Aerobic means that an organism needs oxygen to live. Some microorganisms can live without oxygen and they are called anaerobic. Bacteria are not dependent on oxygen to burn food for energy, but most other living organisms do need oxygen. Fats, proteins, and sugars in the diet of organisms are chemically broken down in the process of digestion to release energy to drive life activities. If oxygen i…
Aerodynamics is the science of air flow over airplanes, cars, buildings, and other objects. Aerodynamic principles are used to find the best ways in which airplanes can get lift, reduce drag, and remain stable by controlling the shape and size of the wing, the angle at which it is positioned with respect to the airstream, and the flight speed. The flight characteristics change at higher altitudes …
Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of…
Africa has more than two thousand languages, representing several thousand cultures, each with its own system of logic. No single aesthetic philosophy characterizes the continent, and any concept of a coherent "Africa" is arbitrary, given such extraordinary diversity. Furthermore, a given culture may possess several aesthetic discourses, as may any artistic genre. Globalization comp…
Culturally, Asia encompasses an enormous range of cultural diversity, with philosophical traditions going back 2,500 years. And aesthetics is the philosophical study of art and the elaboration of criteria of value in arts and in nature, as well as how these two notions overlap with the study of nature and being human. In many Asian traditions value focuses on human well-being. In Daoism and Shinto…
Feminism is broadly defined as the struggle for the liberation of women, and encompasses epistemologies, methodologies, theories, and modes of activism that seek to bring an end to the oppression and subordination of women by men. An individual person espousing feminism is referred to as a feminist, while collective mobilizations of women against the oppression of women are referred to as feminist…
The idea of "Africa" is an exceedingly complex one with multiple genealogies and meanings, which make any extrapolations of "African" identity in the singular or plural, any explorations of what makes Africa "Africa," quite difficult. Both Africans and non-Africans have conceived "Africa" differently in various historical and geographical con…
For four hundred years, African-Americans have been engaged in a fierce struggle, a struggle for freedom, justice and equality, empowerment and self-determination, or social transformation, depending on one's ideology and its discourses. The lived African-American experience, in its class, gender, generational, and regional specificity, and the struggle against black racial oppression in th…
Edward Said's 1978 book, Orientalism, is the harshest critique to date of Western scholarship on the Muslim orient. However, its focus is almost exclusively on the Near and Middle East. The policy statements of William Ponty, the French governor-general in Senegal (1907–1915), quoted below suggest a link between Orientalism, as understood by Edward Said, and colonial scholarship on M…
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…
Ancient Egypt has been offered as a point of origin of African metaphysical speculation. According to the work of Senegalese historian Cheikh Anta Diop, Egypt is to Africa what the Greco-Latin civilization is to the West, and texts such as The Book of the Dead, written more than 3,500 years ago, could play the role of a founding text for a tradition of philosophical thinking on the African contine…
The Universe is approximately 14 billion (14,000,000,000) years old. Its age is measured from the event known as the big bang—an explosion filling all space and generating all of the matter and energy that exist today. Although only in the last 50 years have astronomers been able to estimate the age of the Universe, they have long argued that the Universe must be of finite age, finite size,…
Agent Orange is a defoliant that kills plants and causes the leaves to fall off the dying plants. The name was a code devised by the United States military during the development of the chemical mixture. The name arose from the orange band that marked the containers storing the defoliant. Agent Orange was an equal mixture of two chemicals; 2, 4–D (2,4, dichlorophenoxyl acetic acid) and 2, 4…
Aging is the natural effect of time and the environment on living organisms, and death is its end result. Gerontology is the study of all aspects of aging. No single theory on how and why people age is able to account for all facets of aging. Although great strides have been made to postpone death as the result of certain illnesses, less headway has been made in delaying aging. Life span is specie…
The heyday of agnosticism was in Victorian Britain between the 1860s and the 1890s. Its leading exponents were Herbert Spencer (1820–1903), Thomas Henry Huxley (1825–1895) (who coined the term), Leslie Stephen (1832–1904), John Tyndall (1820–1893), and William Kingdon Clifford (1845–1879). This group all shared a disillusionment with orthodox Christianity; an opp…
The twelve species of agoutis are the best-known members of the family Dasyproctidae (genus Dasyprocta) of the order Rodentia. Agoutis are found from southern Mexico through Central America to southern Brazil, including the Lesser Antilles. They are long-legged, slender-bodied, rabbit-like mammals with short ears and a short tail. The body length of agoutis measures 16-24 in (41.5-62 cm), and adul…
Early farmers quickly learned that a supply of water was essential to farming. Thus, the primary fields of grain were planted alongside the great rivers of the Middle East. However, getting water from the rivers to the fields became a problem. The invention of the shaduf, or chain-of-pots, helped solve this problem. This human-powered primitive device consisted of buckets attached to a circular ro…
AIDS is the abbreviation for acquired immunodeficiency syndrome. The syndrome is caused by several types of a virus that is now known as the human immunodeficiency virus (HIV). AIDS is characterized by the destruction of cells that are vital to the proper operation of the immune system. People afflicted with AIDS can develop opportunistic infections; life-threatening illnesses caused by viruses or…
Acquired immunodeficiency syndrome (AIDS) is a disease characterized by the destruction of the immune system. More than 16,000 new AIDS patients are diagnosed each day. Evidence overwhelmingly supports the view that AIDS is caused by several types of a virus designated as the human immunodeficiency virus (HIV). The immune system is the principle defense system of the body to a variety of infection…
An air mass is an extensive body of air that has a relatively homogeneous temperature and moisture content over a significant altitude. Air masses typically cover areas of a few hundred, thousand, or million square kilometers. A front is the boundary at which two air masses of different temperature and moisture content meet. The role of air masses and fronts in the development of weather systems w…
Air pollution is the presence of chemicals in the earth's atmosphere that are not a normal part of the atmosphere. In other words, air pollution is contaminated air. Air contamination is divided into two broad categories: primary and secondary. Primary pollutants are those released directly into the air. Some examples include dust, smoke, and a variety of toxic chemicals, such as lead, merc…
A technologically advanced cousin of the balloon, airships are streamlined vessels buoyed by gases and controlled by means of propellers, rudders, and pressurized air systems. More commonly referred to as blimps and dirigibles, the airship is comprised of non-rigid, semi-rigid, and rigid types that rely on lighter-than-air gases such as helium and hydrogen for lift. Since the turn of the twentieth…
Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. The scientific meaning of albedo is the ability of a surface to reflect a certain proportion of visible light. A perfect mirror has an albedo of 100%; the polished surface of white metals like aluminum or silver comes close to that figure. Some metals like brass or copper, however, are colored, and they do …
Albinism is a recessive inherited defect in melanin metabolism in which pigment is absent from hair, skin, and eyes (oculocutaneous albinism) or just from the eyes (ocular albinism). Melanin is a dark biological pigment that is formed as an end product of the metabolism of the amino acid tyrosine. When human skin is exposed to sunlight it gradually darkens or tans due to an increase in melanin. Ta…
Chinese alchemy is based on doctrinal principles, first set out in the founding texts of Daoism, concerning the relation between the domains of the Absolute and the relative, or the Dao and the "ten thousand things" (wanwu). Its teachings and practices focus on the idea of the elixir, frequently referred to as the Golden Elixir (jindan), the Reverted Elixir (huandan), or the Medicine…
To a modern observer, alchemy likely connotes only the transmutation of base metals into gold, or perhaps a more metaphorical transformation of the soul. In its roughly two-thousand-year history, however, alchemy's practices and ideas have ranged much more broadly, encompassing everything from the production of dyes, medicines, precious metals, and gemstones to assaying techniques, matter t…
Aldehydes are a class of highly reactive organic chemical compounds that contain a carbonyl group (in which a carbon atom is double-bound to an oxygen atom) and at least one hydrogen atom bound to the alpha carbon (the central carbon atom in the carbonyl group). The aldehydes are similar to the ketones, which also contain a carbonyl group. In the aldehydes, however, the carbonyl group is attached …
Algae (singular: alga) are photosynthetic, eukaryotic organisms that do not develop multicellular sex organs. Algae can be unicellular, or they may be large, multicellular organisms. Algae can occur in salt or fresh waters, or on the surfaces of moist soil or rocks. The multicellular algae develop specialized tissues, but they lack the true stems, leaves, or roots of the more complex, higher plant…
Algebra is often referred to as a generalization of arithmetic. As such, it is a collection of rules: rules for translating words into the symbolic notation of mathematics, rules for formulating mathematical statements using symbolic notation, and rules for rewriting mathematical statements in a manner that leaves their truth unchanged. The power of elementary algebra, which grew out of a desire t…
The word algebra refers to a theory, usually mathematical, which is dominated by the use of words (often abbreviated), signs, and symbols to represent the objects under study (such as numbers), means of their combination (such as addition), and relationships between them (such as inequalities or equations). An algebra cannot be characterized solely as the determination of unknowns, for then most m…
An algorithm is a set of instructions that indicate a method for accomplishing a task. If followed correctly, an algorithm guarantees successful completion even without the use of any intelligence. The term algorithm is derived from the name al-Khowarizmi, a ninth century Arabian mathematician who is credited with discovering algebra. With the advent of computers, which are particularly adept at u…
The notion of alienation is a very unusual one because it is at once an attempt to explain a widespread feeling—a very subjective, somewhat indefinable feeling—and a critique of the nature of any society that regularly produces it. This was not always so. The feeling that one is not at home in the world, the sense of estrangement from one's surrounding, oneself, and other peop…
The first column on the periodic table of the chemical elements is collectively called the alkali metal group: lithium, sodium, potassium, rubidium, cesium, and francium. Because their outer electron structure is similar, they all have somewhat similar chemical and physical properties. All are shiny, soft enough to cut with a knife, and most are white (cesium is yellow-white). All react with water…
The second column on the periodic table of the chemical elements is collectively called the alkaline earth metal group: beryllium, magnesium, calcium, strontium, barium, and radium. Because the outer electron structure in all of these elements is similar, they all have somewhat similar chemical and physical properties. All are shiny, fairly soft—although harder than the alkali metals—…
Most genes exist in more than one form that, when expressed, result in different characteristics. Genes may often exist in more than one form, and these forms are termed alleles of the gene. An allele is one of at least two alternative forms of a particular gene. Alleles provide the genetic instructions for products that, although similar in type, are visibly different (phenotypically different). …
An allergy is an excessive or hypersensitive response of the immune system. The allergic reaction becomes manifest as a pathological immune reaction induced either by antibodies (immediate hypersensitivity) or by lymphoid cells (delayed type allergy). Instead of fighting off a disease-causing foreign substance, the immune system launches a complex series of actions against an irritating substance,…
A striking example of differing physical properties among allotropes is the case of carbon. Solid carbon exists in two allotropic forms: diamond and graphite. Diamond is the hardest naturally occurring substance and has the highest melting point (more than 6,335°F [3,502°C]) of any element. In contrast, graphite is a very soft material, the substance from which the "lead…
A mixture of two or more metals is called an alloy. Alloys are distinguished from composite metals in that alloys are thoroughly mixed, creating, in effect, a synthetic metal. In metal composites, the introduced metal retains its identity within the matrix in the form of fibers, beads, or other shapes. Alloys can be created by mixing the metals while in a molten state or by bonding metal powders. …
An alluvial system consists of sediments eroded, transported, and deposited by water flowing in rivers or streams. The sediments, known as alluvium, can range from clay-sized particles less than 0.002 mm in diameter to boulders greater than 64 mm in diameter, depending on their source and the sediment transport capacity of streams in the system. The term alluvial is closely related to the term flu…
The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged. The element that undergoes "alpha decay" changes into a new element whose atomic number is down two and atomic mass is down four from the original element. Alpha decay occurs when a nucleus has so many protons that the…
Nonrenewable fossil fuels—coal, petroleum, and natural gas—provide more than 85% of the energy used around the world. In the United States, fossil fuels comprise 81.6% of the total energy supply, nuclear power provides 7.7%, and all renewable energy sources provide 7.3%. Wind power, active and passive solar systems, geothermal energy, and biomass are examples of renewable or alternat…
National Institutes of Health classifies alternative medicine as an unrelated group of non-orthodox therapeutic practices, often with explanatory systems that do not follow conventional biomedical explanations or more seriously, based on pseudoscience. Others more generally define it as medical interventions not taught at United States medical schools or not available at United States hospitals. A…
The term altruism was coined by the French philosopher and sociologist Auguste Comte (1798–1857). Derived from the Italian word altrui, meaning "to others" or "of others," "altruism" was introduced as an antonym for "egoism" to refer to the totality of other-regarding instincts in humans. The new terms altruism, altruist, and altruisti…
Altruism refers to animal behavior that benefits other animals of the same species. Living in the company of other animals presents numerous drawbacks, including increased competition for food, nest sites, and mates, and increased visibility to predators, to name just a few. We might expect animals to strive to outdo the competition whenever possible, to take the best food and other resources for …
The amaranth (or pigweed) family is a large group of dicotyledonous flowering plants known to botanists as the Amaranthaceae. It is a relatively large family, having about 65 genera and 900 species. The species in this family are mostly annual or perennial herbs, although a few species are shrubs or small trees. Botanists divide Amaranthaceae into two subfamilies: the Amaranthoideae and the Gomphr…
Species in the amaryllis family are flowering plants, and are mostly long-lived, perennial herbs arising from a bulb or, less commonly, from rhizomes (underground stems). These plants have linear or strap shaped leaves, either crowded around the base of a leafless flowering stem, or arranged in two tight rows along a short stem, as in the common houseplant Clivia. The leaves are usually hairless a…