Issues related to the mind are an important component in contemporary philosophy. While colleagues in psychology, neuroscience, and cognitive ethology do empirical scientific studies of the mind, philosophers tend to focus on more general questions: What is the nature of mind, such as it may be found in any creature or thing? Philosophers tend to concentrate on questions such as: How is mind relat…
The mind is a modern notion. But like many modern notions, it did not emerge from nowhere. What contemporary philosophers mean when they talk about the mind is part of a long tradition, stretching back through the Middle Ages to Greek and Roman antiquity. The mind in its modern sense is best understood in opposition to the body, the extended, flesh-and-blood entity that it seems to inhabit and mov…
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…
A well-established discipline in early-twenty-first-century Western philosophy, the subject known as "the philosophy of religion" has not always been easily demarcated with respect to its nature and scope. The reason for this is historical. The long engagement of philosophy with the claims of religion has manifested itself from antiquity to the early twenty-first century in a wide va…
Phrenology, a science popular from the early to the mid-nineteenth century, was dedicated to the discernment of one's character or traits of personality from reading—that is, feeling the shape and size of—the bumps on one's skull. As formulated by the German physician and anatomist Franz Josef Gall (1758–1828) and as popularized by his student and follower Johann…
It should be understood that a full understanding of the history of physics would include consideration of its institutional, social, and cultural contexts. Physics became a scientific discipline during the nineteenth century, gaining a clear professional and cognitive identity as well as patronage from a number of institutions (especially those pertaining to education and the state). Before the n…
Organic agriculture refers to systems in which crops are grown using natural methods of maintaining fertility of the soil, and methods of pest control other than the use of synthetic pesticides. Compared with conventional agriculture systems that intensively use manufactured fertilizers and pesticides, much smaller environmental costs and damages are associated with organic systems. However, yield…
Like the Enlightenment, Pietism has produced an extremely diverse body of historical scholarship, with opinions ranging from a denial of its existence to precise nationally, geographically, or chronologically defined variants, as well as views that see Pietism essentially as identical with the history of modern Protestantism. Such divergent opinion has led to the introduction of categories such as…
An organism is any individual living entity. Organisms range in size and complexity from microorganisms to multicellular plants and animals. Modern biologists classify Earth's organisms into five kingdoms on the basis of common patterns of the design of life, that is, in their cellular and sub-cellular organization, metabolism, reproduction, and behavior. Listed in order of their earliest a…
Organogensis refers to that period of time during development when the organs are being formed. After an egg has been fertilized, and has been implanted in the uterus, the developing form is known as the embryo. Organogenesis takes place during this embryonic phase. In fact, most organogenesis has begun as early as week five in humans (remember that a normal human pregnancy lasts an average of 40 …
In the human body an organ is composed of two or more different histological types of tissue that work together to carry out a complex function. An organ system consists of a group of organs that perform intricate functions necessary for the survival of an organism. Sometimes an organism can survive with an impaired or nonfunctioning organ. However, when a whole system of organs shuts down, the li…
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…
There is not direct fossil-like evidence of how life originated on Earth, the molecular processes that preceded the appearance of cells do not leave such tangible evidence. However, fossils of single-celled microorganisms are present in rocks 3.0–3.5 billion years old (with some scientific controversy over which rocks contain the oldest true bacterial fossils), and chemical traces in Greenl…
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. …
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…
The true or forest orioles include 28 species of medium-sized birds that make up the family Oriolidae. These birds occur in Africa, Europe, Asia, Southeast Asia, the Philippines, New Guinea, and Australia. Their usual habitats are forests, open woodlands, and savannas. Most species are tropical, but some migratory species occur in temperate regions. Orioles are jay-sized birds with long, pointed w…
Ornithology is the branch of zoology that deals with the study of birds. Birds are any organisms in the class Aves. They are warm-blooded (or homoiothermic) vertebrates that have feathers covering their body; forelimbs modified into wings; stouter hindlimbs used for walking, swimming, or perching; scaly legs and feet; jaws reduced to a toothless beak; and a four-chambered heart. Birds lay hard-she…
Orthopedics is the branch of medicine that specializes in diseases and injuries of bones. It is a specialty devoted to the diagnosis, treatment, rehabilitation and prevention of injuries and diseases of your body's musculoskeletal system. This complex system includes your bones, joints, ligaments, tendons, muscles, and nerves. Once devoted to the care of children with spine and limb deformi…
The oryx (Oryx gazella) is a species of antelope in the family Bovidae. Oryx are native to a rather wide range, extending from the Middle East through much of Africa. There are eight recognized subspecies of oryx, which vary greatly in body and horn shape, and in habitat A herd of scimitar-horned oryx. Photograph by Yav Levy. Phototake. Reproduced by permission. requirements. In some taxo…
In an oscillating chemical reaction, the concentrations of the reactants and products change with time in a periodic or quasi-periodic manner (i.e., they do not move directly or evenly toward their final concentrations). Chemical oscillators exhibit chaotic behavior, in which concentrations of products and the course of a reaction depend on the initial conditions of the reaction. Oscillating react…
An oscillation is a particular kind of motion in which an object repeats the same movement over and over. It is easy to see that a child on a swing and the pendulum on a grandfather clock both oscillate when they move back and forth along an arc. A small weight hanging from a rubber band or a spring can also oscillate if pulled slightly to start its motion, but this repeated motion is now linear (…
An oscilloscope is an instrument that provides a graphical display of electrical signals. It presents a considerable amount of information about the operation of a circuit almost instantly, and the visual nature of the display provides insights that tables of numbers do not offer. Oscilloscopes are extremely useful for monitoring and diagnosing electrical circuits or devices. Though it can plot an…
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. …
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,…
Osmosis is the movement of solvent, such as water, through a barrier from a less concentrated solution into a more concentrated solution. It occurs when two solutions are separated by a semipermeable membrane which allows only the solvent to pass through. Osmosis plays a major role in the chemistry of living things and also has applications in medicine and technology. Osmosis was first described i…
Osmosis is the movement of water across a membrane which is selectively permeable. In osmosis, water moves across a membrane from a region with low solute concentration to a region with high solute concentration. Thus, osmosis tends to equalize the solute concentrations in two separate membrane-enclosed regions. In living cells, water moves by osmosis across membranes between cells or between memb…
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…
Ossification is the process of the synthesis of bone from cartilage. There are two types of ossification—intramembranous and endochondral ossification. Bone may be synthesized by intramembranous ossification, endochondral ossification, or a combination of the two. Intramembranous ossification is the transformation of the mesenchyme, cells of an embryo into bone. During early development of …
Osteoporosis is a condition in which bone mass, and therefore bone strength, is decreased. This results in a greatly increased risk of fracture. Primary osteoporosis is osteoporosis which occurs due to normal, predictable changes within the body during the aging process. Secondary osteoporosis occurs as a result of some other specific disease process which produces osteoporosis as one of its sympt…
Otter shrews are small otter-like aquatic mammals in the family Tenrecidae, with silvery fur. The three species of otter shrews belong to the order Insectivora, and all live in west and central Africa near the equator. The Potamogalidae family includes two genera—Micropotamogale and Potamogale. These genera contain a total of three distinctive species of otter shrew—the giant African…
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…
Otters are small to medium-sized mammals with a long body, flattened head, broad muzzle, and long stiff whiskers. Their tail is strong, long, flattened, and somewhat tapered. Otters have short legs and webbed toes; they are well adapted to a semi-aquatic existence and are skilled swimmers. The outer fur of otters is short, very dense, and highly water resistant. They also have a layer of soft unde…
When weathering and erosion expose part of a rock layer or formation, an outcrop appears. An outcrop is the exposed rock, so named because the exposed rock "crops out." Outcrops provide opportunities for field geologists to sample the local geology—photograph it, hold, touch, climb, hammer, map, sniff, lick, chew, and carry it home. Classes often visit outcrops to see illustra…
The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of the oocyte (egg or ovum). During their reproductive years, nonpregnant females usually experience a cyclical sequence of changes in their ovaries and uterus. Each cycle takes about one month and involves both oogenesis, the process of formation and development of oocyte, and preparation of the uter…
Ovenbirds are 200 species of birds that compose the rather large family Furnariidae, occurring from southern Mexico through Central and all of South America. Ovenbirds occur in a wide variety of habitats, ranging from mature tropical forests to semi-desert, and from coastal lowlands to alpine tundra. In other words, the ovenbird family is very rich in species, and these birds successfully exploit …
Oviparous is a zoological term that refers to animals that lay eggs which then hatch externally. Oviparous animals may fertilize their eggs either externally or internally. External fertilization involves the passage of the sperm to the ova through an ambient medium, usually water. For example, frogs achieve external fertilization of their eggs during amplexus, when the male deposits sperm over th…
Ovoviviparous is a zoological term that refers to animals that produce eggs but retain them inside the female body until hatching occurs, so that "live" offspring are born. The egg-hatching strategy of ovoviviparity occurs in a rather wide diversity of animals, including certain insects, fish, lizards, and snakes. However, ovoviviparity is much less common than the external developme…
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…
Owls comprise two closely related families in the avian order Strigiformes: the barn owls (Tytonidae) and the typical owls (Strigidae). Owls are relatively large birds, with a big head and short neck, a hooked beak, talons adapted to seizing prey, and soft, dense plumage adapted for swift yet almost silent flight. Owls have large eyes located on the front of their face but almost fixed in their so…
Oxalic acid is the more common name of ethanedioic acid. The name ethanedioic acid communicates that the molecule has two carbon atoms (as in ethane) and two acid groups (COOH). It is a white solid used in removal of certain kinds of stains, in removing calcium ions from solutions, and in tanning leather. It occurs naturally and is toxic. The potassium and calcium salts of oxalic acid are found na…
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…
Oxidation-reduction reactions, also known as redox reactions, are chemical processes in which electrons are transferred from one atom, ion, or molecule to another. Explosions, fires, batteries, and even our own bodies are powered by oxidation-reduction reactions. When iron rusts or colored paper bleaches in the sun, oxidation-reduction has taken place. Oxidation-reduction reactions can be thought …
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…
The oxidation state of an atom is a description of how many electrons it has lost or gained from its original state. Each type of atom has a certain number of electrons (which varies from atom to atom) in its elemental form. When an atom forms a bond or otherwise interacts with another atom, it is possible that it will lose or gain an electron. If an atom is electronegative, it is more likely to t…
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…
Oxygen is a non-metallic element of atomic number 8. Its symbol is O, the atomic weight is 15.9994, the specific gravity is 1.10535 (compared to air), the melting point is -361°F (-218.4°C), and the boiling point is -297.4°F (-183°C). Oxygen is a non-metal in group 16 of the periodic table. Its three stable isotopes have atomic weights of 16, 17, and 18. The first is b…
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…
Oystercatchers are six rather similar-looking species of oceanic shorebirds that comprise the family Haematopodidae. Oystercatchers occur widely on sub-arctic, temperate, and tropical seacoasts, on all of the continents except Antarctica. Oystercatchers are relatively large shorebirds, with a body length of 15-21 in (40-53 cm). They have pointed wings, a short tail, short but heavy legs, and three…
In the vicinity of Los Angeles the maximum one-hour concentration of ozone can exceed 500 ppb, and it is typically greater than 100 ppb for at least 15 days per year. In other cities in North America, the annual maximum one-hour concentration is typically 150-250 ppb, and it is typically 90-180 ppb in London, England. Humans and other animals are sensitive to ozone. This gas irritates and damages …
Nova is a Latin word meaning new, and it describes the appearance of a seemingly "new star" in the sky, a brilliant object in a place where there was previously only a very faint star, or perhaps nothing at all. A nova is a phenomenon that happens in a binary star system containing a white dwarf and a stable companion star. A white dwarf is the dead, collapsed core of a star that for…
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…
Procaine hydrochloride was first synthesized by Einhorn in 1905 as a substitute for cocaine, the first local anesthetic. Cocaine, an alkaloid obtained from the leaves of the coca plant, Erythroxylon coca, is highly addictive and toxic. Procaine hydrochloride replaced cocaine as a local anesthetic because it is much less toxic, cheaper and easier to produce, and it is easier to sterilize. In contra…
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…
Ozone is a gas that occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere. The stratosphere is between 5–10.6 mi (8–17 km) to about 31 mi (50 km) above the earth's surface. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation, and thereby …
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 heart is a unique organ that must function continuously to pump blood supplying oxygen to the body. It speeds up during special times of need, as when an individual is running or doing stressful work. It slows at night or during sleep when the demand for blood decreases. This tiny pump, about the size of a fist, squeezes approximately 2.5 fl oz (75 ml) of blood out into the body with each beat…
Nuclear fission is a process in which the nucleus of an atom splits, usually into two pieces. This reaction was discovered when a target of uranium was bombarded by neutrons. Fission fragments were shown to fly apart with a large release of energy. The fission reaction was the basis of the atomic bomb, which was developed by the United States during World War II. After the war, controlled energy r…
Pain, in its most basic form, can be described as an unpleasant physical sensation resulting from a variety of outside stimuli, from a pin prick to a serious burn. However, pain is a complex experience that eludes simple definitions. Not only does the central nervous system play a crucial role in the experience of pain, but psychological factors can also affect how an individual perceives pain. Mo…
Paleobotany endeavors to reconstruct past climates and regional vegetation systems by studying the fossilized remains of plants or preserved pollen samples. Such studies have yielded information regarding global climate change, both natural and man-made, and its effects on specific environments. Paleobotanists aid in the identification of various climatic episodes. By collaborating geological evid…
Organic halides are organic compounds containing a halogen atom bonded to a carbon (C) atom. Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) are all types of halogen atoms. A compound that contains a carbon atom bonded to a fluorine atom (C-F) is called an organofluoride. If the carbon atom is part of a chain of carbon atoms, the organofluoride compound is referred to as an alkyl fluorid…
Paleoclimate studies analyze the variation of the climate in past geologic times, prior to instrumental measurements. Paleoclimate is expressed by its parameters—paleotemperature, precipitation in the past, circulation, sea surface temperature (SST) and sea level. The general state of Earth's climate is dependent upon the amount of energy the Earth receives from the solar radiation, …
Paleoecology is the branch of paleontology that studies ancient organisms and their environments. Paleoecologists study the physical structure and biological functions of organisms, their interactions with each other, and their role in ancient ecosystems. In addition to the basic principles of paleontology, paleoecology research depends on concepts from biology, sedimentology, and geochemistry. Th…
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…
Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of "heavy hydrogen"). In general, the mass of the heavier product nucleus is less than the total mass of the two lighte…
Paleomagnetism is the study of ancient magnetism in rocks. The phenomenon was first discovered by the French physicist Achilles Delesse in 1849. Delesse observed that certain magnetic minerals in rocks were lined up along the earth's magnetic field, just as if they were tiny compasses that had been set in place in the rocks. A related discovery that was even more startling was made by the F…
Nuclear magnetic resonance, NMR, is a process in which the nuclei of certain atoms absorb energy from a magnetic field that gyrates, or has a direction which rotates about some fixed axis. NMR provides a means of measuring nuclear properties using ordinary electromagnetic fields rather than high-energy particles as in a particle accelerator. Its applications range from nuclear measurements to medi…
Paleontology is the study of ancient animal life and how it developed. It is divided into two subdisciplines, invertebrate paleontology and vertebrate paleontology. Paleontologists use two lines of evidence to learn about ancient animals. One is to examine animals that live today, and the other is to study fossils. The study of modern animals includes looking at the earliest stages of development …
Paleopathology is the study of the evidence of trauma, disease, and congenital defects in human remains. Archaeologists, geneticists, and physical anthropologists, conduct paleopathology studies in order to evaluate the effects of disease upon ancient populations. Often, such research is conducted to garner more information about the biological and genetic characteristics of prehistoric or ancient…
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 …
In genetics, the term palindrome refers to a sequence of nucleotides along a DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) strand that contains the same series of nitrogenous bases regardless from which direction the strand is analyzed. Akin to a language palindrome—wherein a word or phrase is spelled the same left-to-right as right-to-left (e.g., the word RADAR or the phrase …
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…
Nuclear medicine is a medical specialty that uses radioactive materials, called radionuclides, to help diagnose and treat a wide variety of diseases, and for biomedical research. The development of nuclear medicine reflects the advances in the fields of nuclear physics, nuclear chemistry, and later, molecular biology. While there was considerable research in the nuclear sciences during the first p…
The palm family is an ancient group of plants dating from at least the late Mesozoic Era, about 85 million years ago. Palms are flowering plants. Flowering plants have been subdivided into two major groups: the monocotyledons which bear only one seed leaf (cotyledon) and the dicotyledons which bear two seed leaves. Palms are among the most diverse of the families of monocotyledons, surpassed in nu…
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…
Palynology is the study of fossil pollen (and sometimes plant spores) extracted from lake sediment, peat bog, or other matrices. The most common goal of palynological research is to reconstruct the probable character of historical plant communities, inferred from the abundance of species in dated portions of the fossil pollen record. Pollen analysis is an extremely useful tool for understanding th…
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…
Nuclear power is any method of doing work that makes use of nuclear fission or nuclear fusion reactions. In its broadest sense, the term refers to both the uncontrolled release of nuclear energy, as in fission or fusion weapons, and to the controlled release of energy, as in nuclear power plants. Most commonly, however, the expression "nuclear power" is reserved for the latter. Appro…
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…
The name "panda" applies to two different species: the familiar and well-loved giant panda (Ailuropoda melanoleuca), and the lesser-known red panda (Ailurus fulgens). Scientists originally thought the two species were closely related because of similarities in their behavior, diet, anatomy, and distribution. However, despite these shared characters of the pandas, researchers have now…
Pangolins, also called scaly anteaters, are subsaharan African and Asian mammals that have horny scales covering the upper parts of their bodies. All seven species belong to one genus, Manis, making up the order Pholidota. A pangolin. Photograph by Nigel Dennis. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. The overlapping brown, gold, olive, or p…
The papaya or pawpaw (Carica papaya) is a tropical tree originally native to the Americas, probably Mexico. This species is easily cultivated, produces large, edible fruits, and now is distributed worldwide to suitable climates where it is grown for subsistence and commercial agriculture. The papaya has large deeply incised, sometimes compound leaves that sprout near the top of the plant. This pla…
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 nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. At the present time, all commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium (the fuel) are broken apart into smaller nuclei, with the release of energy. It is theoretically possible to construct reactors that…
Paper was probably first produced from bamboo and rag fibers about 2,000 years ago. By the eighth century, papermaking technology had spread to the Middle East. By the middle of the twelfth century, the Moors had transplanted the technology to Spain, from where it spread throughout Europe. Rags continued to be the chief source of paper fibers until the introduction of papermaking machinery in the …
Nuclear weapons are explosive devices that release nuclear energy. An individual nuclear device may have an explosive force equivalent to millions of tons (megatons) of trinitrotoluene (TNT, the chemical explosive traditionally used for such comparisons), and is more than enough to inflict devastating physical damage to a city. The destructive power of nuclear weapons derives from the core of the …
Nuclear winter is a theory estimating the global climatic consequences of a nuclear war: prolonged and worldwide cooling and darkening caused by sunlight-blocking smoke and soot entering the atmosphere. The term nuclear winter was first defined and used by American astronomer Carl Sagan (1934–1996) and his group of colleagues in their 1983 article (later referred to as the TTAPS-article, fr…
A parabola is the open curve formed by the intersection of a plane and a right circular cone. It occurs when the plane is parallel to one of the generatrices of the cone (Figure 1). A parabola can also be defined as the set of points which are equidistant from a fixed point (the "focus") and a fixed line (the "directrix") (Figure 2). The "axis" of a parabo…
Two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are found in living things which serve to store, translate, and pass on the genetic information of an organism to the next generation. Nucleic acids are universal to all life, in eukaryotic and prokaryotic cells, as well as in viruses. The mitochondria of eukaryotic cells also contain some DNA, known as mitochondrial DNA. N…
Nucleon is a generic word for the heavy particles that make up the atomic nucleus: the protons and neutrons. It is like the generic word fruit, used to include apples, oranges, and many others, except that the class of nucleons contains only two members. The use of the ending -on for the names of subatomic particles began with electron, a word that was coined in 1891 by the Irish physicist George …
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…
Astronomers must use very indirect methods to measure the distances to stars and other astronomical objects. Measuring a star's parallax is a way to find its distance. This method takes advantage of the apparent shift in position of a nearby star as it is observed from different positions as the earth orbits the Sun. Because the parallax effect depends upon the earth's motion about t…
Two or more lines (or planes) are said to be parallel if they lie in the same plane (or space) and have no point in common, no matter how far they are extended. …
A parallelogram is a plane figure of four sides whose opposite sides are parallel. A rhombus is a parallelogram with all four sides of equal length; a rectangle is a parallelogram whose adjacent sides are perpendicular; and a square is a parallelogram whose adjacent sides are both perpendicular and equal in length. The area of a parallelogram is equal to the length of its base times the length of …
Numbats are the sole members of the family Myrmecobiidae. "Mumbat" is the aborigine name for these small marsupial mammals, otherwise known as banded anteaters. They are slightly larger than rats and weigh about 1 lb (0.5 kg). Considered one of the most attractive marsupials, their general color varies from grayish-brown to reddish-brown, broken by several prominent white bars across…
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…
Number theory is the study of natural, or counting numbers, including prime numbers. Number theory is important because the simple sequence of counting numbers from one to infinity conceals many relationships beneath its surface. Number theory is full of famous formulas that illustrate the relationships between whole numbers from 1 to infinity. Some of these formulas are very complicated, but the …
A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and …
Parity is both an operation and an intrinsic property used to describe particles and their wavefunctions (mathematical representations of one or more particles) in quantum mechanics (a branch of physics focusing on particles smaller than an atomic nucleus). The parity operation is a combination of a left-right trade (mirror reflection) with a top-bottom switch. This combination is also called a sp…
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…
Parkinson disease (PD) is a disease in which cells in regions of the brain involved with muscular coordination and control suffer in impaired ability to synthesize the neurotransmitter dopamine. Parkinson disease or Parkinsonism is named for the English physician James Parkinson. His description of the various manifestations of the disease was published in 1817, in a work entitled "An Essay…
Numeration systems are structured methods or procedures for counting in order to determine the total units in a collection. Numeration systems consist of counting bases (base 2, base 5, base 10, base 20, etc.) and some form of representation. This representation might be as primitive as the hand signals used in aborigine cultures and in the trading pits of stock exchanges, or it could be written o…
A nut is a type of fruit. Like all fruits, a nut develops from the ovary of a mature, fertilized flower. A nut is thick, dry, hard, and partly or entirely enclosed by a husk. A nut is indehiscent, in that it does not open along a naturally occurring seam, and remains closed even when fully mature. A nut is a simple fruit, in that it is derived from the pistil of a single flower. Although a nut con…
Nuthatches are small, short-tailed, large-headed birds in the family Sittidae in the order Passeriformes, the perching birds. There are 25 species of nuthatches, occurring on all continents except South America, Africa, or Antarctica. Most species of nuthatches are forest birds which clamber over the bark of trees seeking insects, spiders, and arthropod eggs. Nuthatches can climb in any direction-…
The nutmeg family, Myristicaceae, order Magnoliales, consists of evergreen trees of the tropical rain forests. The genus Myristica includes about 120 species, the best known of which is the nutmeg tree (Myristica fragrans). The nutmeg tree is native to the Moluccas, a group of islands in eastern Indonesia, also known as the Spice Islands. However, nutmeg is now cultivated in much of southern Asia,…
Parrots, macaws, lories, parakeets, and related birds, known collectively as psittacids, are 328 living species of birds that make up the family Psittacidae. The psittacids and the cockatoos (family Cacatuidae) are the only families in the order Psittaciformes. Species of psittacids occur in Central and South America, Africa, Madagascar, South and Southeast Asia, New Guinea, Australia, and New Zea…
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…
Parthenogenesis in animals refers to reproduction in which a new individual genetically identical to the parent develops from an unfertilized egg. The analogous event in plants, which results in seed formation without fertilization, is called agamospermy. Parthenogenesis is viewed as an aberration of sexual reproduction because animals that reproduce by parthenogenesis evolved from organisms that …
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…
Nutrient deficiency diseases occur when there is an absence of nutrients which are essential for growth and health. Lack of food leading to either malnutrition or starvation gives rise to these diseases. Another cause for a deficiency disease may be due to a structural or biological imbalance in the individual's metabolic system. There are more than 50 known nutrients in food. Nutrients ena…
Nutrients are any chemicals that are required for life. Nutrients can be of two basic types: (1) inorganic substances absorbed by autotrophic organisms such as plants and certain microorganisms for use in their synthetic reactions and metabolism; or (2) biomass ingested as nourishment by animals and heterotrophic microorganisms. Plants absorb a wide range of mineral nutrients, which they utilize i…
Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles inte…
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…
Partridges are species of fowl in the family Phasianidae, which also includes the pheasants, chickens, turkeys, grouse, peafowl, francolins, and quail. Partridges occur naturally in Eurasia, but they have been introduced as game birds to other places as well, including North America. Partridges are medium-sized, stocky birds with short, rounded wings, a short tail, and a short, stout bill, in whic…
Nutrition is the means by which organisms obtain and use nutrients. Nutrition is also the determination of the kinds and quantities of substances (nutrients) needed by organisms to sustain life. Some organisms such as plants require only a supply of light, water, and a few other molecules and ions in order to thrive, and are known as autotrophs, or self nourishers, for they literally build their o…
The nux vomica tree (Strychnos nux-vomica) is a species in the tropical family Loganiaceae. The range of the nux vomica in cultivation extends from Sri Lanka, India, southern China, southeast Asia, and northern Australia. The nux vomica grows as tall as 49.2 ft (15 m). The nux vomica has roundish, opposite leaves and attractive white flowers. The roughly spherical fruits of the nux vomica are larg…
Species of passion vines (Passiflora spp.; family Passifloraceae) that twine upward in tropical, subtropical, and some temperate regions of the world. They occur most often in wet forests, though some species may occur in drier, more open places. These vines have glossy leaves shaped like rounded human hands, and their flowers are often sweetly scented and packed with a ring of colorful filaments.…
Paternity testing and the identification of genetic parents via DNA analysis has, within the last decade, become a highly reliable test upon which courts and medical staff increasingly rely. Paternity or parentage identification is based on the ability to establish the genetic relationship between the parent(s) and their biological offspring. …
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 …
Oaks (Quercus spp.), members of the Beech family (Fagacea), are trees and shrubs having simple, alternate leaves found throughout the world. Characterized by their strong, complex wood, wind-pollinated flowers, fruits called acorns, and their ability to live for centuries, oaks have played an important role in temperate landscapes. Of the 500 species in the genus Quercus, approximately 90 are foun…
Pathogens are organisms, frequently microorganisms, or components of these organisms, that cause disease. Microbial pathogens include various species of bacteria, viruses, and protozoa. Many diseases caused by microbial pathogens. A disease is any condition caused by the presence of an invading organism, or a toxic component, that damages the host. In humans, diseases can be caused by the growth o…
Pathology is the scientific study of disease processes that affect normal anatomy and physiology. Anatomical and physiological changes are pathological changes when they result from an underlying disease process or abnormality. Pathologists play an increasingly important role in diagnosis, research, and in the development of clinical treatments for disease. A specialized branch of pathology, foren…
PCR (polymerase chain reaction) is a technique in which cycles of denaturation, annealing with primer, and extension with DNA polymerase, are used to amplify the number of copies of a target DNA sequence by more than 100 times in a few hours. American molecular biologist Kary Mullis developed the techniques of PCR in the 1970s. For his ingenious invention, he was awarded the 1993 Nobel Prize in ph…
The peafowl are three large, and extraordinarily beautiful species of fowl in the family Phasianidae, which also includes the pheasants, chickens, partridges, francolins, turkeys, guinea fowl, and quail. Species of peafowl are native to Asia and Africa. However, these gorgeous creatures have been kept in captivity as ornamental birds for several thousand years, and they are now found in zoos and a…
Peanut worms are a group of over 300 species of worms, classified as a separate Phylum Sipunculida (Phylum Sipuncula, by some authors), called Sipunculids or Sipunculans in English. They have a simple tubular shape, with "a tube within a tube" body plan and the internal organs inside a body cavity (coelom), that is lined by a fine epithelium called peritoneum. They are non-segmented …
Obesity can be defined as a body weight that is greater than 30% above a favorable target weight. In order to fit the definition of obesity, the excess weight must be due to adipose, or fat, tissue. Muscle mass does not account for the weight attributed to obesity. Therefore, a body-builder with tremendous muscle mass for example is not, by definition, obese. A deleterious condition, obesity is ha…
Peccaries are wild pigs (order Artiodactyla, family Tayassuidae) of the New World which are relatives of the wild pigs of the Old World. Peccaries are the only pigs native to the New World pigs, all other pigs in North and South America are formerly domestic animals that have escaped and become feral. Taxonomists recognize three species of peccaries: the collared peccary (Tayassu tajacu) of the so…
In genetics, a pedigree is a diagram of a family tree showing the relationships between individuals together with relevant facts about their medical histories. A pedigree analysis is the interpretation of these data that allows a better understanding of the transmission of genes within the family. Usually, at least one member of the family has a genetic disease, and by examining the pedigree, clue…
Pelicans are large coastal birds belonging to order Pelecaniformes, along with gannets, tropicbirds, darters, frigatebirds, and cormorants. All of them have throat pouches to one extent or another, but only the pelicans' are so big. They eat nothing but fish, and the pouch is a handy device for catching their food. The pelican has perhaps the most unusual bill in the bird world. It is quite…
The main concern of psychiatrists and therapists who treat people with obsessions is the role they play in a mental illness called obsessive-compulsive disorder (OCD). Obsessions need to be distinguished from compulsions in order to understand how they interconnect with compulsive behavior and reinforce this debilitating illness. In psychiatric literature, obsessions are defined as disruptive thou…
Oceans are large bodies of saltwater that surround Earth's continents and occupy the basins between them. Ocean basins are the part of the seafloor that lies beyond the margins of the continents, generally in water deeper than 600 ft (183 m). Therefore, an ocean is both larger in area and deeper than a sea. It may be that most of the water on Earth today has been cycling between the oceans,…
Penguins are primitive, flightless birds that are highly specialized for marine life. Most species look A gentoo penguin with its chick. JLM Visuals. Reproduced by permission. rather similar, being generally dark-blue or dark-gray on top with a white belly. Some species, however, have a crest on their head and/or patches of color on their head and throat. The legs are set wide apart and t…
A peninsula is a piece of land surrounded by water on three sides and joined to a larger body of land by an isthmus, or neck. A peninsula is a topographic high spot; a dry land range of hills or mountains created during the formation of the earth's crust. It is left visible when the low areas on both sides either subside and become submerged, or when the water level rises and floods the val…
Pentyl or five carbon atom alkyl groups are also referred to as amyl groups. The term amyl is derived from the Latin word for starch, amylum, and is used because the five carbon atom amyl alcohols were first isolated from fermentation products. Amyl or pentyl alcohols consist of a chain of five carbon atoms with a hydroxyl group (-OH) connected to one of the carbon atoms. Each carbon atom is also …
The peony is an attractive flower, much beloved of gardeners. It is in the family Paeonaceae, though in the past it was in the family Ranunculaceae with other flowers such as the buttercup. The generic name is Paeonia and there are some 50 species in this group. The name peony comes from Paeon, a physician in Greek mythology. His teacher was jealous of his skills as a healer and intended to murder…
Pepper, one of the world's most important spices, comes from the fruit (peppercorns) of a flowering shrub, in the genus Piper, family Piperceaea. The pepper plant originates from India, which is still the world's largest producer of pepper. The plant grows in hot, humid regions such as India, Indonesia, Malaysia, and Brazil. The United States is the world's largest importer of…
Proteins are made up of amino acids, which are joined by peptide linkages. Although there are only 20 different naturally occurring amino acids, various combinations of these form the thousands of proteins used in metabolism. All amino acids have a similar structure. There is a central carbon atom, called the alpha-carbon and this is bonded to an amino group on one side and a carboxyl group on the…
Ocean basins are that part of Earth's surface that extends seaward from the continental margins, ranging from an average water depth of about 6,500 ft (2,000 m) down into the deepest trenches. The ocean basins constitute one of the two major topographic features of Earth's surface, the other being the continents. Ocean basins cover about 70% of the total sea area and about half of th…
Percent is a device for expressing hundredths. Thus P% stands for P × 0.01. For example, 6% is equal to six hundredths (6 × 0.01, or 0.06) and 7.2% is equal to 7.2 × 0.01, or 0.072. As shown by these examples, to change from a percent to a number, divide the percent figure by 100. Conversely, to convert hundredths to percent, multiply the number by 100. …
The ocean sunfish (Mola mola), also called the headfish, is so named because of its unique shape: it looks as if it is all head and no body. The ocean sunfish is a very large species that lives in tropical and temperate waters. The ocean sunfish has a flattened, oval body that may measure 11 ft (3.5 m) in length and weigh as much as 2,000 lb (1,000 kg). In contrast to its huge size, it has a verte…
Human perception is the active reception and coordination of information received through our sensory systems in order to make sense of the environment and to behave effectively within it. In contrast with the direct and immediate sensations actually received and transmitted, perception is the transformation of that information into nerve cell activity that is transferred to the brain where furthe…
Perch belong to the class Osteicthyes, whose members have a skeleton of bone rather than cartilage. Bony fish comprise the largest group of vertebrates living today, both in the number of individuals (millions) and in the number of species (about 30,000). Perch occur in both fresh water and sea water throughout the entire world. Perch live at depths in the oceans as great as 7 mi (11.5 km) and in …
The peregrine falcon (Falco peregrinus) is a bird of prey in the family Falconidae. It is one of the most wide-ranging birds in the world with populations in both the Eastern and Western Hemispheres, and occurring on all continents except Antarctica. It is also the world's fastest-flying bird. Unfortunately, beginning in the 1940s, many populations of the peregrine falcon were decimated by …
A perfect number is a whole number which is equal to the sum of its divisors including 1 by excluding the number itself. Thus 6 is a perfect number becauses 1 + 2 + 3 = 6. Likewise 28 is a perfect number because 1 + 2 + 3 + 4 + 7 + 14 = 28. The search for even perfect numbers, then, is the same as the search for Mersenne primes. At the present time 32 are known corresponding to p = 2, 3, 7, 13,...…
Ocean zones are layers within the seas that contain distinctive plant and animal life. They are sometimes referred to as ocean layers or environmental zones. A system of zonation frequently used by oceanographers grew out of suggestions made by Joel Hedgpeth in 1957. According to that system, the ocean environment is first divided into two broad categories, known as realms, the benthic realm, cons…
A periodic function is a function whose values repeat at regular intervals. Given an interval of length t, and a function f, if the value of the function at x + t is equal to the value of the function at x then f is a periodic function. In standard function notation this is written f(x + t) = f(x) (read "f of x plus t equals f of x"). The shortest length t for which the function repe…
Oceanography, the study of the oceans, is a combination of the sciences of biology, chemistry, geology, physics, and meteorology. Ancient explorers of the ocean were sailors and fishermen who learned about marine biology by observing the sea life and discovering when it was most plentiful. They observed the effects of wind, currents, and tides, and learned how to use them to their advantage, or to…
The octet rule allows chemists to predict the placement of electrons around the nucleus (electron orbitals), the identification of electrons added or lost during chemical reactions, and the chemical reactivity of atoms based upon their particular electron configuration. The octet rule is used when drawing Lewis dot structures and disgraming electron configurations. The octet rule is used to descri…
The octopus is an invertebrate in the class Mollusca (the molluscs), which also includes snails, clams, and squid. Octopi are cephalopod molluscs which are generally considered to be the most advanced members of the class. There are about 220 species of octopus. Octopi are found in every ocean of the world, ranging in size from a tiny Philippine species barely an inch across to giant specimens tha…
Ohm's law is a relationship between the voltage across an electric circuit, the electrical resistance in the circuit, and the current in the circuit. This law is named after its discoverer, Georg Simon Ohm. Ohm found that for most electric circuits, the voltage across the circuit was equal to the current flowing through the circuit times the electrical resistance of the circuit. For the sam…
Petroleum is a critically important natural resource. However, petroleum is often mined in places that are far away from the regions where most of its consumption occurs. Accordingly, petroleum must therefore be transported in large quantities, mostly by oceanic tankers, barges on inland waters, and both subsea and overland pipelines. Any of these transportation systems can release pollution throu…
The arrangement of the chemical elements into periods (horizontal rows) and groups (vertical columns) is called the periodic table. The elements in the table are represented by symbols (one, two, or three letters) in individual squares. Above each chemical symbol appears the atomic number of the element. These whole numbers are the number of protons present in the nucleus of that element. Below th…
Oil and natural gas has been found in geologic strata of Earth from the surface to depths exceeding 30,000 ft (9,144 m). Bogs and seeps in the ancient world were the initial source of oil and gas. As advancing economic systems and industries emerged with the development of nations and expanding populations, the need for plentiful and more efficient sources of energy were required. Hydrocarbon base…
Geologists define permafrost as soil layers or rock and soil combinations that remain frozen (i.e., remain below the specific freezing temperature unique to the exact constituents of the formation) for a time period in excess of two years. The exact composition of permafrost can vary widely depending on the unique geology and morphology of the area in which the perafrost forms. Although ice is usu…
The term perpendicular describes a pair of lines or planes that intersect each other at a 90 degree angle. Perpendicularity is an important concept in mathematics, science, and engineering. A line l1 is perpendicular to a line l2 if the two intersect with congruent adjacent angles, which means that the angles are both equal to 90 degrees. Of course, a purely analytical definition of the term exist…
A pesticide is a chemical that is used to kill insects, weeds, and other organisms to protect humans, crops, and livestock. A broad-spectrum pesticide that kills all living organisms is called a biocide. Fumigants, such as ethylene dibromide or dibromochloropropane, used to protect stored grain or sterilize soil fall into this category. Generally, however, we prefer narrower spectrum agents that a…
Pests are any organisms that are considered, from the perspective of humans, to be undesirable in some ecological context. For example, pests could be insects that compete with humans for some common resource, such as agricultural production or timber. Other pests might be associated with diseases of humans, livestock, or agricultural plants. Pests could also be unwanted weeds that compete with ag…
Old-growth forests are natural ecosystems dominated by large, old trees, usually of a mixed species composition, and with all ages present in the community. Old-growth forests also contain many scattered, dead trees, both standing and lying on the forest floor. In the tropics, these forests are threatened by conversion to agriculture and by other disturbances, while old-growth forests in the tempe…
The nuclear age began in mid-July 1945 when an 18.6-kiloton nuclear bomb was detonated at the Trinity test site near Alamogordo, New Mexico. Three weeks later, on 6 August 1945, the world became aware of the existence of nuclear weapons when a U.S. B-29 bomber known as Enola Gay dropped a nuclear bomb on the Japanese city of Hiroshima. That was followed three days later by the dropping of another …
Although the English word nude is derived from the Latin nudus meaning "naked," "bare," it connotes, especially in such phrases as "in the nude" or "The Nude," more than a state of undress; rather it indicates a work of art, a cultural convention, and a socioreligious attitude. The term The Nude signifies a Western cultural ideology while nud…
Objectivity is the idea that the truth can be known independently of viewpoint, perspective, or bias. For example, "Gustav Mahler completed nine symphonies," is an objectively true statement, independent of the person who utters it. "Gustav Mahler was the greatest composer in history" is not an objective truth—some people would agree, others would disagree, depen…
The olive family is a family of flowering plants known to botanists as the Oleaceae. The Oleaceae have about 25 genera and over 500 species. Most species are native to temperate and tropical regions of the Northern Hemisphere. The best known trees of this family are olive and ash, while the most familiar shrubs are privet, lilac, and golden bell, all popular ornamental plants. …
Petrels and shearwaters are wide-ranging oceanic birds with a characteristic tubenose and other specialized features that equip them well for a life spent mostly at sea. Found throughout the world, these long-lived colonial nesting seabirds include some 79 species in four families, all in the Order Procelliformes. These seabirds show a great range in body size, from the giant petrel with a 6 ft (2…
Since Plato wrote of political obligation in his dialogue Crito, obligation in general has been of ongoing interest to philosophers. In that dialogue, Socrates argues that he was under an obligation to obey the laws of Athens and comply with a sentence of death. During the course of the argument, he raises and offers solutions to many of the central issues about obligation that philosophers still …
An omnivore is any animal that is a generalist feeder, consuming a wide variety of foods that can include both animal and plant matter. Because they have attributes of both carnivores and herbivores, omnivores have relatively diverse linkages within ecological food webs. Some examples of omnivorous animals are pig and bear, both of which will eat a remarkably wide range of plant and animal product…
The term Occidentalism refers primarily to the many ways in which non-Western intellectuals, artists, and the general public perceive and present the West. Though it seems to be an inversion of Orientalism, it has acquired some unique aspects defying a simple definition. In fact, the practices and discourses of Occidentalism vary a great deal, from time to time and region to region. If we can arbi…
In mathematics, one-to-one correspondence refers to a situation in which the members of one set (call it A) can be evenly matched with the members of a second set (call it B). Evenly matched means that each member of A is paired with one and only one member of B, each member of B is paired with one and only one member of A, and none of the members from either set are left unpaired. The result is t…
The challenge of reconstructing the history of nonliterate or preliterate societies makes necessary the study and interpretation of oral traditions. Many such societies have gone to great extents to preserve and transmit the knowledge of their past in oral forms. The phrase oral traditions refers to folklore, legends, tales, taboos, and stories through which knowledge of the past is preserved and …
Opahs (Lampris guttatus) are a bony ray-fin oceanic fish, with a world-wide distribution, in the Family Lamprididae of the Order Lampridiformes. Opahs are known only from specimens found stranded on the beach or captured accidentally by commercial travelers because these fish live at depths of 325-1,300 ft (100-400 m). Little is known of their life habits, but they are very fast swimmers and feed …
For the vast majority of human history, the only way people could transmit information has been by speaking, listening, and remembering. Indeed, the capacity for speech and the connected capacities for learning and remembering might be thought of as the defining elements of human consciousness, shared perhaps with other, now extinct, members of the hominid lineage but not shared with any other exi…
Many software programs must be purchased from a vendor. The purchase price entitles the buyer to the benefits of the functional software. The program code that makes these performance features possible (the source code), however, usually remains known only to the company that designed the software. Open source software runs counter to this philosophy. As its name implies, in open source software, …
Organicism refers to the idea that some object or entity shares an important property or quality in common with a living or animate being. It is related to, although remains distinct from, holism, in the sense that organicist doctrines tend to uphold the view that the living creature is an integrated whole containing precisely the range and number of parts necessary for the maintenance of its exis…
The American or common opossum (Didelphis marsupialis) is the only member of the order Marsupialia to occur naturally in North America. The American opossum occurs from southern Ontario through to most of South America. The only other member of its genus is D. azarae, which occurs through much of South America. However, other genera and species of the family Didelphidae, the New World opossums, oc…
In archaeology, petroglyphs and pictographs are terms used to describe forms of "rock art." Petroglyph refers to a rock carving or etching, while the term pictograph is commonly applied to a rock painting. Typically, these features are found on the vertical or overhanging surfaces of large boulders and are sometimes associated with nearby settlements. However, they are often found is…
The Arab-American political activist and professor of European literature Edward Said (1935–2003) durably redefined the term Orientalism with the publication in 1978 of his book by the same name. Before Said's book, Orientalism had two distinct meanings. Both were politically neutral and marginal to the central concerns of the humanities and social sciences. After Said's Orien…
Edward Said's 1978 book, Orientalism, is the harshest critique to date of Western scholarship on the Muslim orient. However, its focus is almost exclusively on the Near and Middle East. The policy statements of William Ponty, the French governor-general in Senegal (1907–1915), quoted below suggest a link between Orientalism, as understood by Edward Said, and colonial scholarship on M…
The very suggestion of a unified tradition implicit in the idea of the word orthodox renders its meaning problematic for the simple reason that conceptions of any religion's traditions are notoriously pluralist. Moreover, the term is hardly universal, since it is seldom applied to indigenous traditions around the world, even though there are some beliefs and practices within each aboriginal…
Opportunistic species of animals or plants are adapted to exploit newly available habitats or resources and are typically found in unpredictable, transient, and variable environments. For example, clear-cut forests create well-lit open areas which are colonized rapidly by the windbone seeds of opportunistic species of plants, many of which are regarded as weeds by farmers and gardeners. Besides pr…
The term orthopraxy means "right practice," and stands as a contrast term for orthodoxy "right belief." It is often said of Asian traditions generally that—in contrast to most Western traditions—right practice is of more importance that right belief, and to a certain extent this is true. For instance, Confucianism is primarily concerned with the rituals an…
Almost from the invention of the laser, researchers were considering the possibilities of optical data storage. Throughout the 1960s and 1970s, a number of companies were at work on optical data storage systems, held back in large part by the cost and performance level of available lasers. In 1982, Sony Corp. revolutionized the music industry with the introduction of the compact disc (CD). CD-ROM …
Orthopraxy or orthopraxis (from Greek orthos, "correct," and praxis, "action") denotes proper action, particularly in a religious context. It is contrasted with orthodoxy (orthos and doxa, "opinion"), which denotes proper belief. The word orthopraxy is of relatively recent invention and has been used above all in connection with Latin American "libe…
All epistemological and hermeneutical investigations are predicated on the observation that the human self, like all objects around a person, gains an understanding of its identity through the binary opposition of self and other. No value can be established for any element in the material and abstract world without the differentiation from "the Other." The famous linguist Ferdinand …
Petroleum is a term that includes a wide variety of liquid hydrocarbons. Many scientists also include natural gas in their definition of petroleum. The most familiar types of petroleum are tar, oil, and natural gas. Petroleum forms through the accumulation, burial, and transformation of organic material, such as the remains of plants and animals, by chemical reactions over long periods of time. Af…
The issues summoned up by the term pacifism are complex and varied because different concepts, traditions, and definitions exist throughout the world, often creating misunderstanding and confusion—sometimes intentionally so. For example, the term may be used pejoratively in political debates by individuals seeking to portray opponents who refuse to support a specific military action, or by …
pH is a measure of the acidity or alkalinity of a solution based upon the dissociation of water. The variability of pH can have a dramatic effect on biological or physical chemical reactions (e.g., geochemical weathering processes). The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868–1939) in 1909 and is generally presented as ranging from 0&#x…
Because it refers neither to a single political ideology nor a clearly discernible philosophical tradition, Pan-Africanism is difficult to define. Many scholars avoid defining it, noting that black internationalism has varied drastically according to time and place. Indeed, various conceptions of Pan-Africanism have been aligned with disparate political and theoretical positions, from largely reli…
Optics is the branch of physics that is concerned with visible light and its properties. Physicists who focus on optics study the properties of light. They also apply these properties to phenomena such as color, mirrors, and lenses. Geometrical optics treats light phenomena (e.g., the determination of focal points, image characteristics, etc.) through calculations derived from the geometry of rays…
Phalangers are a small group of arboreal mammals belonging to the family Phalangeridae, of which 20 species are recognized in six genera. Phalangers, more commonly known as possums and cuscuses, are marsupials but with a vague resemblance to some monkeys. Indeed many early European explorers thought that they were monkeys. These species occur in Australia, New Guinea and adjacent islands west to S…
Pan-Arabism is the concept that all Arabs form one nation and should be politically united in one Arab state. The intellectual foundations of pan-Arabism were laid down in the early decades of the twentieth century, in the context first of Arab alienation from Ottoman rule and later in response to the imperialist partition of the Arab provinces of the Ottoman Empire after World War I. The doctrine…
Pharmacogenetics is the exploration of the relationship between inherited genes and the ability of the body to metabolize drugs. Although research interests are rapidly expanding, by early 2003 pharmacogenetics research concentrated on trying establish connections between the genes carried by an individual (genotype) and specific reactions to drugs (e.g., side effects, toxicities, etc.). Modern me…
Pan-Asianism as a general term refers to a wide range of ideas and movements that called for the solidarity of Asian peoples to counter Western influences in the late nineteenth and early twentieth centuries. In Japan, where Pan-Asianism had a decisive influence on the course of its modern history and served as an ideological justification for its military expansionism through 1945, it is referred…
A term of European origin, pan-Islamism denotes the intellectual and institutional trends toward Islamic unity that emerged among Muslim peoples, starting in the mid–nineteenth century and continuing throughout the twentieth century. The need for a unified Islamic identity was a product of the challenges posed by Western intervention in and domination of Muslim societies during the colonial…
The term Pan-Turkism refers to an intellectual and political movement advocating the union of all Turkic peoples. Although some promoters of this ideology went as far as calling for a political union including all Turkic groups, many others envisioned only a cultural unity. …
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…
Paradigm is the key term in Thomas Kuhn's (1922–1996) very influential book, The Structure of Scientific Revolutions (1962). As is frequently the case when new ideas are presented, Kuhn took an existing term and gave it a specialized meaning. The term paradigm now occurs frequently in every kind of discourse, usually to mean something like "way of thinking" or "a…
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…
The word paradise develops in Western languages from the Greek word paradeisos, the old Persian word pairidaeza, and the modern Arabic and Persian firdaus, all of which originally denoted a walled garden. In the arid environment of the Near East, a garden must be carefully and laboriously constructed with watercourses for irrigation, and its precious flowers and fruits protected from theft by a su…
Orang-utans (Pongo pygmaeus) are large, long-haired, red apes that inhabit lowland primary forests of Borneo and a small area in the mountains of northwest Sumatra. Their distribution was once much more widespread, extending throughout the tropical forests of Southeast Asia. However, these apes are now endangered, mostly because of the clearing of their forest habitat to develop agricultural land,…
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 …
Patriotism is one of a large class of words that are linked to the virtues of membership. To participate in relations of, for example, friendship, community, nationhood, citizenship, or marriage implies normative conventions. In other words, there are value expectations built into such membership. One important dimension of any membership relation is an expectation of loyalty. Fidelity or loyalty …
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…
The peace concept has a long history both in the Western and Eastern intellectual traditions. While Greco-Roman and Judeo-Christian ideas regarding peace have expanded and changed over time, this is not so much the case in Muslim, Hindu, and Buddhist thought. …
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, …
Currently concentrated in Africa, Asia, and Latin America, the peasantry has been defined differently by various scholars, depending on the degree of emphasis placed on any one of several characteristics. Definitions of the peasantry embrace some of the following characteristics: ownership and use of land, production methods, subordination to other social sectors, and the degree of integration int…
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…
Periodization, which became a branch of historical method and the philosophy of history in the twentieth century, has to do with the division of time's arrow—the theoretical timeline of the movement from past to present and future. In Western tradition this speculative aspect of history has its roots in myth and in the Bible—in Hesiod's succession of gold, silver, and b…
An orbit is the path followed by a celestial body moving in a gravitational field. When a single object, such as a planet, is moving freely in a gravitational field of a massive body, such as a star, the orbit is in the shape of a conic section, that is, elliptical, parabolic, or hyperbolic. Most orbits are elliptical. The exact path and position of an object in space can be determined by taking i…
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 …
Notions of boundaries, categories, and periods frame discussions of art and visual culture. The desire to organize visual information and material into clearly defined, manageable units has provided an irresistible impetus for periodization since the emergence of art historical and critical studies in the Renaissance. The application of periods to art and visual culture was extended in the ninetee…
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…
Western European ideas about the person have long centered on the duality between body and soul (in religious discourse) or between body and mind (in the domains of philosophy and psychology). Consequently, early anthropological interest in non-European ideas of the person tended to mirror such deep-seated European conceptualizations, focusing on the origins of the concept of the soul. The preoccu…
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…
What is a person? Two basic types of considerations are apparent in African answers to this question. The first is ontological, the second ethical. Ontologically a person is a combination of a physical constituent, namely the body and a set of two or in some cases three constituents of a rarefied character requiring careful elucidation. The nonbodily constituent on which all the accounts of person…
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 …
In the visual arts, the English word perspective refers to the optical illusion whereby a picture on a flat, two-dimensional plane appears to be three-dimensional; as if the represented objects were actually in a deep space receding behind the picture surface (like looking at them through a window or in a mirror), and in some cases seeming even to project forward in front of the picture. The term …
The many species of orchids comprise one of the largest families of flowering plants, the Orchidaceae, which contains about 1,000 genera and about 20,000 species. Orchids have a worldwide distribution, and they occur in a wide variety of habitats, although their greatest diversity of species is in tropical rain forest. The most species-rich genera of orchids are Dendrobium and Bulbophyllus, each w…
Phenomenology is the study of experience, how things appear to us. The word comes from the Greek but was elaborated in the early nineteenth century on the basis of Immanuel Kant's conception of the world as phenomenon, the world of our experience (as opposed to the world as noumenon, the world as it is "in itself"). Georg Wilhelm Friedrich Hegel famously employed the term in P…
The number 8 can be used in three ways: to tell "how many," to tell "where" in a ranking, and to name someone or something. The girl with the number 8 on her baseball uniform, who is 8th in the batting order, playing on a team that scores 8 runs, is using the same number in each of these ways. When she is 8th in the batting order, she is using the number as an ordinal n…
Active in English usage by the seventeenth century, philanthropy has been shown more recently to have a long, complex, and controversial history displaying multiple meanings of the term. Now philanthropy is normally construed as love of mankind, a voluntary, practical benevolence toward others, or an effort to promote the welfare of humanity by gifts without self-benefit. However, philanthropy ove…
Ancient Egypt has been offered as a point of origin of African metaphysical speculation. According to the work of Senegalese historian Cheikh Anta Diop, Egypt is to Africa what the Greco-Latin civilization is to the West, and texts such as The Book of the Dead, written more than 3,500 years ago, could play the role of a founding text for a tradition of philosophical thinking on the African contine…
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 …
American philosophies are as varied and diverse as the many backgrounds and cultures that collaborate to form American life. Even so, there are some similar emphases that cross these differences. Often, philosophies in America valorize action and the practical aspects of thought; American philosophers are regularly concerned with problems of race, class, gender, and other ethical and political for…
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 term feminism is used both in reference to social movements, such as the late-nineteenth century women's rights movement or the mid-twentieth century women's movement in Europe and the United States, and to theories that identify and critique injustices against women, such as A Vindication of the Rights of Woman by Mary Wollstonecraft (1759–1797) or The Book of the City of…
All Muslims hold the Koran as the very word of God, who provides guidance and understanding of creation and proper human conduct by divine wisdom (hikma). This wisdom is also reflected in the oral statements of the prophet Muhammad collected in the Traditions (hadith) and in the reports of the life and deeds (sunna) of the Prophet. The study of these in the traditional Islamic religious sciences o…
The dialectic between religious and secular philosophy and the split between those who think of philosophy as akin to poetry and those who insist that it be exact and rigorous are not the only oppositions that rend modern philosophy. There is also the perennial tug-of-war between those philosophers who stubbornly hold onto the ancient paradigm of philosophy as the more or less practical search for…
For Plato, the first characteristic of philosophical wisdom is that it meet the needs of rational inquiry. As he suggests at Apology 22, this criterion precludes all types of quotidian knowledge and other homespun verities in favor of genuine philosophical insight. Neither the statesman, the artisan, nor the poet can explain why he is doing what is he doing, for none of them has formulated a clear…
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…
A respected Princeton philosopher keeps a sign on his office door forbidding the discussion therein of any philosophy more than ten years old. At this late stage in his career the restriction includes a good deal of his own work. This may well be the limit case of the antihistorical attitude that prevailed throughout much academic philosophy of the twentieth century, motivated by the view that phi…
Morality consists of two parts: first, morality in the strict sense; and second, morality as custom or mores. The defining principle of the first is the golden rule, that of the second, local utility, idiosyncrasy, or even accident. The first is known to be universal to all human cultures. This has to be so of a necessity, in view of what the rule entails. The golden rule, to follow Harry Gensler,…
Our picture of Socrates derives mainly from several short, inconclusive dialogues by his disciple Plato. In these, Socrates, believing that "the unexamined life is not worth living," typically challenges the conventional beliefs of his fellows, both ordinary people and more sophisticated thinkers, with questions about how human life should be lived. When his inter-locutors prove unab…
Ore is metalliferous rock that can be mined and processed at a profit. Although a broader definition includes nonmetallic rocks like rock salt and gypsum, most geologists classify these materials as industrial rocks and minerals. Unlike products from the forest and farm, ores are a nonrenewable resource. The economic survival of industrial societies is linked to the discovery of new supplies of me…
Moral philosophy in the medieval West derived from two main sources: Christianity and classical ethics. The attempt to reconcile these different traditions and develop a viable synthesis of the two was a central concern of moral philosophy throughout the period. …
An organ is a functional structure of multicellular organisms which consists of a group of several different tissues. Many multicellular organisms have individual cells grouped together into tissues, a group of many associated cells with similar function; tissues grouped together into organs, a group of tissues interacting so as to form a functional unit; and organs grouped together into organ sys…
The moral philosophy of the modern period traditionally included ethics as well as natural-law theories of rights and the normative foundations of state authority. Nowadays, the term moral philosophy tends to be used mainly with reference to ethics proper, while modern natural-law theories and their history are often treated under the headings of political philosophy and philosophy of law. In keep…
Organelles are internal cellular structures that perform dedicated functions. Oraganelles includes tructures such as ribosomes, mitochondria, chloroplasts (the site of photosynthesis in plants and other photosynthesizing organisms), endoplasmic reticulum (ER), Golgi apparatus, and lysosomes. The mitochondrion of all eukaryotes and the chloroplasts of plant cells are the only organelles that have t…
In the works of Aristotle, criticisms of popular misconceptions of divinity and genuine moments of piety are combined. More important for later thinking about theology, however, are Aristotle's arguments for the existence of a divine prime mover of the universe and his account of that entity. At the end of Physics (book 8) and in Metaphysics (book 12), he argues that the impossibility of an…
