Page:EB1911 - Volume 18.djvu/218

 Closely related to the structure of metals is their degree of “plasticity” (susceptibility of being constrained into new forms without breach of continuity). This term of course includes as special cases the qualities of “malleability” (capability of being flattened out under the hammer) and “ductility” (capability of being drawn into wire); but these two special qualities do not always go parallel to each other, for this reason amongst others—that ductility in a higher degree than malleability is determined by the tenacity of a metal. Hence tin and lead, though very malleable, are little ductile. The quality of plasticity is developed to very different degrees in different metals, and even in the same species it depends on temperature, and may be modified by mechanical or physical operations.

What we have called plasticity must not be confused with the notion of “softness,” which means the degree of facility with which the plasticity of a metal can be discounted. Thus lead is far softer than silver, and yet the latter is by far the more plastic of the two. The famous experiments of H. E. Tresca show that the plasticity of certain metals at least goes considerably farther than had before been supposed.

According to Prechtl, the ordinary metals, in regard to the degree of facility or perfection with which they can be hammered flat on the anvil, rolled out into sheet, or drawn into wire, form the following descending series:— To give an idea of what can be done in this way, it may be stated that gold can be beaten out to leaf of the thickness of mm.; and that platinum, by judicious work, can be drawn into wire mm. thick.

By the “hardness” of a metal we mean the resistance which it offers to the file or engraver's tool Taking it in this sense, it does not necessarily measure, e.g. the resistance of a metal to abrasion by friction. Thus, for instance, 10% aluminium bronze is scratched by an ordinary steel knife-blade, yet the sets of needles used for perforating postage stamps last longer if made of aluminium bronze than if made of steel.

Elasticity.—All metals are elastic to this extent that a change of form, brought about by stresses not exceeding certain limit values, will disappear on the stress being removed. Strains exceeding the “limit of elasticity” result in permanent deformation or (if, sufficiently great) in rupture. Referring the reader to the article for the theoretical and to the for the practical aspects of this subject, we give here a table of the “modulus of elasticity,” E (column 2), for millimetre and kilogramme. Hence 1000/E is the elongation in millimetres per metre length per kilo. Column 3 shows the charge causing a permanent elongation of 0·05 mm. per metre, which, for practical purposes, Wertheim takes as giving the limit of elasticity; column 4 gives the breaking strain. These values may vary within certain limits for different specimens.

Specific Gravity.—This varies in metals from ·594 (lithium) to 22·48 (osmium), and in one and the same species is a function of temperature and of previous physical and mechanical treatment. It has in general one value for the powdery metal as obtained by reduction of the oxide in hydrogen below the melting point of the metal, another for the metal in the state which it assumes spontaneously on freezing, and this latter value, in general, is modified by hammering, rolling, drawing, &c These mechanical operations do not necessarily add to the density; stamping, it is true, does so necessarily, but rolling or drawing occasionally causes a diminution of the density. Thus, for instance, chemically pure iron in the ingot has the specific gravity 7·844; when it is rolled out into thin sheet, the value falls to 7·6; when drawn into thin wire, to 7·75. The following table gives the specific gravities of many metals. Where special statements are not made, the numbers hold for the ordinary temperature (15° to 17° or 20° C), referred to water of the same temperature as a standard, and to hold for the natural frozen metal.