Interstellar Matter

Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

Ultraviolet Radiation

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Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 4000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of X rays. Natural ultraviolet radiation is produced principally by the sun. Ultraviolet radiation is produced artificially by electric-arc lamps.

Ultraviolet radiation is often divided into three categories based on wavelength, UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet radiation are more dangerous to living organisms. UV-A has a wavelength from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150 Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to 150 Å and is used to sterilize surfaces because it kills bacteria and viruses.

The earth's atmosphere protects living organisms from the sun's ultraviolet radiation. If all the ultraviolet radiation produced by the sun were allowed to reach the surface of the earth, most life on earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large portion of the vitamin D that humans and animals need for good health is produced when the human's or animal's skin is irradiated by ultraviolet rays.

When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light, become energetic, then shed their excess energy by emitting visible light. As another example, ordinary window glass, transparent to visible light, is opaque to a large portion of ultraviolet rays, particularly ultraviolet rays with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.

In astronomy, ultraviolet-radiation detectors have been used since the early 1960s on artificial satellites, providing data on stellar objects that cannot be obtained from the earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.

Air

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Air, mixture of gases that composes the atmosphere surrounding Earth. These gases consist primarily of the elements nitrogen, oxygen, argon, and smaller amounts of hydrogen, carbon dioxide, water vapor, helium, neon, krypton, xenon, and others. The most important attribute of air is its life-sustaining property. Human and animal life would not be possible without oxygen in the atmosphere. In addition to providing life-sustaining properties, the various atmospheric gases can be isolated from air and used in industrial and scientific applications, ranging from steelmaking to the manufacture of semiconductors. This article discusses how atmospheric gases are isolated and used for industrial and scientific purposes.

GASES IN THE ATMOSPHERE

The atmosphere begins at sea level, and its first layer, the troposphere, extends from 8 to 16 km (5 and 10 mi) from Earth’s surface. The air in the troposphere consists of the following proportions of gases: 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide, and the remaining 0.07 percent is a mixture of hydrogen, water, ozone, neon, helium, krypton, xenon, and other trace components. Companies that isolate gases from air use air from the troposphere, so they produce gases in these same proportions.


PURIFYING AIR

Most larger air-separation plants continue to use cryogenic distillation to separate air gases. Before pure gases can be isolated from air, unwanted components such as water vapor, dust, and carbon dioxide must be removed. First, the air is filtered to remove dust and other particles. Next, the air is compressed as the first step in liquefying the air. However, as the air is compressed, the molecules begin striking each other more frequently, raising the air’s temperature. To offset the higher temperatures, water heat exchangers cool the air both during and after compression. As the air cools, most of its water vapor content condenses into liquid and is removed.

After being compressed, the air passes through beds of adsorption beads that remove carbon dioxide, the remaining water vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and propylene. These compounds all freeze at a higher temperature than do the other air gases. They must be removed before the air is liquefied or they will freeze in the column where distillation occurs.

COMPRESSED AIR

Not all industrial uses of air require it to be separated into its component gases. Compressed air—plain air that has been pressurized by squeezing it into a smaller-than-normal volume—is used in many industrial applications. When air is compressed, the gas molecules collide with each other more frequently and with more force, producing higher kinetic energy. The kinetic energy in compressed air can be converted into mechanical energy or it can be used to produce a powerful air flow or an air cushion. Compressed air is easily transmitted through pipes and hoses with little loss of energy, so it can be utilized at a considerable distance from the compressor or pressure tank.

Alabaster

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Alabaster, varietal name applied to two different minerals. One, Oriental alabaster, was extensively used by the ancient Egyptians. It is a variety of calcite, with a hardness of 3; it is usually white and translucent, but is often banded with dark or colored streaks. The other mineral, true alabaster, is a variety of gypsum, usually snow-white in color with a uniform, fine grain. True alabaster is softer than Oriental alabaster; it has a hardness of 1.5 and is easily carved into intricate shapes. Deposits of fine gypsum alabaster are found in Italy, England, Iran, and Pakistan.

Alkalies

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Alkalies or Alkalis (Arabic al-qili, “ashes of the saltwort plant”), originally the hydroxides and carbonates of potassium and sodium, leached from plant ashes. The term now applies to the corresponding compounds of ammonium, NH4, and the other alkali metals and to the hydroxides of calcium, strontium, and barium. All of these substances produce hydroxide ions, OH-, when dissolved in water. The carbonates and ammonium hydroxide give only moderate concentrations of hydroxide ions and are termed mild alkalis. The hydroxides of sodium and potassium, however, produce hydroxide ions in high enough concentration to destroy flesh; for this reason they are called caustic alkalis. Solutions of alkalis turn red litmus blue, react with and neutralize acids, feel slippery, and are electrical conductors.

Caustic soda, or sodium hydroxide, NaOH, is an important commercial product, used in making soap, rayon, and cellophane; in processing paper pulp; in petroleum refining; and in the manufacture of many other chemical products. Caustic soda is manufactured principally by electrolysis of a common salt solution, with chlorine and hydrogen as important by-products.

Sodium carbonate, Na2CO3, one of the mild alkalis, is manufactured from natural deposits or made from common salt brines by the Solvay process. It is used in the manufacture of glass and as a cleaning agent and water softener.

Alum

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Alum, any of a group of chemical compounds, made up of water molecules and two kinds of salts, one of which is usually aluminum sulfate combined in definite proportions. Potassium alum, also known as common alum, is the most important type of alum.

Potassium alum is a colorless substance that forms large octahedral or cubic crystals when potassium sulfate and aluminum sulfate are dissolved together and the solution is cooled. The solutions of potassium alum are acidic.

Potassium alum is soluble in seven times its weight of water at room temperature and is very soluble in hot water. When crystalline potassium alum is heated, some of the water of hydration becomes chemically separated, and the partly dehydrated salt dissolves in this water, so that the alum appears to melt at about 90°C (about 194°F). When heated to about 200°C (about 392°F), potassium alum swells up, loses all the water and some sulfur trioxide, and becomes a basic salt called burnt alum. Potassium alum has a density of 1.725.

Other types of alums made with aluminum sulfate include sodium alum, ammonium alum, and silver alum. Alums are used for flameproofing textiles and in baking powders, mordants for delicate dyeing operations, and medicines. Potassium alum is a powerful

Alumina

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Alumina or Aluminum Oxide, an oxide found in nature as the minerals corundum, diaspore, gibbsite, and most commonly, bauxite, an impure form of gibbsite. It is the only oxide formed by the metal aluminum. The precious stones ruby and sapphire are composed of corundum colored by small amounts of impurities.

Fused alumina, alumina that has been melted and recrystallized, is identical in chemical and physical properties with natural corundum. It is exceeded in hardness only by diamond and by a few synthetic substances, notably carborundum, or silicon carbide. Both impure natural corundum (emery) and pure synthetic corundum (Alundum) are used as abrasives. At room temperature alumina is insoluble in all ordinary chemical reagents. Its melting point is high, slightly above 2000°C (3632°F), and so alumina is useful as a refractory, for example, for the linings of special furnaces.

Alumina can be purified by fusing it with sodium carbonate. The resulting sodium aluminate is dissolved in water, leaving impurities, such as iron, as an insoluble residue. Hydrated alumina is reprecipitated from the solution by carbon dioxide. Because the alumina contained in bauxite is soluble in sodium hydroxide solution, a less expensive method may be used. By alternately concentrating and diluting the solution, hydrated alumina is precipitated, and the sodium hydroxide may be reused without neutralization. Hydrated alumina, also called aluminum hydroxide or aluminum hydrate, is readily soluble in acids or alkalies and is used as a raw material in the manufacturing process of all aluminum compounds.

Animal Fibers

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All animal fibers are complex proteins. They are resistant to most organic acids (see Acids and Bases) and to certain powerful mineral acids such as sulfuric acid. However, protein fibers are damaged by mild alkalies (basic substances) and may be dissolved by strong alkalies such as sodium hydroxide. They can also be damaged by chlorine-based bleaches, and undiluted liquid hypochloride bleach will dissolve wool or silk.

The principal component of silk is the protein fibroin. Silk is exuded in continuous filaments from the abdomens of various insects and spiders. It is the only natural filament that commonly reaches a length of more than 1000 m (more than 3300 ft). The only silk used in commercial textiles is produced from the cocoons of the silkworm. Several silk filaments can be gathered to produce textile yarn. However, silk is often produced and used in staple form to manufacture spun yarns.

The principal component of hair, wool, and fur is the protein keratin. Individual hairs may be as long as 91 cm (36 in) but are usually no more than 41 cm (16 in). Thus, fibers of hair and wool are not continuous and must be spun into yarn if they are to be woven or knitted into textile fabrics, or they must be made into felt. Any hair fiber can legally be marketed as wool or bear the common English name of the animal from which it was gathered—for example, camel's hair.

The principal hair fiber used to produce textile fabrics is sheep's wool. In wild sheep, the wool is a short, soft underlayer protected by longer, coarser hair. In domesticated sheep bred for their fleece, the wool is much longer. Yarns made of wool are classified as either woolen or worsted. Wool fibers less than 5 cm (less than 2 in) in length are made into fuzzy, soft woolen yarns. Longer fibers are used for the smoother and firmer worsted yarns. Naturally crimped wool fibers produce air-trapping yarns that are used for insulating materials.

Other animals used as sources of hair fibers for textiles include camels, llamas, alpacas, guanacos, vicuñas, rabbits, reindeer, Angora goats, and Kashmīr (or cashmere) goats. Fur fibers from animals such as mink and beavers are sometimes blended with other hairs to spin luxury yarns but are most often found as fur pelts. Horsehair and cow's hair are used for felts and are sometimes spun as yarn, particularly for upholstery and other applications for which durability is important. Even human hair has been spun into yarn and used for textiles.

Keratin

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Keratin, highly fibrous and resistant protein that makes up most of the material in the cells forming the epidermis, hair, nails, scales, feathers, beaks, horns, and hooves of animals. These cells originate from permanent populations of germinal cells, and as they migrate outward they undergo specific patterns of differentiation in a process called keratinization. That is, the cells become increasingly filled with microfibrils of keratin, and the nuclei and organelles of the cells are reabsorbed. Little is understood, however, about how the cells differentiate to form such diverse and efficient structures as the elastic outer layer of the skin of mammals or the stiff scales of fish. An important quality of keratin is its ability to extend and contract reversibly.

Vegetable Fibers

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Vegetable fibers are predominantly cellulose, which, unlike the protein of animal fibers, resists alkalies. Vegetable fibers resist most organic acids but are destroyed by strong mineral acids. Improper use of most bleaches can also weaken or destroy these fibers.

There are four major types of vegetable fibers: seed fibers, which are the soft hairs that surround the seeds of certain plants; bast fibers, the tough fibers that grow between the bark and stem of many dicotyledonous plants (see Dicots); vascular fibers, the tough fibers found in the leaves and stems of monocotyledons (see Monocots); and grass-stem fibers. Other fiber types, of limited utility, include strips of leaf skins, such as raffia; the fiber of fruit cases, such as coir; and palm fibers.

Only two seed fibers, cotton and kapok (see Ceiba), have commercial importance. Cotton fiber, which grows in the seed pod of cotton plants, is the only one that is useful for the manufacture of textiles. Different species of cotton plants produce fibers of different lengths. Long-staple fibers are spun into fine, strong yarns, which are then woven into better-quality fabrics. Short-staple fibers produce coarser yarns for durable fabrics. Cotton yarns can be dyed (see Dyeing) and printed easily, so that they are useful for producing woven fabrics with a multitude of colors and designs. Kapok cannot be spun but is used as upholstery stuffing. Because it is hollow, kapok is buoyant. It was once used in flotation devices such as life preservers, but it has largely been replaced by other materials.

A wide variety of bast fibers are used in applications ranging from fine woven textiles to cordage. Linen cloth is made from flax. Coarser clothes and rope are produced from hemp, jute, ramie, and sunn.

Vascular fibers are used almost exclusively for making cordage. They include agave (sisal), henequen, manila hemp, and yucca. The vascular fibers of pineapple have been used in the production of textiles. Entire stems of some grasses and straws, such as esparto, are woven as fibers for hats and matting.

Antifreeze

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Antifreeze, chemical substance added to a liquid to lower its freezing point. It prevents the freezing of the coolants used in airplane, automobile, and tractor engines, in refrigeration liquids, and in snow-melting and deicing agents.

The ideal antifreeze should be chemically stable, be miscible in the coolant, have low viscosity and electrical conductivity and a high boiling point, be noncorrosive, and have good heat-transfer properties. The most widely used antifreeze materials in automotive engines today are methyl alcohol, ethyl alcohol, and ethylene glycol; most of them contain a phosphate, nitrate, or other anticorrosive agent.

Antimatter

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Antimatter, matter composed of elementary particles that are, in a special sense, mirror images of the particles that make up ordinary matter as it is known on earth. Antiparticles have the same mass as their corresponding particles but have opposite electric charges or other properties related to electromagnetism. For example, the antimatter electron, or positron, has opposite electric charge and magnetic moment (a property that determines how it behaves in a magnetic field), but is identical in all other respects to the electron. The antimatter equivalent of the chargeless neutron, on the other hand, differs in having a magnetic moment of opposite sign (magnetic moment is another electromagnetic property). In all of the other parameters involved in the dynamical properties of elementary particles, such as mass, spin, and partial decay, antiparticles are identical with their corresponding particles.

The existence of antiparticles was first proposed by the British physicist Paul Adrien Maurice Dirac, arising from his attempt to apply the techniques of relativistic mechanics (see Relativity) to quantum theory. In 1928 he developed the concept of a positively charged electron but its actual existence was established experimentally in 1932. The existence of other antiparticles was presumed but not confirmed until 1955, when antiprotons and antineutrons were observed in particle accelerators. Since then, the full range of antiparticles has been observed or indicated. Antimatter atoms were created for the first time in September 1995 at the European Organization for Nuclear Research (CERN). Positrons were combined with antimatter protons to produce antimatter hydrogen atoms. These atoms of antimatter exist only for forty-billionths of a second, but physicists hope future experiments will determine what differences there are between normal hydrogen and its antimatter counterpart.

A profound problem for particle physics and for cosmology in general is the apparent scarcity of antiparticles in the universe. Their nonexistence, except momentarily, on earth is understandable, because particles and antiparticles are mutually annihilated with a great release of energy when they meet. Distant galaxies could possibly be made of antimatter, but no direct method of confirmation exists. Most of what is known about the far universe arrives in the form of photons, which are identical with their antiparticles and thus reveal little about the nature of their sources. The prevailing opinion, however, is that the universe consists overwhelmingly of “ordinary” matter, and explanations for this have been proposed by recent cosmological theory.

Apatite

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Apatite (Greek apate, “deception”), mineral so named because it resembles various other minerals for which it might be mistaken. It consists chiefly of phosphate of lime. Apatite is a distinct mineral of composition in which some or all of the fluorine may be replaced by chlorine (chlorapatite). The mineral crystallizes in the hexagonal system (see Crystal) and has a hardness of 5 and a specific gravity of 3.2. When pure, apatite is colorless and transparent, but it may exhibit various degrees of color and opacity. These mineral phosphates of lime were often used in the preparation of fertilizers, but they have been replaced by phosphate rock.

Aqua Regia

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Aqua Regia (Latin, “royal water”), mixture of concentrated hydrochloric and nitric acids, containing one part by volume of nitric acid to three parts of hydrochloric acid. Aqua regia was used by the alchemists and its name is derived from its ability to dissolve the so-called noble metals, particularly gold, which are inert to either of the acids used separately. It is still occasionally used in the chemical laboratory for dissolving gold and platinum.

Avogadro’s Number

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Avogadro’s Number, the number of molecules that exist in one mole, or gram molecular weight, of any substance. One gram molecular weight is the weight of a substance, in grams, that is numerically equivalent to the dimensionless molecular weight of that substance (see Periodic Law). The number of molecules in one gram molecular weight has been determined to be approximately molecules, as established by various methods currently available to physical chemists.

The Avogadro number is named in honor of the Italian physicist Amedeo Avogadro, who postulated in 1811 that equal volumes of gases, at equivalent temperatures and pressures, contain the same number of molecules (see Avogadro's Law). The theory was significant in the development of chemistry, but the number itself was not calculated until the later 19th century, when the concept was extended to include not only gases but all chemicals. Volume considerations do not apply to liquids or solids, but Avogadro's number itself holds true for all substances, whatever their state.

Avogadro’s Law

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Avogadro’s Law, fundamental law of chemistry stating that under identical conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules. The law was first proposed as a hypothesis by the Italian physicist Amedeo Avogadro in 1811. Italian chemists and physicists continued to develop this hypothesis, and in the 1850s, largely through the efforts of the Italian chemist Stanislao Cannizzaro, Avogadro's law was universally accepted.

Catalysis

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Catalysis, alteration of the speed of a chemical reaction, through the presence of an additional substance, known as a catalyst, that remains chemically unchanged by the reaction. Enzymes, which are among the most powerful catalysts, play an essential role in living organisms, where they accelerate reactions that otherwise would require temperatures that would destroy most of the organic matter.

A catalyst in a solution with—or in the same phase as—the reactants is called a homogeneous catalyst. The catalyst combines with one of the reactants to form an intermediate compound that reacts more readily with the other reactant. The catalyst, however, does not influence the equilibrium of the reaction, because the decomposition of the products into the reactants is speeded up to a similar degree. An example of homogeneous catalysis is the formation of sulfur trioxide by the reaction of sulfur dioxide with oxygen, in which nitrogen dioxide serves as a catalyst. Under extreme heat, sulfur dioxide and nitrogen dioxide react to form sulfur trioxide and the intermediate compound nitric oxide, which then reacts with oxygen to re-form nitrogen dioxide. The same amount of nitrogen dioxide exists at both the beginning and end of the reaction.

Battery

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Battery, also electric cell, device that converts chemical energy into electricity. Strictly speaking, a battery consists of two or more cells connected in series or parallel, but the term is also used for single cells. All cells consist of a liquid, paste, or solid electrolyte and a positive electrode, and a negative electrode. The electrolyte is an ionic conductor; one of the electrodes will react, producing electrons, while the other will accept electrons. When the electrodes are connected to a device to be powered, called a load, an electrical current flows.

Batteries in which the chemicals cannot be reconstituted into their original form once the energy has been converted (that is, batteries that have been discharged) are called primary cells or voltaic cells. Batteries in which the chemicals can be reconstituted by passing an electric current through them in the direction opposite that of normal cell operation are called secondary cells, rechargeable cells, storage cells, or accumulators.

SOLAR BATTERY

Solar batteries produce electricity by a photoelectric conversion process. The source of electricity is a photosensitive semiconducting substance such as a silicon crystal to which impurities have been added. When the crystal is struck by light, electrons are dislodged from the surface of the crystal and migrate toward the opposite surface. There they are collected as a current of electricity. Solar batteries have very long lifetimes and are used chiefly in spacecraft as a source of electricity to operate the equipment aboard. See Solar Energy.

Beryl

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Beryl, mineral and, in certain varieties, a valuable gem material. Chemically it consists of aluminum beryllium silicate and it is the chief commercial ore of beryllium. Pure beryl is colorless and transparent. Emerald, one of the most valuable gems, is a variety that is colored green by minute amounts of chromium. Aquamarine, also a gemstone, is a blue beryl, more common than emerald. Golden beryl and morganite or rose beryl are less valuable. Colorless beryl is occasionally used as a gem under the name goshenite. Beryl has a vitreous luster with little fire or brilliancy, and so its value depends principally on hardness, transparency, and color. It has a hardness of 7.5 to 8 and a specific gravity of 2.75 to 2.8.

Beryl crystallizes in the hexagonal system. Large lettuce-green opaque crystals, some weighing over a ton, are found embedded in a variety of granite called pegmatite. Large, transparent crystals of the colored varieties are occasionally found.

Biological Radiation Effects

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Biological Radiation Effects, effects observed when ionizing radiation strikes living tissue and damages the molecules of cellular matter. Cellular function may be temporarily or permanently impaired from the radiation, or the cell may be destroyed. The severity of the injury depends on the type of radiation, the absorbed dose, the rate at which the dose was absorbed, and the radiosensitivity of the tissues involved. The effects are the same, whether from a radiation source outside the body or from material within.

The biological effects of a large dose of radiation delivered rapidly differ greatly from those of the same dose delivered slowly. The effects of rapid delivery are due to cell death, and they become apparent within hours, days, or weeks. Protracted exposure is better tolerated because some of the damage is repaired while the exposure continues, even if the total dose is relatively high. If the dose is sufficient to cause acute clinical effects, however, repair is less likely and may be slow even if it does occur. Exposure to doses of radiation too low to destroy cells can induce cellular changes that may be detectable clinically only after some years.

Biomass

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Biomass, contraction for biological mass, the amount of living material provided by a given area of the earth's surface. The term is most familiar from discussions of biomass energy, that is, the fuel energy that can be derived directly or indirectly from biological sources. Biomass energy from wood, crop residues, and dung remains the primary source of energy in developing regions. In a few instances it is also a major source of power, as in Brazil, where sugarcane is converted to ethanol fuel, and in China's Sichuan province, where fuel gas is obtained from dung. Various research projects aim at further development of biomass energy, but economic competition with petroleum has mainly kept such efforts at an early developmental stage.

See Synthetic Fuels; Gasohol.

Boric Acid

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Boric Acid, white crystalline powder. Although boric acid is poorly soluble in water at room temperature (1 g dissolving in 18 g water), it dissolves readily in hot water (1 g dissolving in less than 4 g water) and in alcohol and glycerine. It is slightly volatile in steam. A significant amount of boric acid appears in natural steam vents in Tuscany (Toscana), Italy, but free boric acid is not otherwise found in nature. Salts, however, occur in many places. Boric acid can be easily prepared by treating borax with sulfuric acid. Boric acid in solution is only slightly acidic and acts as a nonirritating, slightly astringent antiseptic, mild enough to be used as an eyewash. Commercially, boric acid is used in glazing pottery, in fireproofing cloth, in making electroplating baths and artificial gems, and in hardening steels.

Boson

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Boson, one of the two basic divisions of elementary particles, the basic units of matter and energy. Some bosons, called elementary bosons, are fundamental particles, meaning they cannot be divided into anything smaller. These bosons carry energy between particles of matter, affecting the behavior of matter particles and holding the particles together in larger structures. Mesons are bosons that are made of more than one particle. Bosons are named for Indian physicist Satyendra Bose, who (with German-born American physicist Albert Einstein) developed a set of equations that describe the way bosons behave. See also Elementary Particles.

Bosons fall into two main groups. One group contains the elementary bosons, or bosons that are not made up of other particles. Elementary bosons play a crucial role in transferring energy between the fermions that compose matter. The other group is called the mesons. Mesons are composite particles—that is, they are made up of other particles. Mesons play an important role in holding together the particles in atoms.

Brass

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Brass (alloy), alloy of copper and zinc. Harder than copper, it is ductile and can be hammered into thin leaves. Formerly any alloy of copper, especially one with tin, was called brass, and it is probable that the “brass” of ancient times was of copper and tin. The modern alloy came into use about the 16th century.

The malleability of brass varies with its composition and temperature and with the presence of foreign metals, even in minute quantities. Some kinds of brass are malleable only when cold, others only when hot, and some are not malleable at any temperature. All brass becomes brittle if heated to a temperature near the melting point.

To prepare brass, zinc is mixed directly with copper in crucibles or in a reverberatory or cupola furnace. The ingots are rolled when cold. The bars or sheets can be rolled into rods or cut into strips that can be drawn out into wire.

British Thermal Unit

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British Thermal Unit, in science and engineering, a unit measurement of heat or energy, usually abbreviated as Btu or BTU. One Btu was originally defined as the quantity of heat required to raise the temperature of 1 lb (0.45 kg) of water from 59.5° F (15.3° C) to 60.5° F (15.8° C) at constant pressure of 1 atmosphere; for very accurate scientific or engineering measurements, however, this value was not precise enough. The Btu has now been redefined in terms of the joule as equal to 1055 joules; in engineering, a Btu is equivalent to approximately 0.293 watt-hour.

Bronze

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Bronze, metal compound containing copper and other elements. The term bronze was originally applied to an alloy of copper containing tin, but the term is now used to describe a variety of copper-rich material, including aluminum bronze, manganese bronze, and silicon bronze.

CHARACTERISTICS AND USES

Bronze is stronger and harder than any other common metal alloy except steel. It does not easily break under stress, is corrosion resistant, and is easy to form into finished shapes by molding, casting, or machining.

The strongest bronze alloys contain tin and a small amount of lead. Tin, silicon, or aluminum is often added to bronze to improve its corrosion resistance. As bronze weathers, a brown or green film forms on the surface. This film inhibits corrosion. For example, many bronze statues erected hundreds of years ago show little sign of corrosion. Bronzes have a low melting point, a characteristic that makes them useful for brazing—that is, for joining two pieces of metal. When used as brazing material, bronze is heated above 430°C (800°F), but not above the melting point of the metals being joined. The molten bronze fuses to the other metals, forming a solid joint after cooling.

Lead is often added to make bronze easier to machine. Silicon bronze is machined into piston rings and screening, and because of its resistance to chemical corrosion it is also used to make chemical containers. Manganese bronze is used for valve stems and welding rods. Aluminum bronzes are used in engine parts and in marine hardware.

Bronze containing 10 percent or more tin is most often rolled or drawn into wires, sheets, and pipes. Tin bronze, in a powdered form, is sintered (heated without being melted), pressed into a solid mass, saturated with oil, and used to make self-lubricating bearings.

Positron

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Positron, elementary particle identical to the electron except for its electric charge and its magnetic moment (a property that determines how it behaves in a magnetic field). Positrons are elementary particles, which are fundamental constituents of matter—that is, they cannot be divided into smaller units. Positrons have uses in medicine and in industry, particularly in a form of imaging known as positron emission tomography (PET).

CHARACTERISTICS AND BEHAVIOR

All elementary particles have basic characteristics called mass, charge, and spin (a property analogous to angular momentum). The positron has the same mass—amount of matter—as the electron, and the same spin. The two particles also have the same amount of electric charge, but the positron’s charge is positive and the electron’s is negative. For this reason, the positron is sometimes called a positive electron. Although positrons and electrons have a measurable mass, charge, and spin, they have no measurable size, shape, or structure. Scientists therefore consider them pointlike. Other pointlike elementary particles include neutrinos and quarks.

Every elementary particle has an equal and opposite antiparticle. The positron is the antiparticle of the electron. Just as particles combine to form ordinary matter, antiparticles combine to create antimatter. When a particle and its antiparticle collide, they destroy each other, releasing energy. This feature makes positrons useful in creating PET scans, images of the brain and other soft tissues inside the body. To create a PET scan, positron-emitting substances are injected into the body. Computers track the energy released inside the body by positron-electron collisions and use this information to form images. PET scans are especially helpful in identifying and locating brain tumors and in studying other disorders in the brain. Positrons are also used in industry to reveal defects on metal surfaces and in semiconductors.

Positrons are emitted by certain radioactive substances that scientists create in the laboratory. They are also produced within stars and by collisions of cosmic rays (high energy particles that originate in space). But positrons are short-lived because they soon collide with electrons. In the laboratory, scientists create positrons by a method known as pair production. In this method a gamma ray (particle of electromagnetic energy) interacts with the nucleus of a very heavy atom, producing a positron and an electron.

Calcite

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Calcite, an extremely abundant mineral composed of calcium carbonate. It can form crystals in a wide variety of shapes and colors. It can be a primary or secondary component in sedimentary, igneous, or metamorphic rocks. It often provides the cement that binds particles together in sedimentary rocks. Calcite exhibits several physical properties that make it relatively easy to identify. These properties include its tendency to react with a dilute solution of hydrochloric acid and to break into rhombohedrons. Rhombohedrons are six-sided solids that resemble cubes except that the faces meet at 60° instead of 90°. Calcite crystals and calcite-rich rocks are valuable for a variety of uses that range from components in optical instruments to cement.

Calcite is the third most common mineral in the earth’s crust (behind feldspar and quartz). Because of its abundance, calcite can be found in many rock types.

As a crystal, calcite can take on a variety of forms, also called “habits”. More than 300 different forms of calcite exist. Some especially common shapes are “dogtooth spar” and rhombohedrons. Dogtooth spar crystals are elongated six-sided pyramids, except without a flat bottom. Rhombohedrons resemble cubes except that their faces intersect at 60° instead of 90°. Most large crystals of calcite form in veins by precipitation from hot or cold groundwater as it moves through cracks in rock deep underground. Most large masses of calcite-rich rock form biochemically, in the case of limestone, or as a precipitate, in the case of travertine.

Capacitance

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Capacitance, ability of a circuit system to store electricity. The capacitance of a capacitor is measured in farads and is determined by the formula C = q/V, where q is the charge (in coulombs) on one of the conductors and V is the potential difference (in volts) between the conductors. The capacitance depends only on the thickness, area, and composition of the capacitor's dielectric.

Capacitor

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Capacitor, or electrical condenser, device for storing an electrical charge. In its simplest form a capacitor consists of two metal plates separated by a nonconducting layer called the dielectric. When one plate is charged with electricity from a direct-current or electrostatic source, the other plate will have induced in it a charge of the opposite sign; that is, positive if the original charge is negative and negative if the charge is positive. The Leyden jar is a simple form of capacitor in which the two conducting plates are metal-foil coatings on the inside and outside of a glass bottle or jar that serves as the dielectric. The electrical size of a capacitor is its capacitance, the amount of electric charge it can hold.

Capacitors are limited in the amount of electric charge they can absorb; they can conduct direct current for only an instant but function well as conductors in alternating-current circuits. This property makes them useful when direct current must be prevented from entering some part of an electric circuit. Fixed-capacity and variable-capacity capacitors are used in conjunction with coils as resonant circuits in radios and other electronic equipment. Large capacitors are also employed in power lines to resonate the load on the line and make it possible for the line to transmit more power.

Capacitors are produced in a wide variety of forms. Air, mica, ceramics, paper, oil, and vacuums are used as dielectrics, depending on the purpose for which the device is intended.

Carbon Dioxide

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Carbon Dioxide, colorless, odorless, and slightly acid-tasting gas, sometimes called carbonic acid gas, the molecule of which consists of one atom of carbon joined to two atoms of oxygen. It was called “fixed air” by the Scottish chemist Joseph Black, who obtained it through the decomposition of chalk and limestone and recognized that it entered into the chemical composition of these substances. The French chemist Antoine Lavoisier proved that it is an oxide of carbon by showing that the gas obtained by the combustion of charcoal is identical in its properties with the “fixed air” obtained by Black. Carbon dioxide is about 1.5 times as dense as air. It is soluble in water, 0.9 volume of the gas dissolving in 1 volume of water at 20° C (68° F).

Carbon dioxide is produced in a variety of ways: by combustion, or oxidation, of materials containing carbon, such as coal, wood, oil, or foods; by fermentation of sugars; and by decomposition of carbonates under the influence of heat or acids. Commercially, carbon dioxide is recovered from furnace or kiln gases; from fermentation processes; from reaction of carbonates with acids; and from reaction of steam with natural gas, a step in the commercial production of ammonia. The carbon dioxide is purified by dissolving it in a concentrated solution of alkali carbonate or ethanolamine and then heating the solution with steam. The gas is evolved and is compressed into steel cylinders.

The atmosphere contains carbon dioxide in variable amounts, usually 3 to 4 parts per 10,000, and has been increasing by 0.4 percent a year. It is used by green plants in the process known as photosynthesis, by which carbohydrates are manufactured.

The presence of carbon dioxide in the blood stimulates breathing. For this reason, carbon dioxide is added to oxygen or ordinary air in artificial respiration and to the gases used in anesthesia.

Carbon Disulfide

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Carbon Disulfide, colorless, extremely volatile and flammable compound with a disagreeable, fetid odor. It is used as a solvent for oils, fats, and waxes; as a reagent in the manufacture of regenerated cellulose; as the starting material in the manufacture of carbon tetrachloride; in rayon and cellophane production; and in the vulcanization of rubber. Carbon disulfide is made by heating carbon and sulfur together or by the reaction between methane and sulfur vapor. It freezes at -111.53° C (-168.75° F) and boils at 46.25° C (115.25° F).

Chloroform

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Chloroform, name given to trichloromethane because of its supposed relation to formic acid. A colorless liquid, half again as dense as water and of about the same viscosity, chloroform has a heavy, etherlike odor and a burning sweetness of taste, being about 40 times as sweet as cane sugar. It is almost insoluble in water, but it is freely miscible with organic solvents and is an important solvent for gums, resins, fats, elements such as sulfur and iodine, and a wide variety of organic compounds.

Chloroform may be prepared by the chlorination of ethyl alcohol or of methane, or by the action of iron and acid on carbon tetrachloride; the latter is the principal industrial method in current use.

Chloroform was first prepared in 1831 and was first used as an anesthetic in 1847 in one of the earliest experiments on surgical anesthesia. In the presence of light, however, it tends to decompose, yielding the highly poisonous compound phosgene. Even when pure, it causes fatal cardiac paralysis in about one out of 3000 cases, and so is seldom used for anesthesia.

Chromite

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Chromite, only ore mineral of chromium, consisting of ferrous chromite and belonging to the spinel group. It crystallizes in the isometric system (see Crystal) and has a hardness of 5.5 and a specific gravity, or relative density, of 4.1 to 4.8. It is found in irregular brownish-black or black grains or octahedral crystals. Chromite is one of the first minerals to crystallize from magma. It occurs principally in rocks containing various amounts of ferromagnetic minerals. It also occurs in the minerals serpentine and peridotite, as well as in glacial and alluvian deposits. Large deposits of chromite are found in Kazakhstan, Turkey, and Zimbabwe, and also in Austria, Bosnia and Herzegovina, Serbia and Montenegro (formerly the Federal Republic of Yugoslavia), and the Former Yugoslav Republic of Macedonia.

Cinnabar

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Cinnabar, mineral consisting of mercuric sulfide, the principal commercial source of mercury. It is bright red in color, crystallizes in the hexagonal system (see Crystal), and has perfect prismatic cleavage. The hardness of cinnabar is 2.5, and the specific gravity is 8.10. The mineral is comparatively rare and usually occurs in volcanic vein deposits in sedimentary rocks. Important deposits of cinnabar are found in Spain, Italy, Mexico, and in California and Nevada in the United States. Artificial cinnabar, made from a mixture of sulfur and mercury, is used as the red pigment called vermilion.

Citric Acid

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Citric Acid, white solid, soluble in water and slightly soluble in organic solvents, which melts at 153° C (307° F). Aqueous solutions of citric acid are slightly more acidic than solutions of acetic acid. Traces of citric acid are found in numerous plants and animals, because it is a nearly universal intermediate product of metabolism. Large amounts of the acid are found in the juice of citrus fruits, from which it is precipitated by the addition of lime; the resulting calcium citrate is treated with sulfuric acid to regenerate the citric acid. Fermentation of sugar by the mold Aspergillus niger is the chief commercial source of the acid. It is added to some foods and beverages to produce a pleasant acid flavor; it is also used in medicines, in making blueprint paper, in textile printing, and as a polishing agent for metals.

Ether

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Ether (physics and astronomy), substance once thought to fill all space, but now known not to exist. Scientists of the late 19th and early 20th centuries believed that the ether was the medium, or substance, that allowed light to travel through space. The theory of relativity of German American physicist Albert Einstein showed that light did not need a medium through which to travel, so belief in the existence of the ether was abandoned. See also Quantum Theory.

DEVELOPMENT OF THE ETHER THEORY

Physicists have tried for hundreds of years to determine whether light is a stream of particles or a set of waves. In the 1860s and 1870s Scottish physicist James Clerk Maxwell formulated a theory that linked electricity and magnetism, and light, to waves of electromagnetic energy. His theory predicted that waves of varying electric and magnetic fields travel through space in the form of electromagnetic waves. These waves carry energy from place to place. Maxwell showed that these electromagnetic waves traveled at 300,000 km/s (190,000 mi/s)—the same speed that earlier scientists had measured to be the speed of light. His theory was thus strong evidence that light is carried by waves.

Before Maxwell’s theory was established, all of the types of waves that scientists knew of needed a medium—a substance through which to travel. For example, waves on a rope travel along the rope, and sound waves travel through air or some other substance. Scientists deduced that light waves must also travel through a medium. Scientists knew that light waves reached the earth from distant stars, and so they knew that light could travel through outer space. Physicists reasoned that outer space must be filled with an invisible medium, which they called the luminiferous ether, or just the ether. The function of the ether was to allow light waves to travel through space.

REFUTING THE ETHER THEORY

Scientists believed that the ether did not have mass and was invisible, so it was undetectable by normal chemical and physical means, even though it permeated all matter and all space. Albert A. Michelson, one of the first great American experimental physicists, formulated an experiment in the 1870s to detect the ether by studying its effect on light. Michelson repeated this experiment more accurately in 1887 with American chemist Edward W. Morley, and the experiment is known today as the Michelson-Morley experiment.

Michelson deduced that the earth must move relative to the ether. The earth moves through space around the sun, so if the ether exists and occupies all space, the earth must move through the ether. Michelson and Morley reasoned that the velocity of a light wave should depend on whether it is moving in the same direction as the ether (opposite the earth’s motion) or in the opposite direction of the ether’s movement (the same direction as the earth). Light moving in the same direction as the ether should travel faster than light moving against the ether’s motion. They based this assumption on the behavior of other waves—the speed of waves traveling in water, for example, is related to the speed at which the water is flowing.

The Michelson-Morley experiment separated a beam of light with a special piece of glass, half of which was a mirror, called a beam splitter. Half of the beam of light traveled in the direction of the earth’s motion, and half was reflected by the mirror and traveled perpendicular to the earth’s motion. Each beam bounced off other mirrors and came back to the beam splitter, where the split beams were recombined into one beam. The length of the path that both beams of light traveled was exactly the same. In the original beam, the crests and troughs of the light waves were all lined up, but if one of the split beams had covered the distance more slowly than the other, the crests and troughs would no longer be lined up when the beam was recombined. If the crests and troughs were not lined up, the two beams would interfere with one another, producing a distinctive pattern of light and dark bands. If the beams traveled at the same speed, there would be no interference pattern. Michelson and Morley found no interference pattern, even after rotating their experiment through many orientations and performing the experiment at different times of year. The lack of interference demonstrated that the split beams traveled at the same speed and showed that the speed of light was independent of the motion of the earth. This finding indicated that the ether did not exist.

In 1905 Albert Einstein advanced his theory of relativity, in which the speed of light is a universal constant—the same in all directions. In Einstein’s theory, the existence of the ether is impossible. In today's physics, Einstein's special theory of relativity is completely accepted, and the ether is viewed only as a historical relic that does not actually exist.

Coal

Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is used to manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth’s crust, its outermost layer. Coal, petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.

Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.

Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.

Cobaltite

Cobaltite, mineral, a compound of cobalt, arsenic, and sulfur. It occurs in isometric crystals resembling those of pyrite. It is silver white to red in color and has a metallic luster. The specific gravity of cobaltite is between 6 and 6.4, and the hardness is 5.5. Cobaltite occurs in high-temperature vein deposits or in disseminations in rocks associated with nickel and other cobalt minerals. It is mined as an important ore of cobalt. Large deposits are found in Sweden, Norway, Myanmar (formerly known as Burma), Australia, and Ontario in Canada.

Color

Color, physical phenomenon of light or visual perception associated with the various wavelengths in the visible portion of the electromagnetic spectrum (see Electromagnetic Radiation; Spectrum). As a sensation experienced by humans and some animals, perception of color is a complex neurophysiological process. The methods used for color specification today belong to a technique known as colorimetry and consist of accurate scientific measurements based on the wavelengths of three primary colors.

PRIMARY COLORS

The human eye does not function like a machine for spectral analysis, and the same color sensation can be produced by different physical stimuli. Thus a mixture of red and green light of the proper intensities appears exactly the same as spectral yellow, although it does not contain light of the wavelengths corresponding to yellow. Any color sensation can be duplicated by mixing varying quantities of red, blue, and green. These colors, therefore, are known as the additive primary colors. If light of these primary colors is added together in equal intensities, the sensation of white light is produced. A number of pairs of pure spectral colors called complementary colors also exist; if mixed additively, these will produce the same sensation as white light. Among these pairs are certain yellows and blues, greens and blues, reds and greens, and greens and violets.

Most colors seen in ordinary experience are caused by the partial absorption of white light. The pigments that give color to most objects absorb certain wavelengths of white light and reflect or transmit others, producing the color sensation of the unabsorbed light.

The colors that absorb light of the additive primary colors are called subtractive primary colors. They are magenta (purplish-pink), which absorbs green; yellow, which absorbs blue; and cyan (light greenish-blue), which absorbs red. Thus, if a green light is thrown on a magenta pigment, the eye will perceive black. These subtractive primary colors are also called the pigment primaries. They can be mixed together in varying amounts to match almost any hue. If all three are mixed in about equal amounts, they will produce black. An example of the mixing of subtractive primaries is in color photography and in the printing of colored pictures in magazines, where magenta, yellow, black, and cyan inks are used successively to create natural color. Edwin Herbert Land, an American physicist and inventor of the Polaroid Land camera, demonstrated that color vision depends on a balance between the longer and shorter wavelengths of light. He photographed the same scene on two pieces of black-and-white film, one under red illumination, for long wavelengths, and one under green illumination, for short wavelengths. When both transparencies were projected on the same screen, with a red light in one projector and a green light in the other, a full-color reproduction appeared. The same phenomenon occurred when white light was used in one of the projectors. Reversing the colored lights in the projectors made the scene appear in complementary colors.

Columbite

Columbite, mineral oxide of niobium, tantalum, iron, and manganese with varying proportions of niobium and tantalum. When the proportion of tantalum exceeds that of niobium, it is called tantalite. Columbite is the principal commercial source of tantalum and niobium. It has a hardness of 6 and a specific gravity that varies from about 5 to about 8 depending on the composition. It is black to red in color, often iridescent, and is usually found in the form of orthorhombic crystals in granite and granitic pegmatite. Columbite deposits are found in Australia, Canada, Greenland, Norway, the former Union of Soviet Socialist Republics, and in several states of the United States.

Combustion

Combustion, process of rapid oxidation or burning of a substance with simultaneous evolution of heat and, usually, light. In the case of common fuels, the process is one of chemical combination with atmospheric oxygen to produce as the principal products carbon dioxide, carbon monoxide, and water, together with products such as sulfur dioxide that may be generated by the minor constituents of the fuel (see Chemical Reaction). The term combustion, however, also embraces oxidation in the broad chemical sense, and the oxidizing agent may be nitric acid, certain perchlorates, or even chlorine or fluorine. See separate articles on most of the fuels and chemicals mentioned in this article.

Composite Material

Composite Material, substance that is made up of a combination of two or more different materials. A composite material can provide superior and unique mechanical and physical properties because it combines the most desirable properties of its constituents while suppressing their least desirable properties. For example, a glass-fiber reinforced plastic combines the high strength of thin glass fibers with the ductility and chemical resistance of plastic; the brittleness that the glass fibers have when isolated is not a characteristic of the composite. The opportunity to develop superior products for aerospace, automotive, and recreational applications has sustained the interest in advanced composites. Currently composites are being considered on a broader basis—for applications that include civil engineering structures such as bridges and freeway pillar reinforcement; and for biomedical products, such as prosthetic devices.

Composite materials usually consist of synthetic fibers embedded within a matrix, a material that surrounds and is tightly bound to the fibers. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibers made of a ceramic material such as carbon or glass embedded in a plastic matrix. Typically, the fibers make up about 60 percent of a polymer matrix composite by volume. Metal matrices or ceramic matrices can be substituted for the plastic matrix to provide more specialized composite systems called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.

Critical Point

Critical Point, in physics, point on the temperature or pressure scale, which marks a change in the physical state of a substance. The critical point of a metal alloy is the temperature during the cooling of the substance at which a molecular rearrangement takes place, giving rise to a different form of the substance, usually with the absorption or evolution of heat. The critical temperature of a gas is the maximum temperature at which the gas can be liquefied; the critical pressure is the pressure necessary to liquefy the gas at the critical temperature. Some gases, such as helium, hydrogen, and nitrogen, have low critical temperatures and require intensive cooling before they can be liquefied. Others, such as ammonia and chlorine, have high critical temperatures and can be liquefied at ordinary room temperature by pressure alone. The accompanying table shows critical temperatures and pressures for representative gases.

A third description of the critical point is the critical volume. This is the volume that one mole of gas would occupy at its critical temperature and pressure. These three quantities: critical temperature, pressure, and volume are called, collectively, the critical constants of a substance.

Decomposition

Decomposition, in chemistry, the breaking down of a substance or compound, through a chemical reaction, into its simpler components. Such reduction may yield either elements or compounds as products. A common agent of decomposition in chemistry is heat, which can reduce both inorganic and organic compounds to their constituents. Water, for example, decomposes into hydrogen and oxygen when exposed to an electric current. Also, chemical action, as by the use of acids (see Acids and Bases) or alkalies and as accelerated by catalysis, is used in laboratories to reduce compounds. Decomposition is also caused by bacteria, enzymes, and light. Fermentation, for example, occurs because of enzyme actions.

The term decomposition is also applied to the phenomenon of biological decay, or putrefaction, caused by microorganisms. Natural decomposition can also, however, yield useful products, such as petroleum.

Dielectric

Dielectric, or insulator, substance that is a poor conductor of electricity and that will sustain the force of an electric field passing through it. This property is not exhibited by conducting substances. Two oppositely charged bodies placed on either side of a piece of glass (a dielectric) will attract each other, but if a sheet of copper is instead interposed between the two bodies, the charge will be conducted by the copper.

In most instances the properties of a dielectric are caused by the polarization of the substance. When the dielectric is placed in an electric field, the electrons and protons of its constituent atoms reorient themselves, and in some cases molecules become similarly polarized. As a result of this polarization, the dielectric is under stress, and it stores energy that becomes available when the electric field is removed. The polarization of a dielectric resembles the polarization that takes place when a piece of iron is magnetized. As in the case of a magnet, a certain amount of polarization remains when the polarizing force is removed. A dielectric composed of a wax disk that has hardened while under electric stress will retain its polarization for years. Such dielectrics are known as electrets.

The effectiveness of dielectrics is measured by their relative ability, compared to a vacuum, to store energy, and is expressed in terms of a dielectric constant, with the value for a vacuum taken as unity. The values of this constant for usable dielectrics vary from slightly more than 1 for air up to 100 or more for certain ceramics containing titanium oxide. Glass, mica, porcelain, and mineral oils, often used as dielectrics, have constants ranging from about 2 to 9. The ability of a dielectric to withstand electric fields without losing insulating properties is known as its dielectric strength. A good dielectric must return a large percentage of the energy stores in it when the field is reversed. The fraction lost through so-called electric friction is called the power factor of the dielectric. Dielectrics, particularly those with high dielectric constants, are used extensively in all branches of electrical engineering, where they are employed to increase the efficiency of capacitors. See Capacitor; Electricity; Insulation.

Diffraction

Diffraction, property of wave motion, in which waves spread and bend as they pass through small openings or around barriers. Diffraction is more pronounced when the opening, or aperture, or the barrier is similar in size to or smaller than the wavelength of the incoming wave. Diffraction is a property of the motion of all waves. For example, if a radio is turned on in one room, the sound from the radio can be heard in an adjacent room even from around a doorway. Similarly, whenever water waves pass an object on the surface of the water, such as a jetty or boat dock, waves that pass the object's edge spread out into the region behind the object and directly blocked by it.

To understand this effect, Dutch physicist Christiaan Huygens proposed that each point of a wave on a flat wave front, or crest, acts like a source of secondary, spherical wavelets, or smaller waves. Before reaching a barrier, these secondary wavelets add to the original wave front. When the wave front approaches an aperture or barrier, only the wavelets approaching the unobstructed region can get past the barrier. When the size of the opening or barrier is large compared with the wavelength of the incoming wave, the sum of the wavelets passing through the aperture is nearly flat. The resulting wave front resembles the original wave front, and little bending occurs. However, when the size of the opening is comparable to or smaller than the wavelength of the incoming wave, it appears as though only a few wavelets can get through. These remaining wavelets are then a source of more wavelets that expand in all directions, and the shape of the new wave front is curved. The wavelets of these diffracted, or bent, waves can now travel different paths and subsequently interfere with each other, producing interference patterns. The shape of these patterns depends on the wavelength and the size of the aperture or barrier. According to Huygens's principle, diffraction can be thought of as the interference of a large number of coherent wave sources. Consequently, diffraction and interference are essentially the same phenomenon.

Rectification

Rectification (electricity), process of converting an alternating current (AC), which flows back and forth in a circuit, to direct current (DC), which flows only in one direction. A device known as a rectifier, which permits current to pass in only one direction, effectively blocking its flow in the other direction, is inserted into the circuit for the purpose.

Rectification is carried out at all levels of electrical power, from a thousandth of a watt to detect an AM radio signal, to thousands of kilowatts to operate heavy electrical machinery. The first commercial rectifiers were used in the conversion of alternating to direct current in the operation of electrical motors; these early rectifiers were called mechanical commutators. Today, most rectification is carried out by electronic devices, such as combinations of vacuum-tube diodes, and mercury-arc rectifiers.

Most mechanical rectifiers consist of a rotary switch that is synchronized with the current; the switch is arranged to conduct the current in one direction only. Mechanical rectifiers can be designed and constructed to handle heavy currents (up to thousands of amperes) at levels of several thousand volts, and they are still used in heavy electrical machinery.

Electronic rectifiers conduct current in one direction only by the motion of electrical charges inside the device; they can carry currents as high as 500 amp and withstand voltages up to 1000 V without damage. These rectifiers, therefore, can compete with mechanical rectifiers in many power applications. In low-voltage applications, such as in electronic equipment, either vacuum-tube or semiconductor rectifiers are used almost exclusively.

Photoelectric Effect

Photoelectric Effect, formation and liberation of electrically charged particles in matter when it is irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the photocathode, migrate to the other pole, the anode, under the influence of an electric field.

Study of the external photoelectric effect played an important role in the development of modern physics. Experiments beginning in 1887 showed that the external photoelectric effect had certain qualities that could not be explained by the theories of that time, in which light and all other electromagnetic radiation was considered to behave like waves. For example, as the light shining on a metal becomes increasingly intense, the classical wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, experiments showed that the maximum possible energy of the ejected electrons depends only on the frequency of the incident light, and is independent of the light's intensity.

Photoelectric Cell

Photoelectric Cell, also phototube, electron tube in which the electrons initiating an electric current originate by photoelectric emission. In its simplest form the phototube is composed of a cathode, coated with a photosensitive material, and an anode. Light falling upon the cathode causes the liberation of electrons, which are then attracted to the positively charged anode, resulting in a flow of current proportional to the intensity of the irradiation. Phototubes may be highly evacuated or may be filled with an inert gas at low pressure to achieve greater sensitivity. In a modification called the multiplier phototube, or the photomultiplier, a series of metal plates are so shaped and arranged that the photoelectric emission is amplified by secondary electron emission. The multiplier phototube is capable of detecting radiation of extremely low intensity; hence, it is an essential tool for those working in the area of nuclear research.

The photoelectric cell, popularly known as the electric eye, is employed in operating burglar alarms, traffic-light controls, and door openers. A phototube and a beam of light (which may be infrared or invisible to the eye) form an essential part of such an electric circuit. The light produced by a bulb at one end of the circuit falls on the phototube located some distance away. Interrupting the beam of light breaks the circuit. This in turn causes a relay to close, which energizes the burglar-alarm, or other, circuit. Various types of phototubes are used in sound recording, television, and the scintillation counter (see Particle Detectors).

Center of Mass

Center of Mass, that point at which the entire mass of an object may be considered to be located for purposes of understanding the object's motion. The center of mass of a uniform sphere is the point at the center of the sphere; the center of mass of a uniform rod with a circular cross-section is the point at the center of the cross-sectional slice of the rod that is located at the middle of the rod lengthwise. In some irregularly shaped objects, the center of mass may lie outside the object.

When trying to understand and calculate the motion of an object, focusing attention on the center of mass often simplifies the problem. For example, a rod thrown into the air moves in a complicated manner; the rod moves through the air, and at the same time it tends to rotate. If the motion of a point at the tip of the rod were tracked, the path that point would follow would be very complicated. But if the motion of the rod's center of mass were tracked, the point would follow a parabolic path that can easily be described mathematically. Additionally, the complicated rotation of the rod can be described as simple rotational motion about the center of mass. The center of mass can also be useful when examining the motions of complicated systems that are composed of many objects or particles, such as the motion of the planets around the sun.

Phagocytosis

Phagocytosis (Greek -phagos, “one that eats”; kytos, “cell”), process of ingestion of matter by cells known, in this context, as phagocytes. Single-celled life forms that bodily engulf and ingest foreign matter—whether other cells, bacteria, or nonliving material—display phagocytosis. In multicellular organisms the process is relegated to specialized cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders.

In humans and other higher animals, phagocytes are wandering cells that occur throughout the body. Larger phagocytes, called macrophages, are particularly important in the lymph system, liver, and spleen; amoeboid macrophages also travel throughout the body's tissues, feeding on bacteria and other foreign matter. Smaller phagocytes, which are known as granular leukocytes—a type of white blood cell—are carried throughout the body by the bloodstream. Attracted to sites of infection by chemicals which are emitted by the invading bacteria, they can pass through blood-vessel walls to reach the invaders. The successfulness of the process is related to the nature of the alien material. Proteins in the blood normally coat foreign particles, attracting the phagocytes to adhere and feed. If more-active bacterial forms invade the body, however, they may not be ingested until physically trapped or until coated by particular proteins called antibodies. If still uningested, they may actually be spread throughout the body by the phagocytes.

Glucose

Glucose, monosaccharide sugar, is found in honey and the juices of many fruits; the alternate name grape sugar is derived from the presence of glucose in grapes. It is the sugar most often produced by hydrolysis of natural glycosides. Glucose is a normal constituent of the blood of animals (see Sugar Metabolism).

Glucose is a white crystalline solid, less sweet than ordinary table sugar. Solutions of glucose rotate the plane of polarization of polarized light to the right; hence the alternative name dextrose (Latin dexter, “right”). Glucose crystallizes in three different forms. The degree of rotation of polarized light is different for each form.

Glucose is formed by the hydrolysis of many carbohydrates, including sucrose, maltose, cellulose, starch, and glycogen. Fermentation of glucose by yeast produces ethyl alcohol and carbon dioxide. Glucose is made industrially by the hydrolysis of starch under the influence of dilute acid or, more commonly, under that of enzymes. It is chiefly used as a sweetening agent in the food-processing industries. It is also used in tanning, in dye baths, in making tableted products, and in medicine for treating dehydration and for intravenous feeding.

Selenium

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Selenium (Greek selēnē, “moon”), symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table.

Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulfuric acid residue. It was so called because it was found in association with tellurium (Latin tellus, “earth”).

PROPERTIES AND OCCURRENCE

Chemically, selenium closely resembles sulfur and is related to tellurium. Like sulfur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium. It forms selenious acid and selenic acid, the respective salts of which are called selenites and selenates. Gray selenium melts at 217°C (423°F), boils at about 685°C (about 1265°F), and has a specific gravity of 4.81. The atomic weight of selenium is 78.96.

The element occurs in a few selenide minerals, the most common of which is clausthalite, or lead selenide. It also occurs with free sulfur and in many sulfide ores; it is generally obtained as a by-product in the refining of copper-sulfide ores. The yield from by-product sources, however, is insufficient to supply the rapidly increasing industrial demand for the element.

USES

Gray selenium conducts electricity; it is a better conductor of electricity in light than in darkness, the conductivity varying directly with the intensity of light. It is therefore used in many photoelectric devices. In the form of red selenium or as sodium selenide the element is used to impart a scarlet red color to clear glass, glazes, and enamels. It is also used to a great extent as a decolorizer of glass because it neutralizes the greenish tint produced by iron (ferrous) compounds. Small amounts of selenium are added to vulcanized rubber to increase its resistance to abrasion. Sodium selenate is an insecticide used to combat insects that attack cultivated plants, particularly chrysanthemums and carnations; the insecticide is scattered around the roots and is carried by the sap throughout the plant. Selenium sulfide is used in the treatment of dandruff, acne, eczema, seborrheic dermatitis, and other skin diseases.

Semiconductor

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Semiconductor, solid or liquid material, able to conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminum are excellent conductors, but such insulators as diamond and glass are very poor conductors (see Electrical Conductor; Insulation; Metals). At low temperatures, pure semiconductors behave like insulators. Under higher temperatures or light or with the addition of impurities, however, the conductivity of semiconductors can be increased dramatically, reaching levels that may approach those of metals. The physical properties of semiconductors are studied in solid-state physics.

CONDUCTION ELECTRONS AND HOLES

The common semiconductors include chemical elements and compounds such as silicon, germanium; selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current (See Electricity; Electron). In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal together (See Chemical Reaction). These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or “holes,” are left behind that contribute to the flow of electricity. (These holes are said to be carriers of positive electricity.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole is called the energy gap.

See also: Doping

Semiconductor

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Doping

Another method to produce free carriers of electricity is to add impurities to, or to “dope,” the semiconductor. The difference in the number of valence electrons between the doping material, or dopant (either donors or acceptors of electrons), and host gives rise to negative (n-type) or positive (p-type) carriers of electricity. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n- type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra negative electrons. In p-type silicon, atoms with three valence electrons such as aluminum (Al) lead to a deficiency of electrons, or to holes, which act as positive electrons. The extra electrons or holes can conduct electricity.

When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction. (A diode is a two-terminal device that has a high resistance to electric current in one direction but a low resistance in the other direction.) The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others. See Electronics; Laser; Rectification; Solar Energy; Transistor.

Semiconductor devices have many varied applications in electrical engineering. Recent engineering developments have yielded small semiconductor chips containing hundreds of thousands of transistors. These chips have made possible great miniaturization of electronic devices. More efficient use of such chips has been developed through what is called complementary metal-oxide semiconductor circuitry, or CMOS, which consists of pairs of p- and n-channel transistors controlled by a single circuit. In addition, extremely small devices are being made using the technique of molecular-beam epitaxy.

See also Computer; Integrated Circuit; Microprocessor.

Electrical Conductor

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Electrical Conductor, any material that offers little resistance to the flow of an electric current. The difference between a conductor and an insulator, which is a poor conductor of electricity or heat, is one of degree rather than kind, because all substances conduct electricity to some extent. A good conductor of electricity, such as silver or copper, may have a conductivity a billion or more times as great as the conductivity of a good insulator, such as glass or mica. A phenomenon known as superconductivity is observed when certain substances are cooled to a point near absolute zero, at which point their conductivity becomes almost infinite. In solid conductors the electric current is carried by the movement of electrons; in solutions and gases, the electric current is carried by ions.