A leaf is a plant organ which is an outgrowth of the stem, and has three main parts: the blade, a flattened terminal portion; the petiole, a basal stalk which connects the blade to the stem; and the stipules, small appendages at the base of the petiole. However, the leaves of many species lack one or more of these three parts. Leaves function in photosynthesis, or the biological conversion of ligh…
Leafhoppers are a species of insects in the family Cicadellidae, order Homoptera, a group that also includes the cicadas, whiteflies, aphids, and scale insects. There are about 20,000 species of leafhoppers, including about 2,500 species in North America. Leafhoppers are leaf-feeding herbivores that use their sucking mouthparts to pierce the tissues of plants and feed on their juices, in some case…
Learning is the alteration of behavior as a result of experience. When an organism is observed to change its behavior, it is said to learn. Many theories have been formulated by psychologists to explain the process of learning. Early in the twentieth century, learning was primarily described through behaviorist principles that included associative, or conditioned response. Associative learning is …
Marlins are large fish with elongated, bill-like snouts, fairly high dorsal fins, and a streamlined body. They belong to the order Perciformes and the suborder Scomproidei. Marlins are among the fastest of all fish and are highly valued by sporting fishers. …
A Möbius strip is a twisted surface in space that is made by starting with a rectangular piece of paper, twisting one side through 180° (relative to the opposite side), and then joining it to the opposite side. That is, Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. using the …
A common denominator for a set of fractions is simply the same (common) lower symbol (denominator). In practice the common denominator is chosen to be a number that is divisible by all of the denominators in an addition or subtraction problem. Thus for the fractions 2/3, 1/10, and 7/15, a common denominator is 30. Other common denominators are 60, 90, etc. The smallest of the common denominators i…
Integrated pest management (IPM) is a system that incorporates many methods of dealing with pest problems. IPM systems may include the use of pest-resistant crop varieties; the modification of habitat to make it less suitable for the pest; the use of pest-specific predators, parasites, herbivores, or diseases; and pesticides when necessary. However, because IMP systems do not have an exclusive rel…
Marmosets and tamarins are South and Central American primates of the Amazon Basin. Their family, Callitrichidae, includes 18 species that have been described as "near"-monkeys. All species are considered endangered or threatened by extinction. This plight is mostly caused by deforestation to develop new agricultural land, as well as disturbance of their forest habitat by logging, ro…
Lecithin is a phospholipid which consists of glycerol, two fatty acids, a phosphate group and choline. Lecithin was first found in eggs in 1846, so its name was coined from the Greek word for egg yolk, lekithos. Though lecithin is its common name, chemists refer to it as phosphatidylcholine. It is a yellow-brown fatty substance. In contrast to fats, which function as fuel molecules, lecithin serve…
The integumentary system includes the skin and the related structures that cover and protect the bodies of plants and animals. The integumentary system of plants includes the epidermis, cuticle, plant hairs, and glands. The integumentary system of invertebrates includes shells and exoskeletons as body covering. The integumentary system of vertebrates comprises skin, scales, feathers, hair and glan…
Marmots are species of medium-sized robust, short-legged burrowing herbivorous rodents in the genus Marmota, family Sciuridae, order Rodentia. Marmots are closely related to the ground squirrels and gophers. Marmots live in burrows that they dig themselves, or sometimes in the deep crevices of rock piles and talus slopes beneath cliffs. Most species of marmots occur in alpine or arctic tundra or i…
Interference is the interaction of two or more waves. Waves move along their direction of propagation characterized by crests and troughs. Wherever two or more waves, either from one source by different paths or from different sources, reach the same point in space at the same time, interference occurs. When the waves arrive in-phase (the crests arrive together), constructive interference occurs. …
A LED (light-emitting diode) converts electrical energy to light by means of a semiconductor, made of a solid material, such as silicon, whose electrical conductivity when hot is as great as that of metals and very low when cold. LEDs were commonly referred to early in their history as solid-state lamps. The light produced by LEDs is known as electroluminescence, distinguishing it from incandescen…
Interferometry uses the principles of interference to determine properties about waves, their sources, or the wave propagation medium. Acoustic interferometry has been applied to study the velocity of sound in a fluid. Radio astronomers use interferometry to get accurate measurements of the position and properties of stellar Figure 1. The Michelson interferometer. Illustration by Hans & …
Mars is the fourth planet from the center of the solar system, orbiting the Sun once every 687 (Earth) days at a mean distance of 141 million mi (227 million km). Called the "red planet" for its distinct orange-red color, Mars has been the object of intense interest for over a century. Popularly regarded as a possible source of life, Mars was thought to be barren after the Viking spa…
The Mars Pathfinder probe landed on Mars on July 4, 1997. Pathfinder was second in the Discovery series of robotic spacecraft, which the United States National Aeronautics and Space Administration (NASA) began to develop in the mid 1990s. Costing an average of $150 million per project, the Discovery shift to faster, cheaper, less-ambitious probes was prompted by the catastrophic failure in 1993 of…
Interferons are species-specific proteins that induce antiviral and antiproliferative responses in animal cells. They are a major defense against viral infections and abnormal growths (neoplasms). Interferons are produced in response to penetration of animal cells by viral (or synthetic) nucleic acid and then leave the infected cell to confer resistance on other cells of the organism. Interferons …
Legionnaires' disease is a type of pneumonia caused by Legionella bacteria. The bacterial species re sponsible for Legionnaires' disease is L. pneumophila. Major symptoms include fever, chills, muscle aches, and a cough that is initially nonproductive. Definitive diagnosis relies on specific laboratory tests for the bacteria, bacterial antigens, or antibodies produced by the body�…
Marsupial cats are native carnivores of Australia, in the family Dasyuridae. Like all marsupials, the young of marsupial cats are born when they are still in an embryonic state, and they migrate to a belly pouch (or marsupium) on the female, where they fix onto a nipple and suckle until they are almost fully grown and independent. Marsupial cats fill the ecological roles played by weasels, cats, f…
Marsupial rats and mice are a diverse group of about 40 species of small, native carnivores of Australia, Tasmania, and New Guinea, in the family Dasyuridae. The young of marsupial rats and mice, as with those of all marsupials, are born while still in a tiny, embryonic stage of development. The almost helpless babies migrate to the belly of their mother, where they fix on a nipple and suckle unti…
Marsupials belong to the order Marsupalia, one of three subclasses of mammals (Metatheria). Marsupials are named for the marsupium, which means pouch in Latin; most female marsupials carry their young in pouches. The order Marsupalia includes eight families, 75 genera, and 250 species. Marsupials are divided into two groups based on the number and shape of the incisor teeth. One group has numerous…
The invention and development of the internal combustion engine in the nineteenth century has had a profound impact on human life. The internal combustion engine offers a relatively small, lightweight source for the amount of power it produces. Harnessing that power has made possible practical machines ranging from the smallest model airplane to the largest truck. Electricity is often generated by…
Maser is an acronym for microwave amplification by stimulated emission of radiation. Microwaves correspond to that portion of the electromagnetic spectrum where the radiation has wavelengths of 0.039-12 in (1 mm-30 cm), i.e., between the far infrared and radio frequencies. Crystals can be used as amplifiers of microwave radiation and as sources of radiation having a single wavelength and frequency…
Newton defined the mass of an object as the quantity of matter it possessed. A small rock, for example, has a mass—a fixed, unchanging quantity of matter. If you were to take that rock along with you on a trip to the moon, it would have the same quantity of matter (the same mass) that it had on Earth. Its weight, however, would be less on the moon. The rock's weight on earth was the …
The International Space Station (ISS), formally designated International Space Station Alpha, is a habitable orbital facility that has been under construction since 1998 and is scheduled for completion in 2006. When finished, it will contain about four times as much working space as the Russian space station Mir (1986–2001), the former record holder, and will weigh about one million pounds …
Legumes or beans are species of plants in the family Fabaceae (also known as Leguminoseae). The legume family is very large, containing about 12,000 species and 440 genera with species occurring on all of the habitable continents. The most species-rich groups in the legume family are the milk-vetches (Astragalus spp.) with 2,000 species, indigos (Indigoifera spp.; 500 species), clovers (Trifolium …
Lemmings are small mouselike rodents in the family Muridae, which also includes the voles, gerbils, hamsters, rats, and mice. Lemmings occur in open, northern habitats, especially in alpine and arctic tundra of North America and Eurasia. Lemmings are herbivores, feeding on sedges, grasses, berries, roots, and lichens. Lemmings are ecologically important in their habitat, in part because they are t…
Extinction, the death of all members of a species, is a natural process that has been occurring since the beginning of life on Earth. Nearly all species that have ever existed are now extinct, and extinction is an important process in the evolution of new species. Mass extinction, the death of large numbers of species over a relatively short span of geologic time, is also a natural process, but on…
The International Ultraviolet Explorer satellite (IUE) was a joint project of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Planetary Plasma and Atmospheric Research Center (PPARC) in the United Kingdom. NASA provided the spacecraft, telescope, spectrographs, and one ground observatory facility. ESA created the solar panels for powering the craf…
The mass number of an atom is the total number of protons plus neutrons in its nucleus. To understand this situation, think of the binding energy as the strength of the "glue" that holds the protons and neutrons together as a nucleus. It is, therefore, the amount of energy that would be required to break the "glue" and pull the nucleus apart into its individual neutrons…
Internet messages (e-mail, instant messages, etc.) and file transfers leave an electronic trail that can be traced. Tracing is a process that follows the Internet activity backwards, from the recipient to the user. As well, a users Internet activity on web sites can also be tracked on the recipient site (i.e., what sites are visited and how often, the activity at a particular site). Sometimes this…
Lemurs are primitive primates, or prosimians, found only on the island of Madagascar and nearby small islands off the eastern coast of Africa. Although lemurs, lorises, and tarsiers are all prosimians, or "premonkeys," only the lemurs and lorises have the typical prosimian snout that, like a dog's, remains moist. This wet snout, called a rhinarium, suggests that scent is a par…
In the field of optics, a lens is a device used for focusing or defocusing a beam of light. It is commonly formed from a disk-shaped blank of transparent material, such as glass, plastic, or fused quartz; both sides are ground and polished, with at least one surface being polished with a curve. The word lens is derived from the Latin word for lentil, since the shape of a lens resembles the curved …
Mass production is an entire system of manufacturing products that uses specialized labor, machinery, the smooth and logical flow of materials, and an assembly line to turn out large volumes of the same product at the lowest possible cost. The fullest expression of mass production was probably found at the Ford Motor Company in the early years of the twentieth century, when hundreds of thousands o…
Mass spectrometry is an instrumental method of obtaining structure and mass information about either molecules or atoms by generating ionized particles and then accelerating them in a curved path through a magnetic field. Heavier particles are more difficult for the magnetic field to deflect around the curve, and thus travel in a straighter path than lighter particles. Consequently, by the time th…
On a clear winter night go outside to a dark location and look for the constellation Orion, the hunter. A row of three stars makes up his belt. Hanging from his belt is his sword, a smaller row of three fainter stars. If you look at the center star in the sword with a pair of binoculars or a small telescope, you will see a small fuzzy patch of interstellar gas and dust, called the Orion Nebula. Sp…
Leprosy, also called Hansen's disease, affects 10-12 million people worldwide. Caused by an unusual bacterium called Mycobacterium leprae, leprosy primarily affects Leprosy. CNRI/Phototake NYC. Reproduced by permission. humans. Leprosy is found in tropical areas, such as Africa, South and Southeast Asia, and Central and South America. In the United States, cases of leprosy have bee…
An interval is a set containing all the real numbers located between any two specific real numbers on the number line. It is a property of the set of real numbers that between any two real numbers, there are infinitely many more. Thus, an interval is an infinite set. An interval may contain its endpoints, in which case it is called a closed interval. If it does not contain its endpoints, it is an …
From the Greek words for white (leukos) and blood (hemia), leukemia refers to abnormally shaped and functioning leukocytes (white blood cells). Because the leukocytes multiply at an uncontrolled and rapid rate, leukemia is considered a cancer of the blood. Leukemia is neither contagious nor infectious, nor acquired from the mother prior to or during birth, but some researchers have suggested genet…
Mass transportation is any kind of transportation system in which large numbers of people are carried within a single vehicle or combination of vehicles. Airplanes, railways, buses, trolleys, light rail systems, and subways are examples of mass transportation systems. The term mass transit is commonly used as a synonym for mass transportation. In many parts of the world, mass transit systems are a…
Lewis structures (also called electron-dot structures) are formed when Lewis symbols (also called electron-dot symbols) are combined. Lewis symbols are a simple way of visualizing the valence electrons in an atom. In a Lewis symbol, the symbol for the element is used to represent the atom and its core electrons. Dots placed around the atom are used to indicate the valence electrons. When combined …
Some species of plants, animals, and microorganisms have been spread by humans over much wider ranges than they occupied naturally. Some of these introductions have been deliberate and were intended to improve conditions for some human activity, for example, in agriculture, or to achieve aesthetics that were not naturally available in some place. Other introductions have been accidental, as when p…
Lice are small, wingless, biting or sucking insects, many of which are ecto-parasites. There are about 3,000 species of lice in the orders Mallophaga and Anopleura. The Anopleura are sucking lice, which are parasites of mammals, and which feed only on blood. The Mallophaga are chewing or biting lice, and are primarily pests of birds, feeding on skin and feathers. Most species of lice are specific …
In mathematics a quantity is said to be invariant if its value does not change following a given operation. For instance, multiplication of any real number by the identity element (1) leaves it unchanged. Thus, all real numbers are invariant under the operation of "multiplication by the identity element (1)." In some cases, mathematical operations leave certain properties unchanged. …
Lichens are an intimate symbiosis, in which two species live together as a type of composite organism. Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont. Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color, and biochemistry. Even though lichens are not true "species" in the…
Life history is an ecological term that refers to the significant features of the life cycle of organisms and their relationships with environmental conditions. Life cycle refers to the sequence of discrete developmental stages of an organism from their origin as gametes to their eventual death. Life cycle also refers to the stages through which generations of organisms pass from their own origin …
Mass wasting, or mass movement, is the process that moves Earth materials down a slope, under the influence of gravity. Mass wasting processes range from violent landslides to imperceptibly slow creep. Mass wasting decreases the steepness of slopes, leaving them more stable. While ice formation or water infiltration in sediments or rocks may aid mass wasting, the driving force is gravity. All mass…
An exotic species is one that has been introduced into a habitat it would not normally populate. This introduction can be intentional or unintentional. Exotic species have also been called introduced, nonnative, nonindigenous, or alien species. An invasive species is an exotic species that thrives in its new environment, disrupting the natural ecosystem. The majority of exotic species have been in…
Invertebrates are animals without backbones. This simple definition hides the tremendous diversity found within this group which includes protozoa (single-celled animals), corals, sponges, sea urchins, starfish, sand dollars, worms, snails, clams, spiders, crabs, and insects. In fact, more than 98% of the nearly two million described species are invertebrates, ranging in size from less than a mill…
In inorganic chemistry, ligands are molecules or electrically charged atoms (ions) which are bonded to metal atoms or ions. The ligand changes the metal's ability to dissolve in or react with its surroundings. In biochemistry, ligands are defined as molecules, usually Figure 1. Two ligands (ammonia) each donate a pair of electrons to bond with a silver ion. (N = nitrogen, H = hydrogen, A…
Light can be narrowly defined as the visible portion of the electromagnetic spectrum. A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye. The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does. The next con…
A light-year is the distance that light (or any other form of electromagnetic radiation, such as radio waves) travels in a vacuum in one year. Since light travels at a velocity of 186,171.1 mi/s (299,792.5 km/s), one light-year equals 5,878,489,000,000 miles (9,460,530,000,000 km). The light-year is a convenient unit of measurement to use when discussing distances to the stars in the Milky Way gal…
Lightning is an electrical discharge usually, but not always, produced by well-developed thunderstorms. Although there is a clear-air lightening phenomena,lightning most frequently occurs within a cloud (intra-cloud), between two clouds (inter-cloud), or from the cloud to the ground. A lightning discharge can heat the air as much as five times hotter than the surface temperature of the Sun (about …
Ions are electrically charged atoms or groups of atoms. Ion exchange is the phenomenon of replacing one ion with another of similar charge. Ion exchange resins are solids containing strongly bonded charged atom groups. (Positively charged atoms are called cations; negatively charged, anions.) They occur as natural materials and can Invertebrates make up 97% of the animal kingdom, the largest …
Lilacs (Syringa spp.) are about 10 species of shrubs and small trees in the olive family (Oleaceae). Lilacs are native to Eurasia but have been widely planted elsewhere as ornamental shrubs. The common lilac (Syringa vulgaris) is the most familiar species to most people. The common lilac has shiny green wedge-shaped leaves without teeth on the margins which are arranged alternately on the twigs. …
Lilies are the classic representatives of the monocotyledons—those plants with only one seed leaf. Lilies are mostly perennial, erect herbs arising from a bulb. Some climb, a few are woody, but most arise from underground stems or other structures. The leaves vary in number from one to many, and are arranged on the stem alternately or in whorls. The leaves are flat, linear to lance-shaped, …
Ionization is the process in which one or more electrons are removed from an atom or molecule, thereby creating an ion. The word ionization is also used for the process in which an ionic solid, such as a salt, dissociates into its component ions upon solution. In order to remove an electron from an atom, enough energy must be supplied to break the bond between the negatively charged electron and t…
Ionizing radiation is any energy that causes the ionization of the substance through which it passes. As the radiation is emitted from a source, it detaches a charged particle from an atom or molecule, leaving the atom or molecule with an excess charge. This charged particle is called an ion. To remove an electron from an atom or molecule, the ionizing particles must have a kinetic energy exceedin…
In mathematics the concept of limit formally expresses the notion of arbitrary closeness. That is, a limit is a value that a variable quantity approaches as closely as one desires. The operations of differentiation and integration from calculus are both based on the theory of limits. The theory of limits is based on a particular property of the real numbers; namely that between any two real number…
Limiting factors are environmental influences that constrain the productivity of organisms, populations, or communities and thereby prevent them from achieving their full biological potential which could be realized under optimal conditions. Limiting factors can be single elements or a group of related factors. The environment of organisms must be suitable in many respects. Environmental factors m…
Limpets are a common mollusk of the class Gastropoda. Its shell is generally low, flat, oval, and more bilaterally symmetrical than coiled (like a snail's), and it covers the entire soft body, so that the living animal inside is rarely visible. Limpets adhere strongly to rocks by means of a broad muscular foot. It is important for their survival that they are not dislodged easily, since the…
Irises are plants in the family Iridaceae which contains 1,500-1,800 species and 70-80 genera. The center of diversity of this family is in southern Africa, but species are found on all of the habitable continents. The largest groups in the family are the true irises (Iris spp.) with 200 species and gladiolus (Gladiolus spp.) with 150 species. Many species in the iris family have large, attractive…
Two important concepts emerge in linear algebra to help facilitate the expression and solution of systems of simultaneous linear equations. They are the vector and the matrix. Vectors correspond to directed line segments. They have both magnitude (length) and direction. Matrices are rectangular arrays of numbers. They are used in dealing with the coefficients of simultaneous equations. Using vecto…
An abundance of studies on the subject of intellectuals has focused on societal differences and disparate issues, and featured a wide array of eras, cultures, and practices. A comparison by Raymond Williams between the French tradition of the intellectual engaged in public issues and the more reserved role ascribed to his or her English counterpart attests to this difference among societies. It ha…
Intentionality is that feature of many mental states by which they are directed at or about or of objects and states of affairs in the world. So, for example, if I have a belief, it must be a belief that such and such is the case. If I have a desire, it must be the desire that such and such should be the case. If I have an intention, it must be the intention that I do something. Intentionality is …
Iron is a metallic chemical element of atomic number 26. Its symbol is Fe, atomic weight is 55.847, specific gravity is 7.874, melting point is 2,795°F (1,535°C), and boiling point is 4,982°F (2,750°C). Iron is one of the transition metals, occurring in group 8 of the periodic table. Four naturally occurring isotopes exist with atomic weights of 54 (5.8%), 56 (91.7%), 5…
The notion of intercession (shafa'a) is present within Muslim religious tradition as well as Muslim and Middle Eastern social and political culture. Intercession implies a special intervention or act of patronage providing favors, power, victory, or, in the religious variety, redemption from the terrible fate of sinners. Although there have been some objections, historic and modern, to this…
An irrational number is a number that cannot be expressed as a fraction, that is, it cannot be written as the quotient of two whole numbers. As a decimal, an irrational number is shown by an infinitely long nonrepeating sequence of numbers. Examples of irrational numbers are π (pi, the ratio of circumference to diameter of a circle), e (base of the natural logarithms), and √2 (that n…
The development of innovative and creative ideas in the academy is taking place in the early 2000s largely across departmental and disciplinary divides. Intradisciplinary practice tends to be microfocused and piecemeal. Innovation proceeds intradisciplinarily, where it proceeds at all, mostly in small if not minute increments. Departmental institutionalization itself has been transforming, prompte…
Lipids are a class of natural, organic compounds in plants and animals, defined by a specific way they behave: they are soluble in non-polar solvents. That is, lipids are not soluble in water but dissolve in solvents like gasoline, ether, carbon tetrachloride, or oil. The vast majority of lipids are colorless and mostly fats and oils. Lipids comprise one of the three broad classifications into whi…
The term internal colonialism defines a condition of oppression or subordination, often of one ethnic group over another (as in the subordination of Mexicans in the United States at the conclusion of the U.S.-Mexican War of 1846–1848). Some view the term as a contraposition to the claim that all people in the United States "are immigrants." For example, immigrant does not desc…
Liquid crystals are pure substances in a state of matter that shows properties of both liquids and solids over a specific temperature range. At temperatures lower than this range, the liquid crystals are only like solids. They do not flow and their molecules maintain a regular arrangement. At temperatures above this range, the liquid crystals behave only like liquids. They can flow and the molecul…
Mössbauer effect was first reported by Rudolph Mössbauer in 1958. Three years later, he won the Nobel Prize with his discovery. Since then, it is believed that nuclear γ-ray emission and absorption process can take place in recoil-free fashion. In reality, of course we have both recoil and recoil-free events. Mössbauer also utilized the Doppler (velocity) shift to modu…
The image of a peaceful, orderly world where men and nations resolve their differences without war, where the lion lies down with the lamb, has haunted humankind for millennia. Roman law expressed the gulf between the ideal and the reality concisely by contrasting the condition of man living under the ius naturale, the natural law, a world of peace and harmony, with the human condition as it actua…
Liquefaction of gases is the process by which substances in their gaseous state are converted to the liquid state. When pressure on a gas is increased, its molecules closer together, and its temperature is reduced, which removes enough energy to make it change from the gaseous to the liquid state. …
The problem of interpretation is as old as the written record, even as old as the capacity for human beings to disagree fundamentally. It was raised by Plato. It marked many a conflict in early Christendom. From the Middle Ages to early modernity, cultural texts proliferated. Interpretation was the task of those who were in charge of biblical exegeses and of those who judged a text's origin…
After striking its roots in Egypt and in the far West, Islam was carried into the fringes of black Africa by indigenous tribesmen. Through the foundation of trading centers, the movement of populations, and the affiliation with local ruling elites, Muslim influence in the interior of the region was strongly felt. In their eleventh-century search for gold, Berber nomadic tribesmen reached the area …
A common misconception, based on the fact that Iran is today predominantly Shii, holds that Iran is the historic "homeland" of Shii Islam. However, the truth is that Medina and southern Iraq are the original centers of Shii Islam, which spread from there to other areas in the Muslim world. The heyday of the Shiis was in the tenth and eleventh centuries, when dynasties representing t…
The practice of diverting water from natural resources to crops has been practiced for at least 7,000 years. The earliest methods, as practiced in places like the areas surrounding the Nile river basin, included digging channels to allow water from the river during flood periods to reach cultivated fields along the river's banks. Ancient farmers also built dikes to help retain the water on …
The Muslim population of Southeast Asia is in excess of 250 million. It is the religion of the majority in Indonesia and Malaysia, with significant minorities in Singapore, Thailand, and the Philippines. Islam has a five-hundred-year history in this region. Importantly, it was not imposed by conquest but freely adopted, initially by royal and upper classes and then percolating down to the masses. …
Lithium has been the treatment of choice for manic-depressive illness for several decades. Lithium is a trace element found in plants, mineral rocks, and in the human body. Today, the major source of medical lithium is mines in North Carolina. Lithium is classified as an antimonic medication because of its ability to reverse mania, a mood disorder characterized by extreme excitement and activity. …
The term Sunni is an adjective formed from the noun sunna (plural sunan), an ancient Arabic word meaning "customary practice." Although in the pre-Islamic world of tribal Arabia the word sunna referred generally to the time-tested and widely accepted customs of a tribe, in the Islamic period the term came to refer specifically to the customary practices or "way" of the …
History of science in non-Western societies is often either disregarded—because it is not linked with that in Western societies—or disfigured into a member of a chain of transmission that links antiquity with Latin Christendom. Rather than attempt to fit Islamic science into the history or notions of Western science, this essay examines the relationships between various scientific di…
Famous for its promotion of nonviolence and often paired with Buddhism as one of ancient India's two greatest dissenting religions, Jainism is currently professed by roughly 0.4 percent of the population of India. Its adherents are prominent in business, and some of modern India's wealthiest and most powerful families are Jains. Jain communities are divided between a majority of lay …
From the short-term horizon of the Heisei era (the name Heisei, meaning "peace everywhere," was introduced in 1989 by the emperor Akihito after the death of his father, the emperor Showa [Hirohito]), the other is still the West and more precisely the West as an idealized United States. However, a strong conviction that it is time for Japan to establish a new order has surfaced in the…
Lithography is a method of printing an image by applying patterned layers of color to paper with a series of etched metal or stone plates. This is the process used to print many newspapers and multi-colored lithographs. It is also the general name for the techniques used to fabricate integrated circuits (ICs). In the manufacture of integrated circuits, the silicon wafer that acts as the base and a…
Jihad, in Islam, is an idea of action. The Arabic word literally means "striving." When followed by the modifying phrase fisabil Allah, "in the path of God," or when this phrase is absent but assumed to be in force, jihad has the specific sense of fighting for the sake of God and religion. Other Arabic words are closely related in meaning and usage, including ribat, wh…
An island is an area of land, smaller than a continent, that is entirely surrounded by water. That distinction, although somewhat artificial, suggests different geologic forces acting to create and maintain islands versus continents. Islands further differ from continents in their natural environments—in the biological systems they support, in their rate of response to change, in their abil…
The word lithosphere is derived from the word "sphere," combined with the Greek word "lithos" which means rock. The lithosphere is the solid outer section of Earth which includes Earth's crust (the "skin" of rock on the outer layer of planet Earth), as well as the underlying cool, dense, and fairly rigid upper part of the upper mantle. The lithosphe…
French psychoanalyst Jacques Lacan's (1901–1981) use of the term jouissance, like most other Lacanian concepts, shifts over the years and can be difficult to pin down. Translating from the French, jouissance can be rendered literally as "enjoyment," "both in the sense of deriving pleasure from something, and in the legal sense of exercising property rights"…
Isobars are lines that connect points of equal atmospheric pressure on weather maps. Isobars are similar to height lines on a geographical map, and they are drawn so that they can never cross each other. Meteorologists use isobars on weather maps to depict atmospheric pressure changes over an area and to make predictions concerning wind flow. The term "isobar" originates from the Gre…
As a religion developing over three millennia, Judaism changed, diversified, and acculturated to many cultural and spiritual environments, while maintaining at the same time some basic characteristics. In the following, an attempt is made to describe both the continuities and the variations characteristic of the various forms of Judaism up to 1800. Four main concepts organize the majority of the d…
Isomer is the term used to describe two or more chemical compounds which can be represented by the same chemical formula. There are two main types of isomers: structural isomers which differ from one another by the attachment of atoms on the molecule; and stereoisomers which differ from on another by the location of the atoms in space. Chain isomerism, another type of structural isomerism, occurs …
Judaism has never been monolithically uniform. Made irreducibly complex by 3,500 years of turbulent history, it resists simplification. Its data refuse to be straitjacketed or handled dispassionately. Impartial attempts to understand it are easily spoiled by the partisan sympathies and deeply held antagonisms it activates in observers. Consider a mere sampling of the evidence. Its implications sta…
Virtually everyone becomes involved in disputes about justice at some point. Sometimes our involvement in such disputes is rooted in the fact that we believe ourselves to be victims of some form of injustice, such as job discrimination; sometimes our involvement is rooted in the fact that others believe us to be the perpetrators or at least the beneficiaries of some form of injustice affecting the…
Lithotripsy, extracorporeal shock wave (ESWL), is the first non-invasive (not requiring surgical opening of the body) treatment for eliminating kidney stones by breaking them into sand-like particles, usually by means of high pressure waves generated in water. The particles are then eliminated from the body during urination. The ESWL machine, called a lithotripter, generates shock waves in a reser…
Justice appears as a paradoxical concept in the history of the United States. On the one hand, it has been absent as a rallying cry in the major struggles that have shaped, and continue to shape, the plurality of identities of the American nation. The concept "justice" is not prominent in the Declaration of Independence or in the Constitution. It was not the cornerstone of the abolit…
Isostasy is the term describing the naturally occurring balance of masses of Earth's crust that keeps the planet's gravity in equilibrium. Isostasy is not a force or a process; it is only the term for the phenomenon of adjustments Earth makes to stay balanced in mass and gravity. …
In what follows, central themes and institutions related to justice that appeared within Confucian and Daoist teachings in preimperial China will be explained and evaluated, followed by brief comments about later developments. …
An isotope is one of several kinds of atoms of the same element that have different masses. These atoms have the same number of protons in their nuclei, but different numbers of neutrons, and therefore different mass numbers. The term isotope comes from the Greek isos topos, which means same place, because isotopes of the same element have the same atomic number and therefore occupy the same place…
The philosophy of Immanuel Kant (1724–1804) radically transformed the rationalism and empiricism of the seventeenth and eighteenth centuries and has set many of the problems for epistemology, metaphysics, and philosophy of science, moral and political philosophy, aesthetics, philosophy of history, and philosophy of religion ever since. Almost all philosophy after Kant could be divided into …
A narrow strip of land, an isthmus, connects two wider sections of land. The isthmus of Panama, which connects South America to Central/North America, and the Sinai peninsula, which connects Africa to Asia, illustrate better than any other examples. Moving plates create many isthmi (isthmuses). The earth's outer shell, the crust, breaks into sections, plates, that slowly slide around the ea…
Kinship—which can be initially described as the study of the links between people established on the basis of descent, marriage, or adoption—has been a defining domain of anthropological investigation since the inception of this discipline in the last twenty years of the nineteenth century. The detailed description of the complexities of kinship systems was for many decades considere…
Four words of ancient Greek are important to the first philosophical discussions of knowledge in the Western tradition. In a given context any of them might be translated with the word "knowledge": epistēmē, technē, mētis, and gnōsis. Epistēmē names the most philosophical idea of knowledge: contemplative, disinterested, logical knowledge of t…
Iteration consists of repeating an operation of a value obtained by the same operation. It is often used in making successive approximations, each one more accurate than the one that preceded it. One begins with an approximate solution and substitutes it into an appropriate formula to obtain a better approximation. This approximation is subsequently substituted into the same formula to arrive at a…
The liverworts are one of three classes in the plant phylum Bryophyta. The other two classes are mosses and hornworts. Liverworts are small, green, terrestrial plants. They do not have true roots, stems, or leaves. Instead, they have an above ground leaf-like structure, known as a thallus, and an underground structure, known as a rhizoid. Most liverworts are found in moist environments and they te…
The Oxford English Dictionary defines landscape as both a verb and a noun, signifying not simply its multiple references in vernacular and specialized parlance or its active and passive modes but more importantly the varying perceptions of landscape as an artistic, cultural, and religious entity. Among the definitions of landscape as a noun, the OED proffers first "A picture representing na…
Jacanas are eight species of distinctive birds that inhabit the marshy edges of ponds, lakes, rivers, and swamps, and that make up the family Jacanidae. Jacanas are tropical birds, breeding in central and northern South America, sub-Saharan Africa, Madagascar, South and Southeast Asia, and Australasia. Jacanas do not migrate, but they may wander widely if their local aquatic habitats dry out. Jaca…
Estimates of the number of languages spoken on earth at the turn of the twenty-first century range between four and six thousand. Considering that this number has been rapidly declining for the last couple of centuries, there must at one time have been many more languages, perhaps upwards of ten thousand. Of the languages spoken in the twenty-first century, a few have tens or hundreds of millions …
Jacks, also called scads, trevallys, and crevalles, are marine bony fishes living in open waters. Amberjacks, runners, and pompanos also belong to the same family Carangidae, order Perciformes. Jacks are swift, predatory fishes, found widely in warm and tropical seas. The younger fish tend to travel in vast schools, but the older ones may be solitary. Many jacks are valued for commercial and sport…
Language and writing are not the same (although people now confuse them), and the former is far older than the latter. Human spoken language developed late in the Paleolithic, probably 100,000–120,000 years ago, whereas true writing—the representation of language, element by element, in a permanent medium—was not invented until about 5,200 years ago. …
Jaundice is not a disease but a symptom of an underlying disease or condition. It is caused by too much bilirubin in the blood stream, and is characterized by yellowness of skin, sclera (white of eyes), mucous membranes, and of body fluids such urine and blood plasma. The resulting yellow color (jaune means yellow in French) is also described by the Latin term icterus. Most bilirubin, which is a r…
Only in recent times has philosophy of language been considered a distinct branch of philosophy. But ancient and medieval philosophers had different, sophisticated theories about the relation between language—both individual words and whole sentences—and reality, and the thirteenth century saw one of the most thorough attempts ever to give an abstract, analytical account of the gramm…
Jellyfish, also called medusae, are free-swimming, marine invertebrates in the class Scyphozoa (phylum Coelenterata). They have a gelatinous, translucent, dome-shaped body and occur most commonly in warm, tropical seas, although they are found in all the world's oceans. Jellyfish feed on small planktonic animals or fish which they sting and paralyze with special cells called nematocysts loc…
Although discussions of language in seventeenth-and eighteenth-century philosophy foreshadowed many issues that came to full bloom in the twentieth century, before the twentieth century language was thought to have a secondary role in understanding the special place human beings have in the world. The fundamental concern was the problem of knowledge. How is it possible for human beings to have kno…
Jerboas are small kangaroo-like rodents with large hind legs that make up the family Dipodidae. Three species of jerboas occur in North Africa, and a number of other species occur in Asia. Jerboas are typically pale-colored, with large eyes, immense ears, a long tail, small front legs and paws, and distinctively large hind legs and feet, which are used for jumping. Although the body length of a ty…
The development of law and jurisprudential ideas since the 1970s represents a significant change from the conventional models that had earlier dominated the province of legal theory in the history of ideas. This entry focuses on two salient theoretical emphases that have continued to influence developments of conventional legal theory and postmodern paradigms of legal thought: the analytical tradi…
Islam, like Judaism and unlike Christianity, is a nomocracy, a religion of the law as opposed to theology. It is evident from the Koranic text that sacred law is a crucial feature of the covenants that God establishes, through prophets, with the various nations of mankind. Prophets are united by their main directive, to worship God alone, by their moral exhortations, and by accompanying miracles p…
Although there are seemingly as many descriptions of leadership as there are men and women who fill leadership roles, most meaningful definitions echo the one commonly ascribed to former U.S. president Harry S. Truman: leadership is the ability to get others to willingly move in a new direction in which they are not naturally inclined to move on their own. While the related but distinct concept of…
People have long wondered how best to characterize learning and memory. Most people typically conceive of memory as a place in which information is stored. As Henry Roediger has noted, this spatial metaphor has dominated the study of memory: "We speak of storing memories, of searching for and locating them. We organize our thoughts; we look for memories that have been lost, and if we are fo…
Shang Yang aimed to turn Qin into a powerful state through two parallel and interconnected processes: encouraging agricultural production and strengthening military prowess. To achieve these goals he advocated a clear system of rewards and punishments, according to which aristocratic ranks would be granted for high grain yields and for military merits, while high taxation would be applied against …
Livestock is a collective term for domesticated animals that are kept, mostly for the production of meat, milk, wool, or other products. The most common species are cattle, pigs, sheep, goats, horses, and chickens. The term is not used in reference to animals that are kept as pets or companions. Livestock are domesticated species, which have been genetically modified over time through the artifici…
It is widely agreed that fundamental to liberalism is a concern to protect and promote individual liberty. This means that individuals can decide for themselves what to do or believe with respect to particular areas of human activity such as religion or economics. The contrast is with a society in which the society decides what the individual is to do or believe. In those areas of a society in whi…
The broad definition of liberation theology stresses the interrelatedness of differing structures of oppression and domination. Liberation from oppressive structures necessarily involves political, economic, social, racial, ethnic, and sexual aspects. As a paradigm, liberation theology today places explicit emphasis on assessing different forms of human oppression and suffering, and liberation fro…
Liberty is an integral concept in Western political and social thought. Liberty as an inalienable social and political attribute of individuals emerged in the formation of the modern political discourse in the West. Since Georg Wilhelm Friedrich Hegel (1770–1831) the concept has often been categorized in a threefold manner: moral liberties (freedom of moral choice, such as freedom of consci…
A jet engine is a heat engine that is propelled in a forward direction as the result of the escape of hot gases from the rear of the engine. Two general types of jet engines exist: the air-breathing jet engine and the rocket. In an air-breathing jet engine, air entering the front of the engine is used to burn a fuel within the engine, producing the hot gases needed for propulsion. In a rocket, air…
Throughout recorded history human beings have recognized the qualitative difference between the living and non-living worlds, the animate and inanimate. Placing that recognition on solid, rational footing or giving it a quantitative basis has remained a major challenge, however. What exactly makes a living being so different from one that is nonliving? Living organisms carry out oxidation, for exa…
The jet stream is a narrow, fast, upper atmospheric wind current, flowing at high altitudes around Earth. Although often erroneously applied to all upper-level winds, by definition jet stream wind speeds are in excess of 57 MPH (92 km/h). The jet stream may extend for thousands of miles around the world, but it is only a few hundred miles wide, and usually less than a mile thick. Undulating jet st…
The Sphinx, according to an ancient Greek tale, was a monster with the face of a woman, the body of a lion, and gigantic wings. Sent by the goddess Hera to punish the city of Thebes, she sat on a hilltop and stopped passersby, posing them a riddle: "What has one voice, and is four-footed, two-footed, and three-footed?" Every time the Thebans gave a wrong answer, she devoured one of t…
Juniper is the common name for a large group of evergreen shrubs and trees belonging to genus Juniperus, in the family Cupressaceae (Cypress), order Pinales (pine). There are more than 50 species of Juniperus. They can be low creeping ground cover, broad spreading shrubs, or tall narrow trees. Both low growing and tall varieties are cultivated for ornamental purposes. Junipers have thick, dense fo…
Lobsters are large crustaceans in the order Decapoda, which also includes about 10,000 species of crayfish, crabs, and shrimps. Decapods are characterized by having their carapace fused with their thoracic segments to form a gill chamber above where the legs join the body, and the first three of their eight pairs of thoracic legs are modified into grasping, clawlike structures known as maxillipeds…
Many social historians have argued that adolescence emerged as a distinct life stage only with the advent of industrialization. Using case studies from regions where the historical record is plentiful, such as France, England, and the United States, scholars contend that prior to the industrial revolution, the physical processes of maturity did not necessarily signal a change in life status for th…
The study of the human life cycle is primarily a study of the aging process. The question of why humans age has long intrigued social and biological scientists. While a fountain of youth has yet to be discovered, public health and hygiene interventions have lengthened the human life expectancy greatly over the course of the past hundred years. And yet life expectancy varies greatly within and betw…
"Where word breaks off no thing may be": this is the line from a poem by Stefan George repeatedly cited by Martin Heidegger to indicate his version of the linguistic turn, which affected many philosophers in the early twentieth century—literary scholars already having made the turn, whether consciously or not (Heidegger, p. 60). The phrase "linguistic turn" is ac…
A lock or water lock is an enclosed, rectangular chamber with gates at each end, within which water is raised or lowered to allow boats or ships to overcome differences in water level. Locks have a history of over 2,000 years, and although they are most often used by boats on canals, they also are used to transport massive ships between seas. All locks operate on the simple buoyancy principle that…
When the definitive account of post-1960s intellectual technologies is written, the history of literary movements will constitute a key chapter. Perhaps paradigmatic of the closing decades of the twentieth century in its dramatic shifts and realignments, literary criticism at the opening of the twenty-first century shows all the earmarks of specialized knowledge, professionalization, and market ma…
History traces the passage of men and women through time. Literary history charts their developments and experiments in writing in the hope that global discourse will be stimulated and cultures come to understand one another. It relates, compares, and categorizes the poetry, prose, drama, and reportage of authors at various periods. The process started when the artistic deployment of language (poe…
Almost all senses of the English word literature and its cognates in other Indo-European languages can eventually be traced back to the act of scratching (on a piece of leather or on clay, stone, wood, wax, pottery, lead, or papyrus). But this primitive act very quickly became associated with superior development: civilization. …
A lock is a mechanical device for securing a door, chest, or other receptacle so that it can only be opened by an authorized person. Most locks are opened by a key which is placed in the lock and turned. Combination locks do not use a key but rather have a cylinder that is turned to certain stops. Today, many hotels use special plastic cards with magnetic strips as keys which cause a door to open …
African literature is best understood within the context of Ali Mazrui's categorization of African historical experience as a "triple heritage": Africa as a space produced by endogenous historical traditions, Arab/Islamic influences, and Western Judeo-Christian influences. This triple heritage has produced a literature characterized by a tripodal identity, based on its relatio…
Logic is the study of correct reasoning. A host of philosophical themes have clustered around this central concern: the nature of truth and validity, of possibility and necessity; the semantics of words, sentences, and arguments; and even questions about substances and accidents, free will and determinism. …
This article surveys many of the main positions that have been held in the logic and philosophy of mathematics from around 1800 up to recent times. Most attention is given to symbolic logics of some kind. No position has been definitive; indeed, especially over the last seventy years the variety has continually increased. To compensate for this article's lack of exhaustiveness, the bibliog…
A locus is a set of points that contains all the points, and only the points, that satisfy the condition, or conditions, required to describe a geometric figure. The word locus is Latin for place or location. A locus may also be defined as the path traced out by a point in motion, as it moves according to a stated set of conditions, since all the points on the path satisfy the stated conditions. T…
In the West, probably more has been written about love than any other topic except the nature of God. There is, however, no consensus on what the word love or amor, agape, Liebe, eros, or hundreds of other terms signifies. It is clear, however, that it is more than a simple animal urge to procreate. Harry Harlow, for example, demonstrated that mother love was essential to the normal development of…
Jupiter, the fifth planet from the Sun, is the largest and most massive planet in our solar system. One of the gas giants, it is composed of mostly hydrogen and helium. The Jovian atmosphere provides a rich laboratory for the study of planetary atmospheres. Its most famous feature, the Great Red Spot, has been visible for hundreds of years, and many smaller features are visible in its atmosphere. …
Because logarithms are exponents, they have an intimate connection with exponential functions and with the laws of exponents. The major laws of logarithms and the exponential laws from which they are derived are shown in Table 1. In all these rules, the bases a and b and the arguments x and y are limited to positive numbers. The exponents m, n, p, and r and the logarithms can be positive, negative…
Loyalty is devotion to a cause and is marked by faithfulness, a sense of just purpose, and a willingness to serve in spite of any suffering that may result from service. Dual loyalty involves simultaneous obligations, express or implied, to two parties, with the second party typically constituting a state. Multiple loyalties can threaten the security and survival of a state. Nationality may affect…
The K-T event (Cretaceous-Tertiary event) refers to the mass extinction of the dinosaurs that took place approximately 65 million years ago (mya). In addition to the dinosaurs, most large land animals perished and an estimated 70% of species became extinct. In the early 1980s, a team of physicists and geologists documented a band sedimentary rock in Italy that contained an unusually high level of …
Loons are the only surviving members of an ancient order of birds, the Gaviiformes, which has a fossil record extending back to the Lower Cretaceous, more than 100 million years ago. Loons comprise their own family, the Gaviidae, which consists of 12 extinct and five surviving (extant) species. All of the extant species of loons live in the Northern Hemisphere, where they breed on lakes and ponds,…
Lysenkoism is a censorious name for a movement that demands either censure or faith in agrobiology, as Soviet agronomist Trofim Denisovich Lysenko (1898–1976) called his aggressive creation. He also named it Michurinism, in tribute to the plant breeder Ivan V. Michurin, who won fame with claims of improving fruit while scorning genetics. Obscurantist (mrakobes), the word a geneticist flung …
Kangaroos rats are small burrowing mammals with fur-lined cheek pouches, making up the rodent family Ord's kangaroo rat. Photograph by Larry L. Miller. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. Heteromyidae, found principally in North and Central America. There are five genera of rodents with external pouches in this family but only two…
Machiavellism, a word that goes back to the late sixteenth century, is a name for the theory and practice of amoral politics. In its ideal, simply abstract sense, it is not meant to coincide exactly with the views or practices of any historical individual, even Niccolò Machiavelli (1469–1527) himself. When Machiavelli praises the citizens of the ancient republic of Rome as noble and pu…
Machos are not born; they are made. For the same reason, the term machismo refers to a concept that has been invented and not to a primordial cultural trait of any particular group of people. In the United States, machismo was "discovered" by social scientists and feminists much as the New World was "discovered" by Europeans five centuries earlier: U.S. scholars and fem…
Magic is the performance of acts or rites that are intended to influence a person, object, or event. It can also be performed to counter other magic. Magical acts or rites are usually performed with the assistance of mystical power. People who engage in the different activities magic encompasses can be called magicians, shamans, healers, sorcerers, or priests/priestesses. In some societies the kno…
Manichaeism arose in a highly cosmopolitan culture, in full awareness of antecedent west Asian religions such as Zoroastrianism, Christianity, and various pagan and Gnostic sects, as well as the Hindu, Jain, and Buddhist traditions of South Asia. According to Manichaean teaching, Mani was the last of a series of divinely inspired prophets that included Zoroaster, the Buddha, and Jesus Christ. Thes…
LORAN (Long Range Navigation) is a radio-based navigational aid first used during World War II to locate ships and planes with greater accuracy than could be achieved with conventional techniques. LORAN determines location by comparing accurately-synchronized powerful radio pulses originating from different reference transmitter sites. Pulses from nearby transmitters arrive earlier than pulses fro…
Maoism is not a term that is easy to define. While it is common sense that Maoism refers to the vision, ideology, and political viewpoint of Mao Zedong (1893–1976), it is difficult to pinpoint the specific contents and basic features of Mao's conceptual world in the context of the evolving course of the Chinese Communist revolution. Despite Mao's adoption of Marxist-Leninist t…
Cartography, the art and science of mapmaking, began before the invention of writing and continues to be fundamental to an understanding of the phenomena it represents graphically. Although typically associated with Earth, or parts of this body, its methods are applicable to the delineation of both the microcosm and the macrocosm. Thus, there is mapping of the human brain on the one hand, and the …
Kangaroos and wallabies are pouched mammals, or marsupials, of Australia and nearby islands. Kangaroos and wallabies have hind legs enlarged for leaping. Most species live on the ground, and some in trees. The name kangaroo is usually used for large species, and wallaby for smaller ones. They all belong to the family Macropodidae, meaning "big footed," and they are herbivorous, or pl…
Prior to the nineteenth century, the image of the patriarchal family was a crucial component of both moral injunction (as in the Judeo-Christian fourth commandment to "honor thy father and mother") and political organization. Marriage was the keystone in the arch of social solidarities; it also signaled the creation of a new reproductive unit. For the individuals involved, marriage w…
Karst is a German name for an unusual and distinct limestone terrain in Slovenia, called Kras. The karst region in Slovenia, located just north of the Adriatic Sea, is an area of barren, white, fretted rock. The main feature of a karst region is the absence of surface water flow. Rainfall and surface waters (streams, for example) disappear into a drainage system produced in karst areas. Another fe…
The loris or bushbaby family, Lorisidae, includes 14 species of Asian and African primates. Loris is a Dutch word for clown, given to these amusing creatures by European seaman who saw them. With the lemurs, these attractive little primates make up the group called prosimians, or "pre-monkeys." All lemurs are found on the big island of Madagascar; other members of the loris family ca…
Few sets of ideas are richer and more conflicted than those that have been put forward under the heading of Marxism. Marxism's founder, the German philosopher Karl Marx (1818–1883), had a wide-ranging curiosity about many aspects of humankind and a stamina matching his curiosity. But as the American philosopher Sidney Hook pointed out in his article on Marxism in the 1973 edition of …
A karyotype is a technique that allows geneticists to visualize chromosomes under a microscope. The chromosomes can be seen using proper extraction and staining techniques when the chromosomes are in the metaphase portion of the cell cycle. Detecting chromosomal abnormalities is important for prenatal diagnosis, detection of carrier status for certain genetic diseases or traits, and for general di…
The writings of Karl Marx offer both a critique and a celebration of modern capitalism. On the one hand, Marx presented a devastating moral indictment of the capitalist exploitation of labor and the drowning of all human relationships in "the icy waters of egotistical calculation." On the other, Marx saw capitalism as a necessary and progressive phase of historical development, yield…
Throughout the twentieth century, Latin Americans wrestled with the enduring problems of foreign domination, social inequality, and poverty. Marxist popular movements, political parties, and intellectuals were often key players in these struggles, forming an important basis for trenchant social critique, mass social movement, and revolutionary organization. Even in countries where Marxist ideas, p…
Given that masks and the performance complex of masking, known as the masquerade, are found in practically all cultures at one time or another, there must be some fundamental reasons for the emergence of such a cultural practice. The development of shamanism seems to go hand in hand with masking and masquerading. As humans began to observe nature empirically, including their own behavior, parts of…
Light generation by a process other than by heating is luminescence. For example, an incandescent light bulb, in which the filament is heated until it is literally white-hot, is not luminescent; a fluorescent light tube (which is cool to the touch) is luminescent. Luminescence is generated as part of a process in which atoms or molecules with electrons excited into higher energy states shed energy…
Bony fish are divided into two major groups: rayfinned and fleshy-finned fish. The fleshy-finned fish are further subdivided into two orders: the lungfish, or Dipnoi, and the lobe-finned fish, or Crossopterygii. Although crossopterygian fish are the group that is thought to be close to the ancestors of the land vertebrates, lungfish also display many of these characteristics. In the early stages o…
Johannes Kepler made it his life's work to create a heliocentric (sun-centered) model of the solar system which would accurately represent the observed motion in the sky of the Moon and planets over many centuries. Models using many geometric curves and surfaces to define planetary orbits, including one with the orbits of the six known planets fitted inside the five perfect solids of Pythag…
A keystone species is a particular species that has a great influence on the structure or functioning of its ecological community. This influence is far out of proportion to the relative biomass or productivity of the keystone species within its community. Most keystone species are top predators, although a few are influential because they play a critical role as herbivores or in nutrient cycling.…
Lycophytes are vascular plants in the class Lycopodiopsida, a division of vascular plants known as Pteridophytes (the ferns and their allies). The class Lycopodiopsida is divided into three subclasses: the Lycopodiidae, Selaginellidae, and Isoetidae. Like other pteridophytes, the lycophytes have an alternation of generations, consisting of two generations of morphologically different plants. The l…
Killifish (Fundulus spp.) are small fish tolerant of a wide range of temperatures and salinity, found throughout temperate and tropical waters on every continent except Australia and Antarctica. Not to be confused with the other large group of small fish in the same order (Cyprinodontiformes) known as minnows, killifish differ in having an incomplete lateral line, often not extending past the head…
There are 87 species of kingfishers (family Alcedinidae) which are brightly-colored birds ranging in size from the 4 in (11 cm) long malachite-crested kingfisher, to the laughing kookaburra of Australia, which is 18 in (46 cm) long, weighing 2 lb (0.5 kg). Kingfishers have a stocky body, with a large head equipped with a large, stout, dagger-like bill for grasping their food of fish or other small…
Kinglets are small forest birds in the subfamily Silviinae, family Muscicapidae, within the largest of the avian orders, Passeriformes, the perching birds. Kinglets are the most common North American representatives of their rather large subfamily, which is much more diverse in Europe, Asia, and Australia, and includes some 279 species. There are two species of kinglets in North America, both of w…
The koala (Phascolarctos cinereus) is a tree-living Australian marsupial, or pouched mammal, which early English settlers in Australia called the native bear. The koala is not a bear, but is the only living species in the family Phascolarctidae, though fossils indicate that there were once were a number of species of koala. The name is derived from an Aboriginal word meaning "animal that do…
Lyme disease is an infection transmitted by the bite of ticks carrying the spiral-shaped bacterium Borrelia burgdorferi (Bb). The disease was named for Lyme, Connecticut, the town where it was first diagnosed in 1975, after a puzzling outbreak of arthritis. The organism was named for its discoverer, Willy Burgdorfer. The effects of this disease can be long-term and disabling unless it is recognize…
Kola is a member of the tropical family Sterculiaceae, and it grows as a tree form. Kola nuts from two species, Cola nitida and C. acuminata, have been important objects of trade for at least 1,000 years. These nuts are perhaps most well known now as a constituent of soft drinks. There are over 50 species of kola. Of these seven have edible nuts, but only two have been commercially exploited (C. n…
Korsakoff's syndrome is a memory disorder which is caused by a deficiency of vitamin B1, also called thiamine. In the United States, the most common cause of such a deficiency is alcoholism. Other conditions which cause thiamine deficiency occur quite rarely, but can be seen in patients undergoing dialysis (a procedure during which the individual's blood circulates outside of the bod…
The lymphatic system is the body's network of organs, ducts, and tissues that filter harmful substances out of the fluid that surrounds body tissues. Lymphatic organs include the bone marrow, thymus, spleen, appendix, tonsils, adenoids, lymph nodes, and Peyer's patches (in the small intestine). The thymus and bone marrow are called primary lymphatic organs, because lymphocytes are pr…
The citric acid cycle (also called the tricarboxylic acid cycle) is the common pathway by which organic fuel molecules of the cell are oxidized during cellular respiration. These fuel molecules, glucose, fatty acids, and amino acids, are broken down and fed into the Krebs cycle, becoming oxidized to acetyl coenzyme A (acetyl CoA) before entering the cycle. The Krebs cycle is part of the aerobic de…
Kuiper belt objects (KBOs) are chunks of rock, dust and ice found in the area of the solar system just beyond the orbit of Neptune, starting at about 30 astronomical units (AU) to about 50 AU. In 1992, astronomers proposed that there must be at least 70,000 of these objects with diameters larger than 60 mi (100 km). It is estimated that there are many more such bodies beyond 50 AU, but these are s…
Kuru, a disease once endemic to Papua New Guinea and now virtually extinct, is one of several types of diseases called spongiform encephalopathies, all thought to be caused by abnormal proteins called prions, which riddle the brain with holes. According to proponents of the prion hypothesis, these diseases can arise by direct infection with prions, by inheriting genes that produce faulty proteins,…
Lyrebirds are named for the male's magnificent tail, which spreads in a fan-like display, resembling a lyre, an ancient Greek stringed instrument. The male's body is little longer than 12 in (30.5 cm), but the tail may be longer than 16 in (40.5 cm). The only two species of lyrebird in the world are indigenous to a strip of rugged, hilly bushland along the east coast of the Australia…
Lacewings are insects in the order Neuroptera, sub-order Planipennia. Lacewings are named after the fine, complex, cross-branched venation of their four wings, which presents a beautiful, lacelike pattern. Lacewings are rather poor, fluttery fliers. When at rest, they hold their wings tentlike over their back. Lacewings have a complete metamorphosis, with four life history stages: egg, larva, pupa…
Macaques are medium to large-sized monkeys native to Asia and Africa belonging to the genus Macaca, family Cercopithecidae, order Primates. Macaques are usually various shades of brown, gray, or black in fur color, although golden and white color phases occur rarely. Approximately 20 species are known. Locomotion is mainly quadrupedal, and most species are terrestrial in habit, although they take …
The Mach number is used in fluid mechanics and is especially useful in studies involving supersonic aerodynamics. It is named after Ernst Mach (1838-1916), the Austrian physicist and philosopher who pioneered the study of supersonic projectiles. The Mach number is the ratio of the velocity of a fluid to the velocity of sound in that same fluid. In the case of a body moving through a fluid, the Mac…
The "hydroxy" portion of the name tells chemists that there is an alcohol (OH) group in the molecule, and the "alpha" part of the name means that the alcohol is attached to the carbon atom adjacent to the acid (COOH) group. The "prop" portion of the name indicates that there are three carbon atoms. Lactic acid can also be called 2-hydroxypropanoic aci…
Manned spacecraft are vehicles that can transport human beings outside the Earth's atmosphere. The word "manned," though still used occasionally by the United States National Aeronautics and Space Administration (NASA), is often replaced today in discussions of space travel by the word "crewed," in recognition of the fact that women also travel in space. In its e…
Lagomorphs are herbivorous mammals such as rabbits, hares, and pikas, in the order Lagomorpha. Because they exploit similar ecological niches, lagomorphs and rodents (order Rodentia) are rather similar in many aspects of their morphology and behavior. However, these orders are also different in important respects, and each represents an ancient evolutionary lineage. One distinguishing feature of t…
A machine tool is an electrically powered tool which is used to remove material, usually metal, at a controlled rate to achieve a desired shape or finish. A machine tool typically holds the workpiece and a cutting tool, and moves either the workpiece, tool or both to provide a means of machining the material to the desired shape. Machining, another term for metalcutting, is performed by shaving aw…
Lakes are inland bodies of water—millions of which are scattered over the earth's surface. Lakes are classified on the basis of origin, age, salinity, fertility, and water circulation. Lakes can be formed by glaciers, tectonic plate movements, river and wind currents, and volcanic or meteorite activity. Lakes can also be a phase of evolution in the aging process of a bay or estuary. …
Chevalier de Lamarck was a French naturalist and invertebrate zoologist who lived from 1844-1829. He is best known for a theory of evolution developed in his book, Philosophie zoologique, published in 1809. This theory, known today as Lamarckism, is based on the socalled "inheritance of acquired traits," meaning that characteristics that an organism may develop during its lifetime ar…
Machine vision, also referred to as computer or robot vision, is a term that describes the many techniques by which machines visually sense the physical world. These techniques, used primarily for monitoring industrial manufacturing, are becoming increasingly popular as today's manufacturing environments become more automated and quality control standards increase. Whether the task is to so…
Lampreys and hagfishes are unusual, jaw-less fish that comprise the order Cyclostomata, so named because of the circular shape of the mouth. The 41 species of lampreys are in the superfamily Petromyzontoidea, while the approximately 35 species of hagfishes and slime hags are in the superfamily Myxinoidea. Lampreys and hagfishes lack the scales typical of most fish, and are covered with a slimy muc…
Land and sea breezes are wind and weather phenomena associated with coastal areas. A land breeze is a breeze blowing from land out toward a body of water. A sea breeze is a wind blowing from the water onto the land. Land breezes and sea breezes arise because of differential heating between land and water surfaces. Land and sea breezes can extend inland up to 100 mi (161 km), or manifest as local p…
Land use is a geographical concept that refers to the ways in which parcels of land are utilized by people and society. Land-use planning is an activity that examines the factors that influence the nature and dynamics of land usage and develops ways to optimize those variables to achieve larger social, economic, and ecological benefits. …
The Atlantic mackerel, Somber scombrus, supports one of the most important commercial fisheries and supports a significant sport fishing interest. The fish is a close relative of the tuna. The attraction of mackerel as sport fish is due primarily to the streamlined body, forked tail, pointed head, and high-speed swimming. An unusual characteristic of the mackerel is that it does not possess a swim…
Magic square is an unusual numerical configuration containing consecutive integers in arrangements so that the sum of numbers in any row, column, or diagonal are identical. Such squares were known approximately 4,000 years ago in China. The basic magic square is a square containing consecutive integers starting with number 1. Three of the basic magic squares are shown in Table 1. Other magic …
In geology, magma refers to molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals. Magma has been observed in the form of hot lava and the various rocks made from the solidification of magma. Geologists have created magmas (artificial melts) in the laboratory to learn more about the physical conditions in which magma originated and its composition. Magm…
Magnesium (Mg) is one of the most abundant structural metals (those metals used to build things), ranking third behind iron and aluminum. Magnesium compounds are found in mineral rocks such as dolomite and magnesite, while the Earth's oceans contain a practically unlimited supply of the metal in the form of dissolved magnesium chloride. One cubic mile of seawater holds some six million tons…
The term "sanitary landfill" was first used in the 1930s to refer to the compacting of solid waste materials. Initially adopted by New York City and Fresno, California, the sanitary landfill used heavy earth-moving equipment to compress waste materials and then cover them with soil. The practice of covering solid waste was evident in Greek civilization over 2,000 years ago, but the G…
Magnetic levitation is the phenomenon in which two magnetic objects are repelled from each other in a vertical direction. The phenomenon, also known as MAGLEV, has long been recognized as having some important commercial applications. The most significant of these is the construction of MAGLEV trains which are propelled a few inches above a track at very high rates of speed. …
A landform is a natural sculpture of the surface of the earth. Most landforms are produced by the actions of weathering and erosion, carving away material from higher elevations and depositing it down lower. Different kinds of rock erode at a variety of rates under particular climatic conditions. As softer rock is worn away the more resistant rock is exposed, producing another series of landforms.…
Langurs belong to the primate family Cercopitecidae, of which 13 species are represented in the genus Semnopithecus. This represents one of the largest and most diverse groups of colobine monkeys in Asia, with most species restricted to the south and southeast. Many species are distinguished by their vocalizations and the color of their fur, which ranges from a silver-grey in the common or Hanuman…
Light plays a vital role in the life of all oceans. At the simplest level, it provides one of the basic requirements for photosynthesis and promotes development of a food chain. Some species of fish that live in the darker reaches of the oceans also rely on light for survival. Some of these species, such as lantern fish, have even developed their own artificial means of generating light. Lantern f…
Audiocassette tape recorders are widely used to record and play back music or speech. Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material, such as iron oxide. An electromagnet aligns individual magnetic particles in a pattern that corresponds to the loudness and frequency of incoming sounds. In order to play back the recorded information, the …
Magnetic resonance imaging (MRI) is a medical technique which utilizes a magnetic field and the natural An example of a magnetic resonance imaging system (MRI). Photograph by Mason Morfit. FPG International Corp. Reproduced by permission. resonance of atoms to provide an image of human tissue. While the foundation for its development first took place in the late 1930s, it was not until…
The lanthanides are a series of 14 metallic elements that appear at the bottom of the periodic table. Lanthanum, the element preceding the lanthanides in the periodic table, is usually also included in a discussion of the lanthanides since all 15 elements have very similar properties. When first discovered and isolated, the lanthanides were called the rare earth elements. Many uses have been found…
Larks are 75 species of small, terrestrial songbirds that make up the family Alaudidae. Larks breed on all of the continents except Antarctica. Their usual habitats are all open areas and typically include prairies, savannas, alpine and arctic tundras, heathlands, and some types of agricultural fields. Larks have long, pointed wings, a notched tail, and rather long legs and toes, with the hind, ba…
Laryngitis is an inflammation of the larynx. Located at the upper end of the trachea, or windpipe, the larynx contains the vocal chords that are used to form sounds. Because the larynx plays such an important role in speech, it is sometimes called the "voice box." When the larynx becomes inflamed in laryngitis, it swells and reddens. The major symptom of laryngitis is hoarseness. Oth…
Magnetism is a force generated in matter by the motion of electrons within its atoms. Magnetism and electricity represent different aspects of the force of electromagnetism, which is one part of Nature's fundamental electroweak force. The region in space that is penetrated by the imaginary lines of magnetic force describes a magnetic field. The strength of the magnetic field is determined b…
The magnetosphere is a comet-shaped region of Earth's outer atmosphere in which the behavior of Earth's magnetosphere. Illustration by Hans & Cassidy. Courtesy of Gale Group. charged particles is strongly influenced by magnetic and ionic phenomena. The term was first introduced by the British astronomer Thomas Gold in 1959 although speculation about the existence of suc…
Magnolias are species of trees and woody shrubs that comprise the family Magnoliaceae. This is an ancient and relatively primitive group of dicotyledonous plants with fossil materials known from as early as the Upper Cretaceous. The magnolia family contains about 220 species in 12 genera, including the true magnolias (Magnolia spp.), with about 100 species. Magnolias have seasonally deciduous or e…
Mahogany (Swietenia mahogani) is a member of the family Meliaceae, which contains about 500 other species of trees and shrubs native to tropical forests in the Americas, Africa, and Asia. Other common names for this species are the Spanish or West Indies mahogany. Various other species of trees have also been given the name mahogany, but the true mahogany is Swietenia mahogani. Mahogany is one of …
These are a group of ferns found in tropical and warm temperate regions. They are characterized by having delicate, fan-shaped fronds, arising from a thin black midrib, with small green leaflets. Maidenhair fern belong to the genus Adiantum, and some species are popular as house plants. In North America there are three common species, the northern maidenhair fern (A. pedatum), the southern maidenh…
The laser is a device that uses the principle of stimulated emission to produce light. The qualities of the light generated by a laser are significantly different from that generated by a conventional source such as an incandescent light bulb or fluorescent light tube. These major differences include: divergence: the laser generally emits a pencil thin beam of light whose divergent angle is closel…
Malaria has been described as the world's greatest public health problem. It is caused by one of several strains of the Plasmodium protozoan, a one-celled parasite that is transmitted by the bite of the Anopheles mosquito. Hundreds of millions of persons are struck by malaria each year, resulting in 1–3 million deaths annually, primarily in children under five-years-old in sub-Sahara…
Malnutrition is the condition that develops when the body does not get the right amount of the vitamins, minerals, and other nutrients it needs to maintain healthy tissues and organ function. Malnutrition occurs in people who are either under-nourished or over-nourished. Undernutrition is a consequence of consuming too few essential nutrients or using or excreting them more rapidly than they can b…
The more than 4,000 species of living mammals belong to the vertebrate class Mammalia. This diverse group of animals has certain common features: all have four legs, bodies covered by hair, a high and constant body temperature, a muscular diaphragm used in respiration, a lower jaw consisting of a single bone, a left systemic aortic arch leaving the left ventricle of the heart, and three bones in t…
Laser (light amplification by stimulated emission of radiation) surgery uses an intensely hot, precisely focused beam of light to remove or vaporize tissue and control bleeding in a wide variety of non-invasive and minimally invasive procedures. Laser surgery is used to: The first working laser was introduced in 1960. The device was initially used to treat diseases and disorders of the eye, whose …
Manakins are 53 species of small, tropical birds that comprise the family Pipridae, occurring from southern Mexico to Paraguay. Manakins are species that dwell in mature, tropical forests. Manakins are squat, compact birds, with short, rounded wings and a short tail. However, in some species the tail of the male is greatly lengthened by the occurrence of long, thin extensions, sometimes longer tha…
The concepts of latitude and longitude create a grid system for the unique expression of any location on Earth's surface. Latitudes—also known as parallels—mark and measure distance north or south from the equator. Earth's equator (the great circle or middle circumference) is designated 0° latitude. The north and south geographic poles, respectively, measure 90&#…
The laurels are a family of flowering plants known to botanists as the Lauraceae. Lauraceae contains about 45 genera and 2,000 species, and is the most diverse family in the order Laurales. Most species grow in the tropical forests of Southeast Asia and Central and South America. The best known species is Laurus nobilis, a Mediterranean shrub used by the ancient Greeks to decorate the head of vict…
Mangroves are trees in the family Rhizophoraceae, occurring in tropical and subtropical environments as swampy forests fringing muddy, tidal, estuarine, and oceanic shores. Mangrove forests are generally the first type of woody ecosystem that is encountered when a low-lying tropical shore is approached from the ocean. Mangrove forests comprise a biome, that is, a distinctive ecosystem that occurs …
What makes a bird fly? A person run? A judo expert flip a heavier opponent? Earth orbit the Sun? These and any other motions are governed by three deceptively simple laws first stated by Isaac Newton in the seventeenth century. These three laws of motion when coupled with Newton's law of gravity form the basis for explaining both the motions we see on theearth and the motions of the heavenl…
LCD is short for liquid crystal display. LCDs are devices that use liquid crystals to create images. Liquid crystal images are being used in watch faces, laptop computer screens, camcorder viewers, virtual reality helmet displays, and television screens. The two main LCDs are passive and active. In passive LCDs, once the image has been made, it cannot be changed. In active LCDs, the image can be m…
Leaching usually refers to the movement of dissolved substances with water percolating through soil. Sometimes, leaching may also refer to the movement of soluble chemicals out of biological tissues, as when rainfall causes potassium and other ions to be lost by foliage. Leaching occurs naturally in all soils, as long as the rate of water input through precipitation is greater than water losses by…
Mania is a mood disturbance marked by an abnormal degree of elation or irritability along with a number of other symptoms including restlessness, inflated self confidence, a marked decrease in the need for sleep, rapid and loud speech that is difficult to interrupt, racing thoughts, high distractibility, and a marked increase in certain goal-directed activities. Over time, manic episodes are usual…
Manic-depressive illness, clinically called bipolar disorder, is a major mental illness belonging to the category of illnesses designated as mood disorders. It is estimated that as many as two million Americans suffer from this illness and many more may go undiagnosed or underdiagnosed. Approximately one in five families will be confronted with a family member who may experience a manic episode or…
A map, or mapping, is a rule, often expressed as an equation, that specifies a particular element of one set for each element of another set. To help understand the notion of map, it is useful to picture the two sets schematically, and map one onto the other, by drawing connecting arrows from members of the first set to the appropriate members of the second set. For instance, let the set mapped fr…
A metallic element with atomic number 82. Symbol Pb, atomic weight 207.19, specific gravity 11.35, melting point 621.32°F (327.4°C), boiling point 3,191°F (1,755°C). Lead is in column IVA of the periodic table. It has four naturally occurring stable isotopes, lead-204, lead-206, lead-207, and lead-208. The last three of these are all end products of one or another radio…
The maples are about 150 species of angiosperm trees and shrubs in the genus Acer, family Aceraceae. Most maples occur in temperate forests of the Northern Hemisphere. Maples are characterized by the shape of their leaves, which in most species are broadly palmate with a three- or five-lobed outline, and are arranged in an opposite fashion on their branches. Maples have seasonally deciduous foliag…
The integers are the positive and negative whole numbers... -4, -3, -2, -1, 0, 1, 2,.... The name "integer" comes directly from the Latin word for "whole." The set of integers can be generated from the set of natural numbers by adding zero and the negatives of the natural numbers. To do this, one defines zero to be a number which, added to any number, equals the same nu…
Marfan syndrome is genetic disease that involves defects in the connective tissues of the body with the cardinal collection of abnormalities affecting the skeletal, ocular, and cardiovascular systems. It is inherited as an autosomal dominant disorder, meaning that inheriting only one defective gene from either the paternal or maternal side will lead to the disorder. Although the majority of mutati…
The integral is one of two main concepts embodied in the branch of mathematics known as calculus, and it corresponds to the area under the graph of a function. The area under a curve is approximated by a series of rectangles. As the number of these rectangles approaches infinity, the approximation approaches a limiting value, called the value of the integral. In this sense, the integral gives mean…
Marijuana is the common name for the hemp plant Cannabis sativa. Hemp grows in tropical as well as temperate climates. The dried ground leaves, flowers, and stems of the plant have a long history for their use as drugs. It has been cultivated in different regions of the world throughout centuries for its fiber to produce linen, rope, canvas, and oil. It has also been used as a medicine to relieve …
An integrated circuit (IC) is a single semiconducting chip that contains transistors and sometimes, capacitors, resistors, and diodes. These components are connected to create an electrical circuit. Integrated circuits can be found in almost all electronic devices today, including those in automobiles, microwave ovens, traffic lights, and watches. Just a few years ago, the circuits required to ope…
