METALS very high temperatures, precluding the use of transparent vessels. Silver vapour is blue, potassium vapour is green, many others (mercury vapour, for instance) are colourless. The liquid metals, when cooled down sufficiently, some at lower others at higher temperatures, freeze into compact solids, endowed with the (relative) non-transparency and the lustre of their liquids. These frozen metals in general form compact masses consisting of aggregates of crystals belonging to the regular or rhombic or (more rarely) the quadratic system. But in many cases the crystals are so closely packed as to produce an apparent absence of all structure. Compared with non-metallic solids, they in general are good conductors of heat and of electricity. But their most characteristic, though not perhaps their most general, property is that they combine in themselves the apparently incompatible properties of elasticity and rigidity on the one hand and plasticity on the other. To this remarkable combination of properties more than to anything else the ordinary metals owe their wide applica tion in the mechanical arts. In former times a high specific gravity used to be quoted as one of the characters of the genus ; but this no longer holds, since we have come to know of a whole series of metals which float on water. Let us now proceed to see to what degree the mechanical and physical properties of the genus are developed in the several individual metals. Non-Transparency. This, in the case of even the solid metals, is perhaps only a very low degree of transparency. In regard to gold this has been proved to be so ; gold leaf, or thin films of gold produced chemically on glass plates, transmit light with a green colour. On the other hand, those infinitely thin films of silver which can be produced chemically on glass surfaces are absolutely opaque. Very thin films of liquid mercury, according to Melsens, transmit light with a violet-blue colour ; also thin films of copper are said to be translucent. Other metals, so far as we know, have not been more exactly investigated in this direction. Colour. Gold is yellow ; copper is red ; silver, tin, and some others are pure white; the majority exhibit some modification or other of grey. Reflexion of Light. Polished metallic surfaces, like those of other solids, divide any incident ray into two parts, of which one is refracted while the other is reflected, with this difference, however, that the former is completely absorbed, and that the latter, in regard to polarization, is quite differently affected. 1 The degree of absorption is different for different metals. According to Jamin, the remaining intensity, after one and ten successive perpen dicular reflexions respectively from the metal-mirrors named, is as follows (original intensity = 1 ) : Silver. Speculum Metal. Steel. Red 1 R. 929 905 867 10 R. 478 339 242 1 R. 692 632 599 10 R. 035 010 006 1 R. 609 599 599 10 R. 007 006 006 Yellow Violet This shows the great superiority of silver as a reflecting medium, especially in the case of repeated reflexion. Crystalline Form. Most (perhaps all) metals are capable of crystallization, and in most cases isolated crystals can be produced by judiciously managed partial freezing. The crystals belong to the following systems : regular system silver, gold, palladium, mercury, copper, iron, lead; quadratic system tin, potassium ; rhombic system anti mony, bismuth, tellurium, zinc, magnesium. Stricture. Perhaps all metals, in the shape which they assume in freezing, are crystalline, only the degree of 1 This may be the cause of the peculiarity of metallic lustre. visibility of the crystalline arrangement is very different in different metals, and even in the same metal varies according to the slowness of solidification and other circumstances. Of the ordinary metals, antimony, bismuth, and zinc may be mentioned as exhibiting a very distinct crystalline structure : a bar-shaped ingot readily breaks, and the crystal faces are distinctly visible on the fracture. Tin also is crystalline : a thin bar, when bent, " creaks " audibly from the sliding of the crystal faces over one another ; but the bar is not easily broken, and exhibits an apparently non-crystalline fracture. Class I. Gold, silver, copper, lead, aluminium, cadmium, iron (pure), nickel, and cobalt are practically amorphous, the crystals (where they exist) being so closely packed as to produce a virtually homogeneous mass. Class II. The great contrast in apparent structure between cooled ingots of Class I. and of Class II. appears, however, to be owing chiefly to the fact that, while the latter crystallize in the regular system, metals of Class I. form rhombic or quadratic crystals. Regular crystals expand equally in all directions; rhombic and quadratic ores expand differently in different directions. Hence, supposing the crystals immediately after their formation to be in absolute contact with one another all round, then, in the case of Class II., such con tact will be maintained on cooling, while in the case of Class I. the contraction along a given straight line will in general have different values in any two neighbouring crystals, and the crystals consequently become, however slightly, detached from one another. The crystalline structure which exists on both sides becomes visible only in the metals of the first class, and only there manifests itself as brittleness. 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 " malle ability " (capability of being flattened out under the hammer) and "ductility" (capability of being drawn into wire) ; but it is well at once to point out that 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. A bar of zinc, for instance, as obtained by casting, is very brittle ; but when heated to 100 or 150C. it becomes sufficiently plastic to be rolled into the thinnest sheet or to be drawn into wire. Such sheet or wire then remains flexible after cooling, the originally only loosely cohering crystals having got intertwisted and forced into absolute contact with one another, an explanation supported by the fact that rolled zinc has a somewhat higher specific gravity (7 2) than the original ingot (6 9). The same metal, when heated to 205 C., becomes so brittle that it can be powdered in a mortar. Pure iron, copper, silver, and other metals are easily drawn into wire, or rolled into sheet, or flattened under the hammer. But all these operations render the metals harder, and detract from their plasticity. Their original softness can be restored to them by "annealing," i.e., by heating them to redness and then quenching them in cold water. In the case of iron, however, this applies only if the metal is perfectly pure. If it contains a few parts of carbon per thousand, the annealing process, instead of softening the metal, gives it a " temper," meaning a higher degree of hardness and elasticity (see below).
What we have called plasticity must not be mixed up