Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/75

Rh METALS 65 with the notion of softness, which means the degree of facility with which the plasticity of a metal can be dis counted. Thus lead is far softer than silver, and yet the latter is by far the more plastic of the two. The now famous experiments of Tresca (Comptes Rendus, lix. 754) show that the plasticity of certain metals at least goes considerably farther than had before been supposed. He operated with lead, copper, silver, iron, and some other metals. Hound disks made of these substances were placed in a closely fitting cylindrical cavity drilled in a block of steel, the cavity having a circular aperture of two or four centimetres below. By means of an hydraulic press, applied to a superimposed piston, a pressure of 100,000 kilos was made to act upon the disks, when the metal was seen to &quot; flow &quot; out of the hole like a viscid liquid. In spite of the immense rearrangement of j arts there was no breach of continuity. What came out below was a compact cylinder with a rounded bottom, consisting of so many layers superimposed upon one another. Parallel experiments with layers of dough or sand plus some connecting material proved that the particles in all cases moved along the same tracks as would be followed by a flowing cylinder of liquid. Of the better known metals potassium and sodium are the softest ; they can be kneaded between the fingers like wax. After these follow first thallium and then lead, the latter being the softest of the metals used in the arts. Among these the softness decreases in about the following order : lead, pure silver, pure gold, tin, copper, aluminium, platinum, pure iron. As liquidity might be looked upon as the ne plus ultra of softness, this is the right place for stating that, while most metals, when heated up to their melting points, pass pretty abruptly from the solid to the liquid state, platinum and iron first assume, and throughout a long range of temperatures retain, a condition of viscous semi-solidity which enables two pieces of them to be &quot; welded &quot; together by pressure into one continuous mass. Potassium and sodium might probably be welded if their surfaces could be kept clear of oxide. 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: Hammering. Rolling into Sheet. Drawing into Wire. Lead. Gold. Platinum. Tin. Silver. Silver. Gold. Copper. Iron. Zinc. Tin. Copper. Silver. Lead Gold. Copper. Zinc. Zinc. Platinum. Platinum. Tin. Iron. Iron. Lead. 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 thick ness of -5-5^0&quot; mm -j an d that platinum, by judicious work, can be drawn into wire o o o0 mm. thick. By the hardness of a metal we mean the resistance which it offers to the file or to the 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 per cent, aluminium bronze is scratched by an edge-tool made of ordinary steel as used for knife-blades. And yet it has been found that the sets of needles used for perforating postage stamps last longer if made of aluminium bronze than they do 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 &quot;limit of elasticity&quot; result iu permanent deformation or (if suffi ciently great) in rupture. Where this limit lies is in no case pre cisely known. According to Wertheiin 1 (who has done more for our knowledge of the subject than any one else) and Hodgkinson, 1 Annales de Chimic et de Physique fiii.], vol. xii. the real law seems to be pretty much as indicated by the two curves on the accompanying diagram, where, in reference to a metallic wire, stretched by an appended weight, the abscissa always means the numerical value P of the weight, the ordinate of the upper curve the total elonga tion caused by P, theordinatcof the lower curve that part of the elong ation which re mains when P is removed, so that the piece of the ordinate between the two curves gives the tempor ary (&quot;elastic&quot;) ex pansion. From P-0 up somewhat to a indefi nite point (a or A) &quot; both curves are nearly straight lines, the lower almost coinciding in its beginning with the axis of abscissas ; from that point onwards these two curves approach each other, and at a short distance from the point of rupture they rapidly converge towards intersection. For any value of P which lies fairly on the safe side of A, we have ap proximately IP x= 7 e where X means the elastic (or substantially the total) expansion, / the length, and q the square section of the wire or cylindrical bar operated upon. The reciprocal of 6 (viz. E = l/f) is called the &quot; modulus of elasticity.&quot; Wertheim has determined this constant for a large number of metals and alloys. He used three methods : one was to measure the elongations produced, in a wire of given dimensions, by a succession of charges ; the other two consisted in causing a measured bar to give off a musical note by (a) longitudinal and (b) transversal vibra tion, and counting the vibrations per second. The following table gives some of his results. Column 2 gives the constant E 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 05 mm. per metre, which, for practical purposes, he takes as giving the limit of elasticity ; column 4 gives the breaking strain. Values of E in square brackets [ ] are derived from vibration experiments; the rest from direct measurements of elongations. Numbers in round brackets do not necessarily refer to the same specimen as the other data. Name E. For Wire of 1 Square mm. Section, Weight (in Kilos) causing Permanent Elongation f ?&amp;lt;i^nn- Breakage. 1,803 1,727 [3,923] [4,000] . [5,757] [4,777] 8,132 5 585 0-25 0-20 0-45 0-20 13-5 3-0 11-3 2-6 0-75 1-00 18 under 5 12 under 3 (2G) (14) 32 under 5 2-1 1-8 (2-45) 2-24 27 10 29 16 13 27 40 30 34 23 61 47
 * . &amp;lt;:

2x61 7,358 7,141 9,021 8,735 [9,467] 11,759 9,789 12,449 10,519 17,004 15,518 15,987 15,622 20,869 20,794 7,040 10,700 8,543 10,788 Platininn wire, medium thickness, ) ., annealed ,, annealed Cobalt 3 i um bronze* R - l s5 German silver 6 The above numbers may be assumed to hold for temperatures from 15 to 20 C. Wertheim executed determinations also at other tem peratures; but, as his numbers do not appear to reveal the true 2 From Du Brery. 3 Approximate, by H. St Clair Deville. From deflexion of hammered bar of 5 mm. thickne.-s, charged in the middle, determined by W. IWtmar. 5 Composition, 7.nCu 2 (Wertheim). 6 Composition, Zn 4 CU|jXI 5 (Wertheim). XVI. 9