METAL (Gr. μέταλλον), a term including about 50 elementary substances which possess, either wholly or in part, certain well marked physical and chemical properties, of which the most universal and characteristic is lustre. The peculiar brilliancy and reflective power of the metals, which may be enhanced by polishing, results from their great opacity. The color of the metals is generally white with a grayish, bluish, or pinkish tint; copper and gold are the only exceptions. In extremely thin films some of the metals allow the passage of certain rays of light. Gold leaf transmits light of a faint greenish hue. Most of the metals have a high specific gravity, a property which was regarded as characteristic until the discovery of the alkaline metals, which are lighter than water. With the exception of arsenic, they may all be fused, the temperature required for fusion varying from 100° F. to the highest heat of the oxy-hydrogen blowpipe. Some of the metals may be volatilized. Mercury, the only liquid metal, is solidified at -39° F. Arsenic when heated passes directly into vapor without fusion. Most of the metals possess a certain mobility of particles that allows of their being extended or otherwise altered in form. The two nearly related properties of malleability and ductility, resulting from this, are not possessed by the metals in the same degree. A few of them, as antimony, arsenic, and bismuth, are decidedly brittle. Some assume a plastic condition before complete fusion, notably iron and platinum; on this property depends the operation of welding. The strength of the metals is very dissimilar, iron in the form of wire being about 26 times as tenacious as lead. They are all conductors of heat and electricity, although differing widely in this respect. The metals at present known, with the name of the discoverer and date of discovery of each, together with their atomic weights, are given in the following table:

With a few exceptions, the names and dates in the above list refer to the actual production of the metal. In many instances the metallic compounds were known and studied long before the metal itself was isolated. Some of the rarer metals have never been prepared in a pure form. Pelopium, formerly enumerated among the metals, has been shown to have no existence; and the existence of terbium is doubtful. The last four metals were discovered by means of the spectroscope.—The following tables exhibit the mutual relations of some of the more important metals in physical properties:

The physical properties of the metals are largely dependent on their purity and molecular condition, and on temperature. Hammered, rolled, or drawn metal generally has a higher specific gravity than cast metal. The state of molecular tension often induced by mechanical working, especially when cold, is resolved by annealing, i. e., heating and slow cooling. Most of the metals are more malleable and ductile at high temperatures. Commercial zinc is only malleable between 100° and 150° C.; at 200° it is so brittle that it can be pulverized. The conductivity for electricity is greatly diminished at high temperatures, and also by the presence of impurities in the metal. The addition of a small amount of a foreign substance often makes a metal harder, more rigid, and less susceptible of elongation. This is notably the case with iron, which when pure is soft and stretches considerably under strain before breaking, while steel, which is iron with a small amount of carbon, may be rigid and brittle. If reference be had to the original area of section, the rigid metal will show the greatest strength under a gradually applied tensile strain; but if to the fractured area, the purer metal is the strongest. The tenacity of metals generally decreases as the temperature is raised. The fusing points of the more refractory metals given in the above tables are approximate only, since trustworthy methods for determining high temperatures are wanting. The metals vary greatly in hardness. The alkaline metals are as soft as wax, while some, as chromium, will scratch glass. It is not improbable that extreme hardness in metals is produced by the presence of some foreign body, and is not inherent in the metal itself. Most of the metals are capable of assuming a distinctly crystalline form, generally belonging to the regular or isometric system. Some, as antimony and arsenic, crystallize in rhombohedrons. A few of the metals occur native; these are gold, platinum, palladium, iridium, and rhodium, which are almost exclusively found in the metallic state, and silver, copper, mercury, bismuth, arsenic, and antimony. Generally, however, the metals occur mineralized in combination with oxygen or sulphur. The specific heats of the metals, as will be noticed in the above tables, are inversely as their atomic weights, or, in other words, the specific heats of the atoms of the metals are equal.—Chemically, the metals present very varied characters. As a class they are distinguished by the formation of compounds with oxygen which have basic characters, while the non-metals as a class form oxides which have acid characters. These two classes of oxides are capable of combining to form salts. While the oxides of the non-metals never form bases, the higher oxides of many of the metals have distinctly acid properties, and indeed a few of the metals form only acid oxides. The most stable compounds of tellurium, arsenic, antimony, tungsten, titanium, molybdenum, and vanadium with oxygen are acid in character and capable of combining with basic oxides. Those metals which seem to hold a position intermediate between the two classes have been termed half metals or metalloids. The latter term, as now generally used, includes all the non-metallic elements, viz.: hydrogen, oxygen, bromine, chlorine, iodine, fluorine, boron, nitrogen, phosphorus, selenium, silicon, sulphur, and carbon. Tellurium is closely related to sulphur and selenium, and is often classed with the metalloids; but its metallic appearance, and the analogy which its compounds bear to those of antimony, render its association with the metals equally appropriate. Hydrogen, although a gas and the lightest body known, resembles the metals in its chemical properties, and is capable of replacing them in combination. The formation of salts is regarded in modern chemistry as the replacement of hydrogen in the acid by a metal.—The metals are variously classified. A natural grouping, and one in common use, is: 1, metals of the alkalies; 2, metals of the alkaline earths; 3, metals of the earths proper; 4, oxidable metals proper, whose oxides form powerful bases; 5, oxidable metals, whose oxides form weak bases or acids; 6, metals proper, whose oxides are reduced by heat, called noble metals. The strength of affinity of the different metals for oxygen is the basis of a classification formerly much used. It is embodied in part in the electro-chemical series of Berzelius, which played an important part in the development of chemical science. The alkaline metals oxidize rapidly in the air, and decompose water at ordinary temperatures; others, as iron and zinc, do not oxidize in pure dry air, and decompose water only at a red heat, or in contact with an acid; and others, as the noble metals, do not decompose water at any temperature. The electrical relations of the metals correspond in general to their affinity for oxygen. Thus, the alkaline metals are the most electro-positive, and the noble metals the most electro-negative. The metals likewise fall into groups in which the individual members can replace one another in compounds without change of crystalline form; they are then said to be isomorphous. As examples may be cited magnesium, calcium, manganese, iron, zinc, copper, and aluminum; barium, strontium, and lead; sodium, silver, thallium, gold, and potassium; arsenic, antimony, and bismuth; tin, titanium, tungsten, and molybdenum; platinum, iridium, and osmium. The atomicity of the elements, or their combining values, forms the basis of classification for study in modern chemistry. Metals are thus divided into monads (or those replaceable by or equivalent to one atom of a monogenic element, as hydrogen or chlorine), dyads, triads, tetrads, pentads, and hexads, as follows: monads—lithium, sodium, potassium, rubidium, calcium, silver; dyads—calcium, strontium, barium, glucinum, yttrium, lanthanum, didymium, erbium, thorium, magnesium, zinc, cadmium, copper, mercury; triads gold, thallium; tetrads titanium, tin, aluminum, zirconium, rhodium, ruthenium, palladium, platinum, iridium, osmium, lead, manganese, iron, cobalt, nickel, cerium, indium, uranium; pentads vanadium, arsenic, antimony, bismuth, niobium, tantalum; hexads—chromium, molybdenum, tungsten. A few of the metals possess more than one atomicity, and appear in different compounds with different atomic values. The combinations of the metals with the non-metallic elements may be divided into two classes, those with chlorine, iodine, bromine, and fluorine, and those with oxygen, sulphur, selenium, and tellurium. The former class are saline compounds, while the latter are generally basic, exceptionally acid, as before mentioned. Formerly the distinction was generally observed between haloid and oxygen salts, the former being the combination of a metal with a haloid body, as chloride of sodium, and the latter a combination of a basic oxide with an oxy-acid, as sulphate of soda. In the modern chemistry both characters of salts are regarded as the replacements of hydrogen in the acid by a metal. The combinations of the metals among themselves are known as alloys, or, in case of mercury, as amalgams. (See Alloy, and Amalgam.)