Aardvarks are nocturnal, secretive, termite- and anteating mammals, and are one of Africa's strangest animals. Despite superficial appearances, aardvarks are not classified as true anteaters; they have no close relatives and are the only living species of the order Tubulidentata and family Orycteropodidae. Aardvarks are large piglike animals weighing from 88-143 lb (40-65 kg) and measuring …
The abacus is an ancient calculating machine. This simple apparatus is about 5,000 years old and is thought to have originated in Babylon. As the concepts of zero and Arabic number notation became widespread, basic math functions became simpler, and the use of the abacus diminished. Most of the world employs adding machines, calculators, and computers for mathematical calculations, but today Japan…
Abrasive materials are hard crystals that are either found in nature or manufactured. The most commonly used of such materials are aluminum oxide, silicon carbide, cubic boron nitride, and diamond. Other materials such as garnet, zirconia, glass, and even walnut shells are used for special applications. Abrasives are primarily used in metalworking because their grains can penetrate even the hardes…
An abscess is a circumscribed collection of pus usually caused by microorganisms. Abscesses can occur anywhere in the body—in hard or soft tissue, organs or An amoebic abscess caused by Entameoba histolytica. Phototake (CN) /Phototake NYC. Reproduced by permission. confined spaces. Due to their fluid content, abscesses can assume various shapes. Their internal pressure can cause compr…
Absolute zero, 0 Kelvin, -459.67° Fahrenheit, or -273.15° Celsius, is the minimum possible temperature: the state in which all motion of the particles in a substance has minimum motion. Equivalently, when the entropy of a substance has been reduced to zero, the substance is at absolute zero. Although the third law of thermodynamics declares that it is impossible to cool a substance …
Abyssal plains are the vast, flat, sediment-covered areas of the deep ocean floor. They are the flattest, most featureless areas on the Earth, and have a slope of less than one foot of elevation difference for each thousand feet of distance. The lack of features is due to a thick blanket of sediment that covers most of the surface. These flat abyssal plains occur at depths of over 6,500 ft (1,980 …
The term accelerators most commonly refers to particle accelerators, devices for increasing the velocity of subatomic particles such as protons, electrons, and positrons. Particle accelerators were originally invented for the purpose of studying the basic structure of matter, although they later found a number of practical applications. Particle accelerators can be subdivided into two large sub-gr…
An accretion disk is an astronomical term that refers to the rapidly spiraling matter that is in the process of falling into an astronomical object. In principle, any star could have an accretion disk, but in practice, accretion disks are often associated with highly collapsed stars such as black holes or neutron stars. The matter that serves as the base of the accretion disk can be obtained when …
Accuracy is how close an experimental reading or calculation is to the true value. Lack of accuracy may be due to error or due to approximation. The less total error in an experiment or calculation, the more accurate the results. Error analysis can provide information about the accuracy of a result. …
In the form of vinegar, acetic acid is one of the earliest chemical compounds known to and used by humans. It is mentioned in the Bible as a condiment and was used even earlier in the manufacture of white lead and the extraction of mercury metal from its ores. The first reasonably precise chemical description of the acid was provided by the German natural philosopher Johann Rudolf Glauber in about…
Acetone is a colorless, flammable, and volatile liquid with a characteristic odor that can be detected at very low concentrations. It is used in consumer goods such as nail polish remover, model airplane glue, lacquers, and paints. Industrially, it is used mainly as a solvent and an ingredient to make other chemicals. The International Union of Pure and Applied Chemistry's (IUPAC) systemati…
Acetylcholine is a highly active neurotransmitter acting as a chemical connection between nerves (neu rons). Acetylcholine diffuses across the narrow gap between nerve cells, known as the synapse and thus, plays an important role in connecting nerves to each other. By the early 1900s, scientists had a reasonably clear idea of the anatomy of the nervous system. They knew that individual nerve cells…
Acetylsalicylic acid, commonly known as aspirin, is the most popular therapeutic drug in the world. It is an analgesic (pain-killing), antipyretic (fever-reducing), and anti-inflammatory sold without a prescription as tablets, capsules, powders, or suppositories. The drug reduces pain and fever, is believed to decrease the risk of heart attacks and strokes, and may deter colon cancer and help prev…
"Acid rain" is a popularly used phrase that refers to the deposition of acidifying substances from the atmosphere and the environmental damage that this causes. Acid rain became a prominent issue around 1970, and since then research has demonstrated that the deposition of atmospheric chemicals is causing widespread acidification of lakes and streams, and possibly soil. The resulting …
Chemically, acids and bases may be considered opposites of each other. The concept of acids and bases is so important in chemistry that there are several useful definitions of "acid" and "base" that pertain to different chemical environments, although the definition above is the most common one. …
Acne, also called acne vulgaris, is a chronic inflammation of the sebaceous glands embedded in the skin. These glands secrete sebum, an oily lubricant. Although it may occur at any age, acne is most frequently associated with the maturation of young adult males. Part of the normal maturation process involves the production of—or altered expression of—hormones. During adolescence, hor…
Acorn worms are fragile tube worms that live in sand or mud burrows in the intertidal areas of the world's oceans. Acorn worms are members of the phylum Hemichordata, which includes two classes—the Enteropneusta (acorn worms) and the Pterobranchia (pterobranchs). Acorn worms, also known as tongue worms, belong to one of four genera, Balanoglossus, Glossobalanus, Ptychodera, and Sacco…
Acoustics is the science that deals with the production, transmission, and reception of sound. Sound may be produced when a material body vibrates; it is transmitted only when there is some material body, called the medium, that can carry the vibrations away from the producing body; it is received when a third material body, attached to some indicating device, is set into vibratory motion by that …
Actinides or actinoids is a generic term that refers to a series of 15 chemical elements. Denoted by the generic symbol An, these elements are all radioactive heavy metals, positioned in the seventh period and elaborated upon at the bottom of the periodic table. …
Action potentials are the electrical pulses that allow the transmission of information within nerves. An action potential represents a change in electrical potential from the resting potential of the neuronal cell membrane, and involves a series of electrical and underlying chemical changes that travel down the length of a neural cell (neuron). The neural impulse is created by the controlled devel…
The term activated complex refers to the molecular compound or compounds that exist in the highest energy state, or activated stage, during a chemical reaction. An activated complex acts as an intermediary between the reactants and the products of the reaction. A chemical reaction is the reorganization of atoms of chemically compatible and chemically reactive molecular compounds, called reactants.…
Active galactic nuclei (AGNs) are perhaps the most violently energetic objects in the universe. AGNs are located at the centers of some galaxies—perhaps most galaxies—and emit a tremendous amount of energy, sometimes on the order of trillion times the output of the Sun. An AGN may outshine all the stars in its galaxy by a factor of 100. The energy of a typical AGN is generated in a v…
Acupressure is an ancient method of improving a person's health by applying pressure to specific sites on the body. Acupressure is similar to acupuncture, but does not break the skin. Instead, the acupressure practitioner relies on pressure invoked by fingertip or knuckle to accomplish his purpose. Like acupuncture, acupressure recognizes certain pressure points located along meridians that…
Acupuncture is an ancient method of therapy that originated in China more than 2,000 years ago. It consists of inserting solid, hair-thin needles through the skin at very specific sites to achieve a cure of a disease or to relieve pain. Although it is not part of conventional medical treatment in most of the Western world, a 1998 consensus statement released by the National Institutes of Health (N…
ADA deficiency is an inherited condition that occurs in fewer than one in 100,000 live births worldwide. Individuals with ADA deficiency inherit defective ADA genes and are unable to produce the enzyme adenosine deaminase in their cells. The ADA gene consists of a single 32 kb locus containing 12 exons and is located on the long arm of chromosome 20. The enzyme adenosine deaminase is needed to bre…
An adaptation is any developmental, behavioral, physiological, or anatomical change in an organism that gives that organism a better chance to survive and reproduce. The word "adaptation" also refers to the fitting of a whole species, over time, to function in its particular environment, and to those specific features of a species that make it better-adapted. Adaptations acquired by …
Addiction is a compulsion to engage in unhealthy or detrimental behavior. Human beings can become addicted to many forms of behaviors such as gambling, overeating, sex, or reckless behavior, but the term "addiction" is most commonly used to refer to a physiological state of dependence caused by the habitual use of drugs, alcohol, or other substances. Addiction is characterized by unc…
Addison's disease, also called adrenocortical deficiency or primary adrenal hypofunction, is a rare condition caused by destruction of the cortex of the adrenal gland, one of several glands the endocrine system. Because Addison's disease is treatable, those who develop the illness can expect to have a normal life span. …
Addition, indicated by a + sign, is a method of combining numbers. The result of adding two numbers is called their sum. …
Adenosine diphosphate (ADP) is a key intermediate in the body's energy metabolism—it serves as the "base" to which energy-producing reactions attach an additional phosphate group, forming adenosine triphosphate (ATP). ATP then diffuses throughout the cell to drive reactions that require energy. Structurally, ADP consists of the purine base adenine (a complex, double-rin…
Adenosine triphosphate (ATP) is often described as the body's "energy currency"—energy-producing metabolic ADP is formed during cellular respiration with energy released by the breakdown of glucose molecules. Illustration by Hans & Cassidy. Courtesy of Gale Group. reactions store their energy in the form of ATP, which can then drive energy-requiring synth…
Adhesives bond two or more materials at their surface, and may be classified as structural or nonstructural. Structural adhesives can support heavy loads, while nonstructural adhesives cannot. Most adhesives exist in liquid, paste, or granular form, although film and fabric-backed tape varieties are also commercially available. Adhesives have been used since ancient times. The first adhesives were…
The adrenal glands are a pair of endocrine glands that sit atop the kidneys and that release their hormones directly into the bloodstream. The adrenals are flattened, somewhat triangular bodies that, like other endocrine glands, receive a rich blood supply. The phrenic (from the diaphragm) and renal (from the kidney) arteries send many small branches to the adrenals, while a single large adrenal v…
Aerobic means that an organism needs oxygen to live. Some microorganisms can live without oxygen and they are called anaerobic. Bacteria are not dependent on oxygen to burn food for energy, but most other living organisms do need oxygen. Fats, proteins, and sugars in the diet of organisms are chemically broken down in the process of digestion to release energy to drive life activities. If oxygen i…
Aerodynamics is the science of air flow over airplanes, cars, buildings, and other objects. Aerodynamic principles are used to find the best ways in which airplanes can get lift, reduce drag, and remain stable by controlling the shape and size of the wing, the angle at which it is positioned with respect to the airstream, and the flight speed. The flight characteristics change at higher altitudes …
Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of…
Africa is the world's second largest continent. From the perspective of geologists and paleontologists (scientists studying ancient life forms), Africa also takes center stage in the physical history and development of life on Earth. Africa possesses the world's richest and most concentrated deposits of minerals such as gold, diamonds, uranium, chromium, cobalt, and platinum. It is a…
The Universe is approximately 14 billion (14,000,000,000) years old. Its age is measured from the event known as the big bang—an explosion filling all space and generating all of the matter and energy that exist today. Although only in the last 50 years have astronomers been able to estimate the age of the Universe, they have long argued that the Universe must be of finite age, finite size,…
Agent Orange is a defoliant that kills plants and causes the leaves to fall off the dying plants. The name was a code devised by the United States military during the development of the chemical mixture. The name arose from the orange band that marked the containers storing the defoliant. Agent Orange was an equal mixture of two chemicals; 2, 4–D (2,4, dichlorophenoxyl acetic acid) and 2, 4…
Aging is the natural effect of time and the environment on living organisms, and death is its end result. Gerontology is the study of all aspects of aging. No single theory on how and why people age is able to account for all facets of aging. Although great strides have been made to postpone death as the result of certain illnesses, less headway has been made in delaying aging. Life span is specie…
The twelve species of agoutis are the best-known members of the family Dasyproctidae (genus Dasyprocta) of the order Rodentia. Agoutis are found from southern Mexico through Central America to southern Brazil, including the Lesser Antilles. They are long-legged, slender-bodied, rabbit-like mammals with short ears and a short tail. The body length of agoutis measures 16-24 in (41.5-62 cm), and adul…
Early farmers quickly learned that a supply of water was essential to farming. Thus, the primary fields of grain were planted alongside the great rivers of the Middle East. However, getting water from the rivers to the fields became a problem. The invention of the shaduf, or chain-of-pots, helped solve this problem. This human-powered primitive device consisted of buckets attached to a circular ro…
An agrochemical is any substance that humans use to help in the management of an agricultural ecosystem. Agrochemicals include: (1) fertilizers, (2) liming and acidifying agents, (3) soil conditioners, (4) pesticides, and (5) chemicals used in animal husbandry, such as antibiotics and hormones. The use of agrochemicals is an increasingly prominent aspect of modern, industrial agriculture. The use …
Agronomy can be defined as those branches of agricultural science that deal with the production of both plant and animal crops, and the management of soil. The subject matter of agronomy is quite diverse, but falls into three major categories: (1) crop breeding and the genetic improvement of varieties; (2) methods of cultivation of crops (both plants and animals); and (3) sustainability of the agr…
AIDS is the abbreviation for acquired immunodeficiency syndrome. The syndrome is caused by several types of a virus that is now known as the human immunodeficiency virus (HIV). AIDS is characterized by the destruction of cells that are vital to the proper operation of the immune system. People afflicted with AIDS can develop opportunistic infections; life-threatening illnesses caused by viruses or…
Acquired immunodeficiency syndrome (AIDS) is a disease characterized by the destruction of the immune system. More than 16,000 new AIDS patients are diagnosed each day. Evidence overwhelmingly supports the view that AIDS is caused by several types of a virus designated as the human immunodeficiency virus (HIV). The immune system is the principle defense system of the body to a variety of infection…
An air mass is an extensive body of air that has a relatively homogeneous temperature and moisture content over a significant altitude. Air masses typically cover areas of a few hundred, thousand, or million square kilometers. A front is the boundary at which two air masses of different temperature and moisture content meet. The role of air masses and fronts in the development of weather systems w…
Air pollution is the presence of chemicals in the earth's atmosphere that are not a normal part of the atmosphere. In other words, air pollution is contaminated air. Air contamination is divided into two broad categories: primary and secondary. Primary pollutants are those released directly into the air. Some examples include dust, smoke, and a variety of toxic chemicals, such as lead, merc…
An aircraft is a machine used for traveling through the atmosphere supported either by its own buoyancy or by some sort of engine that propels the ship through the air. Aircraft of the former type are known as lighter-thanair ships, while those of the latter type tend to be heavier-than-air machines. Included in the general term aircraft are specific machines such as dirigibles, gliders, airplanes…
A technologically advanced cousin of the balloon, airships are streamlined vessels buoyed by gases and controlled by means of propellers, rudders, and pressurized air systems. More commonly referred to as blimps and dirigibles, the airship is comprised of non-rigid, semi-rigid, and rigid types that rely on lighter-than-air gases such as helium and hydrogen for lift. Since the turn of the twentieth…
Albatrosses are large, long-lived seabirds in the family Diomedeidae, which contains about 13 species. They are found primarily in the oceans of the Southern Hemisphere. Albatrosses are superb fliers, and may be found far from land, soaring with their wings set in a characteristic bowed position. Together with petrels, shearwaters, and fulmars, albatrosses are grouped in the order Procellariformes…
Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. The scientific meaning of albedo is the ability of a surface to reflect a certain proportion of visible light. A perfect mirror has an albedo of 100%; the polished surface of white metals like aluminum or silver comes close to that figure. Some metals like brass or copper, however, are colored, and they do …
Albinism is a recessive inherited defect in melanin metabolism in which pigment is absent from hair, skin, and eyes (oculocutaneous albinism) or just from the eyes (ocular albinism). Melanin is a dark biological pigment that is formed as an end product of the metabolism of the amino acid tyrosine. When human skin is exposed to sunlight it gradually darkens or tans due to an increase in melanin. Ta…
Alchemy was a system of thinking about nature that preceded and contributed to the development of the modern science of chemistry. It was popular in ancient China, Persia, and western Europe throughout antiquity and the Middle Ages. A combination of philosophy, metallurgical arts, and magic, alchemy was based on a world view postulating an integral correspondence between the microcosm and the macr…
Alcohol is commonly thought of as either rubbing alcohol, the active ingredient in an alcoholic drink, or the additive that makes gasoline into gasohol. Chemists generalize its meaning to include almost any carbon-hydrogen compound with at least one hydroxyl group (symbolized as -OH) in its molecular structure. Categorized by the number and placement of the -OH groups, and the size and shape of th…
Alcoholism is a serious, chronic, potentially fatal condition manifested by a person's powerful addiction to alcoholic beverages. While experts have linked alcoholism to physiological (possibly hereditary), psychological, socioeconomic, ethnic, cultural, and other factors, there is no clear explanation of its genesis. Alcoholism occurs in all economic strata of society, in all age groups, f…
Aldehydes are a class of highly reactive organic chemical compounds that contain a carbonyl group (in which a carbon atom is double-bound to an oxygen atom) and at least one hydrogen atom bound to the alpha carbon (the central carbon atom in the carbonyl group). The aldehydes are similar to the ketones, which also contain a carbonyl group. In the aldehydes, however, the carbonyl group is attached …
Algae (singular: alga) are photosynthetic, eukaryotic organisms that do not develop multicellular sex organs. Algae can be unicellular, or they may be large, multicellular organisms. Algae can occur in salt or fresh waters, or on the surfaces of moist soil or rocks. The multicellular algae develop specialized tissues, but they lack the true stems, leaves, or roots of the more complex, higher plant…
Algebra is often referred to as a generalization of arithmetic. As such, it is a collection of rules: rules for translating words into the symbolic notation of mathematics, rules for formulating mathematical statements using symbolic notation, and rules for rewriting mathematical statements in a manner that leaves their truth unchanged. The power of elementary algebra, which grew out of a desire t…
An algorithm is a set of instructions that indicate a method for accomplishing a task. If followed correctly, an algorithm guarantees successful completion even without the use of any intelligence. The term algorithm is derived from the name al-Khowarizmi, a ninth century Arabian mathematician who is credited with discovering algebra. With the advent of computers, which are particularly adept at u…
The first column on the periodic table of the chemical elements is collectively called the alkali metal group: lithium, sodium, potassium, rubidium, cesium, and francium. Because their outer electron structure is similar, they all have somewhat similar chemical and physical properties. All are shiny, soft enough to cut with a knife, and most are white (cesium is yellow-white). All react with water…
The second column on the periodic table of the chemical elements is collectively called the alkaline earth metal group: beryllium, magnesium, calcium, strontium, barium, and radium. Because the outer electron structure in all of these elements is similar, they all have somewhat similar chemical and physical properties. All are shiny, fairly soft—although harder than the alkali metals—…
Alkaloids are chemical compounds found in plants that can react with acids to form salts. All alkaloids contain the element nitrogen, usually in complex, multi-ring structures. …
Most genes exist in more than one form that, when expressed, result in different characteristics. Genes may often exist in more than one form, and these forms are termed alleles of the gene. An allele is one of at least two alternative forms of a particular gene. Alleles provide the genetic instructions for products that, although similar in type, are visibly different (phenotypically different). …
An allergy is an excessive or hypersensitive response of the immune system. The allergic reaction becomes manifest as a pathological immune reaction induced either by antibodies (immediate hypersensitivity) or by lymphoid cells (delayed type allergy). Instead of fighting off a disease-causing foreign substance, the immune system launches a complex series of actions against an irritating substance,…
A striking example of differing physical properties among allotropes is the case of carbon. Solid carbon exists in two allotropic forms: diamond and graphite. Diamond is the hardest naturally occurring substance and has the highest melting point (more than 6,335°F [3,502°C]) of any element. In contrast, graphite is a very soft material, the substance from which the "lead…
A mixture of two or more metals is called an alloy. Alloys are distinguished from composite metals in that alloys are thoroughly mixed, creating, in effect, a synthetic metal. In metal composites, the introduced metal retains its identity within the matrix in the form of fibers, beads, or other shapes. Alloys can be created by mixing the metals while in a molten state or by bonding metal powders. …
An alluvial system consists of sediments eroded, transported, and deposited by water flowing in rivers or streams. The sediments, known as alluvium, can range from clay-sized particles less than 0.002 mm in diameter to boulders greater than 64 mm in diameter, depending on their source and the sediment transport capacity of streams in the system. The term alluvial is closely related to the term flu…
The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged. The element that undergoes "alpha decay" changes into a new element whose atomic number is down two and atomic mass is down four from the original element. Alpha decay occurs when a nucleus has so many protons that the…
Nonrenewable fossil fuels—coal, petroleum, and natural gas—provide more than 85% of the energy used around the world. In the United States, fossil fuels comprise 81.6% of the total energy supply, nuclear power provides 7.7%, and all renewable energy sources provide 7.3%. Wind power, active and passive solar systems, geothermal energy, and biomass are examples of renewable or alternat…
National Institutes of Health classifies alternative medicine as an unrelated group of non-orthodox therapeutic practices, often with explanatory systems that do not follow conventional biomedical explanations or more seriously, based on pseudoscience. Others more generally define it as medical interventions not taught at United States medical schools or not available at United States hospitals. A…
Altruism refers to animal behavior that benefits other animals of the same species. Living in the company of other animals presents numerous drawbacks, including increased competition for food, nest sites, and mates, and increased visibility to predators, to name just a few. We might expect animals to strive to outdo the competition whenever possible, to take the best food and other resources for …
Aluminum is the metallic chemical element of atomic number 13. Its symbol is Al, its atomic weight is 26.98, its specific gravity is 2.70, its melting point is 1,220.5°F (660°C), and its boiling point is 4,566.2°F (2,519°C). …
Alzheimer disease is the most common form of the brain disorder called dementia. People with dementia experience difficulty in carrying out daily activities because of damage to the regions of their brains that control thought, language, and memory. While many older people have Alzheimer disease, it is not a normal part of aging. Currently, there is no cure. The disease is named after the German p…
The amaranth (or pigweed) family is a large group of dicotyledonous flowering plants known to botanists as the Amaranthaceae. It is a relatively large family, having about 65 genera and 900 species. The species in this family are mostly annual or perennial herbs, although a few species are shrubs or small trees. Botanists divide Amaranthaceae into two subfamilies: the Amaranthoideae and the Gomphr…
Species in the amaryllis family are flowering plants, and are mostly long-lived, perennial herbs arising from a bulb or, less commonly, from rhizomes (underground stems). These plants have linear or strap shaped leaves, either crowded around the base of a leafless flowering stem, or arranged in two tight rows along a short stem, as in the common houseplant Clivia. The leaves are usually hairless a…
The American Standard Code for Information Interchange (ASCII; pronounced "askee") was first introduced in 1968 as a method of encoding alphabetic and numeric data in digital format. Although ASCII code was originally developed for the teletypewriter industry, it has since found widespread use in computer and information-transfer technologies. Because ASCII code is standardized, comp…
The Ames test, named for its developer, Bruce Ames, is a method to test chemicals for their cancer-causing properties. It is used by cosmetic companies, pharmaceutical manufacturers, and other industries that must prove that their products will not cause cancer in humans. Ames, a cancer researcher at the University of California, began development of his method in the late 1950s. He believed an ef…
Two numbers are said to be amicable (i.e., friendly) if each one of them is equal to the sum of the proper divisors of the others, i.e., whole numbers less than the given numbers that divide the given number with no remainder. For example, 220 has proper divisors 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, and 110. The sum of these divisors is 284. The proper divisors of 284 are 1, 2, 4, 71, and 142. Thei…
The synthesis of a protein results in a protein in the formation of an amide bond between adjacent amino acids. Proteins can be considered the most common examples of amides in the natural world. A naturally occurring amide is nicotinamide, one of the B vitamins. A third familiar natural amide is urea, also known as carbamide. Urea is the compound by which otherwise toxic wastes are excreted from …
A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage, between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins. The importance of the amino acids in nature arises from their ability to form proteins, which are the basic build…
Ammonia, composed of three parts hydrogen and one part nitrogen, is a sharp-smelling, flammable, and toxic gas that is very soluble in water, where it acts as a base in its chemical reactions. …
Ammonification is a key component in the nitrogen cycle of ecosystems. The nitrogen cycle consists of a complex of integrated processes by which nitrogen circulates among its major compartments in the atmosphere, water, soil, and organisms. During various phases of the nitrogen cycle, this element is transformed among its various organic and inorganic compounds. …
Amnesia is a dissociative psychological disorder manifested by total or partial loss of memory and usually caused by a trauma. Unlike ordinary forgetfulness (the inability to remember a friend's telephone number), amnesia is a serious threat to a person's professional and social life. Amnesia, which depending on its cause can be either organic and psychogenic, has several types. Glob…
Amniocentesis is an invasive procedure used to obtain amniotic fluid for prenatal diagnosis of a fetus (e.g., assessment of fetal lung maturity). In the 1950s the measurement of bilirubin concentrations present in amniotic fluid in monitoring the rhesus diseases was first reported. Amniocentesis for fetal chromosome analysis was also initiated in the 1950s. The first application was for fetal sex …
Amoebas are single-celled protozoans of the order Amoebida. They consist of a mass of cellular fluid surrounded by a membrane, and containing one or more nuclei (depending upon the species), as well as other cell organelles, such as food vacuoles. The word amoeba is derived from the Greek word ameibein (to change), which describes the amoeba's most easily distinguishable feature, the contin…
Amphetamines are a group of nervous system stimulants that includes amphetamine, dextroamphetamine, and methamphetamine. They are used to induce a state of alert wakefulness and euphoria, and since they inhibit appetite, they also serve as diet pills. After World War II, they were widely prescribed by physicians as diet pills, but they are generally no longer recommended for weight loss programs s…
The vertebrate class Amphibia, to date, includes about 3,500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). There is, however, a much larger number of extinct species, because this ancient group of animals were the first vertebrates to begin exploiting terrestrial environments. Fossil amphibians are known from at l…
An amplifier is a device, usually electronic, that magnifies information to a more powerful signal at the amplifier's output. Amplifiers are usually based on electronic principles but may utilize hydraulics or magnetics. Amplifiers are used when the electrical power of a signal must be increased. Audio amplifiers can increase the microwatts developed by a microphone to more than a million w…
The term amputation refers to the complete or partial severance of a limb or other body part. Surgical amputations may be performed because of an injury, congenital (birth) defect, malignant disease, infection, or vascular disease. Approximately 80% of all surgical amputations are performed on the lower limbs, such as the leg or foot. Artificial limbs (prosthetics) are often used to restore comple…
Anabolism, or biosynthesis, is the process by which living organisms synthesize complex molecules of life from simpler ones. Anabolism, together with catabolism, are the two series of chemical processes in cells that are, together, called metabolism. Anabolic reactions are divergent processes. That is, relatively few types of raw materials are used to synthesize a wide variety of end products. Thi…
The term anaerobic refers to living processes (usually the release of energy from nutrients) that take place in the absence of molecular oxygen. The earliest organisms, the prokaryotic bacteria, lived in an oxygen deficient atmosphere and extracted energy from organic compounds without oxygen (that is, by anaerobic respiration). Most organisms alive today extract their energy from nutrients aerobi…
The Earth's orbit around the Sun is not a perfect circle. It is an ellipse, albeit not a very flattened one, and this leads to a number of interesting observational effects. One of these is the analemma, the apparent path traced by the Sun in the sky when observed at the same time of day over the course of a year. The path resembles a lopsided figure eight, which you sometimes see printed o…
Analgesia is the loss of pain without the loss of consciousness. Techniques for controlling and relieving pain include acupuncture, anesthesia, hypnosis, biofeedback, and the use of analgesic drugs. Acupuncture is the ancient Chinese practice of inserting fine needles along certain pathways of the body and is used to relieve pain, especially in surgery, and to cure disease. In Western medicine the…
A signal is any time-varying physical quantity—voltage, light beam, sound wave, or other—that is used to convey information. Analog signals convey information by analogy (i.e., by mimicking the behavior of some other quantity). Digital signals convey information by assuming a series of distinct states symbolizing numbers (digits). Both analog and digital signals are essential to mode…
Analytic geometry is a branch of mathematics that uses algebraic equations to describe the size and position of geometric figures on a coordinate system. Developed during the seventeenth century, it is also known as Cartesian geometry or coordinate geometry. The use of a coordinate system to relate geometric points to real numbers is the central idea of analytic geometry. By defining each point wi…
Anaphylaxis is a severe, sudden, often fatal bodily reaction to a foreign substance or antigen. C. R. Richet first coined the term to define the puzzling reactions that occurred in dogs following injection of an eel toxin. Instead of acquiring immunity from the toxin as expected, the dogs experienced acute reactions, including often fatal respiratory difficulties, shock, and internal hemorrhaging.…
Anatomy, a subfield of biology, is the study of the structure of living things. There are three main areas of anatomy: cytology studies the structure of cell; histology examines the structure of tissues; and gross anatomy deals with organs and organ groupings called systems. Comparative anatomy, which strives to identify general structural patterns in families of plants and animals, provided the b…
There are many forms of evidence for evolution. One of the strongest forms of evidence is comparative anatomy; comparing structural similarities of organisms to determine their evolutionary relationships. Organisms with similar anatomical features are assumed to be relatively closely related evolutionarily, and they are assumed to share a common ancestor. As a result of the study of evolutionary r…
Anchovies are small, bony fish in the order Clupeiformes, a large group that also includes herring, salmon, and trout. Anchovies are in the family Engraulidae, and all of the more than 100 species are in the genus Engraulis. Anchovies are predominantly marine fish, but are occasionally found in brackish waters and even in freshwater. Species of anchovies are found in the Mediterranean Sea, Black S…
Anemia means literally lack of blood. In fact it is a reduction in the number of red blood cells, plasma, or packed red blood cells to a level that is lower than necessary for normal functioning. This is the result of the inability to replace lost cells or plasma volume at the rate they are being lost. The underlying cause for anemia may be one of several conditions. Although blood cell levels are…
Anesthesia is the loss of feeling or sensation. It may be accomplished without the loss of consciousness, or with partial or total loss of consciousness. Anesthesiology is a branch of medical science that relates to anesthesia and anesthetics. The anesthetist is a specialized physician in charge of supervising and administering anesthesia in the course of a surgical operation. Depending on the typ…
An aneurism is a weak spot in the wall of an artery or a vein that dilates or balloons out, forming a blood-filled sack or pouch. Aneurisms can occur almost anywhere in the body and are found in all age groups, although they occur primarily in the elderly. The foremost cause of aneurisms is atherosclerosis, or fatty deposits in the arteries. If an aneurism bursts, a massive amount of blood is rele…
The word angelfish is a general term that refers to many different kinds of fish. Typically, angelfish have thin bodies that are flattened laterally. They tend to have elongated dorsal, anal, and pelvic fins, and display a wide variety of colors, making them popular aquarium species. The association of these fish with angels may be due to the fins resembling wings. All angelfish belong to the taxo…
Angiography is a medical diagnostic test in which a fluid that is visible on x rays is used to take photographs of the arteries of the heart or other organs. First used in the early 1950s, angiography is now a standard procedure to locate areas where an artery is closed or constricted and interfering with the circulation of blood. Angiography applied to the heart is called coronary angiography. A …
Angiosperm is the name given to those plants that produce flowers during sexual reproduction. The term literally means "vessel seed" and refers to the fact that seeds are contained in a highly specialized organ called an ovary. Flowering plants are the most recently evolved of the major groups of plants, arising only about 130 million years ago. Despite their geological youthfulness,…
An angle is a geometric figure created by two line segments that extend from a single point or two planes which extend from a single line. The size of an angle, measured in units of degrees or radians, is related to the amount of rotation required to superimpose one of its sides on the other. First used by ancient civilizations, angles continue to be an important tool to science and industry today…
Anglerfish are marine fish that attract prey by dangling a fleshy, bait-like appendage (the esca) in front of their heads. The appendage, which resembles a fishing pole, is attached to the end of the dorsal fin's foremost spine (the illicium), which is separated from the rest of the fin. Anglerfish belong to the order Lophiiformes, which includes three suborders, 15 families, and about 215 …
Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms (the others are: Monera or bacteria, Fungi, Protists or protozoans, and Plantae or plants). Animals are multicellular, eukaryotic organisms, with cells that do not have walls made of cellulose. Animals are capable of voluntary, spontaneous movements, often in response to sensory perceptions. For their nutri…
Animal breeding is the selective mating of animals to increase the possibility of obtaining desired traits in the offspring. It has been performed with most domesticated animals, especially cats and dogs, but its main use has been to breed better agricultural stock. The more modern techniques involve a wide variety of laboratory methods, including the modification of embryos, sex selection, and ge…
The chemical causation of cancer is not a simple process. Many, perhaps most, chemical carcinogens do not have the potency to cause cancer in their usual condition. The non-cancer causing form of the chemical is called a procarcinogen. Procarcinogens are frequently complex organic compounds that the human body attempts to dispose of when ingested. Hepatic enzymes chemically change the procarcinoge…
The word anode is used in two different sets of circumstances: with respect to vacuum tubes and with respect to electrochemical cells. …
Anoles are small lizards in the genus Anolis (family Iguanidae), found only in the Americas, mostly in the tropical countries. Because anoles can change the color of their skin according to their mood, temperature, humidity, and light intensity, these animals are sometimes called chameleons. However, none of the more than 300 species of anoles is closely related to the true chameleons (family Cham…
The ant-pipits are 10-11 species of birds that make up the family Conopophagidae. These birds are exclusively South American, occurring in tropical rain forests of Amazonia. The usual habitat of ant-pipits is thick and lush with foliage, and the birds are rather shy. Consequently, these small birds are difficult to see and demanding to study. Therefore, little is known about their biology and ecol…
Of the seven continents on planet Earth—North America, South America, Europe, Africa, Asia, Australia, and Antarctica—the last lies at the southernmost tip of the world. It is the coldest, driest, and windiest continent. Ice covers 98% of the land, and its 5,100,000 sq mi (13,209,000 sq km) occupy nearly one-tenth of the Earth's land surface, or the same area as Europe and the…
The antbirds and gnat-eaters are 231 species of birds that comprise the relatively large family, Formicariidae. These birds only occur in Central and South America, mostly in lowland tropical forests. The antbirds and gnat-eaters are variable in their body form and size. Their body length ranges from 4–14 in (10–36 cm), and they have short, rounded wings, and a rounded tail that can …
Anteaters belong to the family Myrmecophagidae, which includes four species in three genera. They are found in Trinidad and range from southern Mexico to northern Argentina. The spiny anteater (echidna) of Australia is an egg-laying mammal and is not related to the placental anteaters of the New World. The banded anteater (or numbat) of Australia is a marsupial mammal, and not a close relative of …
Antelopes and gazelles belong to the family Bovidae, which includes even-toed hoofed animals with hollow horns and a four-chambered stomach. Sheep, cattle, and goats are also bovids. The family Bovidae in Africa includes nine tribes of antelopes, one of which includes the 12 species of gazelles (Antilopini). Other tribes are the duikers (Cephalophini), dwarf antelopes (Neotragini), reedbuck, kob, …
An antenna is a device used to transmit and receive electromagnetic waves such as radio waves and microwaves. Antenna for a Doppler weather radar site. Doppler radar can be used to detect wind shear and microburst weather conditions. Photograph by Brownie Harris. Stock Market. Reproduced by permission. Antennas provide the transition between a guided wave (flowing in a wire) and a free spa…
Anthrax is the name given to an infection that is caused by the bacterium Bacillus anthracis. The bacterium is common in cattle, sheep, goats, camels, antelopes, and other plant-eating animals. Humans can also become contaminated with the anthrax bacterium. In the past, such human contamination was only associated with farmers or sheepherders, people who worked in close contact with infected anima…
Anthropocentrism is a world view that considers humans to be the most important factor and value in the Universe. In contrast, the biocentric world view considers humans to be no more than a particular species of animal, without greater intrinsic value than any of the other species of organisms that occur on Earth. The ecocentric world view incorporates the biocentric one, while additionally propo…
Anti-inflammatory agents are compounds that reduce the pain and swelling associated with inflammation. Inflammation is a response of the body to injuries such as a blow or a burn. The swelling of the affected region of the body occurs because fluid is directed to that region. The inflammatory response can aid the healing process. In conditions such as rheumatoid arthritis, however, the swelling an…
Antibiotics are natural or synthetic compounds that kill bacteria. Antibiotics are not active against viruses. There are many different antibiotics that have different bacterial targets. Some antibiotics are specific in their activity, affecting only one or a few types (genera) of bacteria. Other antibiotics, such as penicillin, are active against a wide variety of bacteria. Such antibiotics are d…
The antibody and antigen reaction is an important protective mechanism against invading foreign substances. The antibody and antigen reaction, together with phagocytosis, constitute the immune response (humoral immune response). Invading foreign substances are antigens while the antibodies, or immunoglobulins, are specific proteins generated (or previously and present in blood, lymph or mucosal se…
Anticoagulants are complex organic or synthetic compounds, often carbohydrates, that help prevent the clotting or coagulation of blood. The most widely used of these is heparin, which blocks the formation of thromboplastin, an important clotting factor in the blood. Most anticoagulants are used for treating existing thromboses (clots that form in blood vessels) to prevent further clotting. Oral an…
Anticonvulsants are drugs designed to prevent the seizures or convulsions typical of epilepsy or other convulsant disorders. Epilepsy is not a single disease—it is a set of symptoms that may have different causes in different people. There is an imbalance in the brain's electrical activity, which causes seizures. These may affect part or all of the body and may or may not cause a los…
Antidepressant drugs are used to treat serious, continuing mental depression that interferes with a person's ability to function. Everyone feels sad, "blue," or discouraged occasionally, but usually those feelings do not interfere with everyday life and do not need treatment. However, when the feelings become overwhelming and last for weeks or months, professional treatment ca…
Antihelmintics are drugs used to kill parasitic worms (from the Greek word helmins, worm). These preparations are also called vermicides. Worm infestations are among the most common parasitic diseases of man. Often the life cycle of the worm begins when a child playing in dirt ingests the eggs of the worm. The egg hatches in the child's digestive tract and the worms begin their unending que…
Antihistamines are medicines that relieve or prevent the symptoms of hay fever and other kinds of allergy. An allergy is a condition in which the body becomes unusually sensitive to some substance, such as pollen, mold spores, dust particles, certain foods, or medicines. These substances, known as allergens, cause no unusual reactions in most people. But in people who are sensitive to them, exposu…
Antimatter is matter comprising particles that are equal in mass to the particles comprising ordinary matter—neutrons, protons, electrons, and so forth—but with opposite electrical properties. An antiproton has the same mass as the proton, but negative charge, an antielectron (positron) has the same mass as an electron, but negative charge, and an antineutron has the same mass as a n…
Antimetabolites are substances that interfere with the normal metabolism of an organism, thereby causing its death. They are widely used in the medical sciences because they have the ability to kill or inactivate microorganisms that cause disease. Terms such as antibacterials, antifungals, and antivirals are used to describe antimetabolites that act on bacteria, fungi, and viruses, respectively. I…
Antioxidants are molecules that prevent or slow down the breakdown of other substances by oxygen. In biology, antioxidants are scavengers of small, reactive molecules known as free radicals and include intracellular enzymes such as superoxide dismutase (SOD), catalase and glutathione peroxidase. Antioxidants can also be extracellular originating as exogenous cofactors such as vitamins. Nutrients f…
An antiparticle is a subatomic particle identical with more familiar subatomic particles such as electrons or protons, but with the opposite electrical charge or, in the case of uncharged particles, the opposite magnetic moment. For example, an antielectron (also known as a positron) is identical with the more familiar electron, except that the former carries a single unit of positive electrical c…
An antipsychotic drug, sometimes called a neuroleptic, is a prescription medication used to treat psychosis. Psychosis is a major psychiatric disorder characterized by derangement or disorganization of personality and/or by the inability to tell what is real from what is not real, often with hallucinations, delusions, and thought disorders. People who are psychotic often have a difficult time comm…
Antisepsis is the prevention or inhibition of an infection by either killing the organism responsible for the infection, or weakening the organism so that it is unable to cause the infection or survive. This is usually achieved by application of an antiseptic or germicidal preparation. An antiseptic differs from an antibiotic. An antibiotic is specifically directed to a target bacterium or differe…
Antlions or doodlebugs are insects best known by their larvae, which have small, fat bodies with a huge sickle-shaped pair of mandibles. Antlions belong to the family Myrmeleonidae, of the order Neuroptera, which also includes the lacewings. Members of this order are named for the delicate venation on the wings of the adult, but most people are probably more familiar with the larval stage of antli…
Ants are insects in the family Formicidae in the order Hymenoptera, which also includes bees and wasps. The body of ants is divided into three sections: head, thorax, and abdomen. The head bears two long, flexible antennae (for touch and chemical detection), two eyes, and a pair of powerful mandibles (jaws) for feeding and defense. Ants have three pairs of long legs that end with a claw. They are …
Anxiety is an unpleasant emotional state characterized by an often vague apprehension, uneasiness, or dread. Anxiety is often accompanied by physical sensations similar to those of fear such as perspiration, tightness of the chest, difficulty breathing or breathlessness, dry mouth, and headache. Unlike fear, in which the individual is usually aware of its cause, the cause of anxiety is often not c…
Apes are a group of primates that includes gorillas, orang-utans, chimpanzees, and gibbons. These are the primate species that are the most closely related to humans. The hands, feet, and face of an ape are hairless, while the rest of its body is covered with coarse black, brown, or red hair. Apes share some characteristics that set them apart from other primates: they have an appendix, lack a tai…
Apgar score is the assessment of a newborn baby's physical condition based on skin color, heart rate, response to stimulation, muscle tone, and respiratory effort. Each criteria is rated from zero to two with a total score of 10 signifying the best possible physical condition. The assessment determines the need for immediate emergency treatment, helps prevent unnecessary emergency intervent…
Aphasia is a disorder caused by damage to the areas of the brain that direct the ability to speak, interpret, and understand language. Usually, aphasia is caused by a head injury, a brain tumor, a stroke, or a serious infection. In adults, one of the most common causes of aphasia is a cerebrovascular accident—a stroke. A stroke occurs when the blood and oxygen supply to the brain is blocked…
Aphids are insects in the order Homoptera, which are also known as plant lice. Some 3,800 species of aphids have been identified worldwide with 1,300 species occurring in North America, which includes some 80 species that are pests of crops and ornamental plants. Aphids have a distinctive pear-shaped body, and most are soft and green in color. The wings are transparent and are held in a tent-like …
In mathematics, making an approximation is the act or process of finding a number acceptably close to an exact value; that number is then called an approximation or approximate value. Approximating has always been an important process in the experimental sciences and engineering, in part because it is impossible to make perfectly accurate measurements. Approximation also arises because some number…
Apraxia is a disorder of brain function in which a person is unable to perform learned motor acts even though the physical ability exists and the desire to perform them is there. Brain damage to the parietal lobes, particularly in the dominant hemisphere, results in apraxia. Unlike paralysis, movements remain intact but The area of the brain associated with apraxia. Illustration by Hans �…
Although groundwater exists beneath most land surfaces, it is frequently limited in its availability to human users by local hydrogeologic conditions. Those portions of the water-bearing subsurface that are capable, within their hydrogeologic constraints, of yielding significant amounts of that water are called aquifers. Aquifers can store large amounts of water within pore spaces throughout the r…
Arachnids (class Arachnida) form the second largest group of terrestrial arthropods (phylum Arthropoda) with the class Insecta being the most numerous. There are over 70,000 species of arachnids, which include such familiar creatures as scorpions, spiders, harvestmen or daddy longlegs, and ticks and mites, as well as the less common whip scorpions, pseudoscorpions, and sun spiders. Arachnids are m…
The giant of freshwater fishes, the arapaima or pirarucu (Arapaima gigas) is a legend among fish. Weighing up to 440 lb (200 kg), this species, which has only been recorded in the rivers of Brazil and the Guianas, may reach a length of some 16.5 ft (5 m), although most specimens today are less than 10 ft (3 m) long. The origins of the arapaima, which belongs to the bony-tongued fishes (Osteoglossi…
An arc is a segment of a curve, most often a circle. In the strictest definition, an arc is a segment of a curve in a plane. Examples include segments of geometrical forms such as circles, ellipses, and parabolas, as well as irregular arcs defined by analytical functions. Arcs of circles can be classified by size. A minor arc is one whose length is shorter than one-half of the circumference of a c…
Long before the incandescent electric light bulb was invented, arc lamps had given birth to the science of electric lighting. In the early 1800s, when the first large batteries were being built, researchers noticed that electric current would leap across a gap in a circuit, from one electrode to the other, creating a brilliant light. Sir Humphry Davy is credited with discovering this electric arc …
Life on Earth can divided into three large collections, or domains. These are the Eubacteria (or "true" bacteria), Eukaryota (the domain that humans belong to), and Archae. The members of this last domain are the archaebacteria. Most archaebacteria (also called archae) look bacteria-like when viewed under the microscope. They have features that are quite different, however, from both…
Archaeoastronomy is the study of prescientific peo ples' relation to the sky as part of their natural environment. As a formal investigation, the field of archaeoastronomy is relatively young, having begun only in the 1960s. It is often known as cultural astronomy to indicate the multidisciplinary breadth of the field and its emphasis on cultural practices and issues rather than on the �…
By applying modern genetics to population studies, archaeology, and anthropology, scientists are forming a new interpretation of prehistoric migrations. The initial peopling of Europe, Asia, and the Americas is usually explained by basic theories that appeal to reason. For example, scientists consider that groups of prehistoric peoples would periodically migrate into North America via a land bridg…
The term archaeology refers, in part, to the study of human culture and of cultural changes that occur over time. In practice, archaeologists attempt to logically reconstruct human activities of the past by systematically recovering and examining artifacts or objects of human origin. However, archaeology is a multi-faceted scientific pursuit, and includes various specialized disciplines and subfie…
Archaeometallurgy is the study of metal artifacts, the technology that was used to smelt them, and the ways ancient societies acquired ores. In addition to understanding the history of metal technology, archaeometallurgists seek to learn more about the people who made and used metal implements and gain a broader understanding of the economic and social contexts in which the people lived. Archaeome…
Archaeometry is the analysis of archeological materials using analytical techniques borrowed from the physical sciences and engineering. Examples include trace element analysis to determine the source of obsidian used to manufacture arrowheads, and chemical analysis of the growth rings of fossilized sea shells to determine seasonal variations in local temperature over time. Modern archaeometry beg…
Before any excavation is begun at a site, the archeologist must prepare a survey map of the site. Site mapping may be as simple as a sketch of the site boundaries, or as complex as a topographic map complete with details about vegetation, artifacts, structures, and features on the site. By recording the presence of artifacts on the site, the site map may reveal information about the way the site w…
Archeologists are concerned with the activities of people and nature that create evidence of a cultural past. Such evidence, which may include any remnant of human habitation, is referred to as the archeological record. The processes that produce this evidence are called formation processes. There are two types of formation processes: cultural and environmental. Cultural formation processes are th…
Arithmetic is a branch of mathematics concerned with the numerical manipulation of numbers using the operations of addition, subtraction, multiplication, division, and the extraction of roots. General arithmetic principles slowly developed over time from the principle of counting objects. Critical to the advancement of arithmetic was the development of a positional number system and a symbol to re…
Armadillos are bony-skinned mammals native to Central and South America. Armadillos (family Dasypodidae) number 20 species in eight genera. The species include the long-nosed armadillo (six species), the naked-tailed armadillo (four species), the hairy armadillo (three species), the three-banded armadillo (two species), the fairy armadillo (two species), the six-banded or yellow armadillo (one spe…
Arrow worms are small marine planktonic animals of the phylum Chaetognatha found in tropical seas. Most of The anatomy of an arrow worm. Illustration by Hans & Cassidy. Courtesy of Gale Group. the 50 species of arrow worms belong to the genus Sagitta. Arrow worms have a head with eyes and hook-like spines on their jaws that identify them as predators of smaller planktonic anim…
The arrowgrass family (Juncaginaceae) is a family of herbaceous plants whose leaves are grass-like and shaped somewhat like an arrowhead. The arrowgrass family has four genera: Scheuchzeria with two species; Thrighlochin with 12 species; Maundia with one species; and Tetroncium with one species. All species in the arrowgrass family grow in wet or moist habitats in temperate and cold regions of the…
Arrowroot is an edible starch obtained from the underground stems, or rhizomes, of several species of the genus Maranta, family Marantaceae. The most common species of arrowroot is Maranta arundinacea, native to the tropical areas of Florida and the West Indies, and called true, Bermuda, or West Indian arrowroot. Several relatives of true arrowroot are also known locally as arrowroot and have root…
Arteries are blood vessels that transport oxygenated blood from the heart to other organs and systems throughout the body. In humans, healthy arteries are smooth, elastic structures, while diseased arteries may contain bulges due to high blood pressure, hard, inelastic areas, or internal blockages resulting from the accumulation of fatty plaques circulating in the blood. Atherosclerosis is the har…
Arteriosclerosis literally means "hardening of the arteries." As people age, their blood vessel walls naturally grow a bit stiffer and harder, with less flexibility. A common complication of arteriosclerosis is called atherosclerosis. In this condition, plaques (hardened masses composed of lipids, dead cells, fibrous tissue, and platelets) collect in the arteries. If a plaque grows l…
Arthritis is a term that refers to the inflammation of joints (the point where the ends of two bones meet each other). Upwards of 43 million American adults and children (1 out of every 6 citizens) have some form of the more than 100 different types of arthritis. Inflammation is a reaction of the body to injury. Excess fluid is directed to the affected area, which produces swelling. The fluid is m…
Arthropods are invertebrates such as insects, spiders and other arachnids, and crustaceans that comprise the phylum Arthropoda. The phylum Arthropoda includes three major classes—the Insecta, Arachnida, and Crustacea. Arthropods are characterized by their external skeleton, or exoskeleton, made mostly of chitin, a complex, rigid carbohydrate usually covered by a waxy, waterproof cuticle. Th…
Arthroscopic surgery is a type of orthopedic surgery that utilizes an instrument called an arthroscope. An arthroscope is a small fiber-optic device that allows the surgeon to view the inside of a joint without a large incision. Most arthroscopic surgery can be performed requiring only three small incisions, each about 0.25 in (6 mm) long. These incisions are called portals. The word arthroscope i…
Artifacts are often the most intriguing part of archaeological research. Whether priceless or common, artifacts are key to deciphering the archaeological record and garnering information about how people lived in the past. However, most of the information from archaeological excavation is gathered from an artifact's context, or where an artifact is found, and with what other items it is rec…
Most synthetic fibers are polymer-based, and are produced by a process known as spinning. This process involves extrusion of a polymeric liquid through fine holes known as spinnerets. After the liquid has been spun, the resulting fibers are oriented by stretching or drawing. This increases the polymeric chain orientation and degree of crystallinity, and has the effect of increasing the modulus and…
An artificial heart is a manmade device that is intended to replace the heart muscle that pumps approximately 2,000 gal (7,571 L) of blood through the body each day. The heart muscle is composed of several chambers and the blood flow into and out of the chambers is controlled by a system of valves. Valve failure can lead to congestive heart failure, pulmonary edema, and other serious cardiovascula…
Certain tasks can be performed faster and more accurately by traditionally programmed computers than by human beings, particularly numerical computation and the storage, retrieval, and sorting of large quantities of information. However, the ability of computers to interact flexibly with the real world—their "intelligence"—remains slight. Artificial intelligence (AI) is…
Artificial vision refers to the technology (usually, visual implants) that allows blind people to see. The main aim of visual implants is to relay the picture to the brain using either cameras or photoreceptor arrays. There are different types of implants used to stimulate vision; retinal, cortical, optic nerve, and biohybrid implants. None of the currently available technologies restores full vis…
Arums, also called aroids, are flowering plants in the family Araceae. The 2,500 species of arums are distributed worldwide, primarily in tropical and subtropical regions, where they grow in rainforests, mostly on the ground but also commonly as epiphytes. Arums are generally absent from the arctic and deserts. Only 11 species occur in North America and other north temperate regions. Most species …
Asbestos is the general name for a wide variety of silicate minerals, mostly silicates of calcium, magnesium, and iron. Their common characteristics are a fibrous structure and resistance to fire. The two most common families of asbestos minerals are called amphibole and serpentine. The mineral has been known and used by humans for centuries. The ancient Romans, for example, wove asbestos wicks fo…
Sexual reproduction involves the production of new cells by the fusion of sex cells (sperm and ova) to produce a genetically different cell. Asexual reproduction, on the other hand, is the production of new cells by simple division of the parent cell into two daughter cells (called binary fission). Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction…
Asia is the world's largest continent, encompassing an area of 17,177,000 sq mi (44,500,000 sq km), 29.8% of the world's land area. The Himalayan Mountains, which are the highest and youngest mountain range in the world, stretch across the continent from Afghanistan to Burma. The highest of the Himalayan peaks, called Mount Everest, reaches an altitude of 29,028 ft (8,848 m). There a…
An assembly line is a system of mass production in which a product is manufactured in a step-by-step process as it moves continuously past an arrangement of workers and machines. Introduced in the nineteenth century, it provided the basis for the modern methods of mass production of quantities of standardized, relatively low-cost goods available to great numbers of consumers. As one of the most po…
Asses include three of the seven genera that make up the family Equidae, which also includes horses and zebras. Wild asses are completely wary and apt to run swiftly away, so they have been difficult to study. Asses can survive in poor habitat such as scrub and near desert regions. Asses have loud voices, most notable in the raucous bray of the domestic burro and a keen sense of hearing. Male asse…
In algebra, a binary operation is a rule for combining the elements of a set two at a time. In most important examples that combination is also another member of the same set. Addition, subtraction, multiplication, and division are familiar binary operations. A familiar example of a binary operation that is associative (obeys the associative principle) is addition (+) of real numbers. For example,…
Late in 2002, astronomers and the International Astronomical Union (IAU) confirmed the discovery the discovery of an asteroid, designated Asteroid 2002 AA29, in a companion orbit to Earth. It is the first object ever identified to be in a companion orbit around the Sun (i.e., it shares at least some of the same orbital path and space). In another 600 years, the asteroid will technically and tempor…
The asthenosphere is the layer of Earth situated at an average depth of about 62 mi (about 100 km) beneath Earth's surface. It was first named in 1914 by the British geologist Joseph Barrell, who divided Earth's overall structure into three major sections: the lithosphere, or outer layer of rock-like material; the asthenosphere; and the centrosphere, or central part of the planet. Th…
Asthma is a lung disease that affects approximately four million people in the United States. In people with asthma, the airways of the lungs are hypersensitive to irritants such as cigarette smoke or allergens. When these irritants are inhaled, the airways react by constricting, or narrowing. Some people with asthma have only mild, intermittent symptoms that can be controlled without drugs. In ot…
Astrobiology is an area of life science that investigates the origin of life, how the biological components interact to create environment, what makes planets habitable, and searches for life on other planets. Astrobiologists are scientists from many areas of science, including biologists (molecular biologists, microbiologists, ecologists, geneticists), chemists, oceanographers, climatologists, ar…
Astroblemes are the scars left on Earth's surface by the high velocity impact of large objects from outer space. Such colliding bodies are usually meteorites, but some may have been comet heads or asteroids. Few of these impacts are obvious today because our active earth tends to erode meteorite craters over short periods of geologic time. The term astrobleme was coined in 1961 by Robert S.…
At least two forms of the astrolabe have existed. The older form, known as the planispheric astrolabe, consists of two circular metal disks, one representing Earth and the other, the celestial sphere at some particular location (latitude) on the Earth's surface. The first of these disks, called the plate or tympan, is fixed in position on a supporting disk known as the mater. It shows the g…
Astrometry literally means measuring the stars. This type of measurement determines a specific star's location in the sky with great precision. In order to establish a star's location, it is necessary to first establish a coordinate system in which the location can be specified. Traditionally, very distant stars, which show very little motion as viewed from Earth, have been used to e…
An astronomical unit (AU) is a unit of length that astronomers use for measuring distances within the solar system. One astronomical unit is the mean distance between the Earth and the Sun, called the semimajor axis, or 92,919,000 mi (149,597,870 km). Kepler's third law (in a more accurate form derived by Isaac Newton) can be used to calculate a precise value of the AU, if the exact distanc…
Astronomy, the oldest of all the sciences, seeks to describe the structure, movements and processes of celestial bodies. …
Astrophysics describes the processes that give rise to the observable features of our universe in terms of previously developed physical theories. It ties together physics and astronomy by describing astronomical phenomena in terms of the physics and chemistry we are familiar with in our everyday life. …
Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen, and 0.93% argon. The remainder, less than 0.1%, contains many small but important trace gases, including water vapor, carbon dioxide, and ozone. All of these trace gases have important effects on the earth's climate. The atmosphere can be divided into vertical layers determined by the way temperature changes with heigh…
The term weather observation refers to all of the equipment and techniques used to study the properties of the atmosphere. These include such well-known instruments as the thermometer and barometer as well as less familiar devices such as the radiosonde and devices for detecting the presence of trace gases in the atmosphere. One of the first means developed for raising scientific instruments to hi…
Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles. Differential heating causes air to rise in the atmosphere at some locations on the planet and then to sink back to the…
Atmospheric optical phenomena are visual events that take place in Earth's atmosphere as a consequence of the way light is reflected, refracted, and diffracted by solid particles, liquids droplets, and other materials present in the atmosphere. Such phenomena include a wide variety of events ranging from the blue color of the sky itself to mirages and rainbows to sundogs and solar pillars. …
The earth's atmosphere consists of gases that surround the surface, and like any gas, the atmosphere exerts a pressure on everything within it. A gas is made up of molecules that are constantly in motion. If the gas is in a container, some gas molecules are always bouncing off the container walls. When they do so, they exert a tiny force on the walls. With a sufficient number of molecules, …
The temperature of the atmosphere varies with the distance from the equator (latitude) and height above the surface (altitude). It also changes in time, varying from season to season, from day to night and irregularly due to passing weather systems. If these variations are averaged out on a global basis, a pattern of average temperatures emerges for the atmosphere. The vertical temperature profile…
Atomic clocks are the world's most accurate time keepers—more accurate than astronomical time or quartz clocks. Originally, a second was defined as 1/86,400 of a mean solar day. Today it is defined as 9,192,631,770 periods or wavelengths of the radiation absorbed by the cesium-133 atom as it changes between two hyperfine energy levels. The change in definition was the result of the …
The atomic number of an element is equal to the number of protons in the nucleus of its atom. For example, the nucleus of an oxygen atom contains eight protons and eight neutrons. Oxygen's atomic number is, therefore, eight. Since each proton carries a single positive charge, the atomic number is also equal to the total positive charge of the atomic nucleus of an element. The concept of ato…
Atomic spectroscopy is the technique of analyzing the energy emitted by atoms in order to determine the energy levels of the atom's electrons. Electrons can have only certain discrete energies. These energies are characteristic of each element; that is, every atom of an element has the same set of available energies. Normally, electrons in atoms are distributed in the lowest energy levels. …
Atomic theory is the description of atoms, the smallest units of elements. The scientific evidence for the existence of atoms and its even smaller constituents is so vast that most people now consider the existence of atoms to be a fact and not just a theory. …
Atomic weights are actually atomic masses but historically they were called atomic weights because the method used to determine them was called weighing. This terminology has persisted and is more familiar to most people even though the values obtained are actually atomic masses. …
Atoms are the smallest particles of matter that have distinct physical and chemical properties. Each different type of atom makes up an element which is characterized by an atomic weight and an atomic symbol. Since the atomic theory was first proposed in the early nineteenth century, scientists have discovered a number of subatomic particles. This experimental method was suggested by Robert Boyle …
Attention deficit/hyperactivity disorder (ADHD), also known as hyperkinetic disorder (HKD) outside of the United States, is estimated to affect 3–9% of children, and afflicts boys more often than girls. Although difficult to assess in infancy and toddlerhood, signs of ADHD may begin to appear as early as age two or three, but the symptom picture changes as adolescence approaches. Many sympt…
Auks are penguinlike seabirds found in the Northern Hemisphere. These birds spend most of their lives in the coastal waters north of 25°N latitude, coming ashore only to lay their eggs and raise their young. There are 22 species of auks, including the Atlantic puffin, the common murre, the dovekie or lesser auk, and the extinct great auk. Called alcids, the members of the auk family fill an…
Of the seven continents, Australia is the flattest, smallest, and except for Antarctica, the most arid. Including the southeastern island of Tasmania, the island continent is roughly equal in area to the United States, excluding Alaska and Hawaii. Millions of years of geographic isolation from other landmasses accounts for Australia's unique animal species, notably marsupial mammals like th…
Autism is a profound mental disorder marked by an inability to communicate and interact with others. The condition's characteristics include language abnormalities, restricted and repetitive interests, and the appearance of these characteristics in early childhood. The disorder begins in infancy, but typically is not diagnosed until the ages two to five. Although individuals with autism are…
Autoimmunity is a condition where the immune system mistakenly recognizes host tissue or cells as foreign. (The word "auto" is the Greek word for self.) Because of this false recognition, the immune system reacts against the host components. There are a variety of autoimmune disorders (also called autoimmune diseases). An autoimmune disease can be very specific, involving a single or…
The automatic pilot has it roots in the gyroscope, a weighted, balanced wheel mounted in bearings and spinning at high velocity. As early as 1852 the French scientist Jean-Bernard-Léon Foucault had experimented with the gyroscope and found that it tended to stay aligned with its original position and also tended to orient itself parallel to Earth's axis in a north-south direction. Th…
Automation is the use of scientific and technological principles in the manufacture of machines that take over work normally done by humans. This definition has been disputed by professional scientists and engineers, but in any case, the term is derived from the longer term automatization or from the phrase automatic operation. Delmar S. Harder, a plant manager for General Motors, is credited with…
Few inventions in modern times have had as much impact on human life and on the global environment as the automobile. Automobiles and trucks have had a strong influence on the history, economy, and social life of much of the world. Entire societies, especially those of the industrialized countries, have been restructured around the power of rapid, long-distance movement that the automobile confers…
An autotroph is an organism able to make its own food. Autotrophic organisms take inorganic substances into their bodies and transform them into organic nourishment. Autotrophs are essential to all life because they are the primary producers at the base of all food chains. There are two categories of autotrophs, distinguished by the energy each uses to synthesize food. Photoautotrophs use light en…
The concept that a mole of any substance contains the same number of particles arose out of research conducted in the early 1800s by the Italian physicist Amedeo Avogadro (1776-1856). Avogadro based his work on the earlier discovery by Joseph Gay-Lussac that gases combine with each other in simple, whole-number ratios of volumes. For example, one liter of oxygen combines with two liters of hydroge…
The aye-aye (Daubentonia madagascariensis) is a rare tree-dwelling animal that is found only at a few localities along the eastern half of Madagascar, off eastern Africa. It is a member of a group of primitive primates known as Prosimians, most of which are lemurs. The aye-aye is the only surviving member of the family Daubentoniidae; a slightly larger relative (D. robusta) became extinct about 1,…
Babblers are small to medium-sized passerine (perching) birds characterized by soft, fluffy plumage, strong, stout legs, and short rounded wings. Their wings make them poor fliers, and most are largely sedentary birds. Many species, particularly those that stay close to the ground, are gray, brown, or black, while the tree-living (arboreal) species are often green, yellow, or olive. Wren-babblers …
Baboons are ground-living monkeys in the primate family Cercopithecidae and are found in Africa and the Arabian Peninsula. Some taxonomists classify baboons in two genera, while others classify them in three or four. All baboons have a strong torso, a snout-like face, the same dentition with long, sharp canine teeth, powerful jaws, a ground-walking habit, coarse body hair, a naked rump, and a simi…
Bacteria are mostly unicellular organisms that lack chlorophyll and are among the smallest living things on earth—only viruses are smaller. Multiplying rapidly under favorable conditions, bacteria can aggregate into colonies of millions or even billions of organisms within a space as small as a drop of water. The Dutch merchant and amateur scientist Anton van Leeuwenhoek was the first to ob…
Bacteriophage (also known as phages) are viruses that target and infect only bacterial cells. The first observation of what since turned out to be bacteriophage was made in 1896. Almost twenty years later, the British bacteriologist Frederick Twort demonstrated that an unknown microorganism that could pass through a filter that excluded bacteria was capable of destroying bacteria. He did not explo…
Badgers are eight species of robust, burrowing carnivores in the subfamily Melinae of the family Mustelidae, which also includes weasels, mink, marten, otters, and skunks. There are eight species of badgers, distributed among six genera. Badgers have a strong, sturdy body, with short, powerful, strong-clawed legs, and a short tail. The head is slender and triangular-shaped. The fur of badgers is l…
Ball bearings help reduce friction and improve efficiency by minimizing the frictional contact between machine parts through bearings and lubrication. Ball bearings allow rotary or linear movement between two surfaces. As the name indicates, a ball bearing involves a number of balls, typically steel, sandwiched between a spinning inner race (a small steel ring with a rounded grove on its outer sur…
Any missile that lofts an explosive payload which descends to its target as a ballistic projectile—that is, solely under the influence of gravity and air resistance—is a ballistic missile. Missiles that do not deliver a free-falling payload, such as engine powered-cruise missiles (which fly to their targets as robotic airplanes), are not "ballistic." A ballistic missile…
Ballistics is the study of projectile motion. A projectile is an object that has been launched, shot, hurled, thrown, or by other means projected, and continues in motion due to its own inertia. The path of the projectile is determined by its initial velocity (direction and speed) and the forces of gravity and air resistance. For objects projected close to Earth and with negligible air resistance,…
A balloon is a nonsteerable aircraft consisting of a thin envelope inflated with any gas lighter than the surrounding air. The balloon rises from the ground similar to a gas bubble in a glass of soda. The physical principle underlying this ability to ascend is Archimedes' law, according to which any immersed body is pushed upward by a force equal to the weight of the displaced fluid. If thi…