Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/78

Rh 68 METALS mixture. The case of sodium amalgam may be quoted as a forcible illustration. What goes by this name in laboratories is an alloy of two to three parts of sodium with one hundred parts of mercury, which is easily produced by forcing the two components into contact with each other by means of a mortar and pestle, when they unite, with deflagration, into an alloy which after cooling assumes the form of a grey, hard, brittle solid, although mercury is a liquid, and sodium, though a solid, is softer than wax. Similar evidence of chemical action we have in the cases of brass (copper and zinc), bronze (copper and tin), aluminium bronze (copper and aluminium), and in many others that might be quoted. There are indeed a good many alloys the formation of which is not accompanied by any obvious evolution of heat or any very marked change in the mean properties of the components. But in the absence of all precise thermic researches on the subject we are not in a position to assert the absence of chemical action in any case. Indeed our knowledge of the proximate composition of alloys is in the highest degree indefinite we do not even know of a single composite metal which has been really proved to be an unitary compound, and hence the important problem of the relation in alloys between pro perties and composition must be attacked on a purely empirical basis. What has been done in this direction is shortly summarized in the following paragraphs. Colour. Most metals arc white or grey ; so are the alloys of these metals with one another. Gold alloys generally exhibit some thing like the shade of yellow which one would expect from their composition ; its amalgams, however, are all white, not yellow. Copper shows little tendency to impart its characteristic red colour to its alloys with white or grey metals. Thus, for instance, the silver alloy up to about 30 per cent, of copper exhibits an almost pure white colour. The alloys of copper with zinc (brass) or tin (bronze) are reddish-yellow when the copper predominates largely. As the proportion of white metal increases, the colour passes succes sively into dark yellow, pale yellow, and ultimately into white. Aluminium bronze, containing from 5 to 10 per cent, of aluminium, is golden-yellow. Plasticity. This quality is most highly developed in certain pure metals, notably in gold, platinum, silver, and copper. Of platinum alloys little is known. The other three, on uniting with one another, substantially retain their plasticities, but the addition of any metal outside the group leads to deterioration. Thus, for instance, according to Karsten, copper, by being alloyed with as little asO 6 percent, of zinc, loses its capability of being forged at a red heat ; it cracks under the hammer. Antimony or arsenic to the extent of 15 per cent, renders it unfit for being rolled into thin sheet or drawn out into fine wire, and makes it brittle in the heat ; O l per cent, of lead prohibits its conversion into leaf. Hardness, Elasticity, Tensile, Strength. In reference to these qualities, we shall confine ourselves to some very striking changes for the better which the metals (1) gold, (2) silver, (3) copper suffer when alloyed with moderate proportions (10 per cent, or so) of (1) cop per, (2) copper, (3) tin, zinc, or aluminium respectively. Any of these five combinations leads to a considerable increase in the three qualities named, although these are by no means highly developed in the added metals ; most strikingly it does so in the case of alumi nium bronze (copper and aluminium), which is so hard as to be very difficult to file, and is said to be equal in tensile strength to wrought iron. To illustrate this we give in the following table, after Matthiesen, the breaking strains of double wires, No. 23 gauge, in ft avoirdupois, for certain alloys on the one hand and their components on the other. Separate Metals. Alloys. iS^r. . . . . . . . . . &quot;&quot;. . .Vessthan 3 ? } Gun meta1 12 P er cent - of t!n ........ o^ [ standard &amp;lt; 22 carat ) g id ................ 7 - 11 y&amp;gt; * of silver * f p^num ....75- Specific Gravity. This subject has been extensively investigated by Matthiesen, Calvert and Johnson, Kuppfer, and others. In discussing the results it is convenient to compare the values (S) found with the values (S ) calculated on the assumption that the volume of the alloy is equal to the sum of the volumes of the com ponents. Let p lt p%, p 3 ... stand for the relative weights of the com ponents, P for their joint weight, S^S.^Sg...^ their specific gravities, and we have where the expression on the right hand obviously means the con joint volume V of the components ; but the actual volume of the alloy formed by their union is, in general, V = V (l+e), where e means the expansion (or, when negative, the contraction) of unit- volume of mixture. Hence the real value S=S /(l+e) , whence e=(S-S)/S. Matthiesen s investigation (Pogg. Annalcn for 1860, vol. ex. p. 21) extends over a large number of binary alloys derived from the metals named in the following table. He naturally began by procuring pure specimens of these metals and determining their specific gravities. The results (each the mean of a number of determina tions) were as follows : Name. Specific Gravity S at t C. t Adopted Atomic Weight. Antimony 6-713 14 3 122-3 Tin 7 294 12 8 118 Cadmium 8-655 10-5 112 Bismuth 9-823 12-3 208 Silver 10-4G8 13-2 108 Lead 11-376 13-5 207-4 Mercury 13-573 14-5 200 Gold 19-265 12-8 197 In these, as in all the subsequent determinations for the alloys, the weighings were reduced to the vacuum, and the values for S referred to water at 4 C. as unity. From eight metals tventy-eight different kinds of binary alloys can be produced; of these twenty- eight combinations eighteen were selected; in each case the two com ponents were fused together in a variety of properly chosen atomic proportions, and the specific gravities of these alloys were determined. The net results are summarized in the following table, which, for each combination A, B, in the first two columns gives the com position in multiplies of the &quot;atomic-weights&quot; given in the table just quoted, while column 3 gives the values of e as calculated by the writer from Matthiesen s numbers for S and S. Hence, for example, in the accompanying entries the first line shows that the union into an alloy of twice 118 parts of tin and once 197 parts of gold in volves an expansion from 1 volume into 1 004 ; the second that the union of once 118 parts of tin with four times 197 parts of gold involves a contraction from 1 volume into 1 - 028. Tin and Gold. Sn A e 2 1 + 004 1 4 -028 Antimony and Tin. Antimony, Bismuth. Antimony, Lead. SI) Sn e Sb Bi

Sb Pb e 12 to 8 4-2 1 1 1 1 1 1 to 2 3 to 10 20 to 100 + 002 + 006 + 008 + 005

2 Itol2

2 1 2 3 5-25 1 1 1 1 1 + 008 + 006

+ 0067

7 in, Cadmium. Tin, Bismuth. Tin, Silver. Sn Cd e Sn Bi e Sn Ag e 6 4 2 2 1 1 1 to 8 12 + 004 + 005

-001 22 4 3-1 1 1 1 1 1 2 4 to 60

-002 -005 -005

18 9 6 3 2 1 1 1 1 1 1 1 1 1 2 4 -002 -006 -008 -013 -019 024 -047 038 Tin, Gold. Tin, Lead. Cadmium, Bismuth. Sn Au e Sn Pb e Cd Bi e 50 15 6 1 1 1 1 2 1 2 4

-002 + 002 + 004 + 008 + 012 -015 -028 C 4 2 1 1 1 1 1 1 1 2-4 6 + 003 + 002

+ 0015 + 005 + 004 3 1-36

4-2-5 2 3 1 1 1 Cadmium, Lead. Cd Pb e 6 1-36 to 0025 Bismuth, Silver. Bismuth, Gold. Lead, Gold. Bi Ag

Bi Au e Pb Au e 200-2 1 1 1 M 2 4 Oto +002 -003 -006 -007 90 40 20 8 4 2 1 1 1 1 1 1 1 1 1 2

-003 009 -017 -035 -039 -026 10 5 4 3 2 1 1 1 1 1 1 1 1 1 2 4 -004 -009 -008 -009 -016 -018 -004 -Oil