metalwork Useful and decorative objects fashioned of various metals. The oldest technique is hammering. After c. 2500 bc, casting was also used, molten metal being poured into a mold and allowed to cool. Vari¬ ous decorative techniques are used. Gold and silver have been worked since ancient times. Gold and silver objects were in such demand in the 12th century that gold- and silversmiths organized guilds. High-quality gold and silver objects were produced in pre-Columbian America. Copper was worked in ancient Egypt and was widely used for household utensils in 17th-18th-century Europe. Both bronze and brass were widely used in ancient Greece. Pewter plates and tankards were made in the Middle Ages and remained popular until they were superseded by cheaper earthenware and porcelain in the 18th century. Wrought iron has been used for deco¬ rative hinges, gates, and railings since the 16th century. Lead has tradi¬ tionally been used for roof coverings.
metamorphic rock Any of a class of rocks that result from the alter¬ ation of preexisting rocks in response to changing geological conditions, including variations in temperature, pressure, and mechanical stress. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. The structure and mineralogy reflect the particular type of meta¬ morphism that produced the rock and the composition of the parent rock. Metamorphic rocks are commonly classified by type of facies, predict¬ able mineral assemblages associated with certain temperature and pres¬ sure conditions (see, e.g., granulite facies).
metamorphism Mineralogic and structural changes in solid rocks caused by physical conditions different from those under which the rocks originally formed. Changes produced by surface conditions such as com¬ paction are usually excluded. The most important agents of metamor¬ phism are temperature (from 300°-2,200°F, or 150°-1200°C), pressure (from 10 to several hundred kilobars, or 150,000 to several million lbs. per sq in.), and stress. Dynamic metamorphism results from mechanical deformation with little long-term temperature change. Contact metamor¬ phism results from increases in temperature with minor differential stress, is highly localized, and may occur relatively quickly. Regional metamor¬ phism results from the general increase, usually correlated, of tempera¬ ture and pressure over a large area and a long period of time, as in mountain-building processes. See also metamorphic rock.
metamorphosis In biology, any striking developmental change of an animal’s form or structure, accompanied by physiological, biochemical, and behavioral changes. The best-known examples occur among insects, which may exhibit complete or incomplete metamorphosis (see nymph). The complete metamorphosis of butterflies, moths, and some other insects involves four stages: egg, larva (caterpillar), pupa (chrysalis or cocoon), and adult. The change from tadpole to frog is an example of metamor¬ phosis among amphibians; some echinoderms, crustaceans, mollusks, and tunicates also undergo metamorphosis.
metaphor Figure of speech in which a word or phrase denoting one kind of object or action is used in place of another to suggest a likeness or analogy between them (as in “the ship plows the seas” or “a volley of oaths”). A metaphor is an implied comparison (as in “a marble brow”), in contrast to the explicit comparison of the simile (“a brow white as marble”). Metaphor is common at all levels of language and is funda¬ mental in poetry, in which its varied functions range from merely noting a likeness to serving as a central concept and controlling image.
Metaphysical painting Italian Pittura Metafisica Style of paint¬ ing that flourished c. 1910-20 in the works of the Italian painters Gior¬ gio de Chirico and Carlo Carra (1881-1966). The movement began with Chirico, whose dreamlike works with sharp contrasts of light and shadow often had a vaguely threatening, mysterious quality. Chirico, his younger brother Alberto Savinio, and Carra formally established the school and its principles in 1917. Their representational but bizarre and incongruous imagery produces disquieting effects and had a strong influence on Sur¬ realism in the 1920s.
Metaphysical poetry Highly intellectualized poetry written chiefly in 17th-century England. Less concerned with expressing feeling than with analyzing it, Metaphysical poetry is marked by bold and ingenious conceits (e.g., metaphors drawing sometimes forced parallels between apparently dissimilar ideas or things), complex and subtle thought, fre¬ quent use of paradox, and a dramatic directness of language, the rhythm of which derives from living speech. John Donne was the leading Meta¬ physical poet; others include George Herbert, Henry Vaughan, Andrew Marvell, and Abraham Cowley.
metaphysics Branch of philosophy that studies the ultimate structure and constitution of reality—i.e., of that which is real, insofar as it is real. The term, which means literally “what comes after physics,” was used to refer to the treatise by Aristotle on what he himself called “first philoso¬ phy.” In the history of Western philosophy, metaphysics has been under¬ stood in various ways: as an inquiry into what basic categories of things there are (e.g., the mental and the physical); as the study of reality, as opposed to appearance; as the study of the world as a whole; and as a theory of first principles. Some basic problems in the history of metaphysics are the problem of universals —i.e., the problem of the nature of universals and their relation to so-called particulars; the existence of God; the mind-body problem; and the problem of the nature of material, or external, objects. Major types of metaphysical theory include Platonism, Aristotelianism, Thomism, Cartesianism (see also dualism), idealism, realism, and materialism.
metasomatic X.me-to-so-'ma-tikV replacement Process of simulta¬ neous solution and deposition in which one mineral replaces another. It is the method by which wood is petrified (silica replaces the wood fibres), minerals form pseudomorphs (new minerals that preserve the character¬ istic outward form of the earlier mineral that was replaced), or an ore mineral assemblage takes the place of an earlier rock type. Replacement minerals may themselves be replaced, and definite mineral successions have been established. Replacement deposits may be highly valuable ore deposits.
"Study of Hands/' silverpoint height¬ ened with white on pink grounded paper by Leonardo da Vinci; in Wind¬ sor Castle/ Berkshire, Eng.
REPRODUCED BY GRACIOUS PERMISSION OF HER MAJESTY QUEEN ELIZABETH II
© 2006 Encyclopaedia Britannica, Inc.
metastable state ► methanol I 1245
metastable V.me-to-'sta-boU state Excited state (see excitation) of an atom, nucleus, or other system that has a longer lifetime than the ordinary excited states and generally has a shorter lifetime than the ground state. It can be considered a temporary energy trap or a somewhat stable inter¬ mediate stage of a system of which the energy may be lost in discrete amounts. The many photochemical reactions of mercury are a result of the metastable state of mercury atoms, and radiation from metastable oxygen atoms accounts for the characteristic green colour of the aurora borealis and aurora australis.
Metastasio X.ma-ta-'staz-yoV Pietro orig. Antonio Domenico Bonaventura Trapassi (b. Jan. 3, 1698, Rome—d. April 12, 1782, Vienna) Italian poet and opera librettist. His name was changed by his adoptive father, who left the youth enough money to embark on a career as a poet. His first libretto, Didone abbandonata (1724), was so success¬ ful that he was soon known throughout Italy. Important librettos such as Enzio (1728) and Semiramide (1729) soon followed. He was invited to Vienna as court poet by Charles VI. His 27 three-act librettos were set in more than 800 operas in the 18th and early 19th century by composers such as Antonio Vivaldi, George Handel, Christoph Gluck, Joseph Haydn, Wolfgang Amadeus Mozart, and Luigi Cherubini.