METALS. Though each metal is con sidered in a separate article, there are various points regarding the general physical and chemical characters of these bodies, and the method of classifying them, which require notice. A metal, from the chemical point of view, is an element which can replace hydrogen in an acid and thus form a salt. Hydrogen itself is, chemically, considered to be a metal. Those elements which are non metallic in this sense are called metal loids.
The following are the most important of the physical properties of the metals: (1) All metals, unless when they are in a finely-pulverized form, exhibit more or less of the charactistic luster termed metallic. Two of the non-metallic ele ments, iodine and carbon, in some forms also present a metallic luster. (2) All metals are good conductors of heat and electricity, though in very unequal de grees. (3) With the exception of mer cury, all the metals are solid at ordinary temperatures. With the exception of gold, copper, calcium, and strontium, the metals are, when light is only once re flected from them, more or less white, with a tendency to blue or gray. Most of them have been obtained in crystals, and probably all of them are capable of crystallizing under certain conditions.
(4) Metals are remarkable for their opacity, except when they are chemically reduced to extremely thin films. (5) All the metals are fusible, though the tem peratures at which they assume the fluid form are very different, and some of them, as mercury, arsenic, cadmium, zinc, etc., are also volatile. (6) Great weight, or a high specific gravity, is popularly but erroneously regarded as a characteristic of a metal; while plati num, osmium, and iridium (the heaviest bodies known in nature) are more than 20 times as heavy as water, lithium, potassium, and sodium are actually lighter than that fluid. (7) Great dif
ferences are observable in the hardness, brittleness, and tenacity of metals. While potassium and sodium may be kneaded with the finger, and lead may be marked by the finger-nail, most of them possess a considerable degree of hardness. Antimony, arsenic, and bis muth are so brittle that they may be easily pulverized in a mortar; while others, as iron, gold, silver, and copper, require great force for their disintegra tion. Taking iron and lead as represent ing the two extremes of tenacity, it is found that an iron wire will bear a weight 26 times as heavy as a leaden wire of the same diameter.
Various classifications of the metals have been suggested by different chem ists. The following is probably one of the most convenient: (I.) The LIGHT METALS, subdivided into— (1) The metals of the alkalies—viz., potassium, sodium, caesium, rubidium, lithium.
(2) The metals of the alkaline viz., barium, strontium, calcium, magne sium.
(3) The metals of the true earths—a viz., aluminium, glucinum, zirconium, yttrium, erbium, terbium, thorinum, cerium, lanthanum, didymium.
(II.) The HEAVY METALS, subdivided into— (1) Metals whose oxides form power ful bases—viz., iron, manganese, chro mium, nickel, cobalt, zinc, cadmium, lead, bismuth, copper, uranium, thallium.
(2) Metals whose oxides form weak bases or acids—viz., arsenic, antimony, titanium, tantulum, niobium, (or colum bium), tungsten, molybdenum, tin, vana dium, osmium.
(3) Metals whose oxides are reduced by heat—noble metals—viz., mercury, silver, gold, platinum, palladium, iridium, ruthenium, rhodium, osmium. (Several of the rare metals are here omitted.) See MINERAL PRODUCTION, UNITED STATES,