1911 Encyclopædia Britannica/Metal

METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. μέταλλον, in the same sense, probably connected with μεταλλᾶν, to search after, explore, μετὰ, after, ἄλλος, other). Originally applied to gold, silver, copper, iron, tin, lead and bronze, i.e. substances having high specific gravity, malleability, opacity, and especially a peculiar lustre, the term “metal” became generic for all substances with these properties. In modern chemistry, however, the metals are a division of the elements, the members of which may or may not possess all these characters. The progress of science has, in fact, been accompanied by the discovery of some 70 elements, which may be arranged in order of their “metallic” properties as above indicated, and it is found that while the end members of the scale are most distinctly metallic (or non-metallic), certain central members, e.g. arsenic, may be placed in either division, their properties approximating to both metallic and non-metallic. One chemical differentia utilizes the fact that metals always form at least one basic oxide which yields salts with acids, While non-metals usually form acidic oxides, i.e. oxides which yield acids with water. This definition, however, is highly artificial—and objectionable on principle, because when we speak of metals we think, not of their chemical relations, but of a certain sum of mechanical and physical properties which unites them all into one natural family.

All metals, when exposed in an inert atmosphere to a sufficient temperature, assume the form of liquids, which all present the following characteristic properties. They are (at least practically) non-transparent; they reflect light in a peculiar manner, producing what is called “metallic lustre.” When kept in non-metallic vessels they take the shape of a convex meniscus. These liquids, when exposed to higher temperatures, some sooner than others, pass into vapours. What these vapours are like is not known in many cases, since, as a rule, they can be produced only at 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. 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 application 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 now know a series of metals lighter than water.

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, 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.

Colour.—Gold is yellow; copper is red; silver, tin, and some others are pure white; the majority are greyish.

Reflection 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; The following values are due to Rubens and Hagen (Ann. der Phys., 1900, p. 352); they express the percentage of incident light reflected. The superiority of silver is obvious.

Name of Metal.	Violet.	Yellow.	Red.
	λ＝450	λ＝550	λ＝650
Silver	90·6	92·5	93·6
Platinum	55·8	61·1	66·3
Nickel	58·5	62·6	65·9
Steel	58·6	59·4	60·1
Gold	36·8	74·7	88·2
Copper	48·8	59·5	89
Glass backed with silver	79·3–85·7	82–88	83–89
Glass backed with mercury	72·8	71·2	71·5

Crystalline Form and Structure.—Most (perhaps all) metals are capable of crystallization. The crystals belong to the following systems: regular system—silver, gold, palladium, mercury, copper, iron, lead; quadratic system—tin, potassium; rhombic system—antimony, bismuth, tellurium, zinc, magnesium. Perhaps all metals are crystalline, only the degree of 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.

Antimony, bismuth and zinc exhibit 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 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 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 contact 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 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 “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.

A bar of zinc, for instance, as obtained by casting, is very brittle; but when heated to 100° or 150° C. 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 in to 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 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.

He operated with lead, copper, silver, iron and some other metals. Round 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 an hydraulic press a pressure of 100,000 kilos was made to act upon the disks, when the metal was seen to “flow” out of the hole like a viscid liquid. In spite of the immense rearrangement of parts 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 “welded” together by pressure into one continuous mass.

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 thickness of 1/3800 mm.; and that platinum, by judicious work, can be drawn into wire 1/20000 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 Elasticity for the theoretical and to the Strength of Materials 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.

Name of Metal.	E.	For Wire of 1 sq. mm. Section, Weight (in Kilos) causing
		Permanent Elongation of 1/20000.	Breakage.
Lead, drawn	1,803	0·25	2·1
Lead,„ annealed	1,727	0·20	1·8
Tin, drawn	4,148	0·45	2·45
Tin,„ annealed	1,700	0·20
Cadmium	7,070		2·24
Gold, drawn	8,131	13·5	27
Gold,„ annealed	5,585	3·0	10
Silver, drawn	7,357	11·3	29
Silver,„ annealed	7,140	2·6	16
Zinc, pure, cast in mould	9,021
Zinc,„ ordinary, drawn	8,735	0·75	13
Palladium, drawn	11,759	18
Palladium,„ annealed	9,709	under 5	27
Copper, drawn	12,449	12	40
Copper,„ annealed	10,519	under 3	30
Platinum wire, medium thickness, drawn	17,004	26	34
Platinum, annealed	15,518	14	23
Iron, drawn	20,869	32	61
Iron,„ annealed	20,794	under 5	47
Nickel, drawn	23,950		3/2×61
Aluminium	7,200
Nickel,„ bronze	10,700
Brass (ZnCu2)	8,543
German silver (Zn4Cu13Ni4)	10,788

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.

Name of Metal.	Specific Gravity.
Lithium	·594
Potassium	·875
Sodium	·978
Rubidium	1·32
Calcium	1·578
Magnesium	1·743
Caesium	1·88
Beryllium	2·1
Strontium	2·5
Aluminium, pure, ingot	2·583 at 4°
Aluminium,„ ordinary, hammered	2·67
Barium	3·75
Zirconium	4·15
Vanadium, powder	5·5
Gallium	5·95
Lanthanum	6·163
Cerium	6·68
Antimony	6·62
Chromium	6·50
Zinc, ingot	6·915
Zinc,„ rolled out	7·2
Manganese	7·39
Tin, cast	7·29 to 7·299
Tin,„ crystallized by electrolysis from solutions	7·178
Indium	7·42
Iron, chemically pure, ingot	7·844
Iron,„ thin sheet	7·6
Iron,„ wrought, high quality	7·8 to 7·9
Nickel, ingot A	8·279
forged	8·666
Cadmium, ingot	8·546
Cadmium,„ hammered	8·667
Cobalt	8·6
Molybdenum, containing 4 to 5 % of carbon	8·6
Copper, native	8·94
Copper,„ cast	8·92
Copper,„ wire or thin sheet	8·94 to 8·95
Copper,„ electrotype, pure	8·945
Bismuth .	9·823 at 12°
Silver, cast .	10·4 to 10·5
Silver,„ stamped .	10·57
Lead, very slowly frozen	11·254
Lead,„ quickly frozen in cold water	11·363
Palladium	11·4 at 22·5°
Thallium	11·86
Rhodium	12·1
Ruthenium	12·26 at 0°
Mercury, liquid	13·595 at 0°
Mercury,„ solid	14·39 below −40°
Tungsten, compact, by H2 from chloride vapour	16·54
Tungsten,„ as reduced by hydrogen, powder	19·13
Uranium	18·7
Gold, ingot .	19·265 at 13°
Gold,„ stamped .	19·31 to 19·34
Gold,„ powder, precipitated by ferrous sulphate	19·55 to 19·72
Platinum, pure	21·50
Iridium	22·2
Osmium	22·477

Thermal Properties.—The specific heats of most metals have been determined. The general result is that, conformably with Dulong and Petit’s law, the “atomic heats” all come to very nearly the same value (of about 6·4); i.e. atomic weight by specific heat＝6·4. Thus we have for silver by theory 6·4/108=·0593, and by experiment ·0570 for 10° to 100° C.

The expansion by heat varies greatly. The following table gives the linear expansions from 0° to 100° C. according to Fizeau (Comptes rendus, lxviii. 1125), the length at 0° being taken as unity.

Name of Metal.	Expansion 0° to 100°
Platinum, cast	·000 907
Gold, Cast	·001 451
Silver, cast	·001 936
Copper, native, from Lake Superior	·001 708
Copper,„ artificial	·001 869
Iron, soft, as used for electromagnets	·001 228
Iron,„ reduced by hydrogen and compressed	·001 208
Cast steel, English annealed	·001 110
Bismuth, in the direction of the axis	·001 642
Bismuth,„ at right angles to axis	·001 239
Bismuth,„ mean expansion, calculated	·001 374
Tin, of Malacca, compressed powder	·002 269
Lead, cast	·002 948
Zinc, distilled, compressed powder	·002 905
Cadmium, distilled, compressed powder	·003 102
Aluminium, cast	·002 336
Brass (71·5 % copper. 28·5 % zinc)	·001 879
Bronze (86·3% copper, 9·7% tin, 4·0% zinc)	·001 802

The coefficient of expansion is constant for such metals only as crystallize in the regular system; the others expand differently in the directions of, the different axes. To eliminate this source of uncertainty these metals were employed as compressed powders. The cubical expansion of mercury from 0° to 100° C. is ·018153 =1/55·087 (Regnault) (See Thermometry.)

Fusibility and Volatility.—The fusibility in different metals is very different, as shown by the following table, which, besides including all the fusing points (in degrees C.) of metals which have been determined numerically, indicates those of a selection of other metals by the positions assigned to them in the table.

Name of Metal.		Melting Point.	Boiling Point.
Mercury		−38·8	357·3
Caesium		26–27
Gallium		30·1
Rubidium		38·5
Potassium		62·5	719–731
Sodium		95·6	861–954
Iridium		155
Lithium		180·0
Tin		231·9	1450–1600
Bismuth		269·2	1090–1450
Thallium		290
Cadmium		320·7	780
Lead		327·7	1450–1600
Zine		419	929–954
Incipient red heat		525
Antimony		629·5
Magnesium		632·6	about 1100
Aluminium		655
Cherry red heat		700
Calcium		780
Lanthanum		810
Barium		850
Silver		962
Gold		1064
Copper		1082	2100
Yellow heat		1100
Iron		1300–1400
Nickel		1427
Cobalt		1800, (?)
Dazzling white heat		1500–1600
Palladium		1500
Platinum		1760
Rhodium		above Pt.
Iridium		above 2200
Ruthenium		above Ir.
Tantalum	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	In electric
Osmium		furnace

For practical purposes the volatility of metals may be stated as follows:—

1. Distillable below redness: mercury.

2. Distillable at red heats: cadmium, alkali metals, zinc, magnesium.

3, Volatilized more or less readily when heated beyond their fusing points in open crucibles: antimony (very readily), lead, bismuth, tin, silver.

4. Barely so: gold, (copper).

5. Practically non-volatile: (copper), iron, nickel, cobalt, aluminium; 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.—Of these we know little. The following numbers are due to Person—ice, it may be stated, being 80.

Name of Metal.	Latent  Heat.	Name of Metal.	Latent  Heat.
Mercury	2·82	Cadmium	13·6
Lead	5·37	Silver	21·1
Bismuth	12·4	Zinc	28·1

The latent heat of vaporization of mercury was found by Marignac to be 103 to 106.

Conductivity.—Conductivity, whether thermic or electric, is very differently developed in different metals; and, as an exact knowledge of these' conductivities is of great importance, much attention has been given to their numerical determination (see Conduction, Electric; and Conduction of Heat).

The following table gives the electric conductivities of a number of metals as determined by Matthiesen, and the relative internal thermal 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 “vacuum” (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.

Name of Metal.	Relative Conductivities.
	Electric.	Thermic.
Copper, commercial, No. 3	·774  at 18·8°
Copper, commercial, No. 2	·721  at 22·6
Copper, chemically pure, hard drawn	·93[1]
Copper		·748
Gold, pure	·552  at 21·8	·548
Gold, absolutely pure	·73[1]  at 19·0
Brass		·25
Tin, pure	·115  at 21·0	·154
Pianoforte Wire	·144  at 20·4
Iron rod		·101
Steel		·103
Lead, pure	·0777 at 17·3	·079
Platinum	·105  at 20·7	·094
German silver	·0767 at 18·7	·073
Bismuth	·0119 at 13·8
Aluminium	·196  at 19·6
Mercury	·0163 at 22·8
Silver, pure	1·000  at  0	1·000

Magnetic Properties.—Iron, nickel and cobalt are the only metals which are attracted by the magnet and can become magnets themselves. But in regard to their power of retaining their magnetism none of them comes at all up to the compound metal steel. See Magnetism.

Chemical Changes.—Metals may unite chemically both with metals and with non-metals. The compounds formed in the first case, which may be either definite chemical compounds or solid solutions, are discussed under Alloys; in this place only combinations with non-metals are discussed, it being premised that the free metal takes part in the reaction.

Metallic Substances Produced by the Union of Metals with Small Proportions of Non-Metallic Elements.

Hydrogen, as was shown by Graham, is capable of uniting with or being occluded by certain metals, notably with palladium (q.v.), into metal-like compounds.

Oxygen.—Mercury and copper and some other metals are capable of dissolving their own oxides. Mercury, by doing so, becomes viscid and unfit for its ordinary applications. Copper, when pure to start with, suffers considerable deterioration in plasticity. But the presence of moderate proportions of cuprous oxide has been found to correct the evil influence of small contaminations by arsenic, antimony, lead and other foreign metals. Commercial coppers sometimes owe their good qualities to this compensating influence.

Arsenic combines readily with all metals into true arsenides, which latter, in general, are soluble in the metal itself. The presence in a metal of even small proportions of arsenide generally leads to considerable deterioration in mechanical qualities.

Phosphorus.—The remark just made might be said to hold for phosphorus were it not for the existence of what is called “phosphorus-bronze,” an alloy of copper with phosphorus (i.e. its own phosphide), which possesses valuable properties. According to Abel, the most favourable effect is produced by from 1 to 11/2%, of phosphorus. Such an alloy can be cast like ordinary bronze, but excels the latter in hardness, elasticity, toughness and tensile stren

Carbon.—Most metals when molten are capable of dissolving at least small proportions of carbon, which, in general, leads to a deterioration in metallicity, except in the case of iron, which by the addition of small percentages of carbon gains in elasticity and tensile strength with little loss of plasticity (see Iron).

Silicon, so far as we know, behaves to metals pretty much like carbon, but our knowledge of facts is limited. What is known as cast iron is essentially an alloy of iron proper with 2 to 6% of carbon and more or less of silicon (see Iron). Alloys of copper and silicon were prepared by Deville in 1863. The alloy with 12% of silicon is white, hard and brittle. When diluted down to 4·8%, it assumes the colour and fusibility of bronze, but, unlike it, is tenacious and ductile like iron.

Action of the More Ordinary Chemical Agents on Simple Metals.

The metals to be referred to are always understood to be given in the compact (frozen) condition, and that, wherever metals are enumerated as being similarly attacked, the degree of readiness in the action is indicated by the order in which the several members are named—the more readily changed metal always standing first.

Water, at ordinary or slightly elevated temperatures, is decomposed more or less readily, with evolution of hydrogen gas and formation of a basic hydrate, by (1) potassium (formation of KHO), sodium (NaHO), lithium (LiOH), barium, strontium, calcium (BaH₂O₂, &c.); (2) magnesium, zinc, manganese (MgO₂H₂, &c.).

In the case of group 1 the action is more or less violent, and the hydroxides formed are soluble in water and very-strongly basic; metals of group 2 are only slowly attacked, with formation of relatively feebly basic and less soluble hydroxides. Disregarding the rarer elements, the metals not named so far may be said to be proof against the action of pure water in the absence of free oxygen (air).

By the joint action of water and air, thallium, lead, bismuth are oxidized, with formation of more or less sparingly soluble hydroxides (ThHO, PbH₂O₂, BiH₃O₃), which, in the presence of carbonic acid, pass into still less soluble basic carbonates. Iron, when exposed to moisture and air, “rusts”; but this process never takes place in the absence of air, and it is questionable whether it ever sets in in the absence of carbonic acid (see Rust).

Copper, in the present connexion, is intermediate between iron and the following group of metals.

Mercury, if pure, and all the “noble” metals (silver, gold, platinum and platinum-metals), are absolutely proof against water even in the presence of oxygen and carbonic acid.

The metals grouped together above, under 1 and 2, act on steam pretty much as they do on liquid water. Of the rest, the following are readily oxidized by steam at a red heat, with formation of hydrogen gas—zinc, iron, cadmium, cobalt, nickel, tin. Bismuth is similarly attacked, but slowly, at a white heat. Aluminium is barely affected even at a white heat, if it is pure; the ordinary impure metal is liable to be very readily oxidized.

Aqueous Sulphuric or Hydrochloric Acid readily dissolves groups 1 and 2, with evolution of hydrogen and formation of chlorides or sulphates. The same holds for the following group (A): [manganese, zinc, magnesium] iron, aluminium, cobalt, nickel, cadmium. Tin dissolves readily in strong hot hydrochloric acid as SnCl₂; aqueous sulphuric acid does not act on it appreciably in the cold; at 150° it attacks it more or less quickly, according to the strength of the acid, with evolution of sulphuretted hydrogen or, when the acid is stronger, of sulphurous acid gas and deposition of sulphur (Calvert and Johnson). A group (B), comprising copper, is, substantially, attacked only in the presence of oxygen or air. Lead, in sufficiently dilute acid, or in stronger acid if not too hot, remains unchanged. A group (C) may be formed of mercury, silver, gold and platinum, which) are not touched by either aqueous acid in any circumstances.

Hot (concentrated) sulphuric acid does not attack gold, platinum and platinum-metals generally; all other metals (including silver) are converted into sulphates, with evolution of sulphur dioxide. In the case of iron, ferric sulphate, Fe₂(SO₄)₃, is produced; tin yields a somewhat indefinite sulphate of its oxide SnO₂.

Nitric Acid (Aqueous)—Gold, platinum, iridium and rhodium only are proof against the action of this powerful oxidizer. Tin and antimony (also arsenic) are converted by it (ultimately) into hydrates of their highest oxides SnO₂, Sb₂O₅ (As₂O₅)—the oxides of tin and antimony being insoluble in water and in the acid itself. All other metals, including palladium, are dissolved as nitrates, the oxidizing part of the reagent being generally reduced to oxides of nitrogen. Iron, zinc, cadmium, also tin under certain conditions, reduce the dilute acid, partially at least, to nitrous oxide, N₂O, or ammonium nitrate, NH₄NO₃.

Aqua Regia, a mixture of nitric and hydrochloric acids, converts all metals (even gold, the “king of metals,” whence the name) into chlorides, except only rhodium, iridium and ruthenium, which, when pure, are not attacked.

Caustic Alkalis.—Of metals not decomposing liquid pure water, only a few dissolve in aqueous caustic potash or soda, with evolution of hydrogen. The most important of these are aluminium and zinc, which are converted into aluminate, Al(OK, Na)₃, and zincate, Zn(OK, Na)₂, respectively. But of the rest the majority, when treated with boiling sufficiently strong alkali, are attacked at (least superficially; of ordinary metals only gold, platinum, and silver are perfectly proof against the reagents under consideration, and these accordingly are used preferably for the construction of vessels intended for analytical) operations involving the use of aqueous caustic alkalis. For commercial purposes iron is universally employed and works well; but it is not available analytically, because a superficial oxidation of the empty part of the vessel (by the water and air) cannot be prevented. Basins made of pure malleable nickel are free from this drawback; they work as well as platinum, and rather better than silver ones do. There is hardly a single metal which holds out against the alkalis themselves when in the state of fiery fusion; even platinum is most violently attacked. In chemical laboratories fusions with caustic alkalis are always effected in vessels made of gold or silver, these metals holding out fairly well even in the presence of air. Gold is the better of the two. Iron, which stands so well against aqueous alkalis, is most violently attacked by the fused reagents. Yet tons of caustic soda are fused daily in chemical works in iron pots without thereby suffering contamination, which seems to show that (clean) iron, like gold and silver, is attacked only by the joint action of fused alkali and air, the influence of the latter being of course minimized in large-scale operations.

Oxygen or Air.—The noble metals (from silver upwards) do not combine directly with oxygen given as oxygen gas (O₂), although, like silver, they may absorb this gas largely when in the fused condition, and may not be proof against ozone, O₃. Mercury, within a certain range of temperatures situated close to its boiling point, combines slowly with oxygen into the red oxide, which, however, breaks up again at higher temperatures. All other metals, when heated in oxygen or air, are converted, more or less readily, into stable oxides. Potassium, for example, yields peroxide, K₂O₂ or K₂O₄; sodium gives Na₂O₂; the barium-group metals, as well as magnesium, cadmium, zinc, lead, copper, are converted into their monoxides MeO. Bismuth and antimony give (the latter very readily) sesquioxide (Bi₂O₃ and Sb₂O₃, the latter being capable of passing into Sb₂O₄). Aluminium, when pure and kept out of contact with siliceous matter, is only oxidized at a white heat, and then very slowly, into alumina, Al₂O₃. Tin, at high temperatures, passes slowly into oxide, SnO₂.

Sulphur.—Amongst the better known metals, gold and aluminium are the only ones which, when heated with sulphur or in sulphur vapour remain unchanged. All the rest, under these circumstances, are converted into sulphides. The metals of the alkalis and alkaline earths, also magnesium, burn in sulphur vapour as they do in oxygen. Of the heavy metals, copper is the one which exhibits by far the greatest avidity for sulphur, its subsulphide Cu₂S being the stablest of all heavy metallic sulphides in opposition to dry reactions.

Chlorine.—All metals, when treated with chlorine gas at the proper temperatures, pass into chlorides. In some cases the chlorine is taken up in two instalments, a lower chloride being produced first, to pass ultimately into a higher chloride. Iron, for instance, is converted first into FeCl₂, ultimately into FeCl₃, which practically means a mixture of the two chlorides, or pure FeCl₃ as a final product. Of the several products, the chlorides of gold and platinum (AuCl₃ and PtCl₄) are the only ones which when heated beyond their temperature of formation dissociate into metal and chlorine. The ultimate chlorination product of copper, CuCl₂, when heated to redness, decomposes into the lower chloride, CuCl, and chlorine. All the rest, when heated by themselves, volatilize, some at lower, others at higher temperatures.

Of the several individual chlorides, the following are liquids or solids, volatile enough to be distilled from glass vessels: AsCl₃, SbCl₃, SnCl₄, BiCl₃ HgCl₂, the chlorides of arsenic, antimony, tin, bismuth, mercury respectively. The following are readily volatilized in a current of chlorine, at a red heat: AlCl₃, CrCl₃, FeCl₃, the chlorides of aluminium, chromium, iron. The following, though volatile at higher temperatures, are not volatilized at dul redness: KCl, NaCl, LiCl, NiCl₂, CoCl₂, MnCl₂, ZnCl₂, MgCl₂, PbCl₂, AgCl, the chlorides of potassium, sodium, lithium, nickel, cobalt, manganese, zinc, magnesium, lead, silver. Somewhat less volatile than the last-named group are the chlorides (MCl₂) of barium, strontium and calcium.

Metallic chlorides, as a class, are readily soluble in water. The following, are the most important exceptions: silver chloride, AgCl, and mercurous chloride, HgCl, are absolutely insoluble; lead chloride, PbCl2, and cuprous chloride, CuCl, are very sparingly soluble in water. The chlorides AsCl₃, SbCl₃, BiCl₃, are at once decomposed by (liquid) water, with formation of oxide (As₂O₃) or oxychlorides (SbOCl, BiOCl) and hydrochloric acid. The chlorides MgCl₂, AlCl₃, CrCl₃, FeCl₃, suffer a similar decomposition when evaporated with water in the heat. The same holds in a limited sense for ZnCl₂, CoCl₂, NiCl₂, and even CaCl₂. All chlorides, except those of silver and mercury (and, of course, those of gold and platinum), are oxidized by steam at high temperatures, with elimination of hydrochloric acid.

For the characters of metals as chemical elements see the special articles on the different metals.

See generally A. Rossing Geschichte der Metalle (1901); B. Neumann, Die Metalle (1904); also treatises on chemistry.

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1. Published in 1860, and declared by Matthiesen to be more exact than the old numbers.