Pheasants are large species of fowl in the family Phasianidae, which also includes the partridges, peafowl, guinea fowl, francolins, and quail. The greatest diversity of pheasants occurs in Asia, but native species also occur in Africa and Europe. In addition, many species of pheasants have been widely introduced as gamebirds beyond their natural range. Pheasants are also kept as handsome showbird…
In the left-hand representations, the benzene molecule is drawn as a hexagon with alternating single and double bonds. This is a shorthand method of drawing compounds used by chemists. A carbon atom is represented by an intersection of two straight lines or at the end of a line and the correct number of hydrogen atoms to give each carbon atom four bonds is implied but not drawn. The two equivalent…
Phenylketonuria (PKU) is an inherited disorder in which an enzyme (usually phenylalanine hydroxylase) crucial to the appropriate processing of the amino acid, phenylalanine is totally absent or drastically deficient. The result is that phenylalanine cannot be broken down, and it accumulates in large quantities throughout the body. Normally, phenylalanine is converted to tyrosine. Because tyrosine …
Pheromones are volatile chemical compounds secreted by insects and animals. They act as chemical signals between individuals influencing physiology and behavior in a manner similar to hormones. Pheromones are important to a variety of behaviors including mate attraction, territorality, trail marking, danger alarms, and social recognition and regulation. The term pheromone is derived from the Greek…
Phloxes (Phlox spp.) are a group of about 50 species of flowering plants in the family Polemoniaceae, which contains about 300 species in total. Phloxes are herbaceous plants with bright, showy flowers. Each flower has five red, pink, or white petals that are fused at their bases to form a tube, but remain separate at the top of the structure. These flowers are arranged in very attractive groups, …
A phobia is a group of symptoms brought on by an object or situation that causes a person to feel irrational fear. For example, a person terrified by a snake poised to strike only a few feet away on a hiking trail experiences normal fear, while a person terrified by a snake in a glass cage would be said to be having a phobic reaction. A person suffering from a phobia may dwell on the object of his…
The first practical device for recording and reproducing sound was developed by Thomas A. Edison in 1877. He called his device a phonograph, meaning sound writer, because of the crude, mechanically cut impressions, or "writing," it made on the surface of the recording cylinder. The sound reproduction was equally crude. Since the time of Edison's phonograph, the quest for more …
Phoronids are a small group of tube-dwelling marine worms that comprise the phylum Phoronidae. Some 15 species have so far been described. All phoronids are exclusively marine-dwelling and live in shallow waters up to a depth of about 195 ft (60 m) in both tropical and temperate oceans. They are thought to be related to moss animals (phylum Bryozoa) and lamp shells (phylum Brachiopoda). They may o…
Phosphoric acid is a weak acid, with only a small percentage of the molecules in solution ionizing. Phosphoric acid is manufactured by the reaction of sulfuric acid upon phosphate rocks (commonly found in Florida), most notably calcium phosphate, as shown below: The other product of the reaction, calcium sulfate dihydrate is gypsum and is used in drywall in the construction industry. Alternat…
Phosphorus is a chemical element with the atomic number 15 and atomic weight 30.9738. Phosphorus forms the basis of a large number of compounds, by far the most environmentally important of which are phosphates. All plants and animals need phosphates for growth and function, and in many natural waters the production of algae and higher plants is limited by the low natural levels of phosphorus. As …
We live in a world that is constantly recycling materials. All life is composed of the very same matter that exists in the non-living, or abiotic, world. The elements that are found in living things, like carbon, hydrogen, and calcium are also found in abioic compounds of the environment, like soil or rock. Because the quantities of usable sources of materials and elements that compose the living …
Phosphorus (usually in the form of phosphate) is a normal part of the environment. It occurs in the form of phosphate containing rocks and as the excretory and decay products of plants and animals. Human contributions to the phosphorus cycle result primarily from the use of phosphorus-containing detergents and fertilizers. The increased load of phosphorus in the environment as a result of human ac…
The photic zone, also called the euphotic or limnetic zone, is the volume of water where the rate of photosynthesis is greater than the rate of respiration by phytoplankton. Phytoplankton are microscopic plants living suspended in the water column that have little or no means of motility. They are primary producers that use solar energy as a food source. The compensation point, where photosynthesi…
Photochemistry is the study of light-induced chemical reactions and physical processes. A photochemical event involves the absorption of light to create an excited species that may subsequently undergo a number of different reactions. These include unimolecular reactions such as dissociation, ionization, and isomerization; bimolecular reactions, which involve a reaction with a second molecule or a…
Photocopying is the process by which light is used to make copies of book pages and other paper documents. Today the most widely used form of photocopying is xerography ("dry writing"), invented by New York patent attorney Chester Carlson in the 1930s. Indeed, the name of the company founded to develop Carlson's invention, Xerox Corporation, has become synonymous with the proc…
During the latter half of the nineteenth century many scientists and engineers were simultaneously observing a strange phenomenon: electrical devices constructed from certain metals seemed to conduct electricity more efficiently in the daytime than at night. This phenomenon, called the photoelectric effect, had been noted years earlier by the French physicist A. E. Becquerel (1820-1891), who had i…
The process in which visible light, x rays, or gamma rays incident on matter cause an electron to be ejected. The ejected electron is called a photoelectron. …
Photography is the art and science of creating images using light. For most of its history, this has usually meant using silver compounds that darken when exposed to light. With the growth of computers, photography can also be done with electronics that measure light intensities and create images based on them. The invention and perfection of photography has affected many areas of life. Of course,…
Like all other forms of information, photographs and images have entered the electronic age. In 1981, the Sony Corporation unveiled its filmless, electronic camera (termed a still video camera) the Mavica. Mavica is an acronym for Magnetic Video Camera; it uses a still video system to record 50 analog images on a diskette. Although they are recorded on a diskette, they are not digital images. The …
The photon is the basic unit, particle, or carrier of light. The visible light that we see, the x rays that dentists use, and the radio waves that carry music to our radios are all forms of electromagnetic radiation. Other forms include the microwaves which we use to cook food and gamma rays which are produced when radioactive elements disintegrate. Although they seem quite different, all types o…
The overall equation in photosynthetic bacteria is similar, although not identical. In the light reactions of photosynthesis, light energy excites photosynthetic pigments to higher energy levels and this energy is used to make two high energy compounds, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). ATP and NADPH do not appear in the overall equation for…
Phototropism is the orientation of an organism in response to asymmetric illumination. Phototropism is commonly observed in the stems of higher plants, which grow bent toward a light source. Phototropism can be positive (bending toward a light source) or negative (bending away from a light source), depending on the organism and nature of the illumination. Phototropism and other tropisms are differ…
A photovoltaic cell, often called a solar cell, converts the energy in light directly into electrical potential energy using a physical process called the photovoltaic effect. Photovoltaic cells are used to produce electricity in situations where they are more economical than other power generation methods. Occasionally, they are used as photodetectors. The photovoltaic effect has been known since…
Phylogeny is the inferred evolutionary history of a group of organisms. Paleontologists are interested in understanding life through time—not just at one time in the past or present, but over long periods of past time. Before they can attempt to reconstruct the forms, functions, and lives of once-living organisms, paleontologists have to place these organisms in context. The relationships o…
Physical therapy is a medical specialty that provides treatment using various devices or the hands to strengthen muscles and supply flexibility to a part of the body that is subnormal. The need for physical therapy can be the result of a genetic condition, disease, surgery, or a trauma such as a burn or automobile accident. The goal of physical therapy is not necessarily to restore normality but t…
Physics is the science that deals with matter and energy and with the interaction between them. Physics, from which all other sciences derive their foundation, were the first attempts to provide rational explanations for the structure and workings of the Universe. Even in the earliest civilizations, physics allowed a mechanism to understand and quantify nature. An axiom among physicists—sin…
Physiology is the study of how various biological components work independently and together to enable organisms, from animals to microbes, to function. This scientific discipline covers a wide variety of functions from the cellular and subcellular level to the interaction of organ systems that keep more complex biological machines, like humans, running. Physiological studies are aimed at answerin…
While anatomy is the study of the structures of an organism, physiology is the science dealing with the study of the function of an organism's component structures. However, it often is not enough to know what an organ, tissue, or other structure does. Physiologists want to know how something functions. For example, physiological questions might ask: What is the function of human lung tissu…
Phytoplankton are microscopic, photosynthetic organisms that float in the water of the oceans and bodies of freshwater (the word phytoplankton is derived from the Greek for "drifting plants"). The most abundant organisms occurring within the phytoplankton are algae and blue-green bacteria, but this group also includes certain kinds of protists (especially protozoans) that contain sym…
Pi is one of the most fundamental constants in all of mathematics. It is normally first encountered in geometry where it is defined as the ratio of the circumference of a circle to the diameter: π = C/d where C is the circumference and d is the diameter. This fact was known to the ancient Egyptians who used for π the number 22/7 which is accurate enough for many applications. A clo…
Pigeons and doves include about 300 species of birds in the family Columbidae. Most species are found in forests of various types, with fewer species occurring in more open habitats. By far the greatest richness of species of pigeons and doves occurs in moist tropical and sub-tropical forests. Many tropical oceanic islands have endemic species of pigeons and doves that evolved in isolation. Many o…
Pigs, hogs, or swine consist of about eight species of mammals in the family Suidae, which is part of the order Artiodactyla, the cloven-hoofed ungulates. Pigs are closely related to the peccaries (family Tayassuidae) and hippopotamuses (family Hippopotamidae). The natural distribution of pigs includes Africa, Europe, and Asia, but one species, the domestic pig (Sus scrofa), is now found almost wo…
Pike are large carnivorous species of bony fish in the genus Esox in the family Esocidae. Pike occur in static and slowly flowing fresh-water habitats, throughout most of Europe, northern Asia, and North America. Pike have a relatively long, streamlined, fusiform body, adapted to swimming in rapid bursts to catch their prey of smaller fish (including other pike), amphibians, crayfish, small mammal…
On December 18, 1912, Charles Dawson (1865–1916) announced to the Geological Society in London that he had discovered skull fragments and a partial jaw in a gravel formation in Piltdown Common, Fletching, near Lewes, Sussex, England. The skull fragments were accompanied by bones of relatively recent hippopotamus, deer, beaver, and horse, as well as ancient bones of extinct mastodon and rhin…
A pinecone fish has a plump, deep body, measuring about 5 in (12.7 cm) long. The body is covered by heavy, platelike scales that overlap, giving the fish the appearance of a pinecone-hence its name. Under each pinecone fish's lower jaw, there are two phosphorescent organs, giving the impression that the fish itself produces light. The light is actually produced by luminous bacteria that hav…
The pines are species of trees in the genus Pinus, of the family Pinaceae and phylum Coniferophyta, the cone-bearing plants (conifers). Relatives of the pines include other conifers such as fir, Douglas fir, spruce, hemlock, cypress, and redwood. Pines and these other conifers are all considered gymnosperms, because they bear their seeds naked, rather than within an ovary as in the angiosperms (fl…
Pipefish (family Syngnathidae) are slim, elongate fish with large heads and extended, tubular mouths. The extended snout frequently measures more than half of the total head length. The body is enclosed in a tough, segmented skin and the fins, with the exception of the A pipefish (Sygnathus sp.) swimming through the water. Photograph by Herve Chaumeton. Photo Researchers, Inc. Reproduced by p…
In medicine, especially in clinical trials conducted for medical research, a placebo is a substance used as a control in a double-blind study. Half of a group of test subjects are given a medicinal substance being investigated, while the other half is administered an inert material, like a sugar pill, made to look indistinguishable from the medicine. In the optimal double-blind test, neither the r…
At the beginning of the twentieth century, German physicist, Maxwell Planck, proposed that atoms absorb or emit electromagnetic radiation only in certain units or bundles of energy termed quanta. The concept that energy existed only in discrete and defined units seemed counter-intuitive, that is, outside the human experience with nature. Accepting his experimental results regarding the radiation e…
Generally, the term plane, together with point, line, and solid, is considered an undefined term. Every definition in mathematics attempts to use simpler and better understood terms to define more complex ones. As the terms to be defined become ever simpler, this eventually becomes impossible. The simplest terms are so well understood that there is little sense in attempting a formal definition, s…
The Plane family is a family of trees and large shrubs known to botanists as the Platanaceae. This family has a single genus, Platanus, and 7-10 different species. The two most familiar species are the American sycamore (Platanus occidentalis), which is native to eastern and central United States, and the London plane, a hybrid tree species which is commonly planted as an ornamental in the United …
A planet is a relatively cold body that orbits a star. Planets are thought to have formed from the same gas and dust that condensed to make the parent star. They can be seen by eye and telescope because of the light they reflect from their star. The planets themselves often have orbiting moons and dust rings. The nine planets in our solar system that are in elliptical orbits near the ecliptic plan…
Is there another planet beyond Pluto? Prior to 1781 that question could have been asked in regard to Saturn. In that year, Sir William Herschel discovered Uranus, after detecting what he believed to be a comet. Calculations to determine the orbit of Uranus were made, and the planet was found to conform to the "law" of planetary distances suggested by Johann Elert Bode (1747-1826). Ho…
The term planetary atmosphere refers to the envelope of gases that surrounds any of the planets in our solar system. A complete understanding of the properties of a planet's atmosphere involves a number of different areas including atmospheric temperatures, chemical composition of the atmosphere, atmospheric structure, and circulation patterns within the atmosphere. The study of planetary a…
Planetary geology is a branch of geology devoted to the study of structure, composition, processes, and origin of major and minor planetary bodies in our solar system and beyond, and to the effects of interaction between planetary bodies within our solar system. Planetary geology interfaces with many other fields including astronomy, biology, chemistry, and physics. Planetary geologists work in th…
High-density interstellar dust or clouds are referred to as nebulae. These nebulae, both dark and luminous, are equally important since the chemical analyses of these objects contribute significantly to the study of cosmic abundances. Bright or incandescent nebulae, just as dark nebulae, are not self-luminous. It is the star or stars imbedded in these nebulae which produce the luminous objects and…
A peek at Saturn through a small telescope reveals the solar system's jewelry, a breathtaking system of rings. These rings consist of a large number of individual particles orbiting Saturn. The diameter of Saturn's ring system is about 167,670 mi (270,000 km), a little less than the distance between the earth and the Moon. Yet the rings are only a few hundred meters thick. Saturn has…
Plankton are organisms that live in the water column and drift with the currents. Bacteria, fungi, algae, protozoans, invertebrates, and some vertebrates are represented, some organisms spending only parts of their lives (e.g., larval stages) as members of the plankton. Plankton is a relative term, since many planktonic organisms possess some means by which they may control their horizontal and/or…
A plant is an organism in the kingdom Plantae. According to the five-kingdom classification system used by most biologists, plants have the following characteristics: they are multicellular during part of their life; they are eukaryotic, in that their cells have nuclei; they reproduce sexually; they have chloroplasts with chlorophyll-a, chlorophyll-b and carotenoids as photosynthetic pigments; the…
Plant breeding began when early humans saved seeds and planted them. The cultural change from living as nomadic hunter-gatherers, to living in more settled communities, depended on the ability to cultivate plants for food. Present knowledge indicates that this transition occurred in several different parts of the world, about 10,000 years ago. Today, there are literally thousands of different cult…
Like human beings and other animals, plants are subject to diseases. In order to maintain a sufficient food supply for the world's population, it is necessary for those involved in plant growth and management to find ways to combat plant diseases that are capable of destroying crops on a large scale. There are many branches of science that participate in the control of plant diseases. Among…
A plant pigment is any type of colored substance produced by a plant. In general, any chemical compound which absorbs visible radiation between about 380 nm (violet) and 760 nm (ruby-red) is considered a pigment. There are many different plant pigments, and they are found in different classes of organic compounds. Plant pigments give color to leaves, flowers, and fruits and are also important in c…
Plasma is the liquid portion of blood which is about 90% water and transports nutrients, wastes, antibodies, ions, hormones, and other molecules throughout the body. Humans typically have about 1.3-1.5 gal (5-6 l) of blood, which is about 55% plasma and 45% cells-red blood cells, white blood cells, and platelets. The plasma of humans and other vertebrates is nearly colorless, since the red color o…
Plastic surgery is the specialized branch of surgery concerned with repairing deformities, correcting functional deficits, and enhancing appearance. Unlike most surgical specialties, plastic surgery is not confined to one specific anatomical or functional area of the body. Often, plastic surgery is classified as either reconstructive or aesthetic surgery. All plastic surgery procedures seek to res…
In the twentieth century, the term plastic has come to refer to a class of materials that, under suitable conditions, can be deformed by some kind of shaping or molding process to produce an end product that retains its shape. When used as an adjective, the term plastic (from Greek plastikos meaning to mold or form) describes a material that can be shaped or molded with or without the application …
Plate tectonics, is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of Earth's mantle). Plates move and shift their positions relative to one another. Movement of and contact between plates either directly or indirectly accounts for most of the major geologic features at Earth's sur…
The term platonic solids refers to regular polyhedra. In geometry, a polyhedron, (the word is a Greek neologism meaning many seats) is a solid bounded by plane surfaces, which are called the faces; the intersection of three or more edges is called a vertex (plural: vertices). What distinguishes regular polyhedra from all others is the fact that all of their faces are congruent with one another. (I…
The platypus is an egg laying mammal that is well adapted to the water. Physically, it looks like a mole or otter, with a beaver's flattened tail and a duck's bill. It also has short, powerful legs and webbed feet. While the fur on its back is dense, bristly, and reddish or blackish brown, the fur on its underbelly is soft and gray. Its eyes are very small, and it does not have exter…
Plovers are shore birds in the family Charadriidae, order Charadriiformes. Plovers have short, straight bills, with a small swelling towards the tip. Their wings are pointed at the tips, usually with a white wing-stripe on the underside, and the flight of these birds is fast and direct. Plovers and the closely related sandpipers (family Scolopacidae) are affectionately known as "peeps…
The ninth planet from the Sun, Pluto is one of the least well understood objects in the solar system. It is the smallest of the major planets, and has a most unusual orbit. Pluto's companion moon, Charon, is so large that the pair essentially form a binary system. How the Pluto-Charon system formed and how the system acquired its special 2-to-3 orbital resonance with Neptune are unanswered …
Pneumonia is an infection of the lung, and can be caused by nearly any class of organism known to cause human infections, including bacteria, viruses, fungi, and parasites. In the United States, pneumonia is the sixth most common disease leading to death, and the most common fatal infection acquired by already hospitalized patients. In developing countries, pneumonia ties with diarrhea as the most…
Podiatry is a medical specialty that focuses on the diagnosis and treatment of foot disease and deformity. The term is from the Greek word for foot (podos) and means "to heal the foot." Until recent years this specialty was called chiropody, literally meaning "to heal the hand and foot." References to physicians who treated abnormalities or injuries in the foot are foun…
A point is an undefined term in geometry that expresses the notion of an object with position but with no size. Unlike a three-dimensional figure, such as a box (whose dimensions are length, width, and height), a point has no length, no width, and no height. It is said to have dimension 0. Geometric figures such as lines, circles, planes, and spheres, can all be considered as sets of points. …
A point source is a situation where large quantities of pollutants are emitted from a single, discrete source, such as a smokestack, a sewage or thermal outfall into a waterbody, or a volcano. If the emissions from a point source are large, the environment will be characterized by strong but continuous gradients of ecological stress, distributed more-or-less concentrically around the source, and d…
A chemical is said to be a poison if it causes some degree of metabolic disfunction in organisms. Strictly speaking, a toxin is a poisonous chemical of biological origin, being produced by a microorganism, plant, or animal. In common usage, however, the words poison and toxin are often used interchangeably, and in this essay they are also treated as synonyms. It is important to understand that pot…
One of the several systems for addressing points in the plane is the polar-coordinate system. In this system a point P is identified with an ordered pair (r,θ) where r is a distance and θ an angle. The angle is measured counter-clockwise from a fixed ray OA called the "polar axis." The distance to P is measured from the end point O of the ray. This point is called the &…
The polar ice caps cover the north and south poles and their surrounding territory, including the entire continent of Antarctica in the south, the Arctic Ocean, the northern part of Greenland, parts of northern Canada, and bits of Siberia and Scandinavia also in the north. Polar ice caps are dome-shaped sheets of ice that feed ice to other glacial formations, such as ice sheets, ice fields, and ic…
There are three viruses responsible for the infectious disease now called poliomyelitis. It has been called infantile paralysis and is now commonly referred to as polio. While the disease usually afflicts young children, adults can succumb to it also. A notable example of polio in an adult was the case of President Franklin Delano Roosevelt, the thirty-second president of the United States. He con…
Pollen analysis, or palynology, is the study of fossil pollen (and to a lesser degree, plant spores) preserved in lake sediments, bog peat, or other matrices. Usually, the goal of palynology is to reconstruct the probable character of local plant communities in the historical past, as inferred from the abundance of plant species in dated potions of the pollen record. Palynology is a very important…
Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs of A honeybee becomes coated in pollen while gathering nectar and transports the pollen as it goes from flower to flower. Photograph by M. Ruckszis. Stock Market/Zefa Germany. Reproduced by permission. a plant, and it precedes fertilization, the fusion of the male and the female sex cell…
The term pollution is derived from the Latin pollutus, which means to be made foul, unclean, or dirty. Anything that corrupts, degrades, or makes something less valuable or desirable can be considered pollution. There is, however, a good deal of ambiguity and contention about what constitutes a pollutant. Many reserve the term for harmful physical changes in our environment caused by human actions…
Pollution control is the process of reducing or eliminating the release of pollutants into the enviroment. It is regulated by various environmental agencies which establish pollutant discharge limits for air, water, and land. Air pollution control strategies can be divided into two categories, the control of particulate emission and the control of gaseous emissions. There are many kinds of equipme…
Polybrominated biphenyls (or PBBs) are chemicals used to make plastics flame retardant. In Michigan in the early 1970s one type of PBB was accidentally mixed into livestock feed and fed to farm animals, resulting in the sickening and/or death of tens of thousands of animals. A large portion of Michigan's nine million residents became ill as a result of eating contaminated meat or poultry. P…
Polychlorinated biphenyls are a mixture of compounds having from one to 10 chlorine atoms attached to a biphenyl ring structure. There are 209 possible structures theoretically; the manufacturing process results in approximately 120 different structures. PCBs resist biological and heat degradation and were once used in numerous applications, including dielectric fluids in capacitors and transforme…
Polycyclic aromatic hydrocarbons, or polynuclear aromatic hydrocarbons, are a family of hydrocarbons containing two or more closed aromatic ring structures, each based on the structure of benzene. The simplest of these chemicals is naphthalene, consisting of two fused benzene rings. Sometimes there is limited substitution of halogens for the hydrogen of polycyclic aromatic hydrocarbons, in which c…
Polygons are closed plane figures bounded by three or more line segments. In the world of geometry, polygons abound. The term refers to a multisided geometric form in the plane. The number of angles in a polygon always equals the number of sides. Polygons are named to indicate the number of their sides or number of noncollinear points present in the polygon. A square is a special type of polygon, …
A polyhedron is a three-dimensional closed surface or solid, bounded by plane figures called polygons. The word polyhedron comes from the Greek prefix poly- , which means "many," and the root word hedron which refers to "surface." A polyhedron is a solid whose boundaries consist of planes. Many common objects in the world around us are in the shape of polyhedrons. The c…
Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they …
There are various words that are used in conjunction with polynomials. The degree of a polynomial is the exponent of the highest power of x. Thus the degree of The most general form for a polynomial in one variable is Similar definitions apply to polynomials in 3, 4, 5 ellipsevariables but the term "polynomial" without qualification usually refers to a polynomial in one var…
Poppies belong to a small family of flowering plants called the Papaveraceae. Poppies are annual, biennial, or perennial herbs, although three New World genera (Bocconia, Dendromecon, and Romneya) are woody shrubs or small trees. The leaves are alternate, lack stipules, and are often lobed or deeply dissected. The flowers are usually solitary, bisexual, showy, and crumpled in the bud. The fruit is…
The numbers of humans on Earth have increased enormously during the past several millennia, but especially during the past two centuries. By the end of the twentieth century, the global population of humans was 6.0 billion. That figure is twice the population of 1960, a mere 30 years earlier. Moreover, the human population is growing at about 1.5% annually, equivalent to an additional 89 million p…
The number of human beings on Earth has increased enormously during the past several millennia, but especially during the last two centuries: from 1850 to 1950 the human population doubled, from 1.265 billion to 2.516 billion, and has more than doubled from 1950 to the present. Moreover, it is likely that the human population—presently at over 6.215 billion—will continue to increase.…
Two families of rodents are called porcupines. They all have at least some hair modified into quills. The Old World porcupines belong to family Hystricidae of Europe, Asia, and Africa. The New World porcupines are 10 species of forest dwellers of the family Erethizontidae. The most common of these is the North American porcupine (Erthizon dorsatum). The name porcupine means "quill pig,�…
Positive numbers are commonly defined as numbers greater than zero, the numbers to the right of zero on the number line. Zero is not a positive number. The opposite, or additive inverse, of a positive number is a negative number. Negative numbers are always preceded by a negative sign (-), while positive numbers are only preceded by a positive sign (+) when it is required to avoid confusion. Thus …
Positron emission tomography (PET) is a scanning technique used in conjunction with small amounts of radiolabeled compounds to visualize brain anatomy and function. PET was the first scanning method to provide information on brain function as well as anatomy. This information includes data on blood flow, oxygen consumption, glucose metabolism, and concentrations of various molecules in brain tissu…
A postulate is an assumption, that is, a proposition or statement, that is assumed to be true without any proof. Postulates are the fundamental propositions used to prove other statements known as theorems. Once a theorem has been proven it is may be used in the proof of other theorems. In this way, an entire branch of mathematics can be built up from a few postulates. Postulate is synonymous with…
There have been many industrial applications of potassium aluminum sulfate. It is an important part of many products created by the pharmaceutical, cosmetic, and food industries because of its astringency property. It is also used in the manufacture of paper, dyes, glue, and explosives. Additionally, it helps in the water purification process, is used to speed up the hardening of concrete and plas…
Potassium nitrate, also known as saltpeter or niter, is a chemical compound consisting of potassium, nitrogen, and oxygen. While it has many applications, including use as a fertilizer, its most important usage historically has been as a component of gunpowder. Over time its use as an explosive has been made nearly obsolete by dynamite and TNT, but it is still used today in artilleryshell primers,…
The potato is a starchy, red or brown skinned, underground stem called a tuber. Tubers are storage areas for nutrient reserves of plants, such as starch or sugars. A widely cultivated species, the potato plant has the scientific name Solanum tuberosum and is a member of the nightshade family of plants, Solanaceae. Potato plants are widely grown for their familiar edible tubers that are a mainstay …
Man first began making pots at the end of the Stone Age (Neolithic Period), about 12,000 years ago in the Old World, and about 5,000 years ago in the New World. Basketry, including clay-lined baskets, probably served adequately for food storage for awhile. It may have been the accidental burning of a clay-lined basket that led to the discovery that clay, which is malleable when wet, becomes hard a…
A prairie is a natural vegetation type in which perennial herbaceous plants predominate, particularly species of grasses. The word "prairie" comes from the French prérie (later, prairie), meaning meadow. The term was first applied to the swath of mid-continental North American grassland in the 1600s by French Jesuit missionaries and explorers, because the landscape resembled, …
Prairie chickens are two North American species of birds in the grouse family (Phasianidae) in the order Galliformes, the game birds. Both the greater prairie chicken (Tympanuchus cupido) and the lesser prairie chicken (T. pallidicinctus) are brownish birds with a black band on the end of the tail. Male birds have colorful air sacs that are inflated during courtship and a ruff of long feathers tha…
Prairie dogs, or barking squirrels, are ground-dwelling herbivores in the genus Cynomys, in the squirrel family Sciuridae, order Rodentia. Prairie dogs are closely related to the ground squirrels, gophers, and marmots. Prairie dogs are widespread and familiar animals of the open, arid prairies, grasslands, and some agricultural landscapes of the western regions of North America. …
Falcons are very swift birds of prey that hunt during the day. Falcons are in the family Falconidae, of which there are 39 species, all in the genus Falco. The prairie falcon (Falco mexicanus) is a medium-sized, light-brown falcon that breeds in wide-open, semiarid and prairie habitats in the western United States, southwestern Canada, and northern Mexico. Prairie falcons generally breed in the vi…
The praying mantis (plural praying mantids) is a carnivorous insects of the order Mantoidea (or Mantodea) named for its typical stance of an upright body with the two front legs held out in a pose of prayer. The long, thick, spiny, legs and the markedly triangular head with two large compound eyes make the mantis one of the most readily identifiable of all insects. The long neck of the praying man…
The precession of the equinoxes (sometimes simply called precession), is a movement of the celestial equator, the projection of the earth's equator into space, with respect to the fixed stars and the ecliptic, the path of the Sun's motion in space as viewed from the earth. These two great circles in space are inclined to one another by an angle of approximately 23.5°, called t…
Gold, silver, and platinum have historically been valued for their beauty and rarity. They are the precious metals. Platinum usually costs slightly more than gold, and both metals are about 80 times more costly than silver. Precious metal weights are given in Troy ounces (named for Troyes, France, known for its fairs during the Middle Ages) a unit approximately 10% larger than 1 oz (28.35 g). The …
In meteorology, precipitation is water in either solid or liquid form that falls in Earth's atmosphere. Major forms of precipitation include rain, snow, and hail. When air is lifted in the atmosphere, it expands and cools. Cool air cannot hold as much water in vapor form as warm air, and the condensation of vapor into droplets or ice crystals may eventually occur. If these droplets or cryst…
A predator is an organism that hunts and eats its prey. All predators are heterotrophs, meaning they must consume the tissues of other organisms to fuel their own growth and reproduction. The most common use of the term is to describe the many types of carnivorous animals that catch, kill, and eat other animals. There is a great diversity of such predatory animals, ranging in size from small arthr…
Prenatal surgery, also called fetal surgery, is medical treatment of the fetus before birth, while it is still in the womb. Most fetal therapies are "closed" procedures, performed without opening the womb. The rarest type of fetal surgery is known as "open surgery," in which the mother's abdomen and uterus are cut open to reveal the tiny fetus. When doctors began…
Prescribed fire involves the controlled burning of vegetation to achieve some desired management effect. Prescribed burns can be used to encourage a desired type of forest regeneration, to prevent the invasion of prairies by shrubs and trees, to decrease the abundance of pathogens, to prevent catastrophic wildfires by reducing the accumulation of fuel, or to create or maintain habitat for certain …
Pressure is the amount of force applied to a given area. Acrobats and cheerleaders sometimes stand on each other's shoulders to form a human tower. Even with perfect balance, there is a limit to how high such a tower can be built. Ultimately, the ability of the bottom person to bear the pressure, caused by the weight of all the people stacked above, is the limiting factor. Pressure, then, i…
Prey refers to any living entities that are hunted and consumed by predators. Usually the term is used in reference to animals that are stalked, killed, and consumed by other animals, as when a deer is killed by a mountain lion. However, plants may also be considered to be the prey of herbivorous animals, and hosts may be considered the prey of their parasites. Often, predators are important sourc…
Primates are an order of mammals. Most primates are characterized by well-developed binocular vision, a flattened, forward-oriented face, prehensile digits, opposable thumbs (sometimes the first and second digits on the feet are also opposable), five functional digits on the feet, nails on the tips of the digits (instead of claws), a clavicle (or collarbone), a shoulder joint allowing free movemen…
A prime number is any number greater than 1 that is divisible only by itself and 1. The only even prime number is 2, since all other even numbers are at least divisible by themselves, 1, and 2. …
Primroses are perennial, herbaceous plants in the genus Primula, family Primulaceae. There are about 500 species of primroses. Most of these occur in arctic, boreal, and cool-temperate climates, including mountain-tops in tropical latitudes. The greatest species numbers occur Bloom of the shooting star, a member of the primrose family. Photograph by Robert J. Huffman. Field Mark Publications. R…
In photogravure, ink is held in the hollows of a plate rather than on high relief. This method of printing is known as intaglio. The photogravure plate, like the halftone plate, is produced with the aid of a camera and an acid to etch away parts of the metal plate. The acid creates hollows of different depths. The deepest hollows hold the most ink and print the darkest areas in the picture. Shallo…
The term prion (derived from "proteinaceous infectious particle") refers to an infectious agent consisting of a tiny protein that lacks genes, but can proliferate inside the host, causing slowly developing neurodegenerative diseases in animals and humans. Prions are thought to cause several diseases that attack the brain, such as Creutzfeldt-Jakob disease in humans, scrapie in sheep,…
In Euclidean geometry, a prism is a three dimensional figure, or solid, having five or more faces, each of which is a polygon. Polygons, in turn, consist of any number of straight line segments, arranged to form a flat, closed, two-dimensional figure. Thus, triangles, rectangles, pentagons, hexagons, and so on are all polygons. In addition, a prism has at least two congruent (same size and shape) …
Probability theory is a branch of mathematics concerned with determining the long run frequency or chance that a given event will occur. This chance is determined by dividing the number of selected events by the number of total events possible. For example, each of the six faces of a die has one in six probability on a single toss. Inspired by problems encountered by seventeenth century gamblers, …
The proboscis monkey (Nasalis larvatus) of Borneo belongs to the primate family Cercopithecidae. It is grouped with the langurs, leaf monkeys, and colobus monkeys in the subfamily Colobinae. The feature that gives this odd-looking monkey its common name is the large, tongue-shaped nose of the adult male. This nose can be as much as 4 in (10 cm) long. It sometimes hangs down over the mouth, but ext…
Projective geometry is the study of geometric properties which are not changed by a projective transformation. A projective transformation is one that occurs when: points on one line are projected onto another line; points in a plane are projected onto another plane; or points in space are projected onto a plane, etc. Projections can be parallel or central. For example, the Sun shining behind a pe…
Prokaryotes are single-celled organisms such as bacteria that have no distinct nucleus. In addition to the lack of a nucleus, prokaryotes lack many of the other small organelles found in the larger eukaryotic cells. A typical prokaryote is bound by a plasma membrane and a cell wall. Within this double boundary, the fluid material inside the cell (the cytoplasm) is studded with small, rounded bodie…
The pronghorn antelope (Antilocapra americana) is a species of ruminant that is the sole living representative of its family, the Antilocapridae. This family was much more diverse during the Pliocene and early to mid-Pleistocene periods. The Antilocapridae is an exclusively North American family, and pronghorns are not closely related to the true antelopes, which are members of the Bovidae, a …
A proof is a logical argument demonstrating that a specific statement, proposition, or mathematical formula is true. It consists of a set of assumptions, or premises, which are combined according to logical rules, to establish a valid conclusion. This validation can be achieved by direct proof that verifies the conclusion is true, or by indirect proof that establishes that it cannot be false. Each…
Propane is a gas produced primarily from various refinery processes. It is often mixed with butane, a four carbon atomalkane, and sold as bottled gas or liquefied petroleum gas, LPG. The bottled gas is used as an inexpensive fuel for cooking and heating homes not located near natural gas lines. Since liquefied petroleum gas burns very cleanly, it is being used as an alternate fuel for cars, trucks…
Prosimians are the most primitive of the living primates, which also include the monkeys and apes. The name prosimian means pre-monkey. The living prosimians are placed in the suborder Prosimii, which includes four families of lemurs, (the Lemuridae, the Cheirogaleidae, the Indriidae, and the Daubentoniidae), the bush babies, lorises and pottos (family Lorisidae), and the tarsiers (family Tarsiida…
Prosthetics is a branch of surgery that is involved in devising and fabricating a prosthesis for a missing or infirm body part. A prosthesis is an artificial part used to restore some amount of normal body function. The classic example of a prosthesis is a false leg or arm to replace one that has been amputated. A diseased heart valve can be removed and replaced by an artificial one. Artificial bo…
Proteas are evergreen trees and shrubs belonging to the dicotyledonous plant family Proteaceae and, in particular, to members of the genus Protea. They grow mostly in dry regions of the southern hemisphere, especially in Australia and South Africa. The family is divided into five subfamilies, 75 genera, and 1,350 species. The Proteaceae are distinguished from closely related families by having one…
Protected areas are parks, ecological reserves, and other tracts set aside from intense development to conserve their natural ecological values. These areas protect the habitat of endangered species, threatened ecological communities, or representative examples of widespread ecosystems, referred to as indigenous (native) biodiversity values. Some protected areas are intended to conserve places of …
Proteins are linear chains of amino acids connected by chemical bonds between the carboxyl group of each amino acid and the amine group of the one following. These bonds are called peptide bonds, and chains of only a few amino acids are referred to as polypeptides rather than proteins. Different authorities set the protein/polypeptide dividing line at anywhere from 10 to 100 amino acids. Many prot…
Proteome is a complement of proteins expressed in a cell at given time and proteomics means global analysis of this protein complement. Proteomics investigates the global changes of proteins in cells and tissues in response to a stimulus (for example temperature change, drug or nutrient treatment, or growth phase). It also studies protein-protein interactions. Proteomics came into prominence after…
The Kingdom Protista is the most diverse of all six kingdoms. There are more than 200,000 known species of protists with many more yet to be discovered. The protists can be found in countless colors, sizes, and shapes. They inhabit just about any area where water is found some or all of the time. They form the base of ecosystems by making food, as is the case with photosynthetic protists, or by th…
The proton is a positively charged subatomic particle. Protons are one of the fundamental constituents of all atoms. Protons, in addition to neutrons, are found in a very concentrated region of space within atoms referred to as the nucleus. The discovery of the proton, neutron, and electron revolutionized the way scientists viewed the atom. Recent research has shown that protons are themselves mad…
Protozoa are a very varied group of single-celled organisms, with more than 50,000 different types represented. The vast majority are microscopic, many measuring less than 1/200 mm, but some, such as the freshwater Spirostomun, may reach 0.17 in (3 mm) in length, large enough to enable it to be seen with the naked eye. Scientists have even discovered some fossil specimens that measured 0.78 in (20…
Psychiatry is the branch of medicine concerned with the study, diagnosis, and treatment of mental illnesses. The word, psychiatry, comes from two Greek words that mean mind healing. Those who practice psychiatry are called psychiatrists. In addition to their M.D.s, these physicians have post-graduate education in the diagnosis and treatment of behaviors that are considered abnormal. They tend to v…
The term psychoanalysis has three meanings: 1) a theory of personality with an emphasis on motivation, or why we behave the way we do; 2) a method of treatment for various psychological problems; and 3) a group of techniques used to explore human nature or the mind. …
"Psychology" comes from the Greek words psyche, meaning "mind" or "soul," and logos, meaning word. It is the scientific study of human and animal behavior and mental processes. Behavior refers here to easily observable activities such as walking, talking, or smiling. Mental processes, such as thinking, feeling, or remembering, often cannot be directly obse…
Psychometry or psychometrics is a field of psychology which uses tests to quantify psychological aptitudes, reactions to stimuli, types of behavior, etc., in an effort to devlop reliable scientific models that can be applied to larger populations. Currently many new psychometric theories and statistical models are being proposed that will probably lead to changes in test construction. In addition,…
A psychotic state is one in which a person suffering from one of several mental illnesses loses touch with reality. People experiencing psychosis may be diagnosed as schizophrenic, manic-depressive, or delusional. Psychosis can also be induced from drug or alcohol abuse, reaction to medication, from exposure to some toxic substance, or from trauma to the brain. Psychotic episodes have a duration t…
Psychosurgery is the alteration or destruction of brain matter in order to alleviate severe, long-lasting, and harmful psychiatric symptoms that do not respond to psychotherapy, behavioral, physical, or drug treatments. Psychosurgery involves opening up the skull or entering the brain through natural fissures such as the eye sockets, and injecting various tissue-altering solutions, removing or des…
Puberty is the period of sexual maturity when sexual organs mature and secondary sexual characteristics develop. Puberty is also the second major growth period of life—the first being infancy. A number of hormones under the control of the hypothalamus, pituitary, ovaries, and testes regulate this period of sexual growth, which begins for most boys and girls between the ages of nine and 15. …
Puffbirds are 32 species of birds that make up the family Bucconidae. This family is in the order Piciformes, which also contains the woodpeckers, toucans, barbets, jacamars, and honey-guides. Puffbirds are native to lowland tropical forests from southern Mexico, through to Paraguay and northern Argentina in South America. Most species occur in Amazonia. Puffbirds are short, squat birds, with a la…
Puffer fish or globe fish (family Tetraodontidae) are a group of tropical- and warm-temperate-dwelling species that are almost exclusively marine in their habits. A few freshwater species occur in tropical Africa and Asia. Most are typically found in shallow waters, often on coral reefs, in beds of sea grass, and in estuaries, swimming and feeding during daylight. A few species are oceanic. Their …
A pulsar is a celestial object that emits radiation pulses (bursts) of very short (one to a few milliseconds, or thousandths of a second) duration at very regular intervals from a fraction of a second to ten seconds. The first pulsar was discovered in 1967 by Jocelyn Bell and Antony Hewish at Cambridge, England, with radio telescopes equipped to study the twinkling (scintillation) of radio stars. …
Punctuated equilibrium is a theory about how new species evolve that was first advanced by American paleontologists Niles Eldredge and Stephen Jay Gould (1941–2002) in 1972. Although controversial, punctuated equilibrium has stimulated fruitful debate about speciation (the birth of new species) and the fossil record and has, in recent years, won at least partial acceptance among most evolut…
A pyramid is a geometric solid of the shape made famous by the royal tombs of ancient Egypt. It is a solid whose base is a polygon and whose lateral faces are triangles with a common vertex (the vertex of the pyramid). In the case of the Egyptian pyramid of Cheops, the base is an almost perfect square 755 ft (230 m) on an edge, and the faces of triangles that are approximately equilateral. Pyr…
This theorem was likely to have been known earlier to be the Babylonians, Pythagoras is said to have traveled to Babylon as a young man, where he could have learned the famous theorem. Nevertheless, Pythagoras (or some member of his school) is credited with the first proof of the theorem. …
Pythons are nonvenomous constricting snakes in the family Boidae that are found only in the Old World. Like the boas, pythons retain lizard-like features such as paired lungs and the remnants of the hind limbs. Pythons are egg-laying snakes which distinguishes them from boas and sandboas which typically bear live young. Fossil species of pythons are known from Cretaceous period, some 200 million y…
A quadrilateral is a polygon with four sides. Special cases of a quadrilateral are: (1) A trapezium—A quadrilateral Illustration by Hans & Cassidy. Courtesy of Gale Group. Illustration by Hans & Cassidy. Courtesy of Gale Group. with no pairs of opposite sides parallel. (Figure A) (2) A trapezoid—A quadrilateral with one pair of sides parallel. (Figure B) (3) A…
Quail are relatively small species of fowl in the family Phasianidae, which also includes pheasants, partridges, peafowl, turkeys, guinea fowl, and francolins. Like other members of their family, quail have a chunky body with short, rounded wings, and a short, thick, hooked bill, in which the tip of the upper mandible hangs slightly over that of the lower. The legs and feet are stout, and are used…
The value of a material is determined in part by the substances of which it is composed. The operations necessary to determine this composition are known as qualitative analysis. Qualitative analysis is a series of tests; responses to these tests identify the elements and compounds that make up the material. Every substance is unique. Each has, for example, a certain color, texture, and appearance…
Quantitative analysis is a chemical analysis performed to find the amount of each component present in a material. It is done by either a classical or instrumental procedure. A quantitative investigation means that the amount (quantity) or relative amount of each component present is determined. In a pure substance, the entire mass, or 100%, is composed of a single component. In materials composed…
The computers of today are smaller, faster, and more powerful than their predecessors from the 1940s. The underlying philosophy of the ancient computers and their modern cousins, however, is exactly the same. The task remains the same: to manipulate and interpret information that is expressed as either a 0 or a 1. This packaging of information is referred to as the binary bit. Binary bit computers…
Quantum electrodynamics (QED) is a complex and highly mathematical theory regarding the interaction of electromagnetic radiation with matter. The development of QED theory was essential in the verification and development of quantum field theory and it allows physicists to predict how subatomic particles are created or destroyed. QED is a fundamentally important scientific theory that accounts for…
Quantum mechanics is the theory used to provide an understanding of the behavior of microscopic particles such as electrons and atoms. More importantly, quantum mechanics describes the relationships between energy and matter on atomic and subatomic scale. At the beginning of the twentieth century, German physicist Maxwell Planck (1858–1947) proposed that atoms absorb or emit electromagnetic…
A quantum number is a number that specifies the particular state of motion an atom or molecule is in and, usually, the energy of that motion. By 1900, several phenomena were recognized that could not be explained by accepted scientific theories. One such phenomenon was the behavior of light itself. In 1900, however, Max Planck (1858-1947) developed a new theory that successfully described the natu…
Quarks are, according to the modern theory of subatomic particles, one of the three basic building blocks of all matter. The others are the leptons (which include the electron and the three types of neutrinos) and the intermediate vector bosons (which mediate the forces that bind other particles together). The stable particles of which ordinary matter is mostly composed—protons and neutrons…
Quasi-stellar radio sources (quasars) are the most distant cosmic objects observed by astronomers. Although not visible to the naked eye, quasars are also among the most energetic of cosmic phenomena. Although some quasars may be physically smaller in size than our own solar system, some quasars are calculated to be brighter than hundreds of galaxies combined. Quasars and active galaxies appear to…
The quetzal (Pharomachrus mocinno), also known as the resplendent quetzal or magnificent quetzal, is an astonishingly beautiful bird of tropical forests. It is a member of the trogon family (Trogonidae). The quetzal has a body length of 14 in (36 cm); in addition, the male has impressive tail streamers as long as 25 in (64 cm). The mature male has a shining green body color, with a crimson belly, …
Quinine is an alkaloid obtained from the bark of several species of the cinchona tree. Until the development of synthetic drugs, quinine was used as the primary treatment of malaria, a disease that kills over 100 million people a year. The cinchona tree is native to the eastern slopes of the Andes Mountains in South America. Today, the tree is cultivated throughout Central and South America, Indon…
Rabies is a viral brain disease that is almost always fatal if it is allowed to develop and is not prevented with prompt treatment. The disease, which typically spreads to humans from animals through a scratch or a bite, causes inflammation of the brain. The disease is also called hydrophobia (meaning fear of water) because it causes painful muscle spasms in the throat that prevent swallowing. In …
Raccoons are foxlike carnivores of North and South America that belong to the same family (Procyonidae) as the coatis, kinkajou, and the lesser panda. The most common species is the northern raccoon (Procyon lotor), which has numerous subspecies, all with the famous black mask on their faces and rings of dark color on their tails. They are found throughout the United States, in central Canada, and…
Radar (RAdio Detection And Ranging) is an electronic detector system that measures distance or velocity by sending a signal out and receiving its return. It can pierce fog, darkness, or any atmospheric disturbance all the way to the horizon. Within its range, it can show an observer clouds, landmass, or objects such as ships, airplanes, or spacecraft. Radar can measure distance or range to a targe…
Radial keratotomy (RK) is a surgical procedure that reduces myopia (nearsightedness), or astigmatism (diminished focus) by changing the shape of the cornea—the outermost part of the eyeball. The procedure is particularly attractive to individuals who want to avoid wearing glasses or wish to be rid of the inconvenience of contact lenses. RK is a quick, relatively painless procedure that take…
The word radiation comes from the Latin for "ray of light," and is used in a general sense to cover all forms of energy that travel through space from one place to another as "rays." Radiation may be in the form of a spray of subatomic particles, like miniature bullets from a machine gun, or in the form of electromagnetic waves, which are nothing but pure energy and whi…
Radiation detectors are devices which sense and relay information about incoming radiation. Though the name brings to mind images of nuclear power plants and science fiction films, radiation detectors have found homes in such fields as medicine, geology, physics, and biology. The term radiation refers to energies or particles given off by radioactive matter. Mostly, radiation takes the form of alp…
Radiation exposure occurs any time that energy in the form of electromagnetic rays or particles interacts with biological tissue. Ionizing radiation is particularly energetic; examples include: x rays, gamma radiation, and subatomic particles. Biological damages caused by exposure to ionizing range from mild tissue burns to cancer, genetic damage, and ultimately, death. However, there are potentia…
A radical is an uncharged atom or molecule that has an unpaired, or "free," electron. Radicals are formed when a covalent bond in an atom or molecule is split apart and the remaining pieces retain one electron of the original shared pair. These reaction products, called free radicals, are highly reactive entities that can participate in a variety of reactions. In chemical notation, r…
A radical is a symbol for the indicated root of a number, for example a square root or cube root; the term is also synonymous for the root itself. The radical &NA; is the symbol that calls for the root operation; the number or variable under the radical sign is called the radicand. It is common parlance to speak of the radicand as being "under the radical." It is also common to simpl…
Radio is the technology and practice that enables the transmission and reception of information carried by long-wave electromagnetic radiation. Radio makes it possible to establish wireless two-way communication between individual pairs of transmitter and receiver, and it is used for one-way broadcasts to many receivers. Radio signals can carry speech, music, telemetry, or digitally-encoded entert…
Radio waves are a form of electromagnetic radiation with long wavelengths and low frequencies. The radio section of the electromagnetic spectrum covers a fairly wide band and includes waves with frequencies ranging from about 10 kilohertz to about 60,000 megahertz (which correspond to wavelengths between 98,000 ft, or 30,000 m, and 0.2 in, or 0.5 cm). The commercial value of radio waves as a means…
In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth. The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that the earth was con…
The nucleus of each atom has a specific number of protons and neutrons and is either stable or unstable, depending on the relative number of each. The most stable atoms are those that have an equal number of protons and neutrons. Atoms that are unstable are radioactive. An atom that is radioactive can also be called a radionuclide. Of the known nuclides (approximately 2,000), only 264 are stable, …
Radioactive fallout is material produced by a nuclear explosion or a nuclear reactor accident that enters the atmosphere and eventually falls to Earth. This fallout consists of minute, radioactive particles of dust, soil, and other debris. While some fallout results from natural sources, the term is usually used in reference to radioactive particles that were released into the atmosphere by a nucl…
Certain atoms are radioactive, meaning they emit radioactivity during spontaneous transformation from an unstable isotope to a more stable one. Radioactive pollution results from contamination of the environment with such substances, which may represent a significant health risk to humans and other organisms. Radioactive pollution differs from conventional pollution in that it cannot be detoxified…
Radioactive tracers are substances labeled with a radioactive atom to allow easier detection and measurement. They have applications in many fields, but we will focus on their use in medicine. …
Radioactive waste is generated during the production of electricity by nuclear power plants, by the eventual disposal of those facilities, and during the manufacturing and disposal of nuclear weapons and machines used in medical diagnosis and treatments, academic and industrial research, and certain industrial applications. Radioactive waste produces ionizing radiation, which can damage or destroy…
Radioisotopes are extensively used in nuclear medicine to allow physicians to explore bodily structures and functions in vivo (in the living body) with a minimum of invasion to the patient. Radioisotopes are also used in radiotherapy (radiation therapy) to treat some cancers and other medical conditions that require destruction of harmful cells. Radioisotopes, containing unstable combinations of p…
Radiology is a branch of medical science that uses x rays and other forms of technology to image internal structures in the body. For nearly 80 years radiology was based primarily on the x ray, but since the 1970s several new imaging techniques have been developed. Some, like computed tomography, integrates x-ray and computer technology. Others, like ultrasound and magnetic resonance imaging are n…
Radon (usually in the form of the radon-222 isotope) is a colorless and odorless radioactive gas formed from radioactive decay. Denoted by the atomic symbol, Rn. radon has an atomic number of 86 and the atomic weight of its most stable isotope is 222. It is a colorless, odorless gas that emits radioactivity. It is classified as a noble gas based on its location on the periodic table. Radon is the …
Rails are small, shy, marshland birds in the family Rallidae, which includes about 129 species. This family has a worldwide distribution, occurring on all continents except Antarctica. Many species of rails occur only on certain remote, oceanic islands, where many of these isolated species have evolved a flightless condition because of the lack of predators. Unfortunately, this characteristic make…
Water droplets and light form the basis of all rainbows, which are circular arcs of color with a common center. Because only water and light are required for rainbows, one will see them in rain, spray, or even fog. A raindrop acts like a prism and separates sunlight into its individual color components through refraction, as light will do when it passes from one medium to another. When the white l…
Rainforests are temperate or tropical forests, usually occurring as old-growth ecosystems. The world sustains many types of rainforests, which differ geographically in terms of their species composition and the environmental conditions in which they occur. However, the various rainforests have broad ecological similarities. A such, temperate and tropical rainforests are considered to represent bio…
The word "random" is used in mathematics much as it is in ordinary speech. A random number is one whose choice from a set of numbers is purely a matter of chance; a random walk is a sequence of steps whose direction after each step is a matter of chance; a random variable (in statistics) is one whose size depends on events which take place as a matter of chance. Random numbers and ot…
Rangeland is uncultivated land that is suitable for grazing and browsing animals. Rangeland is one of the major types of land in the world. (Other types are: forest, desert, farmland, pasture, and urban/industrial.) Rangelands are the principal source of forage for livestock, and they also provide habitat for a great variety of native plants and animals. Rangelands are also used by people for recr…
Raptors, or birds of prey, are birds having the following three distinctive characteristics: strong grasping feet equipped with sharp talons, a hooked upper beak, and keen vision. Raptors are called birds of prey because these features allow them to be predators that hunt for their food. Many raptors are, in fact, predators. Some raptors actually hunt for and consume other birds. Other members of …
The rare gases, also known as the noble gases, are a group of six gaseous elements found in small amounts in the atmosphere: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Collectively they make up about one percent of the earth's atmosphere. They were discovered by scientists around the turn of the century and because they were so unreactive were initially ca…
The rare genotype advantage hypothesis asserts that genotypes (the set of genes (alleles) carried by an organism) that have been rare in the recent past should have particular advantages over common genotypes under certain new and challenging environmental conditions. Rare genotype advantage can be best illustrated by a host-parasite interaction. Successful parasites are those carrying genotypes t…
A rate is a comparison of the change in one quantity, such as distance, temperature, weight, or time, to the change in a second quantity of this type. The comparison is often shown as a formula, a ratio, or a fraction, dividing the change in the first quantity by the change in the second quantity. When the changes being compared occur over a measurable period of time, their ratio determines an ave…
The ratio of a to b is a way to convey the idea of relative magnitude of two amounts. Thus if the number a is always twice the number b, we can say that the ratio of a to b is "2 to 1." This ratio is sometimes written 2:1. Today, however, it is more common to write a ratio as a fraction, in this case 2/1. At one time, ratios were in common use in solving problems and the terms …
Rational numbers are needed because there are many quantities or measures which natural numbers or integers alone will not adequately describe. Measurement of quantities, whether length, mass, or time, is the most common situation. Rational numbers are needed, for example, if a farmer produces and wants to sell part of a bushel of wheat or a workman needs part of a pound of copper. The reason that…
Rationalization is a process applied most often to the denominators of fractions, such as 5/(1 + √+2). There are two reasons for this. If someone wanted to compute a rational approximation for such an expression, doing so would entail dividing by a many-place decimal, in this case 2.41421... With a calculator it would be easy to do, but if it must be done without a calculator, the process i…
Rats are members of the order Rodentia, which also encompasses beavers, mice, hamsters, and porcupines. Two major families of rats and mice are recognized: the Sigmodontinae; the New World rats and mice, comprising 369 species in 73 genera, and the Murinae, the Old World rats and mice, comprising 408 species in 89 genera. The major taxonomic difference between the two subfamilies is the presence o…
Why is the sky blue? Why are sunsets red? The answer involves Rayleigh scattering. When light strikes small particles, it bounces off in a different direction in a process called scattering. Rayleigh scattering is the scattering that occurs when the particles are smaller than the wavelength of the light. Blue light has a wavelength of about 400 nanometers, and red light has a wavelength of about 7…
Rays are members of the class Chondrichthyes, the cartilaginous fish, that includes sharks, skates, and chimeras. The flattened shape of rays makes them unique among fish. Their pectoral fins are much larger than those of other fish, and are attached the length of the body, from the head to the posterior. Rays, and their relatives the skates, comprise the order Rajiformes, which includes 318 speci…
A real number is any number which can be represented by a point on a number line. The numbers 3.5, −0.003, 2/3, π, and √2 are all real numbers. The real numbers include the rational numbers, which are those which can be expressed as the ratio of two integers, and the irrational numbers, which cannot. (In the list above, all the numbers except pi and the square root of 2 are ra…
The reciprocal of a number is 1 divided by the number. Thus the reciprocal of 3 is 1/3; of 3/2 is 1 ÷ (3/2)= 2/3, of a/b is b/a. If a number a is the reciprocal of the number b, then b is the reciprocal of a. The product of a number and its reciprocal is 1. Thus, 3 × 1/3 = 1, (3/2) × (2/3) = 1, and (a/b) × (b/a) = 1. …
Deoxyribonucleic acid (DNA) is the information blueprint that exists in most living organisms. Some viruses instead contain ribonucleic acid (RNA). Even these viruses require the production of DNA at some stage of their replication. DNA from different organisms of the same species combines together naturally to yield an organism that has traits from both parent organisms. There is also evidence ac…
A rectangle is a quadrilateral whose angles are all right angles. The opposite sides of a rectangle are parallel and equal in length. Any side can be chosen as the Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. base and the altitude is the length of a perpendicular line segment between the base and the opposite side (see Figure 1). A diagonal is either of the line s…
Recycling is a method of reusing materials that would otherwise be disposed in a landfill or incinerator. Discarded materials that contain glass, aluminum, paper, or plastic can be recycled by collecting and processing them into raw materials that are then used to manufacture new products. Recycling has many benefits: it saves money in production and energy costs, helps to conserve stocks of virgi…
A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core and is now using another element such as helium as the fuel for its energy-producing thermonuclear fusion reactions. Hydrogen fusion continues outside the core and causes the star to expand dramatically, making it a giant. Expansion also cools the star's surface, which makes it appear red. Red giant sta…
Red tides are a marine phenomenon in which water is stained a red, brown, or yellowish color because of the temporary abundance of a particular species of pigmented dinoflagellates (these events are known as "blooms"). Also called phytoplankton, or planktonic algae, these single-celled organisms of the class Dinophyceae move using a tail-like structure called a flagellum. They also p…
A redshift is caused by the Doppler effect, which is the change in wavelength and frequency of either light or sound as the source and observer are moving either closer together or farther apart. In astronomy a redshift indicates that the source is moving away, and a blueshift indicates that the source is moving closer to us. Doppler shifts have many important applications in astronomy. They help …
A reflection is one of the three kinds of transformations of plane figures which move the figures but do not change their shape. It is called a reflection because figures after a reflection are the mirror images of the original ones. The reflection takes place across a line called the "line of reflection." Figure 1 shows a triangle ABC and its image A'B'C'. Each individual point and …
Reflexes are set motor responses to specific sensory stimuli. All reflexes share three classical characteristics: they have a sensory inflow pathway, a central relay site, and a motor outflow pathway. Together, these three elements make up the reflex arc. Reflexes can also be characterized according to how much neural processing is involved in eliciting a response. Some reflexes, like the short re…
Refrigerated trucks and railroad cars have had a great impact on the economy and eating habits of Americans. As the United States became more urbanized, the demand for fresh food shipped over long distances increased. Meat products were especially in demand. In the mid-1800s, cattle raised in Texas were shipped by rail to Chicago, Illinois. Although it was more efficient to slaughter the cattle i…
Illness and trauma that lead to disability or functional loss can lead to an individual's need for a changed lifestyle to accommodate his reduced level of ability. A stroke, for example, can lead to partial paralysis; chronic arthritis can result in the inability to stand or to use one's hands; an automobile accident can cause blindness or can result in an individual's confine…
Reinforcement is a term used to refer to the procedure of removing or presenting stimuli (reinforcers) to maintain or increase the frequency or likelihood of a response. The term is also applied to refer to an underlying process that leads to reinforcement or to the actual act of reinforcement, but many psychologists discourage such a broad application of the term. Reinforcement is usually divided…
In mathematics, a relation is any collection of ordered pairs. The fact that the pairs are ordered is important, and means that the ordered pair (a, b) is different from the ordered pair (b, a) unless a = b. For most useful relations, the elements of the ordered pairs are naturally associated or related in some way. More formally, a relation is a subset (a partial collection) of the set of all pos…
The theory of relativity was developed by the German physicist Albert Einstein (1879–1955) in the early twentieth century and quickly became one of the basic organizing ideas of physics. Relativity actually consists of two theories, the special theory (announced in 1905) and the general (1915). Special relativity describes the effects of straight-line, constant-velocity motion on the mass a…
German–American physicist Albert Einstein's (1879–1955) theory of relativity consists of two major portions: the special theory of relativity and the general theory of relativity. Special relativity deals with phenomena that become noticeable when traveling near the speed of light and reference frames that are moving at a constant velocity, inertial reference frames. General r…
Remote sensing is the science and art of obtaining and interpreting information about an object, area, or phenomenon through the analysis of data acquired by a sensor that is not in contact with the object, area, or phenomenon being observed. There are four major characteristics of a remote sensing system, namely, an electromagnetic energy source, transmission path, target, and sensor. The Sun is…
The reproductive system is the structural and physiological network whose purpose is the creation of a new life to continue the species. It is the only body system that is not concerned with supporting the life of its host. Human reproduction is sexual—meaning that both a male and a female are required to produce a life. Gender is determined at conception by the sex chromosome in the sperm …
Reproductive toxicants are substances that adversely affect fertility or a developing embryo or fetus. Toxicants, strictly speaking, are poisons. However, reproductive toxicants loosely include any infectious, physical, chemical, or environmental agent that has a damaging effect on fertility or embryonic development. Some substances that have a beneficial effect on one occasion (such as a dental x…
The class Reptilia includes over 6,000 species grouped into four orders: the turtles (Chelonia), the snakes and lizards (Squamata), the crocodiles and alligators (Crocodilia), and the tuataras (Sphenodonta). Other, now extinct, reptilian orders included Earth's largest terrestrial animals, and some enormous marine creatures. The fishlike ichthyosaurs were large marine reptiles, as were the …
Historically, the term resin has been applied to a group of substances obtained as gums from trees or manufactured synthetically. Strictly speaking, however, resins are complex mixtures, whereas gums are compounds that can be represented by a chemical formula. The word gum was originally applied to any soft sticky product derived from trees; for example, the latex obtained from Hevea trees, which …
There are many instances in which we want to add energy to the motion of an object which is oscillating. In order for this transfer to be efficient, the oscillation and the source of new energy have to be "matched" in a very specific way. When this match occurs, we say that the oscillation and source are in resonance. A simple example of an oscillation that we have all seen is that o…
Natural resources, unlike man-made resources, exist independently of human labor. These resources are, however, not unlimited and must be used with care. Some natural resources are called "fund resources" because they can be exhausted through use, like the burning of fossil fuels. Other fund resources such as metals can be dissipated or wasted if they are discarded instead of being r…
Respiration is the physiological process by which organisms supply oxygen to their cells and the cells use that oxygen to produce high energy molecules. Respiration occurs in all types of organisms, including bacteria, protists, fungi, plants, and animals. In higher animals, respiration is often separated into three separate components: (a) external respiration, the exchange of oxygen and carbon d…
Cellular respiration is the process by which a living cell produces adenosine triphosphate (ATP), carbon dioxide, and water from oxygen and organic fuel. It is a catabolic pathway that involves the release of stored energy from the break down of complex molecules to more simple ones. No single chemical reaction covers the entire process of cellular respiration. Instead it is the cumulative functio…
A respirator is a means to provide needed oxygen to a patient, to infuse medication directly into the lungs, or to provide the power to breathe to someone who is unable to do so on his own. A respirator may be needed following a serious trauma that interferes with the individual's breathing or for a person who has contracted a disease such as poliomyelitis that has affected the nerves that …
There are many different types of respiratory diseases that interfere with the vital process of breathing. Respiratory obstructions arising from diseases can occur in the nasal area, the regions of the throat and windpipe (upper respiratory system), or in the bronchial tubes and lungs (lower respiratory system). The common cold and allergic reactions to airborne pollens block the nasal passages by…
Aerobic organisms take in oxygen from the external environment and release carbon dioxide in a process known as respiration. At the most basic level, this exchange of gases takes place in cells and involves the release of energy from food materials by oxidation. Carbon dioxide is produced as a waste product of these oxidation reactions. The gas exchange in cells is called cellular respiration. In …
Restoration ecology refers to activities undertaken to increase populations of an endangered species or to manage or reconstruct a threatened ecosystem. Ecological restoration is an extremely difficult and expensive endeavor, and only attempted when the population of an endangered species is considered too small to be self-maintaining or the area of a threatened ecosystem is not large enough to al…
Retrograde motion means "moving backward," and describes the loop or Z-shaped path that planets farther from the Sun than Earth appear to trace in the sky over Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. the course of a few months. All the visible planets farther from …
Retroviruses are spherical viruses that contain ribonucleic acid (RNA) as their genetic material. In contrast, most other organisms, including humans, store their genetic information in the form of deoxyribonucleic acid (DNA). Retroviruses are of concern to humans because of their disease causing ability. Examples of retroviruses include human T cell leukemia virus, which causes cancer in humans, …
Reye's syndrome is a serious medical condition associated with viral infection and aspirin intake. It usually strikes children under age 18, most commonly those between the ages of five and 12. Symptoms of Reye's syndrome develop after the patient appears to have recovered from the initial viral infection. Symptoms include fatigue, irritability, and severe vomiting. Eventually, neuro…
Rh factor is a blood protein that plays a critical role in some pregnancies. People without Rh factor are known as Rh negative, while people with the Rh factor are Rh positive. If a woman who is Rh negative is pregnant with a fetus who is Rh positive, her body will make antibodies against the fetus's blood. This can cause Rh disease, also known as hemolytic disease of the newborn, in the ba…
Rhesus monkeys (Macaca mulatta) are macaques belonging to the primate family Cercopithecidae. These medium-sized monkeys are colored from golden-brown to gray-brown. Rhesus monkeys spend most their time on the ground, although they take to trees readily, and have great agility in climbing and leaping. Typica