Metals

Many metals have an appreciable vapour pressure at tempera tures considerably below their boiling points—a property which is made use of in certain industrial processes. Thus it is possible to coat iron objects with zinc by placing them in a mixture of zinc dust and zinc oxide and heating to a temperature below the melt ing point of zinc. The zinc condenses onto the iron.

Thermal Expansion.

As the temperature of a metal is raised the metal expands. This expansion with temperature has been used for many purposes; for example, the tyres of locomotives are sometimes shrunk on to the wheels, by making the tyre slightly smaller than the wheel and expanding it by heat till it can just be forced on to the wheel. On cooling it contracts and grips firmly. In the ordinary mercury thermometer use is made of the expansion of a metal with temperature.

On the other hand, a metal with a zero coefficient of expansion would be invaluable for the construction of apparatus which must keep accurately to size, such as standards of length, pendulum rods, etc. ; although no pure metal approaches this condition, an alloy of iron and nickel (known as Invar) has an extremely low coefficient of expansion.

Thermal and

Electrical Conductivity.—All metals are more or less good conductors of heat and of electricity and in the case of pure metals these properties are closely related. There is little doubt that it is impossible to raise the electrical conductivity of a metal by alloying it and consequently all the metals used for conductors in industry are as pure as is possible consistent with price and strength. The best conductor is silver, closely followed by the cheaper and much used copper, the resistances respectively being 1.62 and 1.69 microhms per centimetre cube. In a few cases, such as trolley wires for electric trams and trains, a small amount of cadmium is added to the copper, as the increase in tensile strength thus gained more than compensates for the lower ing of the conductivity. Next to copper, aluminium is most widely used as a conductor ; its resistivity is considerably greater, being about 2.6 microhms per cu.cm., but its low density renders it preferable in certain cases.

The resistivity of metals falls as the temperature is lowered and in the neighbourhood of absolute zero it drops enormously.

It has been shown that lead at the temperature of liquid helium has a resistance only i/r,000,000,000 of that at o° C. If a cur rent is started in a ring of lead at this temperature it will continue for many hours with but a small decrease in intensity. The resis tivity of lead at ordinary temperatures is 22 microhms per cu.cm. (this is the highest value obtained in the more common metals), but by alloying various metals together it is possible to produce material with resistivities very many times this value. (Values for the resistivities of most of the metals are given in the Table, Col. 6.) Magnetic Properties.—The vast majority of metals are prac tically non-magnetic ; indeed it requires very serrsitive apparatus to discover that they have any magnetic properties at all. A few, however, which are known as the ferro-magnetic elements, are strongly magnetic ; these are iron, nickel and cobalt. It is, how ever, a remarkable fact that certain mixtures of the non-magnetic metals copper, aluminium and manganese are also magnetic.

Crystalline Habit.

Most metals crystallize in what is known as the cubic system (see CRYSTALLOGRAPHY). This means that the atoms, which may be regarded as the bricks out of which the metal is constructed, are so arranged as to be built up into cubes. There are three varieties of this arrangement known as the simple cube, the body-centred cube and the face-centred cube. In the first case the atoms are arranged so as to occupy the corners of a cube ; in the second case there is, in addition, one atom occupying the middle of each cube; whilst the face-centred variety has, in addition to the atom at the corners of the cube, an atom in the middle of each cube face.

No metals are known to crystallize in the simple cubic form, but the majority form either body-centred or face-centred cubes. Several of the metals crystallize in the hexagonal system, which is somewhat more complicated than the cubic, whilst a few assume a tetragonal arrangement which is still more complex. If the metals are arranged in their correct place in the periodic classi fication (q.v.) it will be seen that those in the same sub-group crystallize in the same habit.