Metals

METALS. It is extremely difficult to find a satisfactory definition for a metal but it is probably best specified as a body which possesses many of the following properties : it is solid at ordinary temperatures; it is opaque; when polished it is a good reflector of light; it is a good, or fairly good conductor of heat and electricity; when melted and allowed to cool, it solidifies as a compact mass of crystals. These crystals are generally invisible as such to the unaided eye, but in suitably prepared specimens can be seen under the microscope (see METALLOGRAPHY). It is by no means true to say that every metal possesses all these properties, while many of them, e.g., opacity and crystalline form, are shared with some of the non-metals, and mercury is a liquid.

Metals may be divided into two classes : elemental metals and alloys. The former are chemical elements (q.v.), whilst the latter are made by mixing intimately (generally by melting together) two or more elemental metals. (See ALLOYS.) Metals are only rarely found native in the earth; platinum and gold are usually, and silver is frequently found thus, whilst copper and iron occasionally occur in the native state. The majority of metals, however, are found in the form of ores, which are chemical compounds of the metal with one or more elements such as oxygen, sulphur and carbon. These compounds are generally mixed with other minerals known to the miner as gangue (see MINING). After separating the ore from the gangue, the metal has to be obtained by breaking up the compound, either by chemical means or, more usually, by heat. Examples of the processes used will be found in the articles dealing with the individual metals.

Density.

The density of a metal is usually expressed in terms of the weight of water. Thus if a metal is three times as heavy as an equal volume of water, it is said to have a density of 3. The heaviest known materials are metals, but on the other hand some metals exist which are among the lightest of solid materials. The densest material is osmium which is 22.5 times heavier than water, whilst at the other end of the scale is lithium which is little more than half the weight of water. Among the

more common metals attention may be drawn to aluminium and magnesium which have densities of 2.7 and 1.7 respectively, whilst lead, mercury, gold and platinum have densities of 11.3, 13.5, 19.3 and 21.5. (Col. 3 of the Table gives the densities of most of the known metals.) The property of density is often used in the commercial world. Thus the high density of mercury (which is the only metal that is liquid at ordinary temperatures) renders possible the mercury barometer. The high density of lead is useful when heavy weights are required. On the other hand the low density of aluminium has given it a considerable value both in the pure state and alloyed with other metals. Its uses range from the construction of airships to manufacture of frying pans.

vast majority of metals are of a greyish colour, varying from the blue grey of lead to the so-called white colour of silver. There are, however, certain exceptions to this, such as gold, which is yellow, and copper, which is reddish in appearance.

The colour of a metal can be seen much better if it is viewed by means of light which has been reflected many times off its surface. Thus, the inside of a smooth gold vase appears much richer in colour than the outside.

Melting and Boiling Points.

Pure metals, being chemical elements, melt at a constant temperature. Mercury is liquid at ordinary climatic temperatures, though arctic explorers have to use spirit thermometers, as the temperature to which they may be exposed is often below that at which mercury freezes (-38..9° C). At the other end of the range we have the metal tungsten, which does not fuse until it reaches a temperature of about 3,380° C. (The temperature of the surface of the sun is calculated to be about 6,000° C.) For this reason tungsten is used in the form of fine wires for the filaments of incandescent electric lamps (Col. 5 of the Table gives the melting points of metals.) All metals will boil if the temperature is high enough. Mercury boils at a temperature of C, not much above the melting point of lead, whilst tungsten is said to boil at 5,900° C.