A primitive group of fish, hagfish (order Hyperotreti, family Myxinidae) resemble eels in their external appearance. These fish lack a backbone, jaws, true fins, and scales. Their body is tubelike and often covered in a slimy substance that is secreted from abundant glands in the skin. The body is often a pale fleshy pink, but is occasionally brown-gray above and pink below. They may reach up to 2…
The half-life of a process is an indication of how fast that process proceeds—a measure of the rate or rapidity of the process. Specifically, the half-life is the length of time that it takes for a substance involved in that process to diminish to one-half of its initial amount. The faster the process, the less time it will take to use up one-half of the substance, so the shorter the half-l…
A current-carrying body placed in a magnetic field with the current direction unaligned with the field experiences a force leading to a transient sidewise drift of the charge carriers of the current. This drift continues until the force is balanced by an electric field produced by the charge accumulating at points on the body's surface in the direction of the drift. At points on the body&#x…
Hallucinogens are substances that alter the user's thought processes or mood to the extent that he perceives objects or experiences sensations that in fact have no reality. Many natural and some manmade substances have the ability to bring about hallucinations. In fact, because of the ready market for such chemicals, they are manufactured in illegal chemical laboratories for sale as halluci…
Halogenated hydrocarbons are derivatives of hydrocarbons (that is, organic compounds that only contain carbon and hydrogen atoms) which include some halogen atoms within their chemical structure. The most commonly encountered halogens in halogenated hydrocarbons are fluorine and chlorine, but sometimes bromine or iodine occur, or combinations of any of these. Some halogenated hydrocarbons occur na…
The halogens are a group of chemical elements that includes fluorine, chlorine, bromine, iodine, and astatine. Halogen comes from Greek terms meaning "produce sea salt." None of the halogens occur naturally in the form of elements, but, except for astatine, they are very widespread and abundant in chemical compounds where they are combined with other elements. Sodium chloride, common…
A halosaur is a thin, elongated fish resembling an eel. The largest of halosaurs grows to about 20 in (51 cm) long. Unlike the eel, the halosaur has a backbone composed of many vertebrae. It has somewhat large scales, numbering fewer than 30 horizontal rows on each of its sides. This fish lives close to or on the bottom of the sea and is thus referred to as a benthic fish. It feeds on the ocean fl…
Hamsters are small rodents with dense fur, a short tail, and large cheek pouches. They belong to the mammalian family Muridae, which also includes rats, mice, gerbils, voles, and lemmings. During foraging trips, hamsters use their cheek pouches to carry seeds and grains back to underground food stores that are sometimes quite large. Hamsters mostly eat plant matter, especially seeds, nuts, soft fr…
Hantavirus infections are infections of the lungs caused by hantaviruses. There are five known types of hantaviruses, which differ only slightly from one another. These types are: Hantaan, Seoul, Puumala, Prospect Hill, and Sin Nombre. The Sin Nombre virus was the cause of the 1993 outbreak in the Southwestern United States, which led to a greater understanding of the virus and its transmission to…
Most contemporary understandings of happiness are hedonic: happiness is a state of feeling most precisely defined by the subject of the feeling. Happiness in this sense is subjective and can be of brief duration. Ancient discussions of happiness, however, revolve around the Greek term eudaimonia, and while this word is commonly translated as "happiness," it has a different meaning an…
Hard water is water that contains large amounts of calcium, magnesium, or iron ions. Hard water is undesirable since it often has an unpleasant taste, interferes with the ability of soaps to dissolve (although some synthetic detergents dissolve well in hard water), and can cause scaling (the building up of insoluble precipitates) in pipes and hot water systems. Water hardness is most commonly the …
What makes a note from a musical instrument sound rich? The volume of the sound is determined by the amplitude of the oscillations in a sound wave, the distance individual molecules oscillate. A larger amplitude produces a louder sound and transmits more energy. The pitch of a note is the frequency or number of oscillations per second. A higher frequency produces a higher pitched note. The richnes…
As a concept, hate has several interrelated dimensions. It attempts to provide historical, psychological, and sociocultural depth to the forms of hostility and animosity that the term hate ostensibly defines, and to make the idea clear in terms of its linguistic usages. As such, it faces obstacles that often appear insuperable. Nonetheless, "hate crimes"—criminal acts and beha…
Hawks (family Accipitridae) are one of the major groups of predatory birds that are active during the day. They are members of the order Falconiformes, which also includes the falcons, vultures, and osprey, and like the other Falconiformes, they have the characteristic sharp, strong claws and hooked beak suited for catching and tearing up prey. Found on all continents but Antarctica, hawks are a d…
Hazardous wastes are by-products of human activities that could cause substantial harm to human health or the environment if improperly managed. The United States Environmental Protection Agency (EPA) classifies liquid, solid, and gaseous discarded materials and emissions as hazardous if they are poisonous (toxic), flammable, corrosive, or chemically reactive at levels above specified safety thres…
Hazels or filberts are shrub-sized woody plants in the birch family (Betulaceae) found in temperate forests of North America and Eurasia. Hazels have simple, coarse-toothed, hairy leaves that are deciduous in the autumn. Hazel species native to North America include the American hazel (Corylus americana) of the east and beaked hazel (C. cornuta) of a wider distribution. The giant filbert (C. maxim…
Health and disease seem at first glance to be obvious and opposing concepts. We are either healthy or suffering from some disease. In practice, however, health and disease are neither clearly defined nor mutually exclusive. Asthmatics and diabetics have won Olympic gold medals, and amputees can live to a ripe old age. "Healthy" people in their eighties cannot do things they could eas…
Hearing is the ability to collect, process and interpret sound. Sound vibrations travel through air, water, or solids in the form of pressure waves. When a sound wave hits a flexible object such as the eardrum it causes it to vibrate, which begins the process of hearing. The process of hearing involves the conversion of acoustical energy (sound waves) to mechanical, hydraulic, chemical, and finall…
Heart diseases (cardiovascular disease) is any abnormal organic condition of the heart or the heart and circulation. A number of conditions can lead to the development of heart disease, including angina, atherosclerosis, cardiac arrhythmia, cardiomyopathy, chronic venous insufficiency, diabetes, heart attack, high cholesterol, high homocysteine, high triglycerides, hypertension, insulin resistance…
The developing fetal heart accounts for a large percentage of the volume of the early thorax. About 20 days after fertilization, the heart develops from the fusion of paired endothelial tubes into a single tube. Heart growth subsequently involves the growth, expansion, and partitioning of this tube into four chambers separated by thickened septa of cardiac muscle and valves. Atrial development is …
The heart-lung machine is a device used to provide blood circulation and oxygenation while the heart is stopped. It is a means of keeping a patient alive while his heart is stopped or even removed from his body. Usually called the heart-lung machine, the device also is referred to as cardiopulmonary bypass, indicating its function as a means to substitute for the normal functions of the heart (car…
Heat exchange reflects and drives changes in energy state between two objects—or more generally systems—in thermal contact due to a difference in temperature. Heat flows from a system at higher temperature to one at lower temperature until both systems are at the same temperature. Systems at the same temperature are said to be in thermal equilibrium. The term "heat" is …
Heat capacity (often abbreviated Cp) is defined as the amount of heat required to raise the temperature of a given mass of a substance by one degree Celsius. Heat capacity may also be defined as the energy required to raise the temperature of one mole of a substance by one degree Celsius (the molar heat capacity) or to raise one gram of a substance by one degree Celsius (the specific heat capacity…
The heat index is a measure of how warm an average person feels as a consequence of moisture in the air compared to the actual temperature measured by a thermometer at the same time and location. Generally speaking, the higher the relative humidity, the warmer the temperature will seem to be to a person. The reason for this relationship is that the human body normally loses heat through the proces…
The heath family, or Ericaceae, contains about 100-125 genera of vascular plants comprising 3,000-3,500 species. These plants are widespread in North and South America, Eurasia, and Africa, but are rare in Australasia. Species of heaths are most diverse and ecologically prominent in temperate and subtropical regions. The most species-rich genus in the heath family are the rhododendrons (Rhododendr…
Aspects of heaven and hell cross religious traditions. Paradise can be a city, a palace, a court, a garden, a vision of God, a mystical diagram, or an ineffable concept. Physical, indeed sexual, terms and images express the soul's union with God. In Hell, fires, dragons, serpents, stench, cacophony, torturers, and their paraphernalia abound. Christian, Islamic, Zoroastrian, and Japanese sou…
Hedgehogs are small, often spine-covered members of the insectivore family Erinaceidae. The spiny hedgehogs are 13 species in subfamily Erinaceidae. Most famed is the European hedgehog, Erinaceus europaeus, which is also a resident of New Zealand, where it was introduced. Not all members of the hedgehog family have tough spines. The moonrats, or gymnures, of Southeast Asia have coarse hair instead…
Hedonism is a modern word derived from the Greek hedone, or "pleasure." As a philosophical position, moral hedonism justifies pleasure as a good, or even the good. Its history can be traced back to Hellenistic philosophy. Now, for Plato, Aristotle, and the Stoics, hedone is both the first and the worst candidate. Only the Epicureans, after the Cyrenaics, accept it fully as the condit…
Given the complexity of his thought, it is not surprising that Hegel's philosophy has been interpreted in a number of different and often opposed ways. As such, while many philosophical movements might be described as "Hegelian," there is no univocal sense of this term, nor any unanimity about what the proper interpretation of Hegel's idealism involves. Perhaps because …
Hegemony, from the Greek hegemón (guide, ruler, leader) and hegemonia (rule, leadership), denotes the preeminent influence a state, social class, group, or individual exercises over others. Today it is especially associated with the Italian Marxist and revolutionary Antonio Gramsci (1891–1937), for whom it is the core and organizing concept of his social and political ideas. More recen…
The Heisenberg uncertainty principle first formulated by German physicist Werner Heisenberg (1901–1976), has broad implications for quantum theory. The principle asserts that it is physically impossible to measure both the exact position and the exact momentum of a particle (like an electron) at the same time. The more precisely one quantity is measured, the less precisely the other is know…
The heliocentric theory argues that the Sun is the central body of the solar system and perhaps of the universe. Everything else (planets and their satellites, asteroids, comets, etc.) revolves around it. The prevailing theory of the universe at that time was a geocentric (Earth-centered) one, in which all celestial bodies were believed to revolve around Earth. This was seen as a more plausible th…
Hematology is the study of blood and its basic biological components, including red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (erythrocytes). Hematologists study and help treat a variety of hematological malfunctions and diseases, one of the primary being the various anemias. Anemias, like sickle cell anemia, result in a loss of erythrocytes, which reduces the…
Hemp, or Cannabis sativa, is a tall, annual plant that thrives in temperate and subtropical climates. It is native to central and western Asia, and is one of the oldest cultivated plants. The word "hemp" is derived from the old English word "hanf," and refers to both the plant and the long fibers that are processed from its stems. The most common use of hemp has been as…
Henna, species Lawsonia intermis of the family Lythracea, is a perennial shrub that grows wild in northern Africa and southern Asia. The name henna, which comes from the Arabic word al kenna, refers to both the plant and the dye that comes from the leaves. The henna plant has narrow, grayish green leaves and small, sweet smelling clustered flowers that are white, yellow, or rose in color. One of t…
Hepatitis is inflammation of the liver, a potentially life-threatening disease most frequently caused by viral infections but which may also result from liver damage caused by toxic substances such as alcohol and certain drugs. Hepatitis viruses identified to date occur in five types: hepatitis A (HAV), hepatitis B (HBV), hepatitis C (HCV), hepatitis D (HDV), and hepatitis E (HEV). All types are p…
An herb is an aromatic plant that is used by people most commonly in cooking, but sometimes for medicinal purposes, as an insect repellant, as a source of dye, and sometimes for their attractive aesthetics. Herbs are not necessarily plants that are taxonomically related to each other—what these plants share is a usefulness to humans, not an evolutionary lineage. In general, herbs are non-wo…
Modern medicine has provided many breakthrough treatments for serious diseases. Some conditions, however, have eluded the healing grasp of contemporary western medicine, which emphasizes rigorous scientific investigation of therapies. In addition, rising costs of some treatments have placed modern healthcare beyond the reach of many. The drugs that routinely fill pharmacy shelves of post-industria…
A herbicide is a chemical used to kill or otherwise manage certain species of plants considered to be pests. Plant pests, or weeds, compete with desired crop plants for light, water, nutrients, and space. This ecological interaction may decrease the productivity and yield of crop plants, thereby resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired…
An herbivore is an animal that eats plants as its primary source of sustenance. Examples of herbivores include large mammals such as cattle, deer, sheep, and kangaroos, as well as smaller creatures such as leaf-eating insects and crustaceans that graze upon aquatic algae. However, many animals are not exclusively herbivorous. In addition to feeding mostly upon live plants, omnivorous animals such …
Heresy is derived from a Greek word literally meaning "a choice." St. Irenaeus (c. 120 to 140–c. 200 to 203) defined heresy as deviation from the standard of sound doctrine. This definition provided a model for subsequent conceptions of heresy. Referring to the Greek word, St. Jerome (c. 347–419 or 420) wrote that each one chooses the rule that one judges to be the best…
Traditionally understood as the art of interpretation (ars hermeneutica) that provided rules for the interpretation of sacred texts, hermeneutics today serves to characterize a broad current in contemporary continental philosophy that deals with the issues of interpretation and stresses the historical and linguistic nature of our world-experience. Since this characterization is also valid for cont…
A hernia occurs when an organ or tissue bulges out of its proper location. Hernias can occur in numerous locations throughout the body, including the brain, abdomen, groin, spine, and chest. The most common types will be discussed below. Hernias can be either congenital or acquired. Congenital means that the individual was born with an abnormal opening, allowing the hernia to occur. Such a hernia …
Herons, egrets, and bitterns are large, slender wading birds in the family Ardeidae, order Ciconiiformes (which also includes anhingas, storks, spoonbills, and ibises). Most of the species in the heron family have long legs, necks, and bills. These characteristics are all adaptive to hunting their prey of fish, amphibians, snakes, small mammals, and other animals living in the shallow waters of we…
Herpetology is the scientific study of amphibians and reptiles. The term "herpetology" is derived from the Greek and refers to the study of creeping things. Birds and mammals, for the most part, have legs that lift their bodies above the surface of the ground. Amphibians (class Amphibia) and reptiles (class Reptilia), with the exception of crocodiles and lizards, generally have legs …
One of the most important fisheries in the world is provided by the true herrings, which belong to the bony fish family Clupeidae. This family contains a wide variety of fishes with distinctive habits. Although most of the species are marine, a few are anadromous—that is, they spend their lives in the sea and enter rivers to spawn. Other species remain permanently in freshwater. Herrings ar…
A Hertzsprung-Russell diagram, or H-R diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities, or brightnesses (plotted on the vertical axis). H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar pro…