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…
Bananas, plantains, and their relatives are various species of plants in the family Musaceae. There are about 40 species in this family, divided among only two genera. The most diverse genus is Musa, containing 35 species of bananas and plantains, followed by Ensete, the Abyssinian bananas. The natural range of bananas and plantains is the tropics and subtropics of the Old World, but agricultural …
Australian wildlife holds many surprises, but few as intriguing as the widely distributed bandicoots. These small, rabbit-sized marsupials have a thick set body, short limbs, a pointed muzzle, short neck and short hairy tail. Their teeth are similar to those of insect- and flesh-eating mammals, but their hind feet resemble those of kangaroos and possums. The hindfeet are not only considerably long…
Almost everyone is familiar with the striped bars found on grocery and retail store items. These are bar codes, or more specifically, the Universal Product Code (UPC). UPC codes first appeared in stores in 1973 and have since revolutionized the sales industry. The UPC code consists of ten pairs of thick and thin vertical bars that represent the manufacturer's identity, The parts of the …
Barberries are about 600 species of plants in the genus Berberis, family Berberidaceae, occurring throughout the Northern Hemisphere and South America. Most species of barberry are shrubs or small trees, and many of these have persistent, evergreen leaves. The flowers are small, arranged in clusters, and insect pollinated. The fruits of barberries are multiple-seeded berries. Barberry hybrids are …
Barbets are about 76 species of medium-sized birds, divided among 13 genera. These comprise the family Capitonidae, in the order Piciformes, which also contains the woodpeckers, toucans, and their allies. Barbets are birds of tropical forests, occurring in Central and South America, Africa, and Asia as far south as Indonesia. However, none of the species of barbets occur in more than one continent…
Barbiturates are in the group of medicines known as central nervous system (CNS) depressants. Also known as sedative-hypnotic drugs, barbiturates make people very relaxed, calm, and sleepy. These drugs are sometimes used to help patients relax before surgery. Some may also be used to control seizures (convulsions). Although barbiturates have been used to treat nervousness and sleep problems, they …
Recent reports estimate that the proportion of individuals who are overweight in the United States surpasses 50% of the adult population. Such a staggering statistic has associated with it profound ramifications. Excess weight is a major contributor to serious health conditions that affect millions of people and can result in early death. Aside from tangible diseases, obesity is the root of much p…
Although pure barium is rarely used outside the laboratory, barium's many compounds have a number of practical applications. Perhaps the most familiar is the barium enema. When doctors need to examine a patient's digestive system, a mixture containing barium sulfate is used to coat the inner lining of the intestines. Similarly, to enhance examination of the stomach and esophagus, the…
Barium sulfate occurs in nature as the mineral barite, or baryte, which is mined in Canada and Mexico and, in the United States in Arkansas, Missouri, Georgia, and Nevada. It is also prepared synthetically either by treating a solution of a barium salt with sodium sulfate or as a by-product in the manufacture of hydrogen peroxide. Barium sulfate is used in diagnostic radiology of the digestive sys…
Bark is a protective, outer tissue that occurs on older stems and roots of woody coniferous and angiosperm plants. Bark is generally considered to occur on the outside of the tissue known as wood, or the water-conducting xylem tissues of woody plants. The inner cells of bark, known as phloem, grow by the division of outer cells in a generative layer called the vascular cambium, located between the…
Barley is one of the world's major cultivated crops. It is a member of the grass family (Poaceae). In 1999, approximately142 million acres (57.5 million ha) of barley were grown worldwide and the total production was 147.0 million tons of grain (133.6 million tonnes). Harvesting barley with a combine tractor. Photograph by Holt Studios Limited Ltd. Photo Researchers, Inc. Reproduced by…
The rocky shores of most coastlines are liberally dotted with clusters of barnacles (phylum Arthropoda, class Crustacea). Few people take any notice of these animals, despite their common occurrence. Barnacles are exclusively marine animals: some 900 species have been identified worldwide. Many are tiny organisms measuring just a few centimeters in diameter, while others such as the South American…
A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second makes use of a diaphragm whose size changes as a result of air pressure. The barometer described above is adequate for making …
A barracuda is a long, cylindrical, silvery fish. It has two widely separated dorsal fins, in roughly the same location as the two fins on its belly, and a forked tail. The largest species, the great barracuda, seldom grows longer than 6.5 ft (2 m) and is an aggressive fearsome predator of other fish. All barracudas have an underhung jaw that houses long, incredibly sharp teeth; their teeth are co…
A barrier island is a long, thin, sandy stretch of land oriented parallel to the mainland coast, which protects the coast from the full force of powerful storm waves. Between the barrier island and the mainland is a lagoon or bay. Barrier islands are dynamic systems that migrate under the influence of changing sea levels, storms, waves, tides, and longshore currents. Approximately 2,100 barrier is…
Within the broad field of geology, the term basin can be used to represent a number of features. These include topographic or drainage basins, structural basins, and sedimentary basins. In some cases, a single basin can include aspects of more than one of these types of basins. A topographic, or drainage, basin is a sloping or depressed area from which runoff collects and flows into a channel, str…
Bass is the common name for a number of popular freshwater and saltwater fish, which include the wide mouth bass, the striped bass, groupers, jewfish, and wreckfish, which are some of the finest sports and food fish in the world. Fish known as bass actually belong to different families and are distributed worldwide in tropical and temperate waters. The freshwater family Centrarchidae includes the …
Basswoods are about 30 species of trees in the genus Tilia, in the linden family Tiliaceae. In North America, these trees are generally known as basswoods in forestry, and as lindens in horticulture. Basswoods have simple, long-petioled, coarsely toothed, broadly heart-shaped leaves, arranged alternately on their twigs. The flowers occur in clusters, and emerge from a specialized leaf known as a b…
Throughout human history, the sea has yielded an abundance of resources for man's existence and provided efficient routes for exploration and transportation. In return, it has exacted a toll in terms of human life and property. The fear and respect that it earned from those who ventured out upon its surface was itself a deterrent to learning more about its mysteries. The physical restrictio…
Bats are one of the most diverse and widely distributed groups of mammals on Earth, second only to rodents in the number of species. More than 900 species of bats have been described. They occur in most terrestrial biomes, except for the high Arctic and all of Antarctica. Bats are the only truly flying mammals, and are distinct from the flying lemurs and flying squirrels, which actually glide. Bat…
If two metals are immersed in an aqueous solution that can conduct electricity (electrolyte), they will have different tendencies to dissolve in the solution. A difference in voltage arises because one of the metals appears positive or negative relative to the other. The combination of two metals (electrodes) in an aqueous solution for the purpose of producing electrical energy from chemical energ…
Beach nourishment is the artificial process of adding sediment to a beach for recreational and aesthetic purposes, as well as to provide a buffer to coastal erosion. The sand may be dredged from nearby and pumped onto the beach, or transported in from outside areas. It is considered a soft method of stabilizing the shoreline, as opposed to rock and concrete structures meant to capture sand or prot…
Beardworms are slim, wormlike, deep-sea creatures so named for the thick cluster of long, fine, hairlike tentacles projecting from the front of the first section of a three-segmented body. There are approximately 120 species of beardworms, which belong to the phylum Pogonophora-from the Greek pogon, meaning beard, and phoron, meaning bearer. The front section of the beardworm's body, which …
Bears are large carnivores of the family Ursidae. They are members of the order Carnivora, which also includes dogs, cats, and seals, although these animals are in different families than bears. All of these carnivores have a pair of modified teeth in the upper and lower jaw, called carnassials, that are used to tear meat into smaller chunks during feeding. Bears are not strictly meat-eaters, howe…
The true beavers are robust, aquatic herbivores in the family Castoridae, order Rodentia. Many taxonomists believe that two, closely related species of true beavers exist—the American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). Other taxonomists, however, classify these as closely related variants of the same species, under the name Castor fiber. A few other rodents are a…
Bedrock is the solid rock that is exposed at the earth's surface, or buried beneath one or more layers of loose sediment. It is of igneous, sedimentary or metamorphic origin and forms the upper surface of the rocky foundation that composes the earth's crust. …
Bee-eaters are 24 species of birds that make up the family Meropidae. Bee-eaters occur in open habitats and savannas of the south-temperate and tropical zones, ranging through Africa, southern Europe, southern Asia, Southeast Asia, and many Pacific Islands. Species that breed in temperate habitats migrate to the tropics for the winter. Bee-eaters have large, pointed wings and a long tail, usually …
The beech family is an important group of flowering plants that includes the beeches, oaks, and sweet chestnuts. Most members of the family are deciduous or evergreen trees or shrubs. The leaves are arranged alternately along branches, are leathery in texture, often strongly ribbed, and have margins that are entire, toothed, or deeply lobed. The flowers are unisexual. Male flowers are usually arra…
Bees belong to the insect order Hymenoptera, which includes wasps and ants. Its name is derived from Greek, meaning "winged membrane," and it is the third largest group of insects with more than a hundred thousand species in the order. Ants and bees play vital roles in agriculture, ants being useful in aerating soil and bees in pollinating plants. Wasps play an important part as pred…
Beet belongs to the genus Beta in the goosefoot family, Chenopodiaceae. There are several varieties of beet and all are used as food for either animals or humans. Most species of beet are biennial and are harvested after the first growing season when the roots are most nutritious. The wild beet, Beta maritima, is thought to be the species from which cultivated beets (Beta vulgaris), originate. Wil…
Beetles make up the large, extremely diverse order Coleoptera of the class Insecta, and comprise the largest single group of animals on Earth. There are at least 250,000 species of beetles, compared to the 5,000 known species of mammals. The weevil family of beetles alone contains about 50,000 species, and is the largest family in the animal kingdom. Thus, the order Coleoptera, representing about …
Begonias (genus Begonia) are attractive perennial herbs with soft, succulent stems, and white, pink, red, orange, or yellow flowers. Begonias are members of the begonia family, Begoniaceae, order Violales, subclass Dilleniidae, class Magnoliopsida (dicotyledons), division Magnoliophyta (flowering plants). The begonia family consists of five genera and 920 true species, the majority of which belong…
Behavior is the way that living things respond to their environment. A behavior consists of a response to a stimulus or factor in an individual's internal or external environment. Stimuli include chemicals, heat, light, pressure, and gravity. All living things exhibit behavior. When dust irritates our throats, for example, we respond with coughing behavior. Plants respond with growth behavi…
The bennettites are an extinct group of gym nosperms—seed-bearing plants whose seeds are exposed to the air, not enclosed in the ovary of a flower. Botanists hypothesize that bennettites are related to the cycads, an extant group of gymnosperms, and paleobotanists believe the bennettites originated from the seed ferns (Pteridospermales) about 220 million years ago during the Triassic period…
Benzene is a clear, colorless, highly flammable liquid with a pronounced characteristic odor. It has a freezing point of 41.9°F (5.5°C), a boiling point of 176.2°F (80.1°C), and a density of 0.8787 g/mL. It is only slightly soluble in water (0.18 g/100 mL at 77°F [25°C]), but is completely miscible with alcohol, chloroform, ether, carbon disulfide, carbon …
In its pure form, benzoic acid exists as white needles or scales with a strong characteristic odor. It melts at 252.3°F (122.4°C), although it may also sublime at temperatures around 212°F (100°C). It dissolves only sparingly in cold water [0.4 g/100 g at 77°F (25°C)], but more completely in hot water [6.8 g/100 g at 203°F (95°C)]. Benzoic ac…
Bernoulli's principle states that flowing fluids like air and water press less than still fluids and that pressure decreases quadratically with speed; i.e., with speed squared. …
Beta-blockers are medications used primarily for treating high blood pressure. The usefulness of these medications rests on their ability to block the effects of a nervous system transmitter chemical known as norepinephrine and the related "fight-or-flight" hormone epinephrine. Beta-blockers are also used to treat heart-related chest pain (angina pectoris, or simply angina), abnormal…
The big bang theory is the conceptual and mathematical model that scientists use to describe the origin of the Universe. It states that the Universe began as a tiny, violent explosion about 15 billion years ago. That event produced all of the matter and energy in the universe, including its hydrogen and helium. Some of these light atoms were forged in the cores of stars, over billions of years, in…
Binary stars, often called double stars, refer to pairs of stars sufficiently close to each other in space to be gravitationally bound together. Following the laws of gravitation, each of the components revolves around the common center of mass of the system. At least 50% of stars are found to exist as binary systems, according to conservative statistics. There seems to be no obvious preference fo…
In 1823, a new optical instrument began to appear in French opera houses that allowed patrons in the distant (and less expensive) seats to view the opera as if they were in the front row. Called opera glasses, the device combined telescope lenses with stereoscopic prisms to provide a magnified, three-dimensional view. After many years (but relatively few modifications), opera glasses have evolved …
The question of expanding an equation with two unknown variables called a binomial was posed early in the history of mathematics. One solution, known as Pascal's triangle, was determined in China as early as the thirteenth century by the mathematician Yang Hui. His solution was independently discovered in Europe 300 years later by Blaise Pascal whose name has been permanently associated wit…
Bioaccumulation is the gradual build up over time of a chemical in a living organism. This occurs either because the chemical is taken up faster than it can be used, or because the chemical cannot be broken down for use by the organism (that is, the chemical cannot be metabolized). Bioaccumulation need not be a concern if the accumulated compound is not harmful. Compounds that are harmful to healt…
A bioassay is the use of a living organism to test for the presence of a compound or to determine the amount of the compound that is present in a sample. The organism used is sensitive to the compound for which the test is conducted. Thus, the effect observed is typically the death or deteriorated health of the test organism. Depending on the test organism, soil, air, or liquid samples can be assa…
Oxygen is the molecule used by animals as a final electron acceptor for metabolism. Two electrons (one at a time) from metabolic products can chemically bind each oxygen molecule. While numerous molecules combine with oxygen in the human body, one of the major chemical reactions involving oxygen is the synthesis of the high-energy phosphate bonds in ATP. ATP is the cell's currency for gener…
Biochemistry is the study of the molecular basis of life. The study of biochemistry includes the knowledge of the structure and function of molecules found in the biological world and an understanding of the precise biochemical pathways by which organic molecules are either put together or broken down. Biochemistry seeks to describe the structure, organization, and functions of living matter in mo…
The term biodegradable is used to describe materials that decompose through the actions of bacteria, fungi, and other living organisms. Temperature and sunlight may also play roles in the decomposition of biodegradable plastics and other substances. If such materials are not biodegradable, they remain in the environment for a long time, and, if these same substances are toxic, they may pollute the…
Biodiversity is the total richness of biological variation. The scope of biodiversity is usually considered to range from the genetic variation of individual organisms within and among populations of a species, to different species occurring together in ecological communities. Some definitions of biodiversity also include the spatial patterns and temporal dynamics of populations and communities on…
Bioenergy is energy derived using organic material, especially plant matter, as fuel. The material burned or processed to produce bioenergy (the "feedstock") is called biomass. Biomass has been an energy source for as long as humans have used wood fires to warm themselves and cook food. Wood is still the most commonly used biomass fuel. In some developing countries, crop and logging …
Biofeedback is a means by which a person can mentally influence a natural physiologic process that may or may not be consciously regulated under normal conditions. This could include lowering blood pressure, regulating the heart rate, or influencing the skin temperature. Deliberate control of bodily functions is not a new accomplishment. Many historical accounts exist of Indian yogis who controlle…
A biofilm is a population of bacteria, algae, yeast, or fungi that is growing attached to a surface. The surface can be living or nonliving. Examples of living surfaces where biofilms may grow include the teeth, gums, and the cells that line the intestinal and vaginal tracts. Examples of nonliving surfaces include rocks in watercourses, and implanted medical devices such as catheters. Rudimentary …
Bioinformatics, or computational biology, refers to the development of new database methods to store genomic information, computational software programs, and methods to extract, process, and evaluate this information, and the refinement of existing techniques to acquire the genomic data. Finding genes and determining their function, predicting the structure of proteins and RNA sequences from the …
In biology, the term species refers to all organisms of the same kind that are potentially capable, under natural conditions, of breeding and producing fertile offspring. The members of a species living in a given area at the same time constitute a population. All the populations living and interacting within a particular geographic area make up a biological (or biotic) community. The living organ…
Biological rhythms are often referred to as biological clocks, since they operate on time schedules on a daily, monthly, seasonal, or annual basis. Some biological rhythms even occur on the basis of fractions of seconds. These internal clocks operate independent of the environment, but they are controlled by environmental conditions in changing situations. During times of change, such as seasonal …
Biological warfare is the use of living organisms (e.g., bacteria, virus) or biochemical agents (e.g., chemical neurotoxins) as strategic military weapons to cause harm in humans, animals, or plants. In contrast to bioterrorism, biological warfare is considered the governmentsanctioned use of biological weapons to attack a clearly defined military force or civilian population. These agents can be …
Biology is the scientific study of all forms of life, including plants, animals, and microorganisms. Among the numerous fields in biology are microbiology, the study of microscopic organisms like bacteria; cytology, the study of cells; embryology, the study of development; genetics, the study of heredity; biochemistry, the study of the chemical structures in living things; morphology, the study of…
Bioluminescence is the production of light by living organisms. Some single-celled organisms (bacteria and protista) as well as many multicellular animals and fungi demonstrate bioluminescence. …
Biomagnification (or bioaccumulation) refers to the ability of living organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, non-living environment, or in the case of animals, in the food that they eat. Of course, organisms accumulate any chemical needed for their nutrition. In environmental science, however, the major focus of biomagnification …
Biomass consists of living organisms, or parts of living organisms, as well as waste products and incompletely decomposed remains of living organisms. The term is quite encompassing and includes plants (referred to as phytomass), microbes, and animal material, or zoomass. Biomass density is a distinguishing feature of ecological systems and is usually presented as the amount of dry biomass per uni…
A biome is a major, geographically extensive ecosystem, structurally characterized by its dominant life forms. Terrestrial biomes are usually distinguished on the basis of the major components of their mature or climax vegetation, while aquatic biomes, especially marine ones, are often characterized by their dominant animals. Most of the oceans are considered part of a single biome, although areas…
Biophysics is the integration and application of the principles of physics to explain and explore the form and function of living things. The most familiar examples of the role of physics in biology are the use of lenses to correct visual defects and the use of x rays to reveal the structure of bones. Principles of physics have been used to explain some of the most basic processes in biology such …
Bioremediation is a type of biotechnology in which living organisms or ecological processes are utilized to deal with some environmental problem. The most common use of bioremediation is to metabolically break down or otherwise remove toxic chemicals before or after they have been discharged into the environment. In such uses, bioremediation takes advantage of the fact that certain microorganisms …
The biosphere is the space on and near the earth's surface that contains and supports living organisms and ecosystems. It is typically subdivided into the lithosphere, atmosphere, and hydrosphere. The lithosphere is the earth's surrounding layer composed of solid soil and rock, the atmosphere is the surrounding gaseous envelope, and the hydrosphere refers to liquid environments such …
The Biosphere 2 Project is an experiment in which scientists, engineers, and some intrepid "biospherians" (or dwellers) within the Biosphere have recreated several of the main terrains and habitats of our planet and attempted to co-habit with these environments to the environments' benefit. Many environmentalists see planet Earth as the first, original, and only known biospher…
Biotechnology is the use of any technique involving living organisms to manufacture or change products, to improve the desired characteristics of a plant or animal, or to alter microorganisms for a purpose. Biotechnology has a long history. For example, yeast microorganisms were harnessed to prepare wine by Egyptians some 4,000 years before the birth of Christ. In 1865, Gregor Mendel presented his…
Bioterrorism is the use of a biological weapon against a civilian population. As with any form of terrorism, its purposes include the undermining of morale, creating chaos, or achieving political goals. Biological weapons use microorganisms and toxins to produce disease and death in humans, livestock, and crops. Biological, chemical, and nuclear weapons can all be used to achieve similar destructi…
The birch family is a group of flowering plants of tree or shrub form that includes the birches (Betula), alders (Alnus), hornbeams (Carpinus), and hazels (Corylus). Members of the birch family have simple and alternate leaves that bear appendages (stipules) where they join the branch. The leaves are also deciduous, generally thin and often doubly toothed along the margin. The flowers are densely …
Birds are vertebrate animals in the class Aves. There are approximately 8,800 species of birds, divided among 28 living orders. Of these, slightly more than 900 species are found in North America. There has been considerable disagreement among ornithologists about the appropriate level for differentiating species, leading to multiple classification schemes. But however one distinguishes between sp…
The birds of paradise are some of the most fascinating birds in the world. This is due to the striking coloration of the males of most species, and the wide range of behaviors demonstrated in the group. Researchers of animal behavior are particularly interested in the elaborate mating displays performed by male birds of paradise. Birds of paradise are members of the family Paradisaeidae, which pro…
Birds of prey are predators that catch and eat other animals. These birds are called raptors (from the Latin rapere, meaning to snatch), a reference to their specialized, powerful feet, which are used to seize their prey. Raptorial birds eat birds, small mammals, reptiles, amphibians, fish, and large insects. Birds of prey are members of five avian families within the order Falconiformes. The Acci…
Birth, or parturition, in mammals is the process in which a fully developed fetus is expelled from the mother's uterus by the force of strong, rhythmic muscle contractions. The birth of live offspring is a reproductive feature shared by mammals, some fishes, and selected invertebrates (such as scorpions), as well as some reptiles and amphibians. Animals who give birth to live offspring are …
Birth defects or congenital defects are present at birth. They result from heredity, environmental influences, or maternal illness. Such defects range from the very minor, such as a dark spot or birthmark that may appear anywhere on the body, to more serious conditions that may result in marked disfigurement, impaired functioning, or decreased lifespan. A number of factors individually or in combi…
The American bison (Bison bison) is a large, herbivorous land mammal native to the grasslands and open forests of North America. It is a member of the family Bovidae, which also includes cattle, sheep, and goats. When French explorers first saw these large, shaggy, cow-like animals, they called them boeufs, the French word for "cattle." This later became anglicized into the word �…
Bitterns are about 12 species of wading birds in the subfamily Botaurinae of the family Ardeidae, which also includes herons and egrets. There are two genera: four species of the relatively large and stocky true bitterns (Botaurus spp.), and eight species of the much smaller and more slender, least bitterns (Ixobrychus). Bitterns have brown-and-black, vertically streaked plumage, which renders the…
Bivalve molluscks belong to the class Bivalvia (or Lamellibranchia) of the phylum Mollusca. Known by such common names as clams, mussels, cockles, oysters, and scallops, bivalves are among the most familiar aquatic invertebrates. They occur in large numbers in marine, estuarine, The life cycle of a typical freshwater clam. For species that do not have the parasitic larval stage, the fertilize…
BL Lacertae objects, abbreviated BL Lac, are one subclass of active galactic nuclei (AGN), the extremely energetic nuclei of active galaxies. Roughly 40 BL Lac objects are known. Perhaps the most obvious property of BL Lac objects is that they look like stars. Astronomers originally thought the prototype, BL Lac, was a star. In fact, BL Lacertae is normally a variable star designation, two letters…
A sufficiently intense gravitational field can prevent the escape not only of matter, but even of light. Such gravitational fields are produced by the bodies known as black holes. The maximum intensity of a spherical object's gravitational field is a function both of the amount of matter it contains and of its volume. The more matter is contained in an object and the smaller its volume …
The blackbird family (Icteridae) consists of 94 medium-sized species of birds that occur only in the Americas. Blackbirds are found in widespread habitats, ranging from wetlands, to prairies, to forests. The most common members of the family are various species of blackbirds, grackles, cowbirds, orioles, meadowlarks, bobolink, and others. …
The term blackbody radiation refers to electromagnetic radiation emitted by a completely opaque object. Such an object is referred to as a blackbody since it absorbs all of the radiation that falls on it and thus appears to be colorless, or black. According to Kirchoff's law, any object that qualifies as a blackbody must also be a perfect emitter of radiation. In fact, no real object fits t…
Bleaches are substances that whiten textiles and paper by chemical reaction. These reactions usually involve processes that degrade color. They may destroy or modify chemical bonds or groups that give fabrics their characteristic colors. This process degrades color bodies into smaller, more soluble units that are easily removed in laundering. Conventional bleaching agents, include two types: chlor…
Blennies are small, primarily tropical and subtropical marine fish. They are elongated and often eel-like in shape, with a dorsal fin running from the back of the head almost to the tail fin, and small abdominal fins; the pelvic fin is often completely absent. Many species also lack scales. The blenny's anatomy is well suited for hiding in cracks and crevices along shallow, rocky shorelines…
Blindness is usually considered as an inability to see or a complete loss of vision, although legally, a blind person may retain some vision. In contrast, visual impairment indicates a loss of vision such that there is an impact on daily living, which usually implies partial loss of vision. There are many causes of visual impairment or blindness, and all parts of the eye (cornea, retina, lens, opt…
These tiny, primitive burrowers live underground and forage for ants, termites, soft-bodied insects, and insect larvae. The eyes of most blindsnakes are degenerate; they are covered by scales and do not function. However, the eyes do have light-sensitive cells (rods), so these snakes may not be completely blind. The head is large and the mouth, like a shark's, is below and behind the snout,…
Blood is a liquid connective tissue that performs many functions in the body, including transport of oxygen, Red blood cells alongside the wall of the blood vessel. Photograph by Dennis Kunkel. Phototake NYC. Reproduced by permission. carbon dioxide, nutrients, waste products, and hormones; clotting; and defense against microorganisms. Blood consists of formed elements, or blood cells su…
Blood gas analysis is a means of determining the amount of oxygen or carbon dioxide being carried in the blood, and in some cases, of discovering the identity of a toxic gas, such as carbon monoxide, that may be present. Also, the determination can be made as to whether the blood is too acidic or too alkaline, which may help the physician in his diagnosis. Among other functions, blood carries oxyg…
Blood supply refers to the blood resources in blood banks and hospitals that are critical to the health care community. The blood supply consists of donated blood units (in pints) that are used to replace blood lost during surgery or from trauma. Blood transfusions were attempted as early as 1667 when Jean-Baptiste Denis, a French physician, transfused 12 fl oz (355 ml) of lamb's blood into…
Blotting analysis describes a series of techniques used to determine and describe protein and nucleic acid (e.g., DNA, RNA) sequences. Blotting analysis allows scientists to transfer electrophoretically separated components from a gel to a solid support. This support may then be used for probing with reagents specific for particular sequences of amino acids or nucleotides. In this way, the size an…
The term "blue revolution" refers to the remarkable emergence of aquaculture as an important and highly productive agricultural activity. Aquaculture refers to all forms of active culturing of aquatic animals and plants, occurring in marine, brackish, or fresh waters. Aquaculture has long been practiced in China and other places in eastern Asia, where freshwater fish have been grown …
Bluebirds are small blue-colored perching birds in the thrush family (Turdidae). There are three species of bluebirds in North America. All of these bluebirds nest in natural cavities or nest boxes. They tend to feed from perches, flying down to catch insects as they see them, and sometimes hawking insects in the air. The eastern bluebird (Sialia sialis) occurs in shrub-by habitats, old orchards, …
Boarfish is a common name, so it is not surprising that fish from two different marine families, from two different orders, with superficially similar snout-shaped faces share this name. One species that bears the common name boarfish comprises the few species of the genus Antigonia, which according to some experts is the only genus in the family Caprioidae. Others add the genus Capros to this fam…
Boas are a group of nonvenomous, constricting snakes (family Boidae), most of which are found in tropical America and in Madagascar. Boas bear live young, and in this way they differ from the Old World pythons, which lay eggs. Boas are of ancient derivation, retaining some of the features of their lizard-like ancestors, such as paired lungs (modern snakes have only one), tiny remnants of hind limb…
The Bohr model of atomic structure was developed by Danish physicist and Nobel laureate Niels Bohr (1885–1962). Published in 1913, Bohr's model improved the classical atomic models of physicists J. J. Thomson and Ernest Rutherford by incorporating quantum theory. While working on his doctoral dissertation at Copenhagen University, Bohr studied physicist Max Planck's quantum th…
The boiling point of a liquid substance is the temperature at which the vapor pressure of the liquid equals the external pressure on the liquid. Vapor bubbles form in the liquid, rise to the surface and burst, causing the liquid to boil. At room temperature, in a closed system, there is an equilibrium between the liquid and its vapor phase. For example, if a glass of water is left open, the water …
Bond energy is the strength of a chemical bond between atoms, expressed as the amount of energy required to break it apart. It is as if the bonded atoms were glued together: the stronger the glue is, the more energy would be needed to break them apart. A higher bond energy, therefore, means a stronger bond. Notice that ionic bonds are stronger than covalent bonds. Among covalent bonds, triple bond…
Bony fish (Osteichthyes) are distinguished from other fish species that have a cartilaginous skeleton (Chondrichthyes—sharks, rays and chimaeras, for example) by the presence of true bone—a mixture of calcium phosphates and carbonates—in their skeletons. Other differences between the two groups are modifications in the structure and arrangement of the scales and fins and the p…
Boobies and gannets are nine species of marine birds that make up the family Sulidae, in the order Pelecaniformes, which also includes the pelicans, cormorants, anhingas, tropic birds, and frigate birds. Boobies and gannets have a narrow, cigar-shaped body, a longish, pointed tail, and long, narrow wings. Their feet are fully webbed, and are used in swimming. The beak is strong, pointed, has a ser…
Boolean algebra is often referred to as the algebra of logic, because the English mathematician George Boole, who is largely responsible for its beginnings, was the first to apply algebraic techniques to logical methodology. Boole showed that logical propositions and their connectives could be expressed in the language of set theory. Thus, Boolean algebra is also the algebra of sets. Algebra, in g…
Boric acid has a wide variety of industrial applications. It is used in the manufacture of heat-resistant borosilicate glass and other ceramics, such as crockery, porcelains, enamels, and artificial gemstones. It also used in waterproofing wood and fireproofing textiles. It also finds application as an insecticide for cockroaches and black carpet beetles and as an fungicide on citrus fruits. Its u…
Botany is the study of plants. It is one of the major fields of biology, together with zoology (the study of animals) and microbiology (the study of bacteria and viruses). Specializations within the field of botany include the study of mosses, algae, lichens, ferns, and fungi. Other specialties in botany include plant physiology, the study of the vital processes of plants, such as photosynthesis, …
Botulism is an extremely serious disease caused by the bacterium Clostridium botulinum. C. botulinum release one of the most potent toxins known—one gram of botulinum toxin theoretically can kill one million people. The toxin is swift-acting. It kills by binding to nerve cells, thereby causing paralysis of the muscles used in breathing. First coined in the 1870s, the term botulism comes fro…
As hot magma cools, it undergoes specific reactions. Bowen's reaction series describes the temperature Bowen's reaction series depicts mineral formation in a cooling magma. The discontinuous side depicts mineral formation at decreasing temperatures. The continuous side depicts a solid solution series. As the magma cools there is a trend towards molecular complexity. As the tempe…
The 18 species of bowerbirds are unique in that the males build and decorate a bower, a structure of sticks or grass on the ground, for the purpose of attracting and courting females. Members of the bowerbird family (Ptilonorhynchidae) are found in Australia and New Guinea, and are related to lyrebirds and birds of paradise. Most bowerbirds are about the size of a blue jay or grackle, and as a gro…
The bowfin is a bony fish (Amia calva, family Amiidae) found in eastern North America. It is a relic species—the sole living representative of the order Amiiformes, which first appeared in the Triassic period more than 200 million years ago. Members of this family were common in Europe and Asia, as well as North America, during the Cretaceous and the early part of the Cenozoic. Fossil speci…
Boxfish, also called trunkfish or cowfish, are a small group of shallow-water, marine fish in the family Ostraciontidae (order Tetraodontiformes). The family includes the genera Lactoria, Ostracion, and Tetrosomus and is closely related to the poisonous puffer fish of the family Tetraodontidae. To avoid confusion with these poisonous relatives, some people avoid eating boxfish despite their being …
Brachiopods, or lampshells, are a phylum of small marine animals with a two-valved shell that, at first glance, resemble bivalved mollusks such as clams. The resemblance, however, is quite superficial. The orientation of the shells of brachiopods is very different from that of bivalved mollusks, and brachiopods have two additional structures virtually unique to them, the lophophore (a ciliated fee…
Brackish refers to water with a salinity intermediate to that of fresh water and sea water (the latter has a salt concentration of about 3.5%, or 35 parts per thousand). Brackish waters originate by the mixing of sea water and freshwater, and are most common near the coasts of the oceans. Brackish waters can occur as enclosed systems such as lakes and ponds that receive occasional inputs of oceani…
The brain is a mass of nerve tissue located in an animal's head that controls the body's functions. In simple animals, the brain functions like a switchboard picking up signals from sense organs and passing information to muscles. The brain is also responsible for a variety of involuntary behavior, including keeping the heart beating, and maintaining blood pressure and temperature. I…
Brewing is a multi-stage process during which the brewer encourages a grain such as barley to germinate briefly, steeps the grain in water to release its sugars, and adds yeast to the mixture, which ferments the sugar, turning it into alcohol and carbon dioxide. Around the world, people have brewed grains and starchy vegetables for thousands of years from ingredients as varied as rice, corn, cassa…
Bricks are one of the oldest types of building blocks. They are an ideal building material because they are relatively cheap to make, very durable, and require little maintenance. Bricks are usually made of kiln-baked mixtures of clay. In ancient times, bricks were made of mud and dried in the sun; modern bricks are made from concrete, sand and lime, and glass. The physical and chemical characteri…
Bridges are structures that join two otherwise inaccessible points of land, such as the two shores of a river or lake, or the two sides of a canyon or deep gully. Bridges are designed to carry railroad cars, motor vehicle traffic, or foot travel by pedestrians and/or animals, or to support pipes, troughs, or other conduits used for the movement of goods and materials, such as an oil pipeline or a …
Bristletails are about 300-400 species of small, elongate, terrestrial insects in the order Thysanura. Bristletails have an ancient evolutionary lineage, and they are believed to be relatively primitive, that is, similar in form and function to the most early evolved insects. Bristletails have a simple metamorphosis, with three life-history stages: egg, nymph, and adult. Both the nymphal and adult…
Brittle stars are starfishlike echinoderms (phylum Echinodermata) in the family Ophiodermatidae, whose star-shaped bodies are radially symmetrical and are supported by a hard endoskeleton made of calcium salts. Brittle stars are closely related to basket stars, and more distantly related to starfish, sand dollars, and sea urchins. Brittle stars are named for the ease by which their arms fall off w…
The pineapple family (Bromeliaceae) consists of about 1,500 species of flowering plants. Most species are medium-sized herbs with tightly packed, thick, stiff, spiralling leaves that usually have spiny margins. Some species are semi-woody, and a few rare ones, such as Puya raimondii, are trees that can reach 33 ft (10 m) in height. Most species of bromeliads are epiphytes in rainforests, while oth…
Bronchitis is the inflammation of the bronchi and is a commonly seen winter condition. The bronchi (the air passages leading into the lungs) are formed by the division of the trachea (the main windpipe leading from the larynx [Adam's apple] down through the neck into the chest). The trachea branches left and right into the bronchi which branch to supply lung lobe with the means for air to p…
A brown dwarf is a pseudostar; a body of gas not massive enough for the gravitational pressure in its core to ignite the hydrogen-fusion reaction that powers true stars. The name "brown dwarf" is a play on the name of the smallest class of true stars, "red dwarf," but while red dwarfs are actually red, brown dwarfs are not brown, but purple or magenta. Objects ranging i…
Brownian motion is the constant but irregular zigzag motion of small colloidal particles such as smoke, soot, dust, or pollen that can be seen quite clearly through a microscope. In 1827, Robert Brown, a Scottish botanist, prepared a slide by adding a drop of water to pollen grains. As he watched the tiny particles of pollen under his microscope, Brown noticed that they were constantly jiggling ab…
Brucellosis is a disease caused by bacteria in the genus Brucella. The disease infects animals such as swine, cattle, and sheep; humans can become infected indirectly through contact with infected animals or by drinking Brucella-contaminated milk. In the United States, most domestic animals are vaccinated against the bacteria, but brucellosis remains a risk with imported animal products. Brucella …
Bryophytes include the mosses, liverworts, and hornworts. Bryophytes are the simplest of plants (excluding the algae, which are not considered plants by most botanists). Bryophytes are small, seldom exceeding 6-8 in (15-20 cm) in height, and usually much smaller. They are attached to the substrate (ground, rock, or bark) by rhizoids, which are one or a few-celled, root-like threads that serve only…
As recently as 1984, carbon was thought to exist in only two solid forms. There was graphite, in which the carbon atoms arranged themselves as layered sheets of hexagonally bonded atoms, and there was diamond, in which the carbon atoms formed octahedral structures in which each carbon atom had four nearest neighbors. When compressed to 70% of its initial volume, the buckminsterfullerene is expecte…
Buckthorns are various species of shrubs and small trees in the family Rhamnaceae, a mostly tropical and subtropical family of about 600 species. Most of the buckthorns are in the genus Rhamnus. Buckthorns have a few economic applications, although none of these are very important. A dye known as sap green is made from the fruits of the European buckthorn (Rhamnus cathartica). Another pigment know…
Buckwheat, Fagopyrum esculentum, is not really a wheat at all—it belongs to the family Polygonaceae, and hence is a dicotyledonous plant, not a monocotyledonous species. However, the starchy seeds of buckwheat are utilized in much the same way as the cereal grains of cultivated grasses, such as wheat (Triticum aestivum). The seeds of buckwheat can be used directly as poultry or animal feed.…
Bud is a term used to refer to three different types of undeveloped forms described in this article. …
In chemistry, a buffer is a system, usually an aqueous (water) solution, that resists having its pH changed when an acid or a base is added to it. Normally, the addition of acid to a solution will lower its pH and the addition of a base will raise its pH. If the solution is a buffer, however, its pH will be changed to a much lesser extent than would be expected from the amounts of acid or base tha…
Architects design buildings, but architecture is more than just building design and more than just art on a massive scale. Architecture is about light and space. It is about stimulating emotions in the people who see and inhabit the structure. Architecture creates an environment, whether it is the uplifted spirituality of the Chartres Cathedral, the drama and anticipation of the Schauspielhaus aud…
Bulbuls are about 120 species of medium-sized, perching birds, distributed among 15 genera, and making up the family Pyncnontidae. The most diverse genus is Pycnonotus, with about 50 species. Bulbuls are mostly tropical and subtropical birds, occurring in Africa, Asia, and Southeast Asia. Some relatively northern species are migratory, but most species of bulbuls are local birds. Bulbuls have rath…
Named after the German chemist Robert Wilhelm Bunsen, who contributed to its development, the Bunsen burner was already known to Michael Faraday, who may have created the first design. The idea behind the Bunsen burner is to reduce the considerable loss in heat energy typical in ordinary gas burners. This reduction of energy waste is accomplished by using a mixture of gas and air, the optimal prop…
Fluids such as water or air exert pressure in all directions and the amount of pressure depends on the depth of the fluid. The pressure on the bottom of an object immersed in a fluid will be greater than the pressure on the top of the object. The imbalance of pressure acting on the object creates an upward force called the buoyant force. If the buoyant force is greater than the weight of the objec…
A buret (also spelled burette) is a long glass tube open at both ends, that is used to measure out precise volumes of liquids or gases. Most burets are about 0.04 in (1 mm) in diameter and 30 in (75 cm) long. The bottom of a buret is tapered so that its diameter is only about 0.1 mm in diameter. Burets are most commonly designed to hold volumes of 1 ml or less. Fluid is dispensed form a buret thro…
A burn is damage to the skin. Depending on the type of and severity of the burn, skin may be only superficially damaged, or damage may extend deep within the layers of the skin. Burns can be caused by extreme heat, extreme cold, chemicals, electricity, or radiant energy (i.e., ultraviolet rays from the sun or an artificial source, and x rays). The degree of damage of a burn can be classified in tw…
Bustards are 22 species of tall birds that make up the family Otidae. Bustards occur in relatively open habitats in Africa, central and southern Europe and Asia, Southeast Asia, and Australia. Most species, however, are African. Bustards are large birds, with species ranging in body length from 14.5-52 in (37-132 cm), and in weight from 1-48 lbs (0.6-22 kg). Bustards have a stocky body, a long nec…
Buttercups and crowfoots are about 275 species of plants in the genus Ranunculus, family Ranunculaceae. Buttercups mostly occur in cool and temperate regions of both hemispheres of the world, including mountains in tropical latitudes. Buttercups are annual or perennial, and they are herbaceous plants, dying back to the ground surface before the winter. The leaves of terrestrial species are simple …
Butterflies are insects in the order Lepidoptera, which also includes the moths. Butterflies at rest fold their wings vertically over their head, whereas moths hold their wings horizontally. Most butterflies are active during daylight, while moths are mostly nocturnal. Butterflies undergo complete metamorphosis, that is, their egg hatches to a larva (or caterpillar), which pupates in a chrysalis, …
Butterfly fish (family Chaetodontidae) are some of the most colorful and varied fish of the oceans, the majority of which live on or close to coral reefs. Most species measure from 5-9.5 in (13-24 cm) in length and have deep, flattened bodies that are frequently adorned by extended fins. In some species these may form a large arc over the body. In addition to refinements in the body shape, the col…
bonds to other atoms via an end carbon atom, while the sec-butyl group bonds via an "inner" carbon atom. The boiling points of the butyl alcohols decrease regularly in moving down the above list, from 244°F (118°C) for n-butyl alcohol to 226°F (108°C) for isobutyl alcohol to 212°F (100°C) for sec-butyl alcohol to 180°F (82°C) fo…
Butylated hydroxyanisole is a food additive much more widely known by its abbreviation, BHA. BHA is an aromatic organic compound with the chemical names of 2- and 3-tert-butyl-4-methoxyphenol. It can exist in either of the two isomeric forms or as a mixture of the two isomers. In its pure form, BHA is a waxy white or pale yellow solid with a melting point of 118.4–131°F (48–55…
Butylated hydroxytoluene (BHT) is a derivative of cresol, an aromatic organic compound in which two additional hydrogen atoms in the benzene ring are replaced by tertiary butyl groups. Its technical name is 2,6-di-tert-butyl-p-cresol. In its pure form BHT is a white crystalline solid with a melting point of 158°F (70°C) and a boiling point of 509°F (265°C). It is normal…
The true buzzards are diurnal birds of prey in the genus Buteo, sub-family Buteonidae, family Accipitridae. In North America, buzzards are also commonly known as hawks, although other genera in the family Accipitridae are also given this common name, for example, the Accipiter hawks. There are 25 species of buzzards. Buzzards are in the order Falconiformes, which also includes other types of hawks…
The cactus family or Cactaceae is made up of about 2,000 species of perennial plants with succulent stems, most of which are well-armed with sharp spines. The natural distribution of most cacti is American, ranging from southern British Columbia and southern Ontario in Canada, through much of the United States, to the tip of southern South America. One genus, Rhipsalis, occurs in Africa, Madagasca…
CAD/CAM is an acronym for computer-aided design and computer-aided manufacturing. The use of A CAD system used for Boeing airplanes. © Ed Kashi/Phototake NYC. Reproduced by permission. computers in design and manufacturing applications makes it possible to remove much of the tedium and manual labor involved. For example, the many design specifications, blueprints, material lists, an…
North America's streams, rivers, and lakes are home to more than 1,200 different species of caddisflies, which are aquatic insects in the order Trichoptera. Adaptations to different water conditions and food types allow this group of insects to populate a variety of habitats in America's waters. Caddisflies are best known and most easily identified in their larval stages. Most caddis…
Caecilians are long, worm-like legless amphibians in the order Gymnophiona (sometimes known as Apoda, meaning without legs). There are 163 species of caecilians, in 35 genera. Little is known about these animals, and few species have common names. Most of the caecilians are tropical or sub-tropical, and occur in Central and South America, Africa, and south and southeast Asia. Caecilians grow up to…
Caffeine is an alkaloid found in coffee, tea, chocolate, and other natural foods. It is also a component of cola soft drinks. Caffeine has been a part of the human diet for many centuries and is one of the most widely used central nervous system stimulants worldwide. In recent years, research has raised questions about possible deleterious health effects of caffeine, but no definitive conclusions …
A caisson is a hollow structure made of concrete, steel, or other materials that can be sunk into the earth. It used as the substructure for a bridge, a building, or other large structures. Caissons come in many sizes and shapes depending on their future use. The one shared feature is that their bottom edges are sharp so they easily can be sunk into the ground. These sharp edges are known as the c…
Calcium is a chemical element, a member of the alkaline-earth metals group, represented by the atomic symbol Ca and the atomic number 20. It has an atomic weight of 40.08. In its pure form, calcium is a silvery-white metal, although it is never found in this free state naturally. It is, however, one of the most abundant substances on Earth, comprising approximately 3.64% of the earth's crus…
Calcium carbonate has two major crystalline formstwo different geometric arrangements of the calcium ions and carbonate ions that make up the compound. These two forms are called aragonite and calcite. All calcium carbonate minerals are conglomerations of various-sized crystals of these two forms, packed together in different ways and containing various impurities. The large, transparent crystals …
Calcium oxide (CaO), more commonly known as lime or quick lime, has been studied by scholars as far back as the pre-Christian era. In his book Historia Naturalis, for example, Pliny the Elder discussed the preparation, properties, and uses of lime. Probably the first scientific paper on the substance was Dr. Joseph Black's "Experiments Upon Magnesia, Alba, Quick-lime, and Some Other …
Calcium propionate is used as a food preservative in breads and other baked goods because of its ability to inhibit the growth of molds and other microorganisms. It is not toxic to these organisms, but does prevent them from reproducing and posing a health risk to humans. Propionic acid occurs naturally in some foods and acts as a preservative in those foods. Some types of cheese, for example, con…
Among the many other uses of calcium sulfate are as a pigment in white paints, as a soil conditioner, in Portland cement, as a sizer, filler, and coating agent in papers, in the manufacture of sulfuric acid and sulfur, in the metallurgy of zinc ores, and as a drying agent in many laboratory and commercial processes. …
The calculator is a computing machine. Its purpose is to do mathematics; basic calculators do the basic mathematical functions (addition, subtraction, division, and multiplication) while the more advanced ones, which are relatively new in the history of computing machines, do advanced calculations such as solving polynomials. The odometer, or mileage counter, in your car is a counting machine as i…
Calculus is the branch of mathematics that deals with rates of change and motion. It grew out of a desire to understand various physical phenomena, such as the orbits of planets, and the effects of gravity. The immediate success of calculus in formulating physical laws and predicting their consequences led to development of a new division in mathematics called analysis, of which calculus remains a…
There are three units of time which have a direct basis in astronomy: the day, which is the period of time it takes for the Earth to make one rotation around its axis; the month, which is the period of time it takes for the Moon to revolve around the earth; and the year, which is the period of time it takes for the earth to make one revolution around the Sun. The week has an indirect basis in astr…
Calibration is the process of checking the performance of a measuring instrument or device against some commonly accepted standard. A watch, for example, has to be calibrated so that it keeps correct time, agreeing with the international standard. The dials on a radio must also be calibrated so that the correct frequency or station is actually being received. Calibration provides consistency in a …
A caliper is an instrument used for measuring linear dimensions that are not easily measured by devices such as meter sticks or rulers. Two examples of such measurements include the outer dimensions of a pipe or the internal diameter of a glass tube. Although many kinds of calipers exist, they are all designed on a common principle: two legs are hinged at one end to allow movement of the free ends…
A calorie is the amount of energy required to raise the temperature of 1g of pure water by 34°F (1°C) under standard conditions. These conditions include an atmospheric pressure of one atmosphere, and a temperature change from 60° to 62°F (15.5 to 16.5°C). The calorie is also sometimes designated as a gram-calorie or small calorie (abbreviated: cal), to distingui…
Calorimetry is the measurement of the amount of heat gained or lost during some particular physical or chemical change. Heats of fusion or vaporization, heats of solution, and heats of reaction are examples of the kinds of determination that can be made in calorimetry. The term itself derives the Latin word for heat, caloric, as is the name of the instrument used to make these determinations, the …
Camels and their relatives, the llamas, are longlegged, hoofed mammals in the family Camelidae in order Artiodactyla, whose members have an even number of toes. All camels have a cleft in their upper lip, and all have the ability to withstand great heat and great cold. Camels evolved in North America and spread into South America, Asia, and Africa. Camels in Asia and Africa today have been domesti…
A canal is a man-made waterway or channel that is built for navigation, irrigation, drainage, or water supply. When the word is used today however, it is usually in the context of transport or navigation by boats. Canal transport should not be confused with navigating on a river, because a canal is entirely artificial (although canals are in many cases connected with a natural body of water). Ther…
Cancel refers to an operation used in term mathematics to remove terms from an expression leaving it in a simpler form. For example, in the fraction 6/8, the factor 2 can be removed from both the numerator and the denominator leaving the irreducible fraction 3/4. In this instance the 2 is said to be canceled out of the expression. Canceling is particularly useful for solving algebraic equations. T…
Cancer is not just one disease, but a large group of almost 100 diseases. Its two main characteristics are uncontrolled growth of the cells in the human body and the ability of these cells to migrate from the original site and spread to distant sites. If the spread is not controlled, cancer can result in death. One out of every four deaths in the United States is from cancer. It is second only to …
Canines are species in the carnivore family, Canidae, including the wolves, coyote, foxes, dingo, jackals, and several species of wild dog. The family also includes the domestic dog, which is believed to have descended from the wolf. The Canidae includes 10-14 genera with 30-35 species, depending on the taxonomic treatment. Canines originated in North America during the Eocene era (38-54 million y…
A cantilever, also called a fixed end beam, is a beam supported only at one end. The beam cannot rotate in any direction; thus it creates a solid support. The cantilever is considered the third of the three great structural methods, the other two being post-and-beam construction and arch construction. The cantilever thrusts down which is different from the thrust of an arch which is outward agains…
Capacitance is an electrical effect that opposes change in voltage between conducting surfaces separated by an insulator. Capacitance stores electrical energy when electrons are attracted to nearby but separate surfaces. The voltage across an unchanging capacitance value will stay constant unless the quantity of charge stored is changed. If the voltage across a capacitor is increased until charges…
A capacitor stores electrical energy. It is charged by hooking into an electrical circuit. When the capacitor is fully charged a switch is opened and the electrical energy is stored until it is needed. When the energy is needed, the switch is closed and a burst of electrical energy is released. A capacitor consists of two electrical conductors that are not in contact. The conductors are usually se…
Capillaries are microscopic blood vessels that connect small arteries (arterioles) and small veins (venules). Within the tissues, arterioles terminate into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which merge to form larger and larger veins tha…
Capillary action is the tendency of a liquid to rise in narrow tubes or to be drawn into small openings such as those between grains of a rock. Capillary action, also known as capillarity, is a result of the intermolecular attraction within the liquid and solid materials. A familiar example of capillary action is the tendency of a dry paper towel to absorb a liquid by drawing it into the narrow op…
The frogmouths, oilbird, potoos, owlet frogmouths, and nightjars are five unusual families of birds that make up the order Caprimulgiformes, and are collectively referred to as caprimulgids. Caprimulgids have a large head, with a short but wide beak that can open with an enormous gape, fringed by long, stiff bristles. This apparatus is used by caprimulgids to catch their food of insects in flight.…
In 1973, the Endangered Species Act was passed in the United States to protect species that are rapidly declining due to human influences. Captive breeding and release is one of the tools available to halt or reverse the decline of some species in the wild. Such programs may be carried out by zoos, aquaria, botanical gardens, or conservation organizations. In some cases the efforts have met with a…
Capuchins are New World monkeys characterized by a cap or crown patch of hair that resembles a hood, called a capuche, worn by Franciscan monks. Capuchins belong to the family Cebidae, which includes 31 species in 11 genera. The Cebidae is subdivided into seven subfamilies which include night monkeys, squirrel monkeys, titis, sakis, howlers, spider monkeys, and the capuchins. Monkeys in the family…
Capybaras, also known as carpinchos or water hogs, are large South American rodents in the family Hydrochaeridae. Hydrochaeris hydrochaeris is the larger of the two species of capybaras, and is the world's largest rodent. It can reach a body weight of 110 lb (50 kg), a body length of 4.5 ft (1.3 m), and a height of 1.5 ft (50 cm). Hydrochaeris isthmius is about half this size. H. hydrochaer…
Carbohydrates are naturally occurring compounds composed of carbon, hydrogen, and oxygen. The carbohydrate group includes sugars, starches, cellulose, and a number of other chemically related substances. For the most part, these carbohydrates are produced by green plants through the process known as photosynthesis. Countless varieties of plants use this process to synthesize a simple sugar (glucos…
Carbon has been known since prehistoric times. It gets its name from carbo, the Latin word for charcoal, which is almost pure carbon. In various forms, carbon is found not only on Earth, but in the atmospheres of other planets, in the Sun and stars, in comets, and in some meteorites. On Earth, carbon can be considered to be the most important of all the chemical elements, because it is the essenti…
The carbon cycle describes the movement of carbon in the atmosphere, where it is in the gaseous form carbon dioxide, through organisms, and then back into the atmosphere and the oceans. Carbon is a central element of the huge diversity of organic chemicals found in living things, such as the many kinds of carbohydrates, proteins, and fats. Energy is contained in the chemical bonds that hold the at…
Carbon dioxide was the first gas to be distinguished from ordinary air, perhaps because it is so intimately connected with the cycles of plant and animal life. When we breathe air or when we burn wood and other fuels, carbon dioxide is released; when plants store energy in the form of food, they use up carbon dioxide. Early scientists were able to observe the effects of carbon dioxide long before …
Carbon monoxide is a compound of carbon and oxygen with the chemical formula CO. It is a colorless, odorless, tasteless, toxic gas. It has a density of 1.250 g/L at 32°F (0°C) and 760 mm Hg pressure. Carbon dioxide can be converted into a liquid at its boiling point of -312.7°F (-191.5°C) and then to a solid at its freezing point of -337°F (-205°C). …
Carbon tetrachloride is an organic chemical that is commonly used as a solvent. It is also called tetra chloromethane and is composed of molecules that have one carbon atom and four chlorine atoms bonded together in the shape of a tetrahedron. It is made by combining elemental chlorine with simple carbon compounds like methane or carbon disulfide. It is a liquid at room temperature, with a freezin…
A carbonyl group is a group of atoms that consists of a carbon atom covalently attached to an oxygen atom by a double bond: C = O. The carbon atom, to satisfy its valence of 4, must also be attached by covalent bonds to two other atoms. The simplest type of molecule that contains a carbonyl group is a ketone. Other types of molecules that contain carbonyl groups are aldehydes, acids, esters, and a…
A carboxyl group, also called a carboxy group, is a characteristic group of atoms found in organic molecules. Organic compounds that contain carboxyl groups are called carboxylic acids. The carboxyl group occurs on the end or side of a molecule. The group consists of a carbon atom that forms two chemical bonds to one oxygen atom and one chemical bond to a second oxygen atom. This second oxygen is …
Carboxylic acids are chemical compounds that contain a carboxyl group, which is -COOH. The carboxyl group is attached to another hydrogen atom or to one end of a larger molecule. Examples include formic acid, which is produced by some ants and causes their bites to sting. (In fact, the scientific name for ants, Formica, is what gives formic acid its name.) Another example is acetic acid, which is …
A carcinogen is a substance that causes a normal cell to change into a cancerous cell. The word "car cinogen" is derived from Greek and means in English, cancer-causing. Carcinogens fall into two broad categories, naturally occurring substances that are found in food or soil, or artificial substances created by chemists for various industrial purposes. Although the way carcinogens ca…
The coordinated and rhythmic series of muscular contractions associated with the heart comprise the cardiac cycle. In humans, the cardiac cycle can be subdivided into two major phases, the systolic phase and the diastolic phase. Systole occurs when the ventricles of the heart contract. Accordingly, systole results in the highest pressures within the systemic and pulmonary circulatory systems. Dias…
A measure of the number of elements in a group or a set. For example, the number of books on a shelf can be described by a single cardinal number. Similarly, the set can be assigned the cardinal number 3 because it has only three elements. Since cardinal numbers count the number of elements in a set, they are always positive whole integers. If the elements from two sets have a one-to-one relations…
The cardinals and grosbeaks belong to the subfamilies Cardinalinae, of the finch family (Fringillidae), which is the largest of all North American bird families. (Some researchers include the cardinals and grosbeaks with the Emberizidae, the buntings and tanagers). Cardinals and grosbeaks are New World birds, ranging from central Argentina as far north as central Canada. They live primarily in tem…
The caribou or reindeer (Rangifer tarandus) is a northern species of deer occurring in the boreal and arctic regions of North America and Eurasia. At one time, caribou and reindeer were considered to be separate species, but these animals are fully interfertile and are now considered to be the same species. In North America they are called caribou, whereas in Eurasia they are known as reindeer. Ho…
In the literal sense, a carnivore is any flesh-eating organism. However, in the ecological usage of the word, carnivores kill animals before eating them (that is, they are predators), as opposed to feeding on animals that are already dead (the latter are called scavengers or detritivores). Trophic ecology deals with the feeding and nutritional relationships within ecosystems, and this field has de…
Carnivorous plants are botanical oddities that supplement their requirement for nutrients by trapping, killing, and digesting small animals, mostly insects. Carnivorous plants are photosynthetic, and are therefore fundamentally autotrophic. Still, their feeding relationship with animals represents a reversal of the normal trophic connections between autotrophs and consumers. Carnivorous plants hav…
Carp are fish species in the minnow family (Cyprinidae), one of the major groups of freshwater fish. The most familiar species are the common carp (Cyprinus carpio) and the closely related goldfish (Carassius aureus). The minnow family is characterized by having no teeth in the jaws, although well-developed teeth occur on the pharyngeal bones (located behind the gill chamber) and are used to grind…
Carpal tunnel syndrome results from compression and irritation of the median nerve where it passes through the wrist. In the end, the median nerve is responsible for both sensation and movement. When the median nerve is compressed, an individual's hand will feel as if it has "gone to sleep." The individual will experience numbness, tingling, and a prickly pin like sensation ov…
In genetics, the term carrier describes an organism that carries two different forms (alleles) of a recessive gene (alleles of a gene linked to a recessive trait) and is thus heterozygous for that the recessive gene. Although carriers may act to convey and maintain recessive genes within a population by passing them on to offspring, the carriers themselves are not affected by the recessive trait a…
The carrot family (Apiaceae, or Umbelliferae) is a diverse group of about 3,000 species of plants, occurring in all parts of the world. Most Umbellifers are herbaceous, perennial plants, often with aromatic foliage. Some species have poisonous foliage or roots. The leaves are typically alternately arranged on the stem, and in many species they are compound and divided into lobes. The flowers are s…
Carrying capacity refers to the maximum abundance of a species that can be sustained within a given area of habitat. When an ideal population is at equilibrium with the carrying capacity of its environment, the birth and death rates are equal, and size of the population does not change. Populations larger than the carrying capacity are not sustainable, and will degrade their habitat. In nature, ho…
The Cartesian coordinate system is named after René Descartes (1596–1650), the noted French mathematician and philosopher, who was among the first to describe its properties. However, historical evidence shows that Pierre de Fermat (1601-1665), also a French mathematician and scholar, did more to develop the Cartesian system than did Descartes. To best understand the nature of the Ca…
Cartilaginous fish such as sharks, skates, and rays are vertebrates whose internal skeleton is made entirely of cartilage and contains no ossified bone. Cartilaginous fish are also known as Chondrichthyes and have one or two dorsal fins, a caudal fin, an anal fin, and ventral fins which are supported by girdles of the internal skeleton. Placoid scales, or dermal teeth, are characteristic of the sk…
Cartography is the creation, production, and study of maps. Cartographers are often geographers who specialize in the combination of art, science, and technology to make and study maps. Some cartographers teach mapmaking skills and techniques, some design and produce maps, and some are curators of map libraries. All cartographers, however, focus on maps as the object of their study or livelihood. …
The cashew family (Anacardiaceae) is a group of about 600 species of plants, most of which are tropical in distribution, although some occur in the temperate zone. Maturing pistachio (Pistacia vera) nuts on a tree in California. © Holt Studios International, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. Almost all members of the cashew famil…
In the fall of 1997, the Cassini spacecraft began a seven-year, 2.175 billion mi (3.5 billion km) journey to Saturn. The 22.3 ft (6.8 m) robotic spacecraft is still functioning perfectly. When it arrives on July 1, 2004, it will spend four years probing the Saturnian system. Cassini, the first spacecraft to visit Saturn since Voyager 2 swung past it in 1980 and the first spacecraft ever to take up…
Catabolism is the breakdown of large molecules into small molecules. Its opposite process is anabolism, the combination of small molecules into large molecules. These two cellular chemical reactions are together called metabolism. Cells use anabolic reactions to synthesize enzymes, hormones, sugars, and other molecules needed to sustain themselves, grow, and reproduce. Energy released from organic…
Humans used the process known as catalysis long before, they understood what took place in that process. For example, soap-making, the fermentation of wine to vinegar, and the leavening of bread are all processes that involve catalysis. Ordinary people were using these procedures in their everyday lives without knowing that catalysis was involved. The term catalysis was proposed in 1835 by the Swe…
Catastrophism is the doctrine that Earth's history has been dominated by cataclysmic events rather than gradual processes acting over long periods of time. For example, a catastrophist might conclude that the Rocky Mountains were created in a single rapid event such as a great earthquake rather than by imperceptibly slow uplift and erosion. A new concept, uniformitarianism, grew from the wo…
Catfish include some 2,500 species of mostly freshwater fish characterized by two to four pairs of whiskers or barbels around their mouth. Many species have spines on the dorsal fins and near the gills. In some species these spines may contain poison. Catfish belong to the bony fish order Siluriformes, and are mainly freshwater forms with representatives throughout the world. Most species of catfi…
Catheters are long, flexible tubes that are inserted into the body for various purposes, either to remove an unwanted substance or to instill nourishment or medication. A relatively large catheter can be passed through the nose, down the throat and into the stomach to remove the contents of the stomach; for example, if someone has consumed a poisonous substance, a catheter can be used to remove a …
The cathode is one of the two electrodes that are present in any system in which electricity is entering and leaving a region; the other electrode is called the anode. The electric current enters through one of the electrodes and leaves through the other. Two general kinds of systems employ electrodes: vacuum tubes (also called gas discharge tubes) and electrochemical cells. In a vacuum tube, the …
A cathode ray tube is a device that uses a beam of electrons in order to produce an image on a screen. Cathode ray tubes are also known commonly as CRTs. Cathode ray tubes are widely used in a number of electrical devices, such as computer screens, television sets, radar screens, and oscilloscopes used for scientific and medical purposes. A cathode ray tube consists of five major parts: an envelop…
A cation is any atom or group of atoms that has a net positive charge. While matter is electrically neutral overall, ionic compounds are matter that is composed of positively-charged and negatively-charged particles called ions. An ion is any atom or group of atoms with an overall electrical charge. According to the laws of physics, opposite charges attract, so the oppositelycharged ions attract e…
Cats are mammals in the family Felidae of the order Carnivora, which includes all of the carnivores. The highly predatory instincts of species in the cat family are easily seen in domestic cats, for even well-fed individuals will aggressively hunt small mammals and birds. The cat family includes both large species (jaguar, leopard, lion, and tiger) and small ones (bobcat, lynx, ocelot, and serval)…
Cattails or reedmaces are about 10 species of monocotyledonous plants in the genus Typha, comprising the family Typhaceae. Cattails are tall, herbaceous, aquatic plants, growing from stout rhizomes located in shallow sediments of wetlands. The leaves of cattails are long and strap-like, sheathing at the base of the plant, while the spike-like inflorescence is borne by a long cylindrical shoot. The…
The cattle family, Bovidae, is a widespread group of mammals which also includes the goats, sheep, gazelles, antelopes, and goat-antelopes. Of the 107 species currently recognized within this family, just 12 are wild cattle. Even the large muskox (Ovibos moschatus), which looks quite cow-like, is more closely related to the goats than to cattle. Cattle are generally characterized by their large si…
Cauterization is the application of heat, mechanically or chemically, to prevent or stop bleeding. It is widely used in surgery to hold bleeding to a minimum and speed the surgical process. Birthmark removal by cauterization with an argon laser. Photograph by Alexander Tsiaras. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. Searing areas of bleeding wi…
A cave is a naturally occurring hollow area inside the earth. Most caves are formed by some type of erosional process. The most notable exception is hollow lava tubes such as those in Hawaii. The formation of caves depends upon geologic, topographic, and hydrologic factors. These factors determine where and how caves develop, as well as their structure and shape. The study of caves is called spele…
Many species of fish have evolved to living under strange conditions, but few are more intriguing than those that have adapted to living in complete darkness. Some of these fish have developed a tendency to live at great depths in the ocean where no light penetrates, while others have found refuge in equally dim locations such as caves, wells, and subterranean streams. This specialization to livin…
Celestial coordinates locate objects on the sky, which is considered to be an infinitely large (celestial) sphere. The four conventional celestial coordinate systems are defined. These are based on the earth's rotation, which produces an apparent westward rotation of the celestial sphere around the NCP and SCP. An object's arc distance along its hour circle from the celestial equator…
Modern celestial mechanics began with Isaac New ton's generalization of Kepler's laws published in his Principia in 1687. Newton used his three laws of motion and his law of universal gravitation to do this. The three generalized Kepler's law are: where G is the Newtonian gravitations factor, and from his second law of motion, Newton derived the following general form of …
The celestial sphere is an imaginary projection of the Sun, Moon, planets, stars, and all astronomical bodies upon an imaginary sphere surrounding Earth. The celestial sphere is a useful mapping and tracking remnant of the geocentric theory of the ancient Greek astronomers. Although originally developed as part of the ancient Greek concept of an Earth-centered universe (i.e., a geocentric model of…
The cell is the smallest living component of organisms and is the basic unit of life. In multicellular living things, a collection of cells that work together to perform similar functions is called a tissue; various tissues that perform coordinated functions form organs; and organs that work together to perform general processes form body systems. The human digestive system, for example, is compos…
Like all living things, the various types of cells in plants, animals, and the many different cell types in humans must eventually die. Cell death occurs in one of two ways. Cells can be killed by the effects of physical, biological, or chemical injury. Additionally, cells are induced to kill themselves. Cell suicide is also referred to as apoptosis (from the Greek words apo, meaning from, and pto…
Cell division is the process where a single living cell splits to become two or more distinct new cells. All cells divide at some point in their lives. Cell division occurs in single-celled organisms like bacteria, in which it is the major form of reproduction (binary fission), or in multicellular organisms like plants, animals, and fungi. Many cells continually divide, such as the cells that line…
Electrochemical cells are devices based on the principle that when a chemical oxidation-reduction reaction takes place, electrons are being transferred from one chemical species to another. In one type of electrochemical cell called a voltaic or galvanic cell, these electrons are deliberately taken outside the cell and made to flow through an electric circuit to operate some kind of electrical dev…
The cell is bound by an outer membrane that, in accord with the fluid mosaic model, is comprised of a phospholipid lipid bilayer with proteins—molecules that also act as receptor sites—interspersed within the phospholipid bilayer. Varieties of channels exist within the membrane. There are a number of internal cellular membranes that partially partition the intercellular matrix, and t…
Medical science depends on the staining of cells in tissues to make accurate diagnoses of a wide range of diseases from cholera to sexually transmitted diseases, to parasitic diseases and skin infections. Staining techniques performed routinely in microbiological laboratories include gram's stain, acid-fast stains, acridine orange, calcofluor white, toluidine blue, methylene blue, silver st…
Respiration occurs in three stages. The first stage is glycolysis, which is a series of enzyme-controlled reactions that degrades glucose (a 6-carbon molecule) to pyruvate (a 3-carbon molecule) which is further oxidized to acetylcoenzyme A (acetyl CoA). Amino acids and fatty acids may also be oxidized to acetyl CoA as well as glucose. During electron transport, much of the energy represented by th…
Cellular telephone technology is also called cellular radio. The cellular radio network became fully operational in North America in 1978. This technology relies on the distribution of what are called cell sites over a wide geographical area. Each cell site consists of a radio transceiver and a controller that sends and receives signals from the mobile phones in the area to a telephone switch. The…
Cellulose is a substance found in the cell walls of plants. Although cellulose is not a component of the human body, it is nevertheless the most abundant organic macromolecule on Earth. The chemical structure of cellulose resembles that of starch, but unlike starch, cellulose is extremely rigid (Figure 1). This rigidity imparts great strength to the plant body and protection to the interiors of pl…
Centipedes (phylum Arthropoda, class Chilopoda) occur throughout the world in both temperate and tropical regions where they live in soil and humus and beneath fallen logs, bark, and stones. Because they lack a hard outer skeleton, centipedes are confined to moist environments in order to maintain water balance. Many species are therefore active only at night, remaining sheltered during the day. M…
A centrifuge is a device for separating two or more substances from each other by using centrifugal force. Centrifugal force is the tendency of an object traveling around a central point to continue in a linear motion and fly away from that central point. Centrifugation can be used to separate substances from each other becausematerials with different masses experience different centrifugal forces…
Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are for example clays, spinels, and common window glass. Many ceramic compounds have very high melting points. Ceramics have a wide range of applications. They have been used as refractories, abra…
The Cerenkov effect is the emission of light from a transparent substance like water or glass when a charged particle, such as an electron, travels through the material with a speed faster than the speed of light in that material. The Cerenkov effect was discovered by Russian experimentalist P. A. Cerenkov in 1934 and explained by Russian theorists I. Y. Tamm and I. M. Frank. All three scientists …
Modern biologists believe that life first appeared in the sea; from these marine beginnings, land-dwelling organisms such as mammals gradually evolved. Cetaceans have returned to the marine environment after an ancestral period on land. As evidence of their terrestrial pedigree, consider that a whale fetus possesses four limb buds, a pelvis, tail, and forelimbs with five fingers like any land mamm…
Chachalacas, curassows, and guans are 42 species of birds that make up the family Cracidae. These birds are in the order Galliformes, which also includes the grouse, pheasants, quail, guinea fowl, and turkey. Curassows, chachalacas, and guans (or cracids) are believed to represent a relatively ancient and primitive lineage within this order. Fossil members of this family are known from deposits in…
Chameleons are small, strange-looking lizards in the family Chamaeleonidae. There are 86 species of chameleons, in four genera. The majority of species of A chameleon catching an insect with its tongue. JLM Visuals. Reproduced by permission. chameleon are found in the tropics of Africa and Madagascar, but some species live in southern Spain, Crete, India, and Sri Lanka. Most species of cham…
Chaos theory is used to model the overall behavior of complex systems. Despite its name, chaos theory is used to identify order in complex and otherwise seemingly unpredictable systems. Chaos theory is used to understand explosions, complex chemical reactions (e.g., the Belousov-Zhabotinsky oscillating reaction that yields a red solution that turns blue at varying intervals of time), and many biol…
