Fifty substances are now included in the list of metals; some of them, however, are only _supposed_ to exist, such as _ammonium_, the supposed base of ammonia; and very many are to be viewed rather in the light of chemical curiosities, as from their great rarity they are too expensive for use, even if possessed of valuable properties of which others might be destitute. Several metals have been known from the earliest period of which we have any record; such were iron, gold, silver, copper, lead, tin, mercury, and probably zinc, or at least its ores; for brass, which is an alloy of copper and zinc, is frequently mentioned in the early part of the Old Testament. In the sixteenth century others were discovered, such as antimony and bismuth. In the last century, cobalt, arsenic, platinum, nickel, manganese and chromium, together with several unimportant metals, were discovered by various philosophers; while in the present century, Dr. Wollaston discovered rhodium, the hardest and nearly the most indestructible of all the metals; and a few years later Sir Humphry Davy found that the alkalies, potash and soda, with many of the earths as they were called, had each a metal for its base, to which he gave the Latin name of the alkali or earth, with the termination _um_, as potassi_um_, the base of potassa, sodi_um_ of soda, calci_um_ of calx (lime), etc. Until Sir H. Davy’s discovery of the metals of the alkalies, great specific gravity was regarded as one of the most striking characteristics of a metal, the lightest of them being much heavier than the heaviest earth; but potassium is very much lighter than water, and not much heavier than spirits of wine. The other metals vary from a specific gravity of nearly twenty-one--or twenty-one times heavier than an equal bulk of water--that of platinum, to somewhat less than seven, which is the specific gravity of antimony. When pure, they all have a luster, differing indeed among themselves, but so peculiar that it is called the metallic luster; for instance, gold and copper are yellow and red--nearly all the others white, but of a different shade; still there is no mistaking their metallic character, no other substances at all equaling them in this respect. They are also opaque, although some, like gold, when reduced to thin films, allow light to pass through them. They are all good conductors of heat and electricity, though some possess that property to a greater extent than others. Many of them are what is called malleable, that is, may be extended or spread out by rolling, or beating them with a hammer; and ductile, or have the property of being drawn out into wire. Gold, silver, copper, and iron are the most remarkable in this respect.