodium and potassium, although miscible in various proportions, exhibit little or no tendency to unite in definite quantities; and the same is true of antimony and tin. On the other hand, copper and tin form very stable alloys. In some instances, as in the case of lead and zinc, only very unequal portions can be made to unite, the main bulk of the