The systems in which metals crystallize are given in the Table, Col. 7. It will be noticed that some of the metals are shown as crystallizing in more than one form, so that they are spoken of as having different allotropic modifications. This means that the metal can exist in two or more forms which may differ from each other in their mechanical or physical properties as much as two completely separate metals differ (see ALLOTROPY). Allotropic changes in metals may not be quite as pronounced as with the non-metals, but they are often very marked and are accompanied by a change in the crystalline arrangement. Thus pure iron below 900°C is a highly magnetic material which crystallizes in the face centred cubic form, whilst above 900°C it is non-magnetic and the atoms are arranged in a body-centred cubic "lattice." Another interesting example is to be found in tin which at ordinary tem peratures is a soft ductile metal ; at low temperatures, however, it changes into a grey material which breaks into a powder.
Mechanical Properties.—Among the most remarkable prop erties of metals is their power of resisting deformation of various kinds. This is studied under the heading of elasticity, tensile strength, hardness, ductility, etc. In most cases these properties are found to differ considerably between a single crystal of a metal and an aggregate of crystals of the same metal, showing that the crystal boundaries have an appreciable influence.
The hardness of metals—not easy to define—is usually meas ured either by the pressure required to force a ball or other ob ject to a definite distance into the metal, or by the depth of scratch made in the metal by a diamond point under a given load.
Of the ordinary metals of commerce lead is the softest although lithium is the softest metal of all, whereas possibly manganese is the hardest metal. Chromium was long considered to head the list, but it is now known that this hardness is mainly due to the presence of hydrogen.
Closely related to hardness is the property known as tensile strength. This is measured by finding the weight required to break, by means of a straight pull, a rod of metal of known diameter. Probably the strongest pure metal is tungsten, which as drawn wire has a tensile strength of over 3ookg. per sq.mm.
Many metals possess the property of ductility to a high degree, i.e., when subjected to a tensile load they elongate to a very considerable extent before breaking. Use is made of this property to produce wires, the metal being first rolled into thin rods which are then drawn through holes in a steel plate, known as a die. Each hole through which the wire is drawn reduces its diameter. For very thin wires the dies are made of diamond.
Several metals can also be "extruded," i.e., forced to flow through a hole by means of pressure or they can be rolled or hammered into thin sheets. Gold has been beaten into sheet only i/3,800mm. thick, whilst, by a very special process platinum has been reduced to a wire i/2o,000min. in diameter.