Charge-coupled devices (CCDs) have made possible a revolution in image processing. They consist of a series of light-sensitive elements, called pixels, arranged in a square or rectangular array. When CCDs are exposed to light, an image of the object being observed is formed; this image can be extracted from the CCD and stored on a computer for later analysis. CCDs are used in a variety of modern i…
A chelate is a type of coordination compound in which a single metallic ion is attached by coordinate covalent bonds to a molecule or an ion called a ligand. The term chelate comes from the Greek word chela, meaning "crab's claw." The term clearly describes the appearance of many kinds of chelates, in which the ligand surrounds the central atom in a way that can be compared to…
A chemical bond is any force of attraction that holds two atoms or ions together. In most cases, that force of attraction is between one or more electrons held by one of the atoms and the positively charged nucleus of the second atom. Chemical bonds vary widely in their stability, ranging from relatively strong covalent bonds to very weak hydrogen bonds. …
Chemical evolution describes chemical changes on the primitive Earth that gave rise to the first forms of life. The first living things on Earth were prokaryotes with a type of cell similar to present-day bacteria. Prokaryote fossils have been found in 3.4-million-year-old rock in the southern part of Africa, and in even older rocks in Australia, including some that appear to be photosynthetic. Al…
Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemicaloxygen demand only measures the amount of oxygen consumed by microbial oxidation and is most relevant to waters rich in organic matter. It is important to understand that COD and BOD do not necessarily measure the same types of …
Chemical reactions describe the changes between reactants (the initial substances that enter into the reaction) and products (the final substances that are present at the end of the reaction). Describing interactions among chemical species, chemical reactions involve a rearrangement of the atoms in reactants to form products with new structures in such a way as to conserve atoms. Chemical equation…
Chemical warfare involves the use of natural or synthetic substances to incapacitate or kill an enemy or to deny them the use of resources such as agricultural products or screening foliage. The effects of the chemicals may last only a short time, or they may result in permanent damage and death. Most of the chemicals used are known to be toxic to humans or plant life. Other normally benign (mild)…
Chemistry is the science that studies why materials have their characteristic properties, how these particular qualities relate to their simplest structure, and how these properties can be modified or changed. The term chemistry is derived from the word alchemist, which finds its roots in the Arabic name for Egypt al-Kimia. The Egyptians are credited with being the first to study chemistry. They d…
Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling. In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell, changing the shape of the chemoreceptor. All…
Chestnuts are species of trees in the genus Castanea, family Fagaceae. They are species of temperate hardwood (angiosperm-dominated) forests found in the Northern Hemisphere and are indigenous to eastern North America and Eurasia. Species in the genus Castanea can grow to be 100 ft (30 m) tall. They have simple leaves with a broadly toothed margin and sweet-smelling, yellowish, insect-pollinated, …
A chi-square analysis is best illustrated using an example in which data from a population is categorized with respect to two qualitative variables. Table 1 shows a sample of patients categorized with respect to two qualitative variables, namely, congenital heart defect (CHD; present or absent) and karyotype (trisomy 21, also called Down syndrome, or trisomy 13, also called Patau syndrome). The cl…
Chickenpox, a disease characterized by skin lesions and low-grade fever, is common in the United States and other countries located in areas with temperate climates. The incidence of chickenpox is extremely high-almost everyone living in the United States contracts chicken-pox, usually during childhood, but sometimes in adulthood. In the United States, about 3.9 million people a year contract chic…
Diseases that are more common among children than among adults are referred to as childhood diseases. That is not to say that adults cannot or will not contract these illnesses; but usually children contract these diseases and form the immunity against them that will protect them as adults. In fact, some of these diseases may be quite uncomplicated in children, but may be life-threatening when con…
The chimaeras (order Chimaerae, class Bradyodonti) are a most peculiar looking group of fish that live near the sea bed off continental shelves and in deep offshore waters at a depth of 985-1,640 mi (300-500 m). Collectively these species form a small, cohesive group of about 25 species. They are all exclusively marine species. Closely related to sharks, rays, and dogfish, chimaeras are characteri…
Chimpanzees belong to the order Primates, which includes monkeys, apes, and humans. Chimpanzees are assigned to the family Pongidae, which includes all of the other apes: gorillas, orang-utans, and gibbons. Compared with monkeys, apes are larger, have no tail, and have longer arms and a broader chest. When apes stand upright, their long arms reach below their knees. They also have a great deal of …
Chinchillas and viscachas are seven species of small, South American rodents in the family Chinchillidae. Chinchillas have a large head, broad snout, large eyes, rounded ears, and an extremely fine and dense fur. Their forelimbs are short and the paws small, while the hindlegs and feet are larger and relatively powerful, and are used for a leaping style of locomotion, as well as for running and cr…
Chipmunks are small mammals in the order Rodentia, the rodents. Specifically, they are classified with the squirrel-like rodents, the Sciuridae. Chipmunks are divided into two genera: Gallos and Tamias. North America is home to 17 species of chipmunk, 16 in the West and only one, Tamias striatus, the eastern chipmunk, in the East. The eastern chipmunk is about 5-6 in (12.7-15 cm) long, and the tai…
Chitons are small mollusks, oval in outline, with a broad foot, and a mantle that secretes, and sometimes extends over, the shell. They live on rocky seashores in much the same life-style as limpets. They are easily distinguishable from limpets, however, by their shell made of eight plates (or valves) with transverse sutures. Also, unlike limpets and other snails, the chitons have no tentacles or …
Chlordane is an organochlorine insecticide, more specifically a chlorinated cyclic hydrocarbon within the cyclodiene group. The proper scientific name for chlordane is 1,2,4,5,6,7,8,8-Octachloro-3a,4,7,7a-tetrahydro-4,7-methanoindan. However, the actual technical product is a mixture of various chlorinated hydrocarbons, including isomers of chlordane and other closely related compounds. The first …
A very large and diverse group of organic molecules are chlorinated hydrocarbons. Hydrocarbons are molecules composed entirely of hydrogen and carbon atoms, often derived from carbon-based fossil fuels like petroleum oils and coal. Chlorinated hydrocarbons are specific hydrocarbon molecules that also have atoms of the element chlorine chemically bonded to them. The number of chlorine atoms bonded …
Chlorination is the process by which the element chlorine reacts with some other substance. Chlorination is a very important chemical reaction both in pure research and in the preparation of commercially important chemical products. For example, the reaction between chlorine and methane gas produces one or more chlorinated derivatives, the best known of which are trichloromethane (chloroform) and …
Chlorine is the non-metallic chemical element of atomic number 17, symbol Cl, atomic weight 35.45, melting point -149.8°F (-101°C), and boiling point -29.02°F (-33.9°C). It consists of two stable isotopes, of mass numbers 35 and 37. Ordinary chlorine is a mixture of 75.77% chlorine-35 atoms and 24.23% chlorine-37 atoms. Chlorine is a highly poisonous, greenish yellow ga…
Chlorofluorocarbons (CFCs) are man-made chemical compounds used as refrigerants, cleaning solvents, aerosol propellants, and blowing agents for foam packaging in many commercial applications. CFCs do not spontaneously occur in nature. They were developed by industrial chemists searching for a safer alternative to refrigerants used until the late 1920s. CFCs are non-toxic, chemically non-reactive, …
Chloroform was originally used to calm people suffering from asthma. In 1847, James Y. Simpson, a Professor of Midwifery at the University of Edinburgh, began using chloroform as an anesthetic to reduce pain during childbirth. From this initial experiment, chloroform began to be used as general anesthesia in medical procedures throughout the world. The use of chloroform in this application was eve…
Chlorophyll is a green pigment contained in the foliage of plants, giving them their notable coloration. This pigment is responsible for absorbing sunlight required for the production of sugar molecules, and ultimately of all biochemicals, in the plant. Chlorophyll is found in the thylakoid sacs of the chloroplast. The chloroplast is a specialized part of the cell that functions as an organelle. O…
Chloroplasts are organelles—specialized parts of a cell that function in an organ—like fashion. They are found in vascular plants, mosses, liverworts, and algae. Chloroplast organelles are responsible for photosynthesis, the process by which sunlight is absorbed and converted into fixed chemical energy in the form of simple sugars synthesized from carbon dioxide and water. Chloroplas…
Cholera is one of the most devastating of all human diseases. Although endemic in some areas of the world, cholera is usually associated with massive migrations of people, such as those occurring during war or famine. Cholera is also common in developing countries, where suboptimal sanitation practices are responsible for its spread. If not treated, cholera has a fatality rate of over 60%. Death r…
Cholesterol belongs more specifically to a class of compounds known as the steroids. Most steroids are naturally occurring compounds that play critical roles in plant and animal physiology and biochemistry. Other steroids include sex hormones, certain vitamins, and adrenocorticoid hormones. All steroids share a common structural unit, a four-ring structure known as the perhydrocyclopentanophenanth…
Chordates are a diverse group of animals that comprise the phylum Chordata. There are approximately 44,000 species of chordates, ranging in size from several millimeters to 105 ft (32 m) long. The simplest and earliest chordates are pre-vertebrate animals such as ascidians, tunicates, and Amphioxus. The major group of chordates are the sub-phylum Vertebrata, the vertebrates. Listed more-or-less in…
Chorionic villus sampling (CVS) is a relatively new form of a prenatal testing, completed earlier in gestation than the more traditional testing method, amniocentesis. Through CVS, small samples of the trophoblast are obtained for chromosome or DNA analysis. The use of CVS as a tool for fetal karyotyping at 10 weeks' gestation was introduced in 1969 but it became accepted with the introduct…
Chromatin is the masses of fine fibers comprising the chromosomes in the nucleus of a eukaryotic cell in a nondividing state. During cell division (mitosis or meiosis) the chromatin fibers pull together into thick shortened bodies which are then called chromosomes. Chromatin is present only in cells with a nuclear membrane; it is not found in prokaryotic cells (e.g., bacteria) that lack a nucleus.…
Chromatography is a family of laboratory techniques for separating mixtures of chemicals into their individual compounds. The basic principle of chromatography is that different compounds will stick to a solid surface, or dissolve in a film of liquid, to different degrees. To understand chromatography, suppose that all the runners in a race have sticky shoe soles, and that some runners have sticki…
Chromosome abnormalities describe alterations in the normal number of chromosomes or structural problems within the chromosomes themselves. Both kinds of chromosome abnormalities may result from an egg (ovum) or sperm cell with the incorrect number of chromosomes, or with a structurally faulty chromosome uniting with a normal egg or sperm during conception. Some chromosome abnormalities may occur …
A chromosome is a threadlike structure found in the nucleus of most cells. that carries the genetic material in the form of a linear sequence of deoxyribonucleic acid (DNA). In prokaryotes, or cells without a nucleus, the chromosome represents circular DNA containing the entire genome. In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure. The function of …
Chromosome mapping is the assignment of genes to specific locations on a chromosome. A gene map serves many important functions and is much like understanding the basic human anatomy to allow doctors to diagnose patients with disease. A doctor requires knowledge of where each organ is located as well as the function of this organ to understand disease. A map of the human genome will allow scientis…
Cicadas are insects in the order Homoptera, family Cicadidae. Male cicadas make a well-known, loud, strident, buzzing sound during the summer, so these unusual insects are often heard, but not necessarily seen. Species of cicadas are most diverse in closed and open forests of the temperate and tropical zones. …
The World Health Organization (WHO) has named tobacco one of the greatest public health threats of the twenty-first century. As of 2001, more than 1.2 billion people worldwide smoke, and 3.5 million people are expected to die from causes directly related to tobacco use. This death rate is expected to rise to 10 million by the year 2030. Seventy percent of these deaths will occur in developing coun…
Living things require a circulatory system to deliver food, oxygen, and other needed substances to all cells, and to take away waste products. Materials are transferred between individual cells and their internal environment through the cell membrane by diffusion, osmosis, and active transport. During diffusion and osmosis, molecules move from a higher concentration to a lower concentration. Durin…
The terms circumscribed and inscribed refer, respectively, to geometric figures that have been drawn around the outside of or within some other geometric figure. For example, imagine that a circle is drawn around a triangle so that the circle passes through all three vertices of the triangle. Then the circle is said to be circumscribed around the triangle, and the triangle is said to be inscribed …
Cirrhosis is a degenerative liver disease in which the lobes of the liver become infiltrated with fat and fibrous tissue (fibrous tissue is a type of connective tissue composed of protein fibers called collagen). The word "cir rhosis" is derived from the Greek words kirrhos, meaning "yellowish orange" and osis, meaning "condition," and connotates the appea…
Citric acid is an organic (carbon based) acid found in nearly all citrus fruits, particularly lemons, limes, and grapefruits. It is widely used as a flavoring agent, preservative, and cleaning agent. The structure of citric acid is shown below. The COOH group is a carboxylic acid group, so citric acid is a tricarboxylic acid, possessing three of these groups. Citric acid helps to provide the �…
Citrus trees are various species of trees in the genus Citrus, in the rue family, or Rutaceae. There are 60 species in the genus Citrus, of which about 10 are used in agriculture. The center of origin of most species of Citrus is southern and southeastern Asia. Citrus trees are widely cultivated for their edible fruits in sub-tropical and tropical countries around the world. The sweet orange (Citr…
Small to medium-sized carnivores, civets are in the Viverridae family which includes genets, linsangs, and mongooses. There are 35 species of civets and genets in 20 genera. Their natural distribution is restricted to the warmer regions of the Old World, and they occupy a niche similarly filled by weasels and their relatives found in temperate deciduous forests. Civets vary in size and form, but m…
Climax is a theoretical, ecological notion intended to describe a relatively stable community that is in equilibrium with environmental conditions, and occurring as the terminal, end-point of succession. One of the early proponents of the concept of climax was the American ecologist, Frederic Clements. In an important publication in 1916, he theorized that there was only one true climax community …
Clingfish are about 100 species of small, ray-finned bony fish found primarily in tropical marine waters. They belong to the family Gobiesocidae in the order Gobiesociformes. Clingfish are shaped like tadpoles with a wide, flattened head; they have no scales and are covered with a thick coating of slime that makes them very slippery. Clingfish are characterized by a large suction disc formed by th…
A clone is a molecule (DNA), cell, or organism that is genetically identical to its parental molecule, cell, or organism. There are three types of cloning. One method, gene cloning, utilizes copying fragments of DNA for easier manipulation and study. Another cloning method involves producing genetically identical animals through a process called twinning. The final cloning method involves producin…
A closed curve is one which can be drawn without lifting the pencil from the paper and which ends at the point where it began. In Figure 1, A, B, and C are closed curves; D, E, and F are not. Curve A is a circle. Although the starting point is not indicated, any of its points can be chosen to serve that purpose. Curve B crosses itself and is therefore not a "simple" closed curve, but…
"Closure" is a property which a set either has or lacks with respect to a given operation. A set is closed with respect to that operation if the operation can always be completed with elements in the set. For example, the set of even natural numbers, 2, 4, 6, 8,..., is closed with respect to addition because the sum of any two of them is another even natural number. It is not closed …
All clouds are a form of water. Clouds are condensed atmospheric moisture in the form of minute water droplets or ice crystals. The creation of a cloud begins at ground level. The sun heats the earth's surface, the warm ground heats the air, which rises. The air contains variable amounts of water, as vapor, that has evaporated from bodies of water and plants. Air at ground level is denser t…
Club mosses, also called lycophytes, are flowerless and seedless plants in the family Lycopodiaceae, that belong to an ancient group of plants of the division Lycophyta. The lycophytes were one of the dominant plants during the Coal age (360-286 million years ago) and many were shrubs or large trees. By 250 million years ago, most of the woody species had died out. Between 10 and 15 living genera …
Coal is a naturally occurring combustible material consisting primarily of the element carbon, but with low percentages of solid, liquid, and gaseous hydrocarbons and other materials, such as compounds of nitrogen and sulfur. Coal is usually classified into the sub-groups known as anthracite, bituminous, lignite, and peat. The physical, chemical, and other properties of coal vary considerably from…
The coast and beach, where the continents meet the sea, are dynamic environments where agents of erosion vie with processes of deposition to produce a set of features reflecting their complex interplay and the influences of changes in sea level, climate, sediment supply, etc. "Coast" usually refers to the larger region of a continent or island that is significantly affected by its pr…
Coatis are raccoon-like mammals in the family Procyonidae, which have a long ringed tail, typically held perpendicular to the body, and a flexible, upturned, elongated snout. Coatis are also distinctive socially. The females live together in highly organized groups called bands, composed of 5-12 allied individuals, while adult males are solitary. The difference in the social patterning of the sexe…
The coca plant, genus Erythroxylum, family Erythroxylaceae, order Linales, is native to the Andean slopes of South America. The genus Erythroxylum comprises approximately 250 species, of which the most cultivated species are Erythroxylum coca (southern Peru and Bolivia) and Erythroxylum novogranatense ( Colombia and northern coastal Peru). The coca plant is a shrub, growing to about 15 ft (5 m). C…
Cocaine is a colorless or white narcotic crystalline alkaloid derived from the leaves of the South American coca plant—Erythroxylum coca. Aside from its use as a local anesthetic, which has largely been supplanted by safer drugs, its medical applications failed to live up to the hopes of physicians and chemists of the late nineteenth century. They administered cocaine to themselves and othe…
Cockatoos are species of birds in the family Cacatuidae, in the order Psittaciformes, which also contains the typical parrots (family Psittacidae). Parrots and cockatoos all have powerful, curved bills, short legs, and strong, dexterous feet with two toes pointing forward and two backward. These birds also have specialized feathers known as powder down, which disintegrates into a powder that is us…
Cockroaches are insects in the order Blattaria. They are somewhat flat, oval shaped, leathery in texture, and are usually brown or black in color. Cockroaches range in body size from 0.1 to 2.3 in (2.5 to 60 mm), and are rampant pest insects in human inhabited areas, as well as common outdoor insects in most warm areas of the world. These insects were formerly classified in the order Orthoptera, w…
Codeine is a type of medication belonging to a class of drugs known as opioid analgesics, which are derived from the Papaver somniferum, a type of poppy flower, or are manufactured to chemically resemble the products of that poppy. In Latin, Papaver refers to any flower of the poppy variety, while somniferum translates to mean "maker of sleep." The plant has been used for over 6,000 …
Codfish (family Gadidae) are a family of bottom-feeding fish that live in cool or cold seas, mostly in the Northern Hemisphere. There are about 21 genera and 55 species of codfishes. The most commonly utilized marine habitats are inshore waters and continental shelves, generally in depths of less than about 300 ft (100 m), but sometimes considerably deeper. Codfishes are voracious predators of sma…
Information for the genetic code is stored in a sequence of three nucleotide bases of DNA called base triplets, which act as a template for which messenger RNA (mRNA) is transcribed. A sequence of three successive nucleotide bases in the transcript mRNA is called a codon. Codons are complimentary to base triplets in the DNA. For example, if the base triplet in the DNA sequence is GCT, the correspo…
A coefficient is a constant multiplier of variables and any part of an algebraic term, Thus, in the expression has 4 as a coefficient of In many formulas, especially in statistics, certain numbers are considered coefficients, such as correlation coefficients in statistics or the coefficient of expansion in physics. …
The coelacanth (Latimeria chalumnae) is the only living representative of an ancient order of fishes, until recently thought to have become extinct 70 million years ago, at about the same time as the dinosaurs. In 1938, however, scientists were astonished when living coelacanths were discovered (this is described later). The coelacanth is a sarcoptergian, or lobe-finned fish, distantly related to …
The coffee tree, genus Coffea, family Rubiaceae (Madder family), is native to Ethiopia. The name coffee also refers to the fruit (beans) of the tree and to the beverage brewed from the beans. Coffee is one of the world's most valuable agricultural crops. There are about 30 species of Coffea, but only two species provide most of the world market for coffee. Coffea arabica is indigenous to Et…
Cogeneration is the simultaneous generation of two forms of energy, usually heat and electricity, from one energy source. Traditional energy generating systems produce only heat or electricity by burning a fuel source. In both cases, burning the fuel generates a lot of heat and the exhaust gases can be hotter than 932°F (500°C). Traditionally, this "waste heat" would be…
Cognition is a complex mental process whereby an individual gains knowledge and understanding of the world. While cognition cannot be neatly dissected into constitutive processes, psychologists point out that it reveals the interplay of such critical psychological mechanisms as perception, attention, memory, imagery, verbal function, judgment, problem-solving, decision-making, with the admixture o…
The common cold, also often referred to as an upper respiratory infection, is caused by some 200 different viruses, and has defied both cure and vaccine for centuries. The United States alone will have about a half a billion colds a year, or two for each man, woman, and child. Dedicated researchers have searched for a cure or even an effective treatment for years. The pharmaceutical company that d…
Collagen is a protein found abundantly throughout the bodies of animals, including humans. In fact, collagen makes up about one-third of the total body weight. Collagen is an important component of the body's connective tissues, which perform a variety of functions in the body. These tissues provide the framework, or internal scaffolding, for various organs such as the kidneys and lymph nod…
A colloid is a type of particle intermediate in size between a molecule and the type of particles we normally think of, which are visible to the naked eye. Colloidal particles are usually from 1 to 1,000 nanometers in diameter. When a colloid is placed in water, it forms a mixture which is similar in some ways to a solution, and similar in some ways to a suspension. Like a solution, the particles …
Colobus monkeys and the closely related langurs and leaf monkeys are Old World monkeys in the sub-family Colobinae of the family Cercopithecidae. The primates in this subfamily share a common trait—they lack thumbs or have only small, useless thumbs. (The name colobus comes from a Greek word meaning mutilated.) However, lack of a thumb does not stop them from nimbly moving among the branche…
Color is a complex and fascinating subject. Several fields of science are involved in explaining the phenomenon of color. The physics of light, the chemistry of colorants, the psychology and physiology of human emotion are all related to color. Since the beginning of history, people of all cultures have tried to explain why there is light and why we see colors. Some people have regarded color with…
The condition known as color blindness is a defect in vision that causes problems in distinguishing between certain colors. The condition is usually passed on genetically, and is more common in men than in women. About 6% of all men and about 0.6% of women inherit the condition. Individuals can also acquire the condition through various eye diseases. There is no treatment for color blindness. …
A colugo is a furry mammal with a thin neck, a slender body, and large eyes. It is about the size of an average house cat, measuring between 15-16.5 in (38-42 cm) long with a tail adding another 8-10 in (20-25 cm). Also known as a flying lemur, the colugo neither truly flies nor is it a lemur. A gliding mammal, it is able to give the appearance of flight with the help of a membrane that stretches …
Coma, from the Greek word koma, meaning deep sleep, is a state of extreme unresponsiveness in which an individual exhibits no voluntary movement or behavior. In a deep coma, stimuli, even painful stimuli, are unable to effect any response. Normal reflexes may be lost. Coma lies on a spectrum with other alterations in consciousness. The level of consciousness which you, the reader, are currently en…
Combinatorics is the study of combining objects by various rules to create new arrangements of objects. The objects can be anything from points and numbers to apples and oranges. Combinatorics, like algebra, numerical analysis and topology, is a important branch of mathematics. Examples of combinatorial questions are whether we can make a certain arrangement, how many arrangements can be made, and…
Combustion is the chemical term for a process known more commonly as burning. It is certainly one of the earliest chemical changes noted by humans, at least partly because of the dramatic effects it has on materials. Today, the mechanism by which combustion takes place is well understood and is more correctly defined as a form of oxidation that occurs so rapidly that noticeable heat and light are …
In the spring of 1997, the night sky provided a spectacular light show as Comet Hale-Bopp, one of the brightest comets of the century, traversed the heavens. The comet was more closely studied than any before it, and scientists continue to make discoveries based on the data gathered by terrestrial and orbital telescopes and instrumentation. Originating primarily from the Oort Cloud, a belt of stel…
A comet is an object with a dark, solid core (the nucleus) some miles in diameter. The core is composed mostly of water ice and frozen gas and is surrounded—whenever the comet is close enough to the Sun for part of the core to vaporize—by a cloud of glowing vapor (the coma). Together, the core and coma comprise the comet's head, which appears as a bright, well-defined cloud. A…
Commensalism is a type of symbiosis, specifically, a biological relationship in which one species benefits from an interaction, while the host species is neither positively or negatively affected to any tangible degree. For example, epiphytic plants (which grow on other plants but are not parasitic) gain an enormous ecological benefit from living on larger plants, because they gain access to a sub…
Within the science of ecology, a community is a set of organisms coexisting within a defined area. Community ecology, then, is the study of the interactions that occur among groups of species coexisting within a region. For example, a community ecologist might consider the ways in which plants and animals within a forest affect one another's growth. Contrary to popular usage, the term ecolo…
"Commutativity" is a property an operation between two numbers (or other mathematical elements) may or may not have. The operation is commutative if it does not matter which element is named first. For example, because addition is commutative, 5 + 7 has the same value as 7 + 5. Subtraction, on the other hand, is not commutative, and the difference 5 - 7 does not have the same value a…
In 1978, Philips and Sony together launched an effort to produce an audio compact disc (CD) as a method of delivering digital sound and music to consumers. The two companies continued to cooperate through the 1980s and eventually worked out standards for using the CD technology to store computer data. These recommendations evolved into the CD-ROM technology of today. The CD-ROM (compact disc-read …
Competition is a biological interaction among organisms of the same or different species associated with the need for a common resource that occurs in a limited supply relative to demand. In other words, competition occurs when the capability of the environment to supply resources is smaller than the potential biological requirement so that organisms interfere with each other. Plants, for example,…
Complementary deoxyribonucleic acid (DNA) is DNA in which the sequence of the constituent molecules on one strand of the double stranded structure chemically matches the sequence on the other strand. A useful analog is to picture a key and a lock. While there are many different types of keys, only one design matches the contours of the lock and so will fit into the lock. The different chemical mol…
A complex is a species in which the central atom is surrounded by a group of Lewis bases that have covalent bonds to the central atom. The Lewis bases that surround the central atom are generally referred to as ligands. Complexes are so named because when they were first studied, they seemed unusual and difficult to understand. Primarily, transition metals form complexes and their most observable …
Typical complex numbers are 3 - i, 1/2 + 7i, and -6 - 2i. If one writes the real number 17 as 17 + 0i and the imaginary number -2.5i as 0 - 2.5i, they too can be considered complex numbers. Complex numbers are so called because they are made up of two parts which cannot be combined. Even though the parts are joined by a plus sign, the addition cannot be performed. The expression must be left as an…
The composite or aster family (Asteraceae) is one of the largest families of plants, containing about 20,000 species, distributed among more than 1,000 genera, and occurring widely on all continents, except Antarctica. This family is commonly regarded by modern botanists as the most advanced of the plant families, because of the complex, highly evolved structure of its multi-flowered, composite re…
A composite material is a microscopic or macroscopic combination of two or more distinct materials with a recognizable interface between them. For structural applications, the definition can be restricted to include those materials that consist of a reinforcing phase such as fibers or particles supported by a binder or matrix phase. Other features of composites include the following: (1) The distr…
Composting is the process of arranging and manipulating organic wastes so that they are gradually broken down, or decomposed, by soil microorganisms and animals. The resulting product is a black, earthy-smelling, nutritious, crumbly mixture called compost or humus. Compost is usually mixed with other soil to improve the soil's structural quality and to add nutrients for plant growth. Compos…
A compound is a substance composed of two or more elements chemically combined with each other. Historically, the distinction between compounds and mixtures was often unclear. Today, however, the two can be distinguished from each other on the basis of three primary criteria. First, compounds have constant and definite compositions, while mixtures may exist in virtually any proportion. A sample of…
The Compton effect (sometimes called Compton scattering) occurs when an x ray collides with an electron. In 1923, Arthur H. Compton did experiments bouncing x rays off the electrons in graphite atoms. Compton found the x rays that scattered off the electrons had a lower frequency (and longer wavelength) than they had before striking the electrons. The amount the frequency changes depends on the sc…
The main concern of psychiatrists and therapists who treat people with compulsions is the role they play in a mental illness called obsessive-compulsive disorder (OCD). Compulsions need to be distinguished from obsessions in order to understand how they interconnect with compulsive behavior and reinforce this debilitating illness. In psychiatric literature, compulsions are defined as repetitive be…
A digital computer employs physical device states as symbols; an analog computer employs them as models. An analog computer models the behaviors of smoothly varying mathematical variables—usually representing physical phenomena such as temperatures, pressures, or velocities—by translating these variables into (usually) voltages or gear movements. It then manipulates these physical qu…
A digital computer is a programmable device that processes information by manipulating symbols according to logical rules. Digital computers come in a wide variety of types, ranging from tiny, special-purpose devices embedded in cars and other devices to the familiar desktop computer, the minicomputer, the mainframe, and the supercomputer. The fastest supercomputer, as of early 2003, can execute u…
A computer language is the means by which instructions and data are transmitted to computers. Put another way, computer languages are the interface between a computer and a human being. There are various computer languages, each with differing complexities. For example, the information that is understandable to a computer is expressed as zeros and ones (i.e., binary language). However, binary lang…
Physical and virtual memory are forms of memory (internal storage of data). Physical memory exists on chips (RAM memory) and on storage devices such as hard disks. Before a process can be executed, it must first load into RAM physical memory (also termed main memory). Virtual memory is a process whereby data (e.g., programming code,) can be rapidly exchanged between physical memory storage locatio…
Computers are built of electronic components encased in a sturdy metal container and attached to the outside of the container. Examples of hardware include electrical connections, circuit boards, hard and disk drives, viewing monitor, and printer. The components and the container are referred to as hardware. On their own, computer hardware is functionally useless. For the electronic circuitry to b…
A computer virus is a program or segment of executable computer code that is designed to reproduce itself in computer memory and, sometimes, to damage data. Viruses are generally short programs; they may either stand-alone or be embedded in larger bodies of code. The term "virus" is applied to such code by analogy to biological viruses, microorganisms that force larger cells to manuf…
Computerized axial tomography (CAT) is a diagnostic procedure that employs x rays in a unique manner. The CAT scan machine is computer controlled to assure accuracy in placement of the x-ray beam. Axial refers to the fact that the x-ray tubes are arranged in an arc about an axis. Tomography is a combination of tomo, from the Greek meaning "to cut," and graph, "to draw,"…
Concentration is a ratio of how much of one ingredient is present in a mixture, compared to the whole mixture or compared to the main ingredient, often the solvent. The amounts of each substance can be expressed in mass or volume units, and many different units can be used. The components of the mixture can be gases, liquids, or solids. Earth's atmosphere, for example, is a mixture of gases…
Concrete, from the Latin word concretus meaning "having grown together," generally consists of Portland cement, water, and a relatively unreactive filler called aggregate. The filler is usually a conglomerate of gravel, sand, and blast-furnace stony matter known as slag. When Portland cement is mixed with water, the various ingredients begin to react chemically with the water. For a …
Conditioning is a term used in psychology to refer to two specific types of associative learning as well as to the operant and classical conditioning procedures which produce that learning. Very generally, operant conditioning involves administering or withholding reinforcements based on the performance of a targeted response, and classical conditioning involves pairing a stimulus that naturally e…
Condors are New World vultures that are among the largest of flying birds. There are only two species, the Andean condor (Vultur gryphus) and the critically endangered California condor (Gymnogyps californianus). They are related to the smaller vultures of the Americas, including the king vulture (Sarcoramphus papa) and turkey vulture (Cathartes aura), which also belong to family Cathartidae. In t…
The term congenital is used to describe a condition or defect that exists at birth. Congenital disorders are inborn. They are present in the developing fetus. Sickle cell disease, Down syndrome, and congenital rubella syndrome are three examples of congenital conditions in humans. Congenital disorders result from abnormalities in the fetus's genetic inheritance, conditions in the fetal envi…
Two triangles are congruent if they are alike in every geometric respect except, perhaps, one. That one possible exception is in the triangle's "handedness." There are only six parts of a triangle that can be seen and measured: the three angles and the three sides. The six features of a triangle are all involved with congruence. If triangle ABC is congruent to triangle DEF, th…
A conic section is the plane curve formed by the intersection of a plane and a right-circular, two-napped cone. Such a cone is shown in Figure 1. The cone is the surface formed by all the lines passing through a circle and a point. The point must lie on a line, called the "axis," which is perpendicular to the plane of the circle at the circle's center. The point is called the …
Conifer (common name for phylum Pinophyta) is a type of tree that thrives in temperate and boreal climates. Characterized by seed-bearing cones, conifers typically have narrow, needle-like leaves covered with a waxy cuticle, and straight trunks with horizontal branches. These trees are usually evergreen, meaning they do not shed their leaves all at once, and can photosynthesize continually. There …
Connective tissue is found throughout the body and includes fat, cartilage, bone, and blood. The main functions of the different types of connective tissue include providing support, filling in spaces between organs, protecting organs, and aiding in the transport of materials around the body. Connective tissue is composed of living cells and protein fibers suspended in a gel-like material called m…
Conservation is the philosophy that natural resources should be used cautiously and rationally so that they will remain available for future generations of people. American conservationist thought has evolved from its inception in the mid 1850s, when naturalists, businesspeople and statesmen alike foresaw environmental, economic and social peril in the unregulated use and abuse of North America&#x…
Conservation laws refer to physical quantities that remain constant throughout the multitude of processes which occur in nature. If these physical quantities are carefully measured, and if all known sources are taken into account, they will always yield the same result. The validity of the conservation laws is tested through experiments. However, many of the conservation laws are suggested from th…
A constellation is a group of stars that form a long-recognized pattern in the sky. The names of many constellations are Greek in origin and are related to ancient mythology. The stars that make up a constellation may be at very different distances from the earth and from one another. The pattern is one that we as humans choose to see and has no physical significance. Novice stargazers are often t…
Much of Euclidean geometry is based on two geometric constructions: the drawing of circles and the drawing of straight lines. To draw a circle with a compass, one needs to know the location of the center and some one point on the circle. To draw a line segment with a straightedge, one needs to know the location of its two end points. To extend a segment, one must know the location of it or a piece…
The presence of toxic and radioactive chemicals in soil at concentrations above trace levels poses potential risks to human health and ecological integrity. Soil can be contaminated by many human actions, including discharge of solid or liquid materials to the soil surface, pesticide and fertilizer application, subsurface release from buried tanks, pipes, or landfills, and deposition of atmospheri…
Contamination generally refers to the occurrence of some substance in the environment. The contaminant may be present in a larger concentration than normally occurs in the ambient environment. However, contamination is only said to occur when the concentration is smaller than that at which measurable biological or ecological damage can be demonstrated. Contamination is different from pollution, wh…
A continent is a large land mass and its surrounding shallow continental shelf. Both are composed of felsic crust. Continents, as by-products of plate tectonic activity, have grown to cover about one-third of Earth's surface over the last four billion years. Continents are unique to Earth, as plate tectonics does not occur on the other planets of our solar system. Continent interiors consis…
The relative movement of the continents is explained by modern theories of plate tectonics. The relative movement of continents is explained by the movement Diagram illustrating formation of continental margins. (a) Materials in Earth's mantle move up (often through volcanos) expanding the continental crust and causing rifting. (b) As it divides, the continental crust thins, and contin…
The continental margin is that portion of the ocean that separates the continents from the deep ocean floor. For purposes of study, the continental margin is usually subdivided into three major sections: the continental shelf, the continental slope, and the continental rise. In addition to these sections, one of the most important features of the continental margin is the presence of very large su…
The continental shelf is a gently sloping and relatively flat extension of a continent that is covered by the oceans. Seaward, the shelf ends abruptly at the shelf break, the boundary that separates the shelf from the continental slope. The shelf occupies only 7% of the total ocean floor. The average slope of the shelf is about 10 ft per mi (1.9 m per km). That is, for every one kilometer of dista…
The continuity of space or time, considered by Zeno and others, is represented in mathematics by the continuity of points on a line. As late as the seventeenth century, mathematicians continued to believe, as the ancient Greeks had, that this continuity of points was a simple result of density, meaning that between any two points, no matter how close together, there is always another. This is true…
One of the earliest methods of conservation tillage came to be known as contour plowing, or "plowing on the contour." Tilling the soil along the gentle slopes of a piece Contour rice farming in Arkansas. Photograph by Paul Logsdon. Phototake NYC. Reproduced by permission. of cropland, instead of up and down the gradient, prevents fertile topsoil from being carried downhill by fl…
In the United States, about 64% of women between 15 and 44 years of age used contraception in 1995, a total of about 60 million. Worldwide, contraceptive use increased 10-fold from 1963 to 1993. However, contraception remains controversial, with some religious and political groups opposed to the distribution of contraceptives. …
Convection is the vertical transfer of mass, heat, or other properties in a fluid or substance that undergoes fluid-like dynamics. Convection takes place in the atmosphere, in the oceans, and in Earth's molten subcrustal asthenosphere. Convective currents of air in the atmosphere are referred to as updrafts and downdrafts. In addition to heat transfer, convention can be driven by other prop…
A coordination compound is formed when groups of atoms, ions, or molecules chemically bond with each other by donating and accepting pairs of electrons. Groups donating electron pairs are called ligands. They are usually Lewis bases. Groups accepting electron pairs are often transition metal cations. Figure 1. Formation and representations of a coordinate-covalent bond. Illustration by Ha…
Copepods are pale or translucent crustaceans, measuring between 0.04 mm to several millimeters long. They have adapted to many different habitats; while they usually live in salt water, copepods can live in lakes and ponds as well. Furthermore, they have different modes of locomotion: some can swim purposefully but others are planktonic, floating with the current. Scientists generally A copep…
Copper is the metallic chemical element of atomic number 29, symbol Cu, atomic weight 63.55, specific gravity 8.96, melting point 1,985°F (1,085°C), and boiling point 4,645.4°F (2,563°C). It consists of two stable isotopes, of mass numbers 63 (69.1%) and 65 (30.9%). Copper is one of only two metals that are colored, Copper is reddish brown, while gold is...gold—a…
Coral reefs are highly diverse ecosystems, supporting greater numbers of fish species and other organisms than any other marine ecosystem. Coral reefs are located in warm, shallow, tropical marine waters with enough light to stimulate the growth of the reef organisms. The primary reef-building organisms are invertebrate animals known as corals; corals secrete the bulk of the calcium carbonate (lim…
The Coriolis effect is a mechanical principle demonstrating that, on a rotating solid body, an inertial force acts on the body at right angles to its direction of motion. The Coriolis effect (also called the Coriolis force) is based on the classic laws of motion introduced to the world by Sir Issac Newton (1642–1727). A rotating body not only moves according to Newtonian motion, but it is a…
Cork is the outer, regenerative bark of the cork oak tree, Quercus suber, family Fagaceae. Unlike other oak species, the cork oak is an evergreen tree and dates from the Oligocene epoch of the Tertiary period. The oldest cork fossil, dating 10 million years old, was found in Portugal and is identical to modern cork. Today cork oak trees grow exclusively around the edge of the Mediterranean, primar…
A corm is a modified, upright, swollen, underground stem base of a herbaceous plant. Corms serve as a perennating organ, storing energy and producing new shoots and flowering stems from one or more buds located in the axils of the scale–like leaves of the previous year. Corms differ from superficially similar bulbs in that their leaves are thin rather than fleshy, and they are entirely comp…
Cormorants or shags are long-necked, generally black or dark grey, aquatic birds in the family Phalacrocoracidae. These birds occur in most temperate and tropical marine coasts, and on many large lakes. There are 29 species of cormorants with fewer species occurring at higher latitudes. The plumage of cormorants is not completely waterproof, since these birds lack an oil gland for preening, so the…
According to Native American legends of the American Southwest, the Indian people have occupied four (some say five) worlds since the creation of man. The present world is the fourth world and each of the three former worlds lies under the one succeeding it. Some legends say that maize, or corn as Europeans came to call it, was already present in the first world, at the time the first humans were …
Coronal mass ejections (CMEs) are explosive and violent eruptions of charged, magnetic field-inducing particles and gas (plasma) from the Sun's outer coronal layer. The ejection from the Sun's corona can be massive (e.g., estimates of CME mass often range in the billions of tons. Ejections propel particles in specific directions, some directly crossing Earth's orbital position…
In geology, the term correlation refers to the methods by which the age relationship between various strata of Earth's crust is established. Such relationships can be established, in general, in one of two ways: by comparing the physical characteristics of strata with each other (physical correlation); and by comparing the type of fossils found in various strata (fossil correlation). Correl…
Correlation refers to the degree of correspondence or relationship between two variables. Correlated variables tend to change together. If one variable gets larger, the other one systematically becomes either larger or smaller. For example, we would expect to find such a relationship between scores on an arithmetic test taken three months apart. We could expect high scores on the first test to pre…
Corrosion is the deterioration of metals and other materials by chemical reaction. Corrosion of metals is the most common type of corrosion and is a process involving an exchange of electrons between two substances, one of them being the metal. In this process, the metal usually loses electrons, becoming oxidized, while the other substance gains electrons, becoming reduced. For this reason, corros…
In 1965, Arno Penzias and Robert Wilson announced the discovery of microwave radiation which uniformly filled the sky and had a blackbody temperature of about 3.5K. The pair had been testing a new radio amplifier that was supposed to be exceptionally quiet. What better way to do such a test than to tune the radio so that it should hear nothing at all? After many attempts to account for all extrane…
The term cosmic ray refers to tiny particles of matter that travel through space. Cosmic rays generally possess an electromagnetic charge and are highly energetic. Physicists divide cosmic rays into two categories: primary and secondary. Primary cosmic rays originate far outside Earth's atmosphere. Secondary cosmic rays are particles produced within Earth's atmosphere as a result of …
Cosmology is the study of the origin, structure and evolution of the universe. The origins of cosmology predate the human written record. The earliest civilizations constructed elaborate myths and folk tales to explain the wanderings of the Sun, Moon, and stars through the heavens. Ancient Egyptians tied their religious beliefs to celestial objects and Ancient Greek and Roman philosophers debated …
Cotingas are a highly diverse group of birds that make up the family Cotingidae. Species of cotingas occur widely in tropical forests of South and Central America. Cotingas are fly-catching birds, and are similar in many respects to species of tyrant flycatchers (family Tyrannidae), although these families are not closely related. Species of cotingas are extremely variable in size, shape, color, b…
Cotton is a fiber obtained from various species of plants, genus Gossypium, family Malvaceae (Mallow), and is the most important and widely used natural fiber in the world. Cotton is primarily an agricultural crop, but it can also be found growing wild. Originally cotton species were perennial plants, but in some areas cotton has been selectively bred to develop as an annual plant. There are more …
A coulomb (abbreviation: C) is the standard unit of charge in the metric system. It was named after the French physicist Charles A. de Coulomb (1736-1806) who formulated the law of electrical force that now carries his name. …
Every set that can be counted is countable, but this is no surprise. The interesting case for countable sets comes when we abandon finite sets and consider infinite ones. An infinite set of numbers, points, or other elements is said to be "countable" (also called denumerable) if its elements can be paired one-to-one with the natural numbers, 1, 2, 3, etc. The term countable is somewh…
Coursers and the closely related pratincoles are 17 species of birds that comprise the family Glareolidae, in the order Charadriiformes, which also contains the plovers, sandpipers, and other families of waders and shorebirds. The pratincoles occur in southern Europe and Asia, Africa, Southeast Asia, and Australasia, but coursers only occur in Africa, the Middle East, and India. Coursers and prati…
Courtship is a complex set of behaviors in animals that leads to mating. Courtship behavior communicates to each of the potential mates that the other is not a threat. It also reveals information to each animal that the species, gender, and physical condition of the other are suitable for mating. Pre-mating activities are for the most part ritualistic. They consist of a series of fixed action patt…
The coypu or nutria (Myocastor coypu) is a species of semi-aquatic, dog-sized rodent in the family Capromyidae. These animals are native to central and southern South America, but they have become widely established elsewhere, mostly as a result of animals that have escaped from fur farms or that have been deliberately released. Coypus have a stout, 17-25 in (43-64 cm) long body, with a roughly tr…
Crabs are some of the best known arthropods—a terms that means jointed foot (Greek: arthron, joint; pous, foot). They are among the most successful of all living species (about 4,500 species have been described), with members adapted to living on land and in water; some species even succeed in living in both habitats. The majority, however, live in the marine environment. Unlike lobsters (t…
The first true cranes, founded on the principles of levers and counterweights, used a pulley system fixed to a single mast or boom. Lifting power was provided by humans or draft animals operating a treadmill or large wheel. Eventually, a second mast and guy wires were added to increase the strength and stability of this early form of crane. One of the most significant developments in crane design,…
Cranes are tall, wading birds known for their beauty, elaborate courtship dances, and voices that boom across their wetland habitat. Their family Gruidae, is among the oldest on Earth. Today 15 crane species are found throughout the world, except in South America and Antarctica. Two species, the whooping crane (Grus americana) and the sandhill crane (G. canadensis) are found in North America. Cran…
Crayfish are freshwater crustaceans of the order Decapoda, which includes crabs, shrimps, lobsters, and hermit crabs. Crayfish are nocturnally active, live in shallow freshwater habitats, and feed on aquatic plant and animal life, as well as dead organic matter. Their natural predators include fish, otters, turtles, and wading birds. Crayfish are particularly vulnerable to predation during their p…
Crestfish, also called unicornfish, are a small family (Lophotidae) of deepwater, marine bony fish in the order Lampridiformes. These rare fish have unusual boxlike heads with protruding foreheads and ribbon-shaped silvery bodies with crimson fins. The prominent dorsal fin extends from the tip of the head to beyond the tail; the first rays of this fin form a crest above the head, giving these fish…
Creutzfeldt-Jakob disease is a rare encephalopathy, or brain disease, that causes a swift, progressive dementia and neuromuscular changes. It was first described by German psychiatrist Alfons Maria Jakob (1884–1931) in 1921. He gave credit to Hans Gerhard Creutzfeldt (1885–1964), also a German psychiatrist, for describing the syndrome first without realizing he had stumbled onto a re…
Crickets (order Orthoptera, family Grillidae) are found throughout the world except for the polar regions. More than 900 species have been described. Often heard, but more seldom seen, at first glance crickets are quite similar to grasshoppers and bush crickets—also known as long-horned grasshoppers or katydids—but may be distinguished from these insects by their much longer, thread-…
All species have particular requirements for their ecological habitat. These specific needs are known as critical habitat, and they must be satisfied if the species is to survive. Critical habitat can involve specific types of food, a habitat required for breeding (as is the case of species that nest in tree cavities), or some other crucial environmental requirement. Some critical habitat features…
The crocodile order (Crocodylia) consists of several families of large, unmistakable, amphibious reptiles: the crocodiles (Crocodylidae), gavials (Gavialidae), and the alligators and caimans (Alligatoridae). Although these animals look superficially like lizards, they are different in many important respects, and are believed by biologists to be the most highly evolved of the living reptiles. Croc…
Crop rotation is a method of maintaining soil fertility and structure by planting a particular parcel of agricultural land with alternating plant species. Most crop rotation schedules require that a field contain a different crop each year, and some schemes incorporate times when the field remains uncultivated, or lies fallow. Farmers rotate crops to control erosion, promote soil fertility, contai…
Crops are any organisms that humans utilize as a source of food, materials, or energy. Crops may be utilized for subsistence purposes, to barter for other goods, or to sell for a cash profit. They may be harvested from wild ecosystems, or they may be husbanded and managed, as occurs with domesticated species in agriculture. In general, the purpose of management is to increase the amount of crop pr…
If two fractions are equal, say then it is always true that the products of the numbers given by are also equal or ad = bc. This is the most common form of cross multiplication. That implies ad = bc can be shown by multiplying both sides of by the common denominator bd and canceling. Cross multiplying is a common first step in solving proportions. is equivalent to The…
In solid geometry, the cross section of a three-dimensional object is a two-dimensional figure obtained by slicing the object perpendicular to its axis and viewing it end on. Thus, a sausage has a circular cross section, a 4 × 4 fence post has a square cross section, and a football has a circular cross section when sliced one way and an elliptical cross section when sliced another way. More…
The members of the crow family (Corvidae) are among the world's most intelligent birds. The family has recently undergone taxonomic expansion, brought about by evidence gathered through genetic testing, and now includes such diverse species as birds-of-paradise, orioles, and drongos. Crows and jays belong to the subfamily Corvinae. The corvids comprise 113 species in 25 genera, which includ…
The crustacea (subphylum Mandibulata, class Crustacea) are a diverse group of animals. This class includes some of the more familiar arthropods, including barnacles, copepods, crabs, prawns, lobsters, and wood lice. More than 30,000 species have been identified, the majority of which are marine-dwelling. Terrestrial species such as woodlice and pill bugs are believed to have evolved from marine sp…
Cryobiology is the scientific study of the effects of freezing and sub-freezing temperatures on biological fluids, cells, and tissues. It is an extension of cryogenics, which is the study of the properties of matter at very low temperatures. Cryobiological techniques have application in genetic research, livestock breeding, infertility treatment, and organ transplantation. A related field, cryogen…
Cryogenics is the science of producing and studying low-temperature environments. The word cryogenics comes from the Greek word "kryos," meaning cold; combined with a shortened form of the English verb "to generate," it has come to mean the generation of temperatures well below those of normal human experience. More specifically, a low-temperature environment is termed …
Cryptography is a division of applied mathematics concerned with developing schemes and formula to enhance the privacy of communications through the use of codes. Cryptography allows its users, whether governments, military, businesses, or individuals, to maintain privacy and confidentiality in their communications. Encryption is any form of coding, ciphering, or secret writing. Encryption of data…
A crystal is a solid in which the particles that make up the solid take up a highly ordered, definite, geometric arrangement that is repeated in all directions within the crystal. In ancient Greece, Archimedes made a study of regular solids, and Plato and Aristotle speculated on the relationship between regular solids and the elements. In the sixteenth century, the German naturalist, Giorgius Agri…
Every cubic equation has either three real roots as in our first example or one real root and a pair of (conjugate) complex roots as in our second example. There is a formula for finding the roots of a cubic equation that is similar to the one for the quadratic equation but much more complicated. It was first used by Geronimo Cardano in 1545, even though he had obtained the formula from Niccolo Ta…
Cuckoos, coucals, anis, malkohas, and roadrunners are approximately 127 species of birds that make up the family Cuculidae. These birds are mostly tropical in distribution, but some species also breed in the temperate zones. Many species are parasitic breeders, laying their eggs in the nests of other species of birds. Species of the cuckoo family occupy a great diversity of habitats, ranging from …
Curare (pronounced cue-rah'-ree) is a general term for certain chemical substances found in different plants throughout the world's rainforests. These plants produce a harmless sap which for centuries the natives of the rainforests have refined into a deadly poison. The way of refining and delivering the poison from certain types of plants is similar for natives occupying equatorial …
Curlews are large, brownish shorebirds (family Scolopacidae) with long legs and lengthy, downward curving bills, adapted for probing into sediment and soil for their food of invertebrates. Although neither species of North American curlew is common, the most abundant ones are the long-billed curlew (Numenius americanus) and the whimbre, or Hudsonian curlew (N. phaeopus). The long-billed curlew bre…
Currents are steady, smooth movements of water following a specific course; they proceed either in a cyclical pattern or as a continuous stream. In the Northern Hemisphere, currents generally move in a clockwise direction, while in the Southern Hemisphere they move counterclockwise. There are three basic types of ocean currents: surface currents; currents produced by long wave movements or tides; …
Informally, one can picture a curve as either a line, a line segment, or a figure obtained from a line or a line segment by having the line or line segment bent, stretched, or contracted in any way. A plane curve, such as a circle, is one that lies in a plane; a curve in three dimensional space, such as one on a sphere or cylinder, is called a skew curve. A closed curve is a curve with no endpoint…
In the early twentieth century, Harvey Cushing, (1869–1939), an American neurosurgeon, described a set of symptoms that he identified as a syndrome later called Cushing syndrome or disease. The cause of the syndrome at the time was unknown, but since then, a better understanding of the underlying causes of Cushing syndrome have been elucidated. Cushing syndrome is a disorder that is caused …
Cuttlefish are squid-like cephalopod mollusks of the family Sepiidae, in the order Sepioidea. Cephalopod literally means "head-footed animal" and is the name given to advanced mollusks (such as cuttlefish, squid and octopus) whose heads are circled with tentacles. Cuttlefish have a relatively well-developed brain, sensitive organs of smell and hearing, highly developed eyes, and a re…
Cybernetics is a term that was originated by American mathematician Norbert Wiener (1894–1964) in the late 1940s. Based on common relationships between humans and machines, cybernetics is the study and analysis of control and communication systems. As Wiener explains in his 1948 book, Cybernetics: or Control and Communication in the Animal and the Machine, any machine that is "intell…
The cycads are a relatively small phylum (Cycadophyta) in the plant kingdom Plantae. The cycads are considered to be gymnosperms, because they bear their seeds naked on modified leaves called sporophylls. In contrast, the evolutionarily more recent angiosperms (flowering plants) bear their seeds inside of ovaries. Cycads grow in tropical and subtropical regions of the world. Cycads are sometimes r…
University of Illinois graduate student Michael Sveda first synthesized cyclamate in 1937. Some say that he discovered its sweet taste by chance when he accidentally got some on the cigarette he was smoking. The university eventually transferred patent rights to Abbott Laboratories, which brought the sweetener to market in 1950. Another reason for combining saccharin with cyclamate is that the swe…
The terms cyclone and anticyclone are used to describe areas of low and high atmospheric pressure, respectively. Air flowing around one or the other of these areas is said to be moving cyclonically in the first case and anticyclonically in the second. In the northern hemisphere, cyclonic winds travel in a counterclockwise direction and anticyclonic winds, in a clockwise direction. When a cyclone o…
Cyclosporines are drugs used in the field of immunosuppressant medicine to prevent the rejection of transplanted organs. They were discovered by Jean F. Borel in 1972. The cyclosporine used for transplant surgery is called cyclosporine A (CsA) and in 1984 it was added to the group of medicines used to prevent transplant rejection. Cyclosporine A is the most common form of the Norwegian fungus Toly…
A cyclotron is a type of particle accelerator designed to accelerate protons and ions to high velocities and then release them so as to strike a target. Observations of such collisions yield information about the nature of atomic particles. In contrast to the enormous particle accelerators used in particle physics today, the first cyclotron, built in 1930 by U.S. physicist E. O. Lawrence (1901…
Cystic fibrosis (CF) is genetic disease characterized by defects in the transport of a molecule called chloride. Abnormalities in CF have been described in several organs and tissues, including the airways, pancreas, bile ducts, gastrointestinal tract, sweat glands, and male reproductive. Lung function is often normal at birth; however, airway obstruction and inflammation as well as bacterial colo…
Cytochromes are electron-transporting protein pigments concerned with cell respiration that contain an iron-containing molecule called heme, allied to that of hemoglobin. When the iron of heme accepts an electron, it changes from the oxidized ferric (Fe III) state to the reduced ferrous (Fe II) state. The oxidation of cytochromes to molecular oxygen and their subsequent reduction by oxidizable sub…
Cytology is the branch of biology that studies cells, the building blocks of life. The name for this science is translated from kytos, the Greek term for "cavity." Cytology's roots travel back to 1665, when British botanist Robert Hooke, examining a cross-section of cork, gave the spaces the name "cells," meaning "little rooms" or "cavities.&…
Dams are structures designed to restrict the flow of a stream or river, thus forming a pond, lake, or reservoir behind the wall. Dams are used for flood control, for production of hydroelectric power, to store and distribute water for agriculture and human populations, and as recreation sites. …
Damselflies are the smaller and more delicate members of the insect order Odonata, which includes the dragonflies. The damselfly suborder Zygoptera is characterized by similar fore and hind wings, which are both narrow at the base. Most damselflies can be easily distinguished from their larger and heavier dragonfly relatives in the field by their fluttering flight, and when at rest by their holdin…
Dark matter is the term astronomers use to describe material in the Universe that is non-luminous—that is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears t…
Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted …
Dichlorodiphenyl-trichloroacetic acid (or DDT) is a chlorinated hydrocarbon that has been widely used as an insecticide. DDT is virtually insoluble in water, but is freely soluble in oils and in the fat of organisms. DDT is also persistent in the environment. The combination of persistence and lipid solubility means that DDT biomagnifies, occurring in organisms in preference to the nonliving envir…
Deafness is the lack of functional sense of hearing in both ears. Loss of hearing can result from environmental or genetic causes and it can be temporary or permanent. Environmental loss of hearing results from occupational noise, noise pollution, accidents, or intake of certain drugs. Inherited loss of hearing can be caused by mutations in any of over a hundred of genes known to affect hearing, a…
A decimal fraction is a numeral that uses the numeration system, based on 10, to represent numbers that are not necessarily whole numbers. The numeral includes a dot, called the decimal point. The digits to the right of the decimal point extend the place-values to tenths, hundredths, thousandths, and so forth. For example, the decimal fraction 5.403 means "5 ones, 4 tenths, 0 hundredths, an…
Fungus creates circular patterns on decomposing maple leaves. CORBIS/Gary Braasch. Reproduced by permission. Decomposition is the natural process by which large organic materials and molecules are broken down into simpler ones. The ultimate products of decomposition are simple molecules, such as carbon dioxide and water. Sometimes misunderstood as being undesirable, decomposition is actuall…
Deer are members of the order Artiodactyla, the even-toed ungulates. This order also includes the antelopes, bovines, and giraffes. Deer are generally slender and long-legged, and their most striking characteristic is the presence of antlers, which are often used to differentiate species. The deer family, Cervidae, includes about 45 species, which are divided among 17 genera and five subfamilies: …
The deer mouse (Peromyscus maniculatus) is a small, native rodent with an almost ubiquitous distribution in North America. The deer mouse ranges from the subarctic boreal forest, through wide areas of more southern conifer and mixed-wood forests, to drier habitats as far south as some regions of Mexico. The deer mouse is highly variable in size and color over its range. Its body length ranges from…
Deforestation refers to a longer-term conversion of forest to some other kind of ecosystem, such as agricultural or urbanized land. Sometimes, however, the term is used in reference to any situation in which forests are disturbed, for example by clear-cut harvesting, even if another forest subsequently regenerates on the site. Various human activities result in net losses of forest area and theref…
Dehydroepiandrosterone (DHEA) is one of the androgens secreted by the adrenal cortex. An androgen is a hormone that stimulates masculine characteristics and is present in both males and females. The adrenal glands are small structures located at the tops of the kidneys. The adrenal medulla is the central portion of the adrenal gland and the adrenal cortex is the outer portion. The adrenal glands p…
A delta is a low-lying, almost flat landform, composed of sediments deposited where a river flows into a lake or an ocean. Deltas form when the volume of sediment deposited at a river mouth is greater than what waves, currents, and tides can erode. Deltas extend the coastline outward, forming new land along the shore. However, even as the delta is constructed, waves, currents, or tidal activity ma…
Dementia is a decline in a person's ability to think and learn. To distinguish true dementia from more limited difficulties due to localized brain damage, the strict medical definition requires that this decline affect at least two distinct spheres of mental activity; examples of such spheres include memory, verbal fluency, calculating ability, and understanding of time and location. Some d…
Dengue fever is an illness caused by four closely related viruses (DEN-1, DEN-2, DEN-3, and DEN-4). Even though these viruses are closely related, they are recognized by the immune system as being different from each other. Thus, an infection with one virus does not provide immune protection against infections with the remaining three viral types. A person can have four bouts of dengue fever in hi…
The density of an object is defined simply as the mass of the object divided by the volume of the object. For a concrete example, imagine you have two identical boxes. You are told that one is filled with feathers and the other is filled with cement. You can tell when you pick up the boxes, without looking inside, which is the box filled with cement and which is the box filled with feathers. The b…
Dentistry is the medical activity focused on treating the teeth, the gums and the oral cavity. This includes treating teeth damaged due to accidents or disease, filling teeth damaged due to tooth decay, and replacing damaged or injured teeth with replacement teeth. Major disciplines of dentistry include orthodontics, which focuses on the correction of tooth problems such as gaps between the teeth,…
Deoxyribonucleic acid (DNA), "the master molecule," is a natural polymer which encodes the genetic information required for the growth, development, and reproduction of an organism. Found in all cells, it consists of chains of units called nucleotides. Each nucleotide unit contains three components: the sugar deoxyribose, a phosphate group, and a nitrogen-containing amine or base wit…
A deposit is an accumulation of Earth materials, usually loose sediment or minerals, that is laid down by a natural agent. Deposits are all around you—the sand on the beach, the soil in your backyard, the rocks in a mountain stream. All of these consist of earth materials transported and laid down (that is, deposited) by a natural agent. These natural agents may include flowing water, ice, …
Depression is a psychoneurotic disorder characterized by lingering sadness, inactivity, and difficulty in thinking and concentration. A significant increase or decrease in appetite and time spent sleeping, feelings of dejection and hopelessness, and sometimes suicidal tendencies may also be present. It is one of the most common psychiatric conditions encountered, and affects up to 25% of women and…
Depth perception is the ability to see the environment in three dimensions and to estimate the spatial distances of objects from ourself and from each other. Depth perception is vital for our survival, being necessary to effectively navigate around and function in the world. Without it we would be unable to tell how far objects are from us, and thus how far we would need to move to reach or avoid …
In mathematics, the derivative is the exact rate at which one quantity changes with respect to another. Geometrically, the derivative is the slope of a curve at a point on the curve, defined as the slope of the tangent to the curve at the same point. The process of finding the derivative is called differentiation. This process is central to the branch of mathematics called differential calculus. …
Desalination, also called desalting, is the removal of salt from seawater. It provides essential water for drinking and industry in desert regions or wherever the local water supply is brackish. In 1991, about 3.5 billion gallons of desalinated water were produced in about 4,000 desalination plants worldwide. Most of this water was produced through distillation. However, other methods, including r…
A desert is an arid land area where more water is lost through evaporation than is gained from precipitation. Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses, coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland. Most desert regions are the result of large-sca…
Desertification is the gradual degradation of productive arid or semi-arid land into biologically unproductive land. The French botanist, André Aubreville, coined the term in 1949 to describe to the transformation of productive agricultural land in northern Africa to desert-like, uncultivable fallowland. Loss of biological and ecological viability occurs when natural variations, like extend…
A determinant, signified by two straight lines ||, is a square array of numbers or symbols that has a specific value. For a square matrix, say, A, there exists an associated determinant, |A|, which has elements identical with the corresponding elements of the matrix. When matrices are not square, they do not possess corresponding determinants. For determinants of third order, that is, n = 3, or th…
Developmental processes are the series of biological changes associated with information transfer, growth, and differentiation during the life cycle of organisms. Information transfer is the transmission of DNA and other biological signals from parent cells to daughter cells. Growth is the increase in size due to cell expansion and cell division. Differentiation is the change of unspecialized cell…
The dew point is that temperature below which the water vapor in a body of air cannot all remain vapor. When a body of air is cooled to its dew point or below, some fraction of its water vapor shifts from gaseous to liquid phase to form fog or cloud droplets. If a smooth surface is available, vapor condenses directly onto it as drops of water (dew). The dew point of a body of air depends on its wa…
Diabetes mellitus is a group of diseases characterized by high levels of glucose in the blood resulting from defects in insulin production (insulin deficiency), insulin action (insulin resistance), or both. Insulin is a hormone produced by the pancreas. When eaten, foods are converted to a type of sugar called glucose that enters the bloodstream. Insulin is needed to move glucose into the body cel…
Diagnosis, from gnosis, the Greek word for knowledge, is the process of identifying a disease or disorder in a person by examining the person and studying the results of medical tests. The diagnosis begins when the patient is presented to the doctor with a set of symptoms or perceived abnormalities such as pain, nausea, fever, or other untoward feeling. Often the diagnosis is relatively simple, an…
Dialysis is a process by which small molecules in a solution are separated from large molecules. The principle behind the process was discovered by the Scottish chemist Thomas Graham in about 1861. Graham found that the rate at which some substances, such as inorganic salts, pass through a semipermeable membrane is up to 50 times as great as the rate at which other substances, such as proteins, do…
Diamond is a mineral with the same carbon composition as graphite, but with different structure. Diamonds are formed in the compression of coal. Photograph by Rick Gayle. Stock Market. Reproduced by permission. Diamonds are a globally traded commodity used for a variety of industrial and artistic purposes. In December 2000, the United Nations General Assembly unanimously adopted a resolutio…
Algae are a very diverse group of simple, nucleated, plant-like aquatic organisms that are primary producers. Primary producers are able to utilize photosynthesis to create organic molecules from sunlight, water, and carbon dioxide. Ecologically vital, algae account for roughly half of photosynthetic production of organic material on earth in both freshwater and marine environments. Algae exist ei…
Dielectric materials are substances that have very low conductivity. That is, they are electrical insulators through which an electrical current flows only with the greatest of difficulty. Technically, a dielectric can be defined as a material with electrical conductivity of less than one millionth of a mho (a unit of electrical conductance) per centimeter. In theory, dielectrics can include solid…
Diesel engines are a class of internal combustion engine in which the fuel is burned internally and the combustion products are used as the working fluid. Unlike the spark-ignited (SI) engines found in the majority of today's automobiles in which the premixed fuel-air mixture is ignited by an electric spark, diesel engines are characterized by a spontaneously initiated combustion process wh…
The substance diethylstilbestrol (DES) is a synthetic, nonsteroidal estrogen, which was first made in 1938. Initially the substance was seen as a great scientific breakthrough, drawing on research that documented the importance of naturally occurring estrogen in women. Wide-scale use of DES by pregnant women to prevent miscarriage beginning in the 1940s ended in 1971 when researchers discovered th…
Diffraction is the deviation from a straight path that occurs when a wave such as light or sound passes around an obstacle or through an opening. The importance of diffraction in any particular situation depends on the relative size of the obstacle or opening and the wavelength of the wave that strikes it. The diffraction grating is an important device that makes use of the diffraction of light to…
A diffraction grating is an optical device consisting of many closely spaced parallel lines or grooves. In a transmission type of grating, light passes through the narrow transparent slits that lie between the dark lines on a glass or plastic plate. In a reflecting grating, light is reflected by the many parallel, narrow, smooth surfaces and absorbed or scattered by the lines cut in the reflecting…
Diffusion is the movement of molecules along a concentration gradient, from an area of high concentration to one of low concentration. Diffusion proceeds until the two concentrations are equal. Diffusion occurs in both gases and liquids. Diffusion takes place not only in liquid solutions, but in gases. The odor of bread wafting through a house is an example of the diffusion of bread-smell chemical…
The digestive system is a group of organs responsible for the conversion of food into absorbable chemicals which are then used to provide energy for growth and repair. The digestive system is also known by a number of other names, including the gut, the digestive tube, the alimentary canal, the gastrointestinal (GI) tract, the intestinal tract, and the intestinal tube. The digestive system consist…
Digital recording is a technique for preserving audio signals and video or visual images as a series of pulses that can be stored on magnetic tapes, optical discs (compact discs), or computer diskettes. These pulses are stored in the form of a series of binary digits (that is, zeros and ones). To make the recording, an analog-to-digital converter transforms the sound signal or visual image into di…
Digitalis is a drug that has been used for centuries to treat heart disease. The active ingredient in the drug is glycoside, a chemical compound that contains a sugar molecule linked to another molecule. The glycoside compound can be broken down into a sugar and nonsugar compound. Though current digitalis drugs are synthetic, that is, man-made, early forms of the drug were derived from a plant. Di…
Dik-diks (genus Madoqua) are small (dog-sized) African antelopes belonging to the family of Bovidae, which includes cattle, sheep, and goats, as well as antelopes, gazelles, and impalas. Like all bovids, dik-diks have even-toed hooves, horns, and a four-chambered stomach. There are five species of dik-dik—Kirk's (the largest), Günther's, Salt's, Red-bellied, and …
Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria that lived from about 225 million years ago to 66 million years ago, during the Mesozoic era. Species of dinosaurs ranged from chicken-sized creatures such as the 2 lb (1 kg) predator Compsognathus to colossal, herbivorous animals known as "sauropods," which were larger than any terrestrial animals tha…
A diode is an electronic device that has two electrodes arranged in such a way that electrons can flow in only one direction. Because of this ability to control the flow of electrodes, a diode is commonly used as a rectifier, a device that connects alternating current into direct current. In general, two types of diodes exist. Older diodes were vacuum tubes containing two metal components, while n…
Chlorinated dioxins are a diverse group of organic chemicals. TCDD, or 2,3,7,8-tetrachlorodibenzo-p-dioxin, is a particular dioxin that is toxic to some species of animals in extremely small concentrations. As such, TCDD is the most environmentally controversial of the chlorinated dioxins, and the focus of this entry. …
Diphtheria is a serious disease caused by the bacterium Corynebacterium diptheriae. Usually, the bacteria initially infect the throat and pharynx. During the course of the infection, a membrane-like growth appearing on the throat can obstruct breathing. Some strains of this bacterium release a toxin, a substance that acts as a poison in the body. This toxin, when released into the bloodstream, tra…
Dipole, literally, means "two poles," two electrical charges, one negative and one positive. Dipoles are common in atoms whenever electrons (-) are unevenly distributed around nuclei (+), and in molecules whenever electrons are unevenly shared between two atoms in a covalent bond. When a dipole is present, the atom or covalent bond is said to be polarized, or divided into negative an…
If one quantity increases (or decreases) each time another quantity increases (or decreases), the two quantities are said to vary together. The most common form of this is direct variation in which the ratio of the two amounts is always the same. For example, speed and distance traveled vary directly for a given time. If you travel at 4 mph (6.5 kph) for three hours, you go 12 mi (19.5 km), but at…
Disease can be defined as a change in the body processes that impairs its normal ability to function. Every day the physiology of the human body demands that oxygenation, acidity, salinity, and other functions be maintained within a very narrow spectrum. A deviation from the norm can be brought about by organ failure, toxins, heredity, radiation, or invading bacteria and viruses. Normally the body…
Dissociation is the process by which a molecule separates into ions. It may also be called ionization, but because there are other ways to form ions, the term dissociation is preferred. Substances dissociate to different degrees, ranging from substances that dissociate very slightly, such as water, to those that dissociate almost completely, such as strong acids and bases. The extent to which a su…
Distance has two different meanings. It is a number used to characterize the shortest length between two geometric figures, and it is the total length of a path. In the first case, the distance between two points is the simplest instance. In the case of parallel lines, the distance between the two lines is the length of a perpendicular segment connecting them. If two figures such as line segments,…
Distillation is one of the most important processes for separating the components of a solution. The solution is heated to form a vapor of the more volatile components A typical laboratory distillation setup. Illustration by Hans & Cassidy. Courtesy of Gale Group. in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and c…
The distributive property states that the multiplication "distributes" over addition. Thus a × (b + c) = a × b + a × c and (b + c) × a = b × a + c × a for all real or complex numbers a, b, and c. The distributive property is behind the common multiplication algorithm. For example, 27 × 4 means 4 × (2 tens + 7 ones). To complete …
A forest in Homestead, Florida, that was destroyed by Hurricane Hugo. JLM Visuals. Reproduced by permission. In the ecological context, disturbance is regarded as an event of intense environmental stress occurring over a relatively short period of time and causing large changes in the affected ecosystem. Disturbance can result from natural causes or from the activities of humans. Disturbanc…
Diurnal cycles refer to patterns within about a 24-hour period that typically reoccur each day. Most daily cycles are caused by the rotation of Earth, which spins once around its axis about every 24 hours. The term diurnal comes from the Latin word diurnus, meaning daily. Diurnal cycles such as temperature diurnal cycles, diurnal tides, and solar diurnal cycles affect global processes. Diurnal tid…
Division is the mathematical operation that is the inverse of multiplication. If one multiplies 47 by 92 then divides by 92, the result is the original 47. In general, (ab)/b = a. Likewise, if one divides first then multiplies, the two operations nullify each other: (a/b)b = a. This latter relationship can be taken as the definition of division: a/b is a number which, when multiplied by b, yields …
Genetic, genomic, or DNA fingerprinting is the term applied to a range of techniques that are used to show similarities and dissimilarities between the DNA present in different individuals. Genetic fingerprinting is an important tool in the arsenal of forensic investigators. Genetic fingerprinting allows for positive identification, not only of body remains, but also of suspects in custody. Geneti…
DNA, short for deoxyribonucleic acid, is a double-stranded, helical molecule that forms the molecular basis for heredity. For DNA replication to occur, this molecule must first unwind, or "unzip," itself to allow the information-encoding bases to become accessible. The base pairing within DNA is of a complementary nature and, consequently, when the molecule unzips, due to the action …
Deoxyribonucleic acid (DNA) synthesis is a process by which strands of nucleic acids are created. In a cell, DNA synthesis takes place in a process known as replication. Using genetic engineering and enzyme chemistry, scientists have also developed man-made methods for synthesizing DNA. The DNA molecule was discovered by Francis Crick, James Watson, and Maurice Wilkins. In 1953, Watson and Crick u…
DNA technology has revolutionized modern science. Deoxyribonucleic acid (DNA), or an organism's genetic material—inherited from one generation to the next—holds many clues that have unlocked some of the mysteries behind human behavior, disease, evolution, and aging. As technological advances lead to a better understanding of DNA, new DNA-based technologies will emerge. Recent …
The use of a vaccine constructed of a protein has traditionally been to induce the formation of an antibody to the particular protein. Antibodies are crucial to an or ganism's attempt to stop an infection caused by a microorganism. In the early 1990s, scientists observed that plasmid DNA (DNA that is present in bacteria that is not part of the main body of DNA) could affect test animals. Wo…
Dobsonflies are species of medium- to large-sized insects in the order Neuroptera, family Corydalidae. The life cycle of dobsonflies is characterized by a complete metamorphosis, with four developmental stages: egg, larva, pupa, and adult. Adult dobsonflies are usually found near freshwater, especially streams, either resting on vegetation or engaged in an awkward, fluttering flight. Sometimes adu…
Dogwood refers to certain species of trees and shrubs in the dogwood family (Cornaceae). The dogwoods are in the genus Cornus, which mostly occur in temperate and boreal forests of the Northern Hemisphere. Species in the dogwood family have seasonally deciduous foliage. The leaves are simple, usually untoothed, and generally have an opposite arrangement on the twig. The flowers of dogwoods develop…
The domain of a relation is the set that contains all the first elements, x, from the ordered pairs (x,y) that make up the relation. In mathematics, a relation is defined as a set of ordered pairs (x,y) for which each y depends on x in a predetermined way. If x represents an element from the set X, and y represents an element from the set Y, the Cartesian product of X and Y is the set of all possi…
Domestic donkeys, members of the order Perissodactyla, are large single-hoofed horse-like mammals with elongated heads. Donkeys usually stand between 9.5 and 11 hands high measured at the withers, that is, 38-44 in (95-110 cm) tall. Because of the large amount of interbreeding among different donkey species, donkeys differ markedly in appearance. They can be brown, gray, black, roan (a mixture of …
Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning, and behavior. It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the…
The Doppler effect was named after Johann Christian Doppler (1803-1853). This Austrian physicist observed and explained the changes in pitch and frequency of sound and light waves, as well as all other types of waves, caused by the motion of moving bodies. The general rule of the Doppler effect is that the wave frequencies of moving bodies rise as they travel toward an observer and fall as they re…
Dories are bony fish in the family Zeidae. A dory has an oval body with a back that rises so that the main part of the body is higher than the head. The body itself is relatively thin and compressed and appears oval in side view. Another distinguishing mark of the dories is a dark spot on each side of the body surrounded by a yellow ring. Dories typically are found in the middle depths of the seas…
Dormice are approximately ten species of rodents that make up the family Gliridae. Dormice typically live in trees, bushes, hedgerows, gardens, and rock piles. Dormice have a superficial resemblance to squirrels (family Sciuridae), but they are smaller and differ in many other anatomical and behavioral characters. Dormice have soft fur, and a long, bushy tail. Their forefeet have four digits, the …
New drugs undergo double-blind testing to determine whether they are effective. The test is called double-blind because neither the doctor who is administering the medication nor the patient who is taking it knows whether the patient is getting the experimental drug or a neutral substitute, called a placebo. Getting a new drug approved is a long, complex process in order to ensure the drug is safe…
The double helix refers to DNA's "spiral staircase" structure, consisting of two right-handed helical polynucleotide chains coiled around a central axis. Genes, which are specific regions of DNA, contain the instructions for synthesizing every protein. Because life cannot exist without proteins, the discovery of DNA's structure unveiled the secret of life: protein synth…
Down syndrome is the most common cause of mental retardation. It can be caused by the presence of an extra chromosome. Chromosomes contain sequences of DNA called genes, which represent the genetic information that exists within almost every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central DNA-containing structure) of each cell. Whe…
Dragonflies are large flying insects in the order Odonata. Dragonflies can be as large as 3 in (7.5 cm) in length, with a wing span of up to 8 in (20 cm). The fossilized remains of a huge dragonfly-like insect that had a wingspread of more than 2 ft (70 cm) is known from the Carboniferous period, some 300 million years ago. Dragonflies are very distinctive insects, with large eyes that almost cove…
Drift nets are lengthy, free-floating, 26-49 ft (8-15 m) deep nets, each as long as 55 mi (90 km). Drift nets are used to snare fish by their gills in pelagic, open-water situations. Because drift nets are not very selective of species, their use results in a large by-catch of non-target fish, sharks, turtles, seabirds, and marine mammals, which are usually jettisoned, dead, back to the ocean. Dri…
Drongos are 20 species of handsome birds that make up the family of perching birds known as Dicruridae. Drongos occur in Africa, southern and southeastern Asia, and Australasia. Their usual habitats are open forests, savannas, and some types of cultivated areas with trees. Drongos are typically black colored with a beautiful, greenish or purplish iridescence. The wings of these elegant, jay-sized …
Throughout the last century, the fruit fly Drosophila melanogaster, has been the workhorse for genetic studies in eukaryotes. These studies provided the basis of much of scientists' understanding of fundamental aspects of eukaryotic genetics. Cloned fruit fly genes have led to the identification of mammalian cognates. Discoveries have shown that the conservation between the fruit fly and ma…
Drought is characterized by low precipitation compared to the normal amount for the region, low humidity, high temperatures, and/or high wind velocities. When these conditions occur over an extended period of time, drought causes low water supplies that are inadequate to support the demands of plants, animals, and people. Drought is a temporary condition that occurs in moist climates. This is in c…
Ducks are waterfowl in the order Anseriformes, in the family Anatidae, which also includes geese and swans. Ducks occur on all continents except Antarctica, and are widespread in many types of aquatic habitats. Almost all ducks breed in freshwater habitats, especially shallow lakes, marshes, and swamps. Most species of ducks also winter in these habitats, sometimes additionally using grain fields …
Duckweeds are small, floating to slightly submerged species of flowering plants in the genus Lemna. The simple body is leaf-like, generally flat on top and convex below, lacks stems or leaves, is oval to tear-dropped in shape, and has one unbranched root that lacks vascular (conducting) tissue. The upper surface of the plant is covered with waxy compounds so as to shed water. Duckweeds are abundan…
Duikers are small African antelopes in the large family of Bovidae. This family of hoofed animals includes antelope, gazelles, cattle, sheep, and goats. Like all bovids, duikers have even-toed hooves, horns, and a four-chambered stomach structure that allows them to digest a diet of plants. Duikers are found throughout sub-Saharan Africa. These small antelopes range in size from 22 in (55 cm) to a…
A dune is a wind-blown pile of sand. Over time, dunes become well-sorted deposits of materials by wind Dunes form different characteristic shapes depending on the amount of sand, amount of moisture, and the strength and prevailing direction of the wind (i.e., windward to leeward). Illustration by Argosy. The Gale Group. or water that take on a characteristic shape and that retain that genera…
Along with squaring the circle and trisecting an angle, duplication of a cube is considered one of the three "unsolvable" problems of mathematical antiquity. According to tradition, the problem of duplication of the cube arose when the Greeks of Athens sought the assistance of the oracle at Delos in order to gain relief from a devasting epidemic. The oracle told them that to do so th…
A dust devil is a relatively small, rapidly rotating wind that stirs up dust, sand, leaves, and other material as it moves across the ground. Dust devils are also known as whirlwinds or, especially in Australia, willywillys. In most cases, dust devils are no more than 10 ft (3 m) in width and less than 300 ft (100 m) in height. Dust devils form most commonly on hot dry days in arid regions such as…
In 1995, Philips and Sony introduced the digital video disc (DVD), which had the same dimensions as a standard compact disk (CD), but was able to store up to 4.7 gigabytes of data, such as high-definition digital video files. This is more than three times the capacity of a CD. DVD players use a higher-power laser than that used for CDs, which enables smaller pits (0.4 micrometre) and separation tr…
These small antelopes belong to the ruminant family Bovidae, and are grouped with the gazelles in the subfamily Antilopinae. The 13 species of dwarf antelopes are in the tribe Neotragini. Dwarf antelopes range from extremely small (3.3-4.4 lb or 1.5-2 kg) hare-sized royal antelopes and dik-diks to the medium-sized oribi and beira weighing from 30-50 lb (10-25 kg). Dwarf antelopes engage in territo…
Color scientists use the term "colorant" for the entire spectrum of coloring materials, including dyes and pigments. While both dyes and pigments are sources of color, they are different from one another. Pigments are particles of color that are insoluble in water, oils, and resins. They need a binder or to be suspended in a dispersing agent to impart or spread their color. Dyes are …
Dysentery is an infectious disease that has ravaged armies and prisoner-of-war camps throughout history. The disease still is a major problem in tropical countries with primitive sanitary facilities. Refugee camps in Africa resulting from many civil wars are major sinks of infestation for dysentery. …
Dyslexia is a disorder that falls under the broad category of learning disabilities. It is often described as a neurological syndrome in which otherwise normal people have difficulty reading and writing. Frequently, dyslexia is defined by what it is not—dyslexia is not mental retardation, a psychiatric or emotional disorder, or a vision problem. Dyslexia is not caused by poverty, psychologi…
Dysplasia is a combination of two Greek words; dys, which means difficult or disordered; and plassein, to form. In other words, dysplasia is the abnormal or disordered formation of certain structures. In medicine, dysplasia refers to cells that have acquired an abnormality in their form, size, or orientation with respect to each other. Dysplasia may occur as the result of any number of stimuli. Su…
Dystrophinopathies are progressive hereditary degenerative diseases (often called muscular dystrophies) of skeletal muscles due to an absence or deficiency of the protein dystrophin. Dystrophin and the associated proteins form a complex system that connects the intracellular cytoskeleton to the extracellular matrix. The normal operation of this system is critical for maintaining the integrity of t…
The number e, like the number pi, is a useful mathematical constant that is the basis of the system of natural logarithms. Its value correct to nine places is 2.718281828... The number e is used in complex equations to describe a process of growth or decay. It is therefore utilized in the biology, business, demographics, physics, and engineering fields. …
Eagles are large, diurnal birds of prey in the subfamily Buteonidae, which also includes buzzards and other broad-winged hawks. The buteonids are in the order Falconiformes, which also includes falcons, osprey, goshawks, and vultures. Like all of these predatory birds, eagles have strong, raptorial (or grasping) talons, a large hooked beak, and extremely acute vision. Eagles are broadly distinguis…
The human ear is the anatomical structure responsible for hearing and balance. The ear consists of three parts: the outer, middle, and inner ears. The outer ear collects sounds from the environment and funnels them through the auditory system. The outer ear is composed of three parts, the pinna (or auricle), the external auditory canal (or external auditory meatus), and the tympanic membrane (or e…
Earth is our home planet. Its surface is mostly water (about 70%), and it has a moderately dense nitrogen-and-oxygen atmosphere that supports life—the only known life in the Universe. Rich in iron and nickel, Earth is a dense, rocky ball orbiting the Sun with its only natural satellite, the Moon. A complete revolution of the earth around the Sun takes about one year, while a rotation on its…
Befitting a dynamic Earth, the study of Earth science embraces a multitude of subdisciplines. At the heart of Earth science is the study of geology. Literally meaning "to study the Earth," traditional geological studies of rocks, minerals, and local formations have within the last century, especially in the light of the development of plate tectonic theory, broadened to include studi…
It is approximately 3,950 mi (6,370 km) from Earth's surface to its center. Geologists understand the structure and composition of the surface by direct observation and by analysis of rock samples raised by drilling projects; however, the depth of drill holes and, therefore, the depth limit of scientists' ability to directly observe Earth's interior is severely limited. Even t…
Earth acts as a dipole magnet with the positive and negative magnetic poles near, but not aligned exactly with, the north and south geographic poles. Because of this difference, detailed maps commonly distinguish between true north and magnetic north. The difference, known as magnetic declination, must be taken into account when navigating with a magnetic compass. Its magnetic field also molds the…
All objects in the universe and our solar system move in space. The earth moves in two ways. It rotates like a top on its axis, an imaginary line through the north and south poles, and revolves in an orbit around the Sun. Centrifugal force results from the earth's rotation; without gravity, centrifugal force could cause objects to fly into space. Because the force of the earth's grav…
An earthquake is the shaking or vibration of Earth's surface as the result of sudden movement along a fault, the movement of molten rock within the Earth, or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth's surface directly above the focus …
Earwigs are long-bodied insects with chewing mouthparts and many-jointed antennae in the order Dermaptera. Earwigs have small, vestigial forewings modified into a wing case, but their membranous hind-wings are large, folded, and functional, although they are not often used for flying. Earwigs hatch into nymphs which closely resemble the adults, only they are much smaller. Metamorphosis in earwigs …
Eating disorders are psychological conditions that involve either overeating, voluntary starvation, or both. No one is sure what causes eating disorders, but researchers think that family dynamics, biochemical abnormalities, and society's preoccupation with thinness may all contribute. Eating disorders are virtually unknown in parts of the world where food is scarce and within less affluent…
The Ebola virus is one of a number of viruses that cause a devastating disease in humans and closely related species such as monkeys, gorillas, and chimpanzees. The disease is known as a hemorrhagic fever because of the massive internal bleeding caused by the viral infection. Most strains of Ebola hemorrhagic fever progresses quickly from the initial appearance of symptoms to resolution, which is …
Ebony (Diospyros spp., family Ebenaceae) are species of tropical hardwood trees favored for their hard and beautiful wood. Only the black or brown heartwood is used commercially. There are more than 300 species of ebony, ranging in size from shrubs to trees taller than 100 ft (30 m). The best commercial ebony comes from India, Madagascar, Nigeria, Zaire, and the Celebes Islands. Most species of eb…
Echiuroid worms, or echiurans, commonly called spoon worms, are soft-bodied, unsegmented, marine animals of worldwide distribution. The approximately 125 species in the phylum Echiura occur mostly in the shallow intertidal zone of oceans. Most burrow or form tubes in sand or mud. Some live in discarded shells of sea urchins and sand dollars. Others inhabit cracks and crevices in rocks or coral fra…
In the animal kingdom, echolocation is an animal's determination of the position of an object by the interpretation of echoes of sounds produced by the animal. Echolocation is an elegant evolutionary adaptation to a low-light niche. The only animals that have come to exploit this unique sense ability are mammals—bats, dolphins, porpoises, and toothed whales. It is now believed that t…
It is a coincidence of nature that the apparent size of the Sun and the Moon in the sky are about the same. Thus on those rare occasions when the orbital motion of Earth and Moon cause them to align with the sun, as seen from points on Earth, the Moon will just cover the surface of the Sun and day will suddenly become night. Those who are located in the converging lunar shadow that just reaches Ea…
Ecological integrity is a relatively new concept that is being actively discussed by ecologists. However, a consensus has not yet emerged as to the definition of ecological integrity. Clearly, human activities result in many environmental changes that enhance some species, ecosystems, and ecological processes, while at the same time causing important damage to others. The challenge for the concept…
Governments everywhere are increasingly recognizing the fact that human activities are causing serious environmental and ecological damage. To effectively deal with this environmental crisis, it is important to understand its dimensions and dynamics. What, specifically, are the damages, how are they changing over time, and what are the best means of prevention or mitigation? To develop answers to …
Ecological productivity refers to the primary fixation of solar energy by plants and the subsequent use of that fixed energy by plant-eating herbivores, animal-eating carnivores, and the detritivores that feed upon dead biomass. This complex of energy fixation and utilization is called a food web. Ecologists refer to the productivity of green plants as primary productivity. Gross primary productiv…
Ecological pyramids are graphical representations of the trophic structure of ecosystems. Ecological pyramids are organized with the productivity of plants on the bottom, that of herbivores above the plants, and carnivores above the herbivores. If the ecosystem sustains top carnivores, they are represented at the apex of the ecological pyramid of productivity. A fact of ecological energetics is th…
Ecology can be defined as the study of the relationships of organisms with their living and nonliving environment. Most ecologists are interested in questions involving the natural environment. Increasingly, however, ecologists are concerned about degradation associated with the ecological effects of humans and their activities. Ultimately, ecological knowledge will prove to be fundamental to the …
The notion of ecosystem (or ecological system) refers to indeterminate ecological assemblages, consisting of communities of organisms and their environment. Ecosystems can vary greatly in size. Small ecosystems can be considered to occur in tidal pools, in a back yard, or in the rumen of an individual cow. Larger ecosystems might encompass lakes or stands of forest. Landscape-scale ecosystems comp…
An ecotone is a zone of transition between distinct ecological communities or habitats. Usually, the word is used to refer to relatively sharp, local transitions, also known as edges. Because many physical and chemical changes in the environment tend to be continuous, ecological transitions are often similarly gradual. For example, climate and precipitation change steadily across continents and up…
Ecotourism refers to outdoor recreation, sightseeing, and guided natural history studies in remote or fragile natural areas, or archeological and cultural sites. Ecotourism usually involves travel to engage in activities such as trekking and hiking, diving, mountaineering, biking, and paddling, while exploring a region's natural highlights, observing native animals, and learning about the a…
Edema is the accumulation of fluid in any given location in the body. Edema can result from trauma, as in a sprained ankle, or from a chronic condition such as heart or kidney failure. The word edema is from the Greek and means "swelling." The presence of edema can be an important diagnostic tool for the physician. A patient who is developing congestive heart failure often will devel…
Eel grasses (or eelgrasses) are 18 species of herbaceous aquatic plants in the family Zosteraceae, 12 species of which are in the genus Zostera. However, some plant systematists have treated the eel grasses as a component of a much larger family, the pondweeds or Potamogetonaceae. Eel grasses have long, strap-like leaves that emerge from a thin rhizome growing in the surface sediment of the shallo…
El Niño and La Niña are disruptions of the oceanic and atmospheric systems of the equatorial Pacific Ocean that have far reaching effects on Earth's weather patterns. El Niño and La Niña do not change with the regularity of the seasons; instead, they repeat about every two to seven years. They are the extremes in an aperiodic, or irregular, cycle called the El Ni…
Eland (Taurotragus oryx) are the largest African antelopes, weighing up to a 2,205 lb (1,000 kg) and standing 6.6 ft (2 m) at the shoulder. They belong to the family Bovidae in the order Artiodactyla, the even-toed hoofed mammals. Eland belong to the tribe Tragelaphini, a closely-related group of spiral-horned antelopes, whose members are not territorial. Both sexes posses long horns, and females …
Elapid snakes are extremely venomous snakes such as cobras, mambas, kraits, tiger snakes, and coral snakes in the family Elapidae. The elapids are about 120 species in the subfamily Elapinae. The sea snakes (subfamily Hydrophiinae) and subfamily Laticaudinae make up the other two subfamilies in the Elapidae. Elapid snakes have a wide distribution from warm temperate climates to tropical climates, …
Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed. Greater forces may cause permanent changes of shape or size, called plastic deformation. In ordinary language, a …
An electric arc is a high-current, low-voltage electrical discharge between electrodes in the presence of gases. In an electric arc, electrons are emitted from a heated cathode. Arcs can be formed in high, atmospheric, or low pressures, and in various gases. They have wide uses as highly luminous lamps, as furnaces for heating, cutting and welding, and as tools for spectrochemical analysis. …
Rub a balloon or styrofoam drinking cup against a wool sweater. It will then stick to a wall (at least on a dry day) or pick up small bits of paper. Why? The answer leads to the concept of electric charge. Electromagnetic forces are one of the four fundamental forces in nature. The other three are gravitational, strong nuclear, and weak nuclear forces. The electromagnetic force unifies both electr…
An electric circuit is a system of conducting elements designed to control the path of electric current for a particular purpose. Circuits consist of sources of electric energy, like generators and batteries; elements that transform, dissipate, or store this energy, such as resistors, capacitors, and inductors; and connecting wires. Circuits often include a fuse or circuit breaker to prevent a pow…
An electric conductor is any material that can efficiently conduct electricity, such as a metal, ionic solution, or ionized gas. Usually, this term refers to the current-carrier component of an electric circuit system. …
Electric current is the result of the relative motion of net electric charge. In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second. A current as small as a…
An electric motor is a machine used to convert electrical energy to mechanical energy. Electric motors are extremely important to modern-day life, being used in many different places, e.g., vacuum cleaners, dishwashers, computer printers, fax machines, video cassette recorders, machine tools, printing presses, automobiles, subway systems, sewage treatment plants and water pumping stations. The maj…
Electric vehicles (EV), vehicles whose wheels are turned by electric motors rather than by a mechanical gasoline-powered drivetrain, have been long touted as saviors of the environment due to their low pollution and high fuel efficiency. However, they have yet to take over the highways and byways. Thomas Davenport is credited with building the first practical EV in 1834, which was quickly followed…
Conductivity is the term used to describe the ability of a material medium to permit the passage of particles or energy. Electrical conductivity refers to the movement of charged particles through matter. Thermal conductivity refers to the transmission of heat energy through matter. Together, these are the most significant examples of a broader classification of phenomena known as transport proces…
An electrical power supply is a device that provides the energy needed by electrical or electronic equipment to perform their functions. Often, that energy originates from a source with inappropriate electrical characteristics, and a power supply is needed to change the power to meet the equipment's requirements. Power supplies usually change alternating current into direct current, raise o…
The electrical resistance of a wire or circuit is a way of measuring the resistance to the flow of an electrical current. A good electrical conductor, such as a copper wire, will have a very low resistance. Good insulators, such as rubber or glass insulators, have a very high resistance. The resistance is measured in ohms, and is related to the current in the circuit and voltage across the circuit…
Electricity is a natural phenomenon resulting from one of the most basic properties of matter, electrical charge. Our understanding of electrical principles has developed from a long history of experimentation. Electrical technology, essential to modern society for energy transmission and information processing, is the result of our knowledge about electrical charge at rest and electrical charge i…
The electrocardiogram, ECG or EKG, directly measures microvoltages in the heart muscle (myocardium) occurring over specific periods of time in a cardiac, i.e., a heartbeat, otherwise known as a cardiac impulse. With each heartbeat, electrical currents called action potentials, measured in millivolts (mV), travel at predictable velocities through a conducting system in the heart. The potentials ori…
An electroencephalogram, usually abbreviated EEG, is a medical test that records electrical activity in the brain. During the test, the brain's spontaneous electrical signals are traced onto paper. The electroencephalograph is the machine that amplifies and records the electrical signals from the brain. The electroencephalogram is the paper strip the machine produces. The EEG changes with d…
Electrolysis is the process of causing a chemical reaction to occur by passing an electric current through a substance or mixture of substances, most often in liquid form. Electrolysis frequently results in the decomposition of a compound into its elements. To carry out an electrolysis, two electrodes, a positive electrode (anode) and a negative electrode (cathode), are immersed into the material …
An electrolyte is a substance that will allow current to flow through the solution when dissolved in water. Electrolytes promote this current flow because they produce positive and negative ions when dissolved. The current flows through the solution in the form of positive ions (cations) moving toward the negative electrode and negative ion (anions) moving the positive electrode. Electrolytes can …
An electromagnetic field is an area in which electric and magnetic forces are interacting. It arises from electric charges in motion. Electromagnetic fields are directly related to the strength and direction of the force that a charged particle, called the "test" charge, would be subject to under the electromagnetic force caused by another charged particle or group of particles, call…
Electromagnetic induction is the generation of an electromotive force in a closed electrical circuit. It results from a changing magnetic field as it passes through the circuit. Some of the most basic components of electrical power systems—such as generators and transformers—make use of electromagnetic induction. …
The electromagnetic spectrum encompasses a continuous range of frequencies or wavelengths of electromagnetic radiation, ranging from long wavelength, low energy radio waves to short wavelength, high frequency, high-energy gamma rays. The electromagnetic spectrum is traditionally divided into regions of radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, x rays, and gamma …
Electromagnetism is a branch of physical science that involves all the phenomena in which electricity and magnetism interact. This field is especially important to electronics because a magnetic field is created by an electric current. The rules of electromagnetism are responsible for the way charged particles of atoms interact. Some of the rules of electrostatics, the study of electric charges at…
In an electric circuit, electromotive force is the work done by a source on an electrical charge. Because it is not really a force, the term is actually a misnomer; it is more commonly referred to by the initials EMF. EMF is another term for electrical potential, or the difference in charge across a battery or voltage source. For a circuit with no current flowing, the potential difference is calle…
All atoms found in nature have a positively charged nucleus about which the negatively charged electrons move. The atom is electrically neutral and thus the positive electrical charge on the nucleus has the same magnitude as the negative charge due to all the electrons. The electrons are held in the atom by the attractive force exerted on them by the positively charged nucleus. They move very rapi…
The term electron cloud is used to describe the area around an atomic nucleus where an electron will probably be. It is also described as the "fuzzy" orbit of an atomic electron. An electron bound to the nucleus of an atom is often thought of as orbiting the nucleus in much the same manner that a planet orbits a sun, but this is not a valid visualization. An electron is not bound by …
Electronics is a field of engineering and applied physics that grew out of the study and application of electricity. Electricity concerns the generation and transmission of power and uses metal conductors. Electronics manipulates the flow of electrons in a variety of ways and accomplishes this by using gases, materials like silicon and germanium that are semiconductors, and other devices like sola…
Electrophoresis is a technique used for the separation of biological molecules based on their movement due to the influence of a direct electric current. The technique was pioneered in 1937 by the Swedish chemist Arne Tiselius for the separation of proteins. It has now been extended to the separation of many other different classes of biomolecules including nucleic acids, carbohydrates and amino a…
Electrostatics is the study of the behavior of electric charges that are at rest. The phenomenon of static electricity has been known for well over 2,000 years, and a variety of electrostatic devices have been created over the centuries. In the late sixteenth century, William Gilbert (1544-1603) began experimenting with static electricity, pointing out the difference between static electric attrac…
A chemical element is a substance made up of only one kind of atom (atoms having the same atomic number). A compound, on the other hand, is made up of two or more kinds of atom combined together in specific proportions. The atomic number of an element is the number of protons found in the nucleus of each atom of that element; the number of protons in the nucleus equals the number of electrons that…
A family of chemical elements usually consists of elements that are in the same group (the same column) on the periodic table. The term is also applied to certain closely related elements within the same period (row). Just as the individual members in a human family are all different but have common characteristics, like hair color, so to do the elements in a chemical family have certain propertie…
A transuranium (beyond uranium) element is any of the chemical elements with atomic numbers higher than 92, which is the atomic number of uranium. Ever since the eighteenth century when chemists began to recognize certain substances as chemical elements, uranium had been the element with the highest atomic weight; it had the heaviest atoms of all the elements that could be found on Earth. The gene…
Elements are identified by the nuclei of the atoms of which they are made. For example, an atom having six protons in its nucleus is carbon, and one having 26 protons is iron. There are over 80 naturally occurring elements, with uranium (92 protons) being the heaviest (heavier nuclei have been produced in reactors on Earth). Nuclei also contain certain neutrons, usually in numbers greater than the…
Elephants are large, four-legged, herbivorous mammals. They have a tough, almost hairless hide, a long flexible trunk, and two ivory tusks growing from their upper jaw. Only two species of elephant exist today, the African elephant (Loxodonta africana) and the Asian or Indian elephant (Elephas maximus), both of which are threatened or endangered. African elephants are the largest of all land anima…
Elephant shrews are relatively small mammals in the family Macroscelididae, order Macroscelidea. Elephant shrews have a characteristic long, narrow snout that is broad at the base, and very sensitive and flexible but not retractile. This snout is movable in circular manner at the base, and has nostrils at the end. There are five genera with some 185 species of elephant shrews, living in continenta…
Elephant snout fish belong to a diverse group of fishes that comprise the family Mormyridae. All are freshwater species that are confined to tropical parts of the African continent. Some 150 species have been described so far. The group takes its common or English name from the animals' extended snout. This adaptation is taken to the extreme in the genus Gnathonemus which has a pendulous, t…
Elephantiasis is an extreme symptom of human infection by a type of roundworm or nematode. It involves massive swelling of a limb or of the scrotum. The leg of an individual suffering from elephantiasis can become enlarged to two or three times normal diameter. The actual name of the disease or infection which causes elephantiasis is lymphatic filariasis. Lymphatic filariasis is an important paras…
An elevator is an enclosed car that moves in a vertical shaft between the multi-story floors of a building carrying passengers or freight. All elevators are based on the principle of the counterweight, and modern elevators also use geared, electric motors and a system of cables and pulleys to propel them. The world's most often used means of mechanical transportation, it is also the safest.…
An ellipse is a kind of oval. It is the oval formed by the intersection of a plane and a right circular cone-one of the four types of conic sections. The other three are the circle, the hyperbola, and the parabola. The ellipse is symmetrical along two lines, called axes. The major axis runs through the longest part of the ellipse and its center, and the minor axis is perpendicular to the major axi…
Elms are trees (occasionally shrubs) of flowering plants in the genus Ulmus. Elm leaves possess stipules, and often have a nonsymmetrical leaf, that is, one half is larger than the other so that the bottom ends do not meet where they are attached to the mid-rib. Elms flower in the spring. Their flowers lack petals, form reddish brown clusters in the tops of the trees, appear before the leaves have…
Embiids are small, cylindrical, soft-bodied insects in the order Embioptera that spin tubular galleries of silk, an ability that gives them the common name web-spinners. They have chewing mouthparts, and undergo paurometabolism, or gradual metamorphosis, exhibiting a definite egg, nymph, and adult stage. In the phylogeny, or evolutionary history of the class Insecta, embiids are thought to be most…
An embolism is the sudden blockage of a blood vessel by a blood clot that has been brought to that location by the bloodstream. The clot, called an embolus, from the Greek word meaning plug, is a blood clot that has formed inside the circulatory system and is floating in the bloodstream. It will remain on the move until it encounters a blood vessel too small for it to fit through, where it will pl…
An embryo is a stage directly after fertilization that signifies the early stages of growth and development of an organism. In humans, this stage ends during the third month of pregnancy, and is then called a fetus. Plants and invertebrate as well as vertebrate animals have an embryonic stage of development. For example, the embryo of the common North American leopard frog, Rana pipiens is from a …
Developments in reproductive technology are occurring at a rapid rate in animal science as well as in human biology. In vitro fertilization, embryo culture, preservation of embryos by freezing (cryopreservation), and cloning technology yield embryos that are produced outside of the female reproductive system. Embryo transfer permits continued survival of embryos by insertion into the female reprod…
Embryology is the study of the development of organisms. This is as true of plants as it is of animals. Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat, and another part sometimes called the endosperm. While plants are extraordinarily important for survival of animal life, animal embryology is described here. The dictionary definitio…
In the context of ecology and environmental science, the word emission generally refers to a release of a A nickel smelting facility emitting pollutants that carry for hundreds of miles in Russian Siberia. Photograph by Josef Polleross. Stock Market. Reproduced by permission. substance or energy to the environment. Often, emissions refer to substances or energy that are ecological stresso…
Emphysema is an incurable lung disease that results in the destruction of air sacs in the lungs. It is brought about almost exclusively by smoking. In the past, the majority of its victims were male, but the disease has become more common in women as more women smoke. Emphysema is also called chronic obstructive pulmonary (or lung) disease (COPD or COLD). Chronic bronchitis, that is, inflammation …
An emulsion is a two phase system of immiscible liquids in which one liquid is dispersed in the other in the form of microscopic droplets. This dispersion is achieved through the use of emulsifying agents, known as surfactants, which act as chemical bridges between the two liquids. Emulsions provide a variety of benefits such as the ability to deliver water insoluble active materials from water ba…
Encephalitis is an inflammatory disease of the brain. It is caused by a virus that either has invaded the brain, or a virus encountered elsewhere in the body that has caused a sensitivity reaction in the brain. Most cases of encephalitis are considered secondary to a virus infection that stimulates an immune reaction. An infection that involves the membranes associated with the spinal cord is call…
An endangered species of plant, animal, or microorganism is at risk of imminent extinction or extirpation in all or most of its range. Extinct species no longer occur anywhere on Earth, and once gone they are gone forever. Extirpated species have disappeared locally or regionally, but still survive in other regions or in captivity. Threatened species are at risk of becoming endangered in the fores…
Endemic is a biogeographic term referring to a distinct race or species that originated in a local place or region, and that has a geographically restricted distribution. Endemic species tend to occur in certain ecological contexts, being especially frequent in places that are ecologically isolated, and that have not been affected by a regional-scale, catastrophic disturbance for a very long time.…
The endocrine system is the body's network of nine glands and over 100 hormones which maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes: in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glan…
The phylum Endoprocta is a group of about 60 species that closely resemble moss animals or members of the phylum Bryozoa. With the exception of the genus Urnatella all endoprocts are marine-dwelling species. Like moss animals they are sessile, being attached to a wide range of submerged objects such as rocks, shells, sponges, corals, and other objects. These tiny animals—the largest measure…
Endoscopy is the use of a thin, lengthy, flexible scope that can be inserted into the body for the diagnosis and treatment of various conditions. Until the last third of the twentieth century, one of the limiting factors in the treatment of internal injuries or diseases was the need to perform open surgery on the patient. That meant putting him under anesthesia, carrying out the operation, sewing …
The term endothermic has two distinct meanings, depending on the context in which it is used. In chemistry, endothermic means that a chemical reaction or phase transition absorbs heat. (A phase transition is the transformation of matter from a gas, liquid, or solid into a different one of these states.) In physiology, the term endothermic refers to organisms which metabolically generate heat to ma…
Energy is a state function commonly defined as the capacity to do work. Since work is defined as the movement of an object through a distance, energy can also be described as the ability to move an object through a distance. As an example, imagine that a bar magnet is placed next to a pile of iron filings (thin slivers of iron metal). The iron filings begin to move toward the iron bar because magn…
An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components. Solar electromagnetic radiation is the major input of energy to Earth. This external sourc…
Energy efficiency refers to any process by which the amount of useful energy obtained from some process is increased compared to the amount of energy put into that process. As a simple example, some automobiles can travel 40 mi (17 km) by burning a single gallon (liter) of gasoline, while others can travel only 20 mpg (8.5 km/l). The energy efficiency achieved by the first car is twice that achiev…
Energy transfer describes the changes in energy (a state function) that occur between organisms within an ecosystem. Living organisms are constantly changing as they grow, move, reproduce, and repair tissues. These changes are fueled by energy. Plants, through photosynthesis, capture some of the Sun's radiant energy and transform it into chemical energy, which is stored as plant biomass. Th…
Engineering is the art of applying science, mathematics, and creativity to solve technological problems. The accomplishments of engineering can be seen in nearly every aspect of our daily lives, from transportation to communications, and entertainment to health care. And, although each of these applications is unique, the process of engineering is largely independent. This process begins by carefu…
Engraving and etching are processes used to make intaglio prints. An intaglio print is made from a plate, usually a metal one, which has been had lines drawn into its surface. These lines trap ink when the ink is rolled across the surface of the plate. When the surface of the plate is wiped with a cloth, the lines retain their ink. A piece of damp paper is placed on the plate, and the two are run …
Enterobacteria are bacteria from the family Enterobacteriaceae, which are primarily known for their ability to cause intestinal upset. Enterobacteria are responsible for a variety of human illnesses, including urinary tract infections, wound infections, gastroenteritis, meningitis, septicemia, and pneumonia. Some are true intestinal pathogens; whereas others are merely opportunistic pests which at…
Entropy is a physical quantity that is primarily a measure of the thermodynamic disorder of a physical system. Entropy has the unique property in that its global value must always increase or stay the same; this property is reflected in the second law of thermodynamics. The fact that entropy must always increase in natural processes introduces the concept of irreversibility, and defines a unique d…
Ethics is a branch of philosophy that primarily discusses issues dealing with human behavior and character. Ethics attempts to establish a basis for judging right from wrong and good from bad. Environmental ethics employs concepts from the entire field of philosophy, especially aesthetics, metaphysics, epistemology, philosophy of science, and social and political philosophy in an effort to relate …
In the United States, the National Environmental Policy Act (NEPA) requires federal agencies to file an Environmental Impact Statement (EIS) for any major project or legislative proposal that may have significant environmental effects. According to the U.S. Environmental Protection Agency (EPA), "NEPA requires federal agencies to integrate environmental values into their decision making pro…
The introduction of the techniques of modern molecular biology, beginning in the 1970s, have made possible the genetic engineering of proteins. New proteins can be created and existing proteins altered. Enzymes are proteins that function in chemical reactions by making the reaction occur more easily than if the enzyme was absent. Enzymatic engineering includes a wide variety of techniques related …
Enzymes are biological catalysts, agents which increase the rate of chemical reactions without being used up in the reaction. They are proteins which possess special binding sites for a certain biochemicals. Weak binding interactions with the biochemical allow enzymes to accelerate specific reaction rates millions of times. Enzyme kinetics is the study of enzyme reactions and mechanisms. Enzyme in…
An epidemic is an outbreak of a disease among members of a specific population that exceeds the extent of occurrence of the disease normally found in that population. Epidemics affect those members of the population who do not have an acquired or inherent immunity to the disease. Although most epidemics are caused by infectious organisms, the term can be applied to an outbreak of any chronic disea…
Epidemiology is the study of the occurrence, frequency, and distribution of diseases in a given population. As part of this study, epidemiologists—scientists who investigate epidemics (widespread occurrence of a disease that occurs during a certain time)—attempt to determine how the disease is transmitted, and what are the host(s) and environmental factor(s) that start, maintain, and…
Though the cause of epilepsy remains unknown, the manner in which the condition is demonstrated indicates the area of the brain that is affected. Jacksonian seizures, for example, which are localized twitching of muscles, originate in the frontal lobe of the brain in the motor cortex. A localized numbness or tingling indicates an origin in the parietal lobe on the side of the brain in the sensory …
An episome is a portion of genetic material that can exist independent of the main body of genetic material (called the chromosome) at some times, while at other times is able to integrate into the chromosome. Examples of episomes include insertion sequences and transposons. Viruses are another example of an episome. Viruses that integrate their genetic material into the host chromosome enable the…
Epstein-Barr virus (EBV) is part of the family of human herpes viruses. Infectious mononucleosis (IM) is the most common disease manifestation of this virus which, once established in the host, can never be completely eradicated. Very little can be done to treat EBV; most methods can only alleviate resultant symptoms. Sleep and rest—complete bedrest in severe cases—is still the best …
Chemical equations reveal the chemical species involved in a particular reaction, the charges and weight relationships among them, and how much heat of reaction results. Equations tell us the beginning compounds, called reactants, and the ending compounds, called products, and which direction the reaction is going. Equations are widely used in chemical engineering, they serve as the basis for chem…
Chemical equilibrium is the final condition of a chemical reaction after the reacting substances have completey reacted. Depending on the reaction, they may reach this condition quickly or slowly, but eventually they will come to a condition in which there are definite, unchanging amounts of all the relevant substances. Chemical equilibrium is one of the most important features of chemical process…
The Latin meaning of equinox is "equal night," the times of the year when day and night are equal in length. In astronomy, the equinox is the point at which the Sun appears to cross the equator as a result of Earth's rotation around the Sun. The vernal equinox, which occurs as the Sun moves from south to north across the equator, takes place around March 21 and marks the begin…
Erosion is a group of processes that, acting together, slowly decompose, disintegrate, remove, and transport materials on the surface of Earth. Among geologists, there is no general agreement on what processes to include as a part of erosion. Some limit usage to only those processes that remove and transport materials. Other geologists also include weathering (decomposition and disintegration). Th…
Error is the amount of deviation in a physical quantity that arises as a result of the process of measurement or approximation. Another term for error is uncertainty. Physical quantities such as weight, volume, temperature, speed, or time must all be measured by an instrument of one sort or another. No matter how accurate the measuring tool—be it an atomic clock that determines time based o…
Escherichia coli is one of the most well-known and intensively studied bacteria. Often shortened to E. coli, the bacterium was discovered in 1885 by the German bacteriologist Dr. Theodor Escherich. Initially, the bacterium was termed Bacterium coli, but later the name was changed to honor Dr. Escherich. Escherichia coli inhabits the intestinal tract of humans and other warm-blooded mammals. It con…
Ester is an organic functional group that forms many sweet-smelling compounds. The chemical structure of an ester is represented by the general formula, R-CO-OR', where a central carbon atom that has two bonds to an oxygen atom (the carbonyl group), C=O, a single bond to another carbon atom represented by R, and a single bond to an oxygen atom connected to a carbon atom represented by R�…
Esterification is the chemical process for making esters, which are compounds of the chemical structure R-COOR', where R and R' are either alkyl or aryl groups. The most common method for preparing esters is to heat a carboxylic acid, R-CO-OH, with an alcohol, R'-OH, while removing the water that is formed. A mineral acid catalyst is usually needed to make the reaction occur a…
Ethanol is an alcohol fuel that is manufactured by fermenting and distilling crops with a high starch or sugar content, such as grains, sugarcane, or corn. In the energy sector, ethanol can be used for space and water heating, to generate electricity, and as an alternative vehicle fuel, which has been its major use to date. Worldwide, ethanol is the mostly widely used alternative liquid fuel. Etha…
Ether was first used as an anesthetic to kill pain by W. T. G. Morton (1819-1868), a Boston dentist. Morton had learned about ether from a chemist named Charles T. Jackson (1805-1880). Eventually Morton convinced Dr. J. C. Warren (1778-1856) to let him use ether as an anesthetic on one of his patients. In Massachusetts General Hospital on October 16, 1846, Morton put a Mr. Abbott to sleep with eth…
Ethnoarchaeology, a subfield of archaeology, is the study of contemporary cultures in order to interpret social organization within an archeological site. Traditionally, archaeology has been concerned with the identification, classification, and chronological ordering of remains. Archaeologists were able to describe a civilization according to its artifacts, but not to fully understand its culture…
Ethnobotany is the study of the relationships between plants and people. Most often, however, the term is used in relation to the study of the use of plants by aboriginal people living relatively simple, pre industrial lifestyles. Plants have always played a central role within indigenous cultures. Plant products are used as food, as sources of medicine, and as raw materials for the weaving of fab…
Ethane is a gas at room temperature and burns very easily. The word ethane is derived from aithein, the Greek word for to blaze or to kindle. Ethane makes up about 15% of natural gas and can also be isolated from crude oil. It is used by industries for the production of ethylene and ethyl chloride. The incorporation of an ethyl group or chain of two carbon atoms into a molecule's structure …
Ethylene glycol is an organic (carbon based) molecule most widely used as antifreeze in automobile engines and as an industrial solvent, a chemical in which other substances are dissolved. The addition of ethylene glycol to water raises the boiling point of the engine coolant and reduces the chances of a car's radiator "boiling over." The name ethylene glycol communicates much…
Ethylenediaminetetra-acetic acid, typically shortened to EDTA, is a chemical compound with the ability to form multiple bonds with metal ions, making it an important chemical to analytical scientists and industry alike. The compounds used to create EDTA include ethylenediamine, formaldehyde, and sodium cyanide. When these compounds are mixed in an appropriate fashion, a series of chemical reaction…
Etiology is study of the cause of disease. More specifically, etiology is the sum of knowledge regarding a disease, or knowledge of all pathological processes leading to a disease. There may be just one causative agent, but no one cause of disease. For example, as stated in Boyd's Introduction To The Study Of Disease, we know tuberculosis is caused by the tubercle bacillus. Many people are …
The eubacteria are the largest and most diverse taxonomic group of bacteria. Some regard this as an artificial assemblage, merely a group of convenience rather than a natural grouping. The eubacteria are all easily stained, rod-shaped or spherical bacteria. They are generally unicellular, but a small number of multicellular forms do occur. They can be motile or non-motile and the motile forms are …
Eugenics is the study of improving the human race by selective breeding. Its rationale is to remove bad or deleterious genes from the population, increasing the overall fitness of humanity as a result. Campaigns to stop the criminal, the poor, the handicapped, and the mentally ill from passing on their genes were supported in the past by such people as British feminist Marie Stopes and Irish playw…
Eukaryotae, or eukaryotic cells, are large and complex cells bounded by an outer plasma membrane. They contain many organelles within their cytoplasm and a nucleus separated from the cytoplasm by the nuclear membrane. Fossils of eukaryotic cells are present in rocks dated as 1.5 billion years old. All living things on Earth, except bacteria and blue-green algae (cyanobacteria), which are Prokaryot…
The continent of Europe is a landmass bounded on the east by the Ural mountains, on the south by the Mediterranean Sea, and on the north and west by the Arctic and Atlantic Oceans. Numerous islands around this landmass are considered a part of Europe. Europe is also the westernmost part of the Eurasian supercontinent (several continental masses joined together). Europe holds a unique place among t…
The process of heightened biological productivity in a body of water is call eutrophication. The major factors controlling eutrophication in a body of water, whether large, small, warm, cold, fast-moving, or quiescent, are nutrient input and rates of primary production. Not all lakes experience eutrophication. Warmth and light increase eutrophication, (which in Greek means "well nourished&#…
Evaporation is a process that is commonly used to concentrate an aqueous solution of nonvolatile solutes and a volatile solvent. In evaporation, a portion of the solvent is vaporized or boiled away, leaving a thick liquid or solid precipitate as the final product. The vapor is condensed to recover the solvent or it can simply be discarded. A typical example is the evaporation of brine to produce s…
Evapotranspiration refers to the vaporization of water from both non-living and living surfaces on a landscape. Evapotranspiration is a composite of two words: evaporation and transpiration. Evaporation refers to the vaporization of water from surface waters such as lakes, rivers, and streams, from moist soil and rocks, and any other substrates that are non-living. Transpiration refers to the vapo…
An even number is an integer that is divisible by 2 and an odd number is an integer that is not divisible by 2. Thus 2, 4, 6, 8... are even numbers and 1, 3, 5, 7... are odd numbers. Any even number can be written as 2n where n is an integer and any odd number can be written as 2n + 1 or as 2n - 1, where n is an integer. The sum, difference, or product of even numbers is an even number. On the oth…
In early 2001, data gathered by NASA's Hubble Space Telescope and the Chandra X-ray Observatory independently provided strong evidence of an event horizon, the observable boundary region surrounding an unobservable black hole. The size of the event horizon surrounding a black hole is termed the Schwarzschild radius, named after the German astronomer Karl Schwarzschild (1873–1916), wh…
Evolution refers to biological change. Biological evolution involves change in the genetic constitution of populations over time such that complexity is achieved due to the formation of new genes or gene-encoded functions rather than harmful mutations. These changes are passed on from parents to their offspring, but biological evolution does not involve individual organisms. Individuals develop, b…
Convergent evolution represents a phenomenon when two distinct species with differing ancestries evolve to display similar physical features. Environmental circumstances that require similar developmental or structural alterations for the purposes of adaptation can lead to convergent evolution even though the species differ in descent. These adaptation similarities that arise as a result of the sa…
Divergent evolution occurs when a group from a specific population develops into a new species. In order to adapt to various environmental conditions, the two groups develop into distinct species due to differences in the demands driven by the environmental circumstances. A good example of how divergent evolution occurs is in comparing how a human foot evolved to be very different from a monkey&#x…
Evidence of evolution can be observed in numerous ways, including distribution of species (both geographically and through time), comparative anatomy, taxonomy, embryology, cell biology, molecular biology, and paleontology. The English naturalist Charles Darwin (1809–1882) formulated the theory of evolution through natural selection in his ground breaking publication The Origin of Species b…
Parallel evolution occurs when unrelated organisms develop the same characteristics or adaptive mechanisms due to the nature of their environmental conditions. Or stated differently, parallel evolution occurs when similar environments produce similar adaptations. The morphologies (or structural form) of two or more lineages evolve together in a similar manner in parallel evolution, rather than div…
Rates of evolution change vary widely over time, among characteristics, and among species. Evolutionary change can be estimated by examining fossils and species that are related to each other. The rate of change is governed by the life span of the species under examination, short-lived species are capable of changing more quickly than those that have a longer life span and reproduce less often. Ye…
Evolution is the process of biological change over time. Such changes, especially at the genetic level are accomplished by a complex set of evolutionary mechanisms that act to increase or decrease genetic variation. Evolutionary theory is the cornerstone of modern biology, and unites all the fields of biology under one theoretical umbrella to explain the changes in any given gene pool of a populat…
Archeological excavation involves the removal of soil, sediment, or rock that covers artifacts or other evidence of human activity. Early excavation techniques involved destructive random digging and removal of objects with little or no location data recorded. Modern excavations often involve slow, careful extraction of sediments in very thin layers, detailed sifting of sediment samples, and exact…
The Pauli exclusion principle states that no two electrons in the same atom can have the same set of quantum numbers. The principle was first stated by the great Austrian-Swiss physicist Wolfgang Pauli (1900-1958) in 1925. …
The excretory system removes cellular wastes and helps maintain the salt-water balance in an organism. In providing these functions, excretion contributes to the body's homeostasis, the maintenance of constancy of the internal environment. When cells break down proteins, they produce nitrogenous wastes, such as urea. The excretory system serves to remove these nitrogenous waste products, as…
By definition, exercise is physical activity that is planned, structured, and repetitive for the purpose of conditioning any part of the body. Exercise is utilized to improve health, maintain fitness and is important as a means of physical rehabilitation. Exercise is used in preventing or treating coronary heart disease, osteoporosis, weakness, diabetes, obesity, and depression. Range of motion is…
Glands in the human body are classified as exocrine or endocrine. The secretions of exocrine glands are released through ducts onto an organ's surface, while those of endocrine glands are released directly into the blood. The secretions of both types of glands are carefully regulated by the body. Exocrine gland secretions include saliva, perspiration, oil, earwax, milk, mucus, and digestive…
Explosives are substances that produce violent chemical or nuclear reactions. These reactions generate large amounts of heat and gas in a fraction of a second. Shock waves produced by rapidly expanded gasses are responsible for much of the destruction seen following an explosion. The power of most chemical explosives comes from the reaction of oxygen with other atoms such as nitrogen and carbon. T…
Exponents have a number of useful properties: Property (6) shows how the operation of exponentiation is extended to the rational numbers. Note that unit-fraction exponents, such as 1/3 or 1/2, are simply roots; that is, 125 to the 1/3 power is the same as the cube root of 125, while 49 to 1/2 power is the same as the square root of 49. By keeping properties (1) through (6) as centra…
Extinction is the death of all members of a species and thus, of the species itself. Extinction may occur as a result of environmental changes (natural or human-caused) or competition from other organisms. A species confronted by environmental change or competitors may (1) adapt behaviorally (e.g., shift to a different diet), (2) adapt by evolving new characteristics, or (3) die out. At the presen…
The eye is the organ of sight in humans and animals which transforms light waves into visual images and provides 80% of all information received by the human brain. These remarkable organs are almost spherical in shape and are housed in the orbital sockets in the skull. Sight begins when light waves enter the eye through the cornea (the transparent layer at the front of the eye) pass through the p…
