It has been customary to divide the elements into two classes, the metals and the non-metals. As we have seen, this classification is a completely arbitrary one; for there are some elements capable of existing in both states. The name "metal" was originally given to seven substances, all alike in possessing that bright lustre known as "metallic." These were gold, silver, mercury, copper, iron, lead, and tin. But in the Middle Ages bismuth and antimony were isolated in a fairly pure state, and these, together with zinc, were at first not received into the class, but were regarded as spurious ; for they were brittle and easily oxidisable. Although there is no reason for retaining the division, yet it is often convenient. Bodies which possess metallic lustre have the power of conducting electricity better than transparent bodies, and they are also relatively good conductors of heat.

The elements exist in various physical states. Those which are gases at the ordinary temperature, however, have all been condensed to the liquid state by sufficient reduction of temperature. The lowering of temperature is most easily produced by means of liquid air, now a cheap commodity. To liquefy air, it is compressed by a pump to a pressure of 150 atmospheres; it then traverses a coil of copper pipe, and escapes from an orifice at the lower end. Now, compressed air has some resemblance to a liquid, for when it expands, as when a liquid changes to gas, heat is absorbed. The rapidly escaping air becomes cold, and in passing up over the coil of tube through which it has descended, it cools the pipe, so that the air passing down becomes colder and colder ; finally, it is so cooled that it liquefies, and escapes from, the orifice in a liquid state. It may be poured from one vessel to another, with little loss by evaporation ; and if other gases be allowed to stream into a tube cooled by its aid, they too are liquefied. The principle of liquefying hydrogen is the same, for its boiling-point lies so low that it cannot be liquefied by the aid of liquid air. That of helium is still lower ; it has never been liquefied.

The elements which are gases at the ordinary temperature are hydrogen, helium, neon, argon, krypton, xenon, nitrogen, oxygen and ozone, fluorine, and chlorine. The first seven are colourless, both in the gaseous and the liquid state. Oxygen is a colourless gas, but forms a pale blue liquid; gaseous ozone has a blue colour; fluorine is pale yellow; and chlorine has a greenish-yellow colour. It forms a white solid, which, however, melts to a bright green liquid. Bromine is a dark red liquid at atmospheric temperature, but above its boiling-point, 59°, it is a deep red gas. Iodine is a blue-black solid, melting to a black liquid at 114% and giving off a violet vapour. Ozone and the " halogens," as fluorine, chlorine, bromine, and iodine are called, have all a powerful odour, and act on the skin in a corrosive manner. Chlorine and bromine are soluble in water.

Among the other non-metallic elements are boron, a black, dusty, infusible powder ; carbon, in its ordinary form an amorphous non-crystalline) black substance, of which the most familiar variety is charcoal; carbon does not fuse, but at the enormously high temperature of the electric arc it volatilises ; silicon, a blackish-brown powder, melting at bright redness to a lustrous liquid, which solidifies in shining white metallic lumps ; phosphorus, a waxy, pale yellow solid, melting at 44.40; sulphur and selenium, yellow and brown-red solids, the former melting at 1150 to a brown liquid, and boiling at 4460 ; the latter forming a black liquid at 2170, and a black vapour at 6650.

The metals of the alkalies, as they are usually called, lithium, sodium, potassium, rubidium, and caesium, are soft white metals, at once attacked by water, and oxidised readily by air, caesium, indeed, taking fire spontaneously. To protect them from oxidation, they must be kept under rock-oil or ligroin, a compound which contains no oxygen. Of these, caesium has the lowest and lithium the highest melting-point. The metals calcium, strontium, and barium are sometimes named the " metals of the alkaline earths." They are hard white bodies, also like those of the sodium group oxidising readily on exposure to air, and at once attacked by water. Magnesium, zinc, and cadmium are noteworthy, inasmuch as their temperature of ebullition is not so high that it cannot be reached in an ordinary furnace; they can therefore be distilled. Magnesium and zinc are hard and brittle; cadmium is softish, like lead, and of a somewhat greyer tint.

The remaining elements may be classed under the headings, "hard," "soft," "brittle," etc. This implies only their behaviour at ordinary temperatures; at higher or lower temperatures the properties are materially changed. Mercury, for example, below -40% is malleable; lead is brittle.

(a) Malleable metals :-

(1) White, ductile, moderately hard :-beryllium, aluminium, gallium, indium, tin, silver, nickel. Red, copper. Tellow, gold.

(2) Grey-white, ductile, and moderately hard:-iron, manganese, cobalt.

(3 ) Grey-white and soft ; ductile :-thallium, lead; somewhat harder, and fusible only at a very high temperature :- rhodium, ruthenium, palladium, platinum, iridium.

(3) Liquid metal:-mercury.

(c) Brittle metals:-

(1) White, hard:-antimony, bismuth, tellurium, zirconium, didymium (a mixture), osmium, germanium. Less hard, arsenic.

(2) Grey, hard:-lanthanum, cerium, yttrium, uranium. ( 3 ) Grey powders, acquiring metallic lustre under the burnisher :-thorium, niobium, tungsten.

(4) Black powders :-tantalum, titanium. The elements scandium, samarium, gadolinium, and radium have not been prepared.

Although the external properties of the elements do not show any obvious relation to their order in the periodic table (see Part I.), yet it may be generally remarked that the density increases as each column is descended. Among the lightest of the elements are lithium, beryllium, magnesium, and aluminium, at least in the solid state ; whereas osmium, iridium, platinum, and gold are among the heaviest. But much more must be ascertained regarding their properties before a satisfactory comparison can be made.

Another gaseous substance, termed radium "emanation," and which is in all probability an element, has recently been discovered by Messrs. Rutherford and Soddy. It is given off in extremely minute amount when a compound of the newly discovered radium (see p. 198) is heated. It reveals itself by its power to discharge an electroscope. That it belongs to the argon group is almost certain, for it resists the action of the most powerful oxidising agents, as well as that of red hot magnesium or calcium. This mysterious gas gradually loses its discharging power, and what is still more curious, the heated radium regains its property of evolving gas when heated, so that the supply is practically inexhaustible, although the amount given off at any one operation is inconceivably minute.