Kids Research Express

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

. 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

. 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

. 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 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 o

. 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

. 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 w

. 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 hair

. 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 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

. 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, 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 Adr

. 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 (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, 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 itse

. 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, 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

. 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 ac

. 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, 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 li

. 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, 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, 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

. 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, 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, 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. Whe

. 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

. 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

. 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, 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 fro

. 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 acid

. 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, 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 paralys

. 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, 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, 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 (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 electroma

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 overburd

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, 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 primar

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, 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, 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 embe

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.

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, 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

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

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 con

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 s

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 e

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

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 bacter

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

. 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

. 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 ; se

.. 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 t

. 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.