Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/77

Rh METALS 67 4. Barely so : gold, (copper). 5. Practically non-volatile : (copper), iron, nickel, cobalt, alu minium; also lithium, barium, strontium, and calcium. In the oxyhydrogen flame silver boils, forming a blue vapour, while platinum volatilizes slowly, and osmium, though infusible, very readily. Latent Heats of Liquefaction Qf these we know little. The fol lowing numbers are due to Person ice, it may be stated, being 80. Metal. Latent Heat. Metal. Latent Heat. 2-82 Cadmium 13-6 Lead 5-37 21-1 12-4 ( Zinc 28 1 Of the latent heats of vaporization only that of mercury has been determined, by Marignac, who found it to be 103 to 106 units. Conductivity. Conductivity, whether thermic or electric, is very differently developed in different metals ; and, as an exact know ledge of these conductivities is of great scientific and practical importance, much attention has been given to their numerical determination. The following are the modes in which the two conductivities have been defined as quantities. 1. Thermic. Imagine one side (1) of a metallic plate, D units thick, to be kept at the constant temperature t lt the other (II) at ? 2 . After a sufficient time each point between I and II will be at a constant intermediate temperature, and in every unit of time a constant quantity Q of heat will pass from any circumscribed area S on I to the opposite area S on II, according to the equation I is called the (internal) conductivity of the metal the plate is made of. It is, strictly speaking, a function of ^ and &amp;lt; 2 ; but within a given small interval of temperatures it may be taken as a constant. 2. Electric. When a given constant battery is closed successively by different wires of the same sort, then, according to experience, the strength I of the current (as measured for instance by the heat- equivalent of the electricity flowing through the circuit in unit of time) is in accordance with the equation A/i-i+nls , where I is the length and s the square section of the wire, while A is a constant which, for our purpose, need not be denned in regard to its physical meaning; r measures the specific resistance of the particular metal. Supposing a certain silver wire on the one hand and a certain copper wire on the other, when substituted for each other, to produce currents of the same strength, we have r l ! l /s l -r 2 r 2 /s 2 , whence r 1 /r 2 =* 1 / 2 /( 2 / 1 )=* where k is the computed value of the ratio on the right-hand side. Hence, taking r 2, the resistance of copper, as unity, we have r 1 = k, i.e., k gives us the specific resistance of silver, that of copper being taken = 1. In this relative manner resistances are usually measured, silver generally being taken as the standard of compari son. Supposing the relative resistance of a metal to be R, the re ciprocal 1/R is called its &quot;electric conductivity.&quot; For the same metal R varies with the temperature, the higher temperature cor responding to the higher resistance. The following table gives the electric conductivities of a number of metals as determined by Matthiesen, and the relative internal thermic conductivities of (nominally) the same metals as determined by Wiedemann and Franz, with rods about 5 mm. thick, of which one end was kept at 100 C., the rest of the rod in a &quot; vacuum&quot; (of 5 mm. tension) at 12 C. Matthiesen s results, except in the two cases noted, are from his memoir in Pogg. Ann., 1858, ciii. 428. Metals. Relative Conductivities. Electric. Thermic. Copper, commercial, No. 3 774 at 18-8 721 22-6 93 552 21-8 73 &amp;gt; 19 115 21-0 144 204 0777 17-3 105 ,, 20-7 74S 54S 25 154 101 103 079 094 073 1-000 No. 2 chemically pure, hard drawn Copper Gold, pure ., absolutely pure Brass Tin, pure. Pianoforte wire Iron rod Steel Lead, pure Platinum German silver 0767 18-7 Bismuth Oil!) ,, 13-8 196 ,, 19-6 01G3 22-8 Aluminium Mercuiy Silver, pure 1-000 1 Published in 1860, and declared by Matthiesen to be more exact than the old numbers. Going by Matthiesen s old numbers, we find them to n^ree fairly with Wiedemann and Franz s thermic conductivities, which supports an ^bvious and pretty generally received proposition. Matthie sen s new numbers for gold and copper, however, destroy the har mony. Magnetic Properties. Iron, nickel, and cobalt are the only metals which are attracted by the magnet and can become magnet s themselves. But in regard to their power of retaining their mag netism none of them comes at all up to the compound metal steel See MAGNETISM. Chemical Changes. The chemical changes which metals are liable to may be classified according to the loss of metallicity involved in them. We will adopt this principle and begin with the action of metals on metals, which, as experience shows, always leads to the formation of truly metallic compounds. Any two or more metals when mixed together in the liquid state unite chemically, or at least molecularly, in this sense that, although the mixture, on standing (hot), may separate into layers, each layer is a homogeneous solution or &quot;alloy&quot; of, in general, all the components in one another. With binary combinations the following two cases may present themselves : (1) the two metals mix permanently in any proportion ; or (2) either of the two metals refuses to take up more than a certain limit-proportion of the other ; hence a random mixture of the two metals will, in general, part into two layers, one a solution of A in B, the other a solution of B in A. The first case presents itself very frequently; it holds, for instance, for gold and silver, gold and copper, copper and silver, lead and tin, and any alloy of these two and bismuth. Many other cases might be quoted. A good example of the second case is lead and zinc, either of which dissolves only a very small percentage of the other. In the preparation of an alloy we need not start with the components in the liquid state; the several metals need only be heated together in the same crucible when, in general, the liquid of the more readily fusible part dissolves the more refractory compo nents at temperatures far below their fusing points. Molten lead, for instance, as many a tyro in chemical analysis has come to learn to his cost, readily runs through a platinum crucible at little more than its own fusing point. A homogeneous liquid alloy, when solidified suddenly, say by pouring it drop by drop into cold water, necessarily yields an equally homogeneous solid. But it may not be so when it is allowed to freeze gradually. If, in this case, we allow the process to go a certain way, and then pour off the still liquid portion, the frozen part generally presents itself in the shape of more or less distinct crystals; whether this happens or not, the rule is that its composition differs from that of the mother liquor, and consequently from that of the original alloy. This phenomenon of &quot; liquation,&quot; as it is called, is occasionally utilized in metallurgy for the approximate separation of metals from one another ; - but in the manipulation of alloys made to be used as such it may prove very inconvenient. It does so, for instance, in the case of the copper-silver alloy which our coins are made of ; in a large ingot of such sterling silver the core may contain as much as 3 per cent, of silver more than the outer shell. The existence of crystallized alloys, as the phenomenon of liquation generally, strongly suggests the idea that alloys generally are mixtures, not of their elementary com ponents, but of chemical compounds of these elements with one another, associated possibly with uncombined remnants of these. This notion is strongly supported by the fact that the formation of many alloys involves an obvious evolution of heat and a decided modification in what one would presume to be the properties of the corresponding - A good illustration is afforded by the process of Pattinsou as used for concentrating the silver in argentiferous lead. See LKAD.