It is their weight, lustre, malleability, conductivity, tenacity, ductility, fusibility, power of solidifying, hardness, and softness that make the metals so extremely useful. A short account of their properties is given below.

1 All alloys vary within certain limits, but the above may be taken as typical.

Weight

-It is not necessary to explain what is meant by the weight of a metal. But a useful figure in connexion with calculation of weights is the"Specific Gravity"or"Relative Density". This figure expresses the number of times any given volume of metal is heavier than an equal volume of water. For the exact conditions under which the comparison is made the reader is referred to a textbook of physics. Suffice it here to point out that when we say the relative density of iron or lead is 7.4 or 11.3 we mean iron is 7.4 and lead 11.3 times as heavy as water. An example is given of the use of this figure in calculating weight. It must be remembered that a cubic foot of water weighs 1000 oz. or 62 1/2 lb. (nearly). An iron plate 6 x 4 ft. x 1 in. has in it 72 x 48 x 1 cub. in., i.e.72 x 48 x1 /1728 = 2 cub. ft. This would weigh 2000 oz. or 125 lb.

if it were water, and therefore since it is iron 125 x 7.4 = 925 lb.

If it had been lead the weight would have been 125 x 11.3. The same plate of cast steel would have weighed 125 x 7.8. See table on p. 125.

Lustre

The power of reflecting the rays of light is possessed in a much higher degree by metals than by non-metallic substances. The metals and alloys which exhibit the greatest lustre for the longest time are those which are not quickly attacked by the oxygen and carbonic acid in the air and are sufficiently hard to receive a high polish. Gold and platinum have considerable lustre, because though not very hard they are not affected by the constituents of the atmosphere. Metals when highly polished are less easily tarnished because gases have then less tendency to condense on their surfaces.

Malleability is the property of being able to be beaten out by hammering, or the possessing of an internal mobility by which the shape may be altered by pressure without cracking or breaking. The malleability of a metal is affected by the temperature of the metal at the time of the hammering or working, also by the structure of the metal itself, or what is termed its"molecular condition". For example, a piece of gold or copper which has been hammered a great deal becomes hard or brittle, and if the hammering is continued it will eventually crack. But it will, however, regain its malleability on being heated to a certain degree; this heating is called annealing. The method of cooling is also important, as copper may be cooled from a red heat by being plunged into water, but steel by this method would become very hard, and to be softened steel must be heated to a red heat and then cooled very gradually. The crystalline or noncrystalline structure of the metal is also a factor to be considered. When crystalline a metal is not very malleable, but all malleable metals become hardened by hammering or working, so that they have to be annealed during the process of manufacture. A fibrous metal is generally very malleable and tenacious.

Some metals undergo changes of structure in process of time. Brass wire becomes brittle when kept in a state of tension, or in a damp storeroom; wrought-iron chains used in carrying loads have occasionally to be annealed to restore their malleability, as by the constant strain they become crystalline. Gold when hammered out into the form known as gold leaf is 1/280000 part of an inch in thickness, and is semi-transparent, showing a green colour by transmitted light.

Conductivity is a power that metals possess of conducting heat and electricity. Metals that are the best conductors of heat are also the best conductors of electricity, and in both cases the conductivity is seriously impaired by the presence of even small quantities of other metals or impurities.

Tenacity is the property which enables a metal to withstand rupture by pulling. The influence of impurities in metals upon their tenacity is very variable. The enormous increase in tenacity produced by combining a small proportion of carbon with iron is well known, and may be contrasted with the relatively greater reduction in strength caused by a trace of bismuth in gold.

Ductility is the property of being able to be drawn out into fine wire or tubing. The ductility of metals is not in the exact ratio of their malleability; thus iron is very ductile and can be drawn into very fine wire, but it cannot, except in a very pure state, be rolled into such thin sheets as either copper or tin.

Fusibility is the property of becoming liquid when heated. Although the property of becoming liquid at high temperatures is not confined to the metals, it is one of those qualities which contribute very largely to their utility, for it enables the founder to produce a large number of objects from a given pattern, with only a small expenditure of time and labour. It also offers to the worker in metals a ready means of joining together in a durable manner the separate pieces of his work.