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Possible definitions for hydroa
hydra
Any of 20-30 species of freshwater cnidarians (genus Hydra). The polyp-type body is a thin, usually translucent tube that measures up to slightly more than 1 in. (25 mm) long. Food is ingested and wastes are ejected from the open, tentacled end. Individuals are usually hermaphroditic (see hermaphroditism). Reproduction by budding is also common. Species differ in color, tentacle length and number, and gonad position and size. All hydras feed on other small invertebrates (e.g., crustaceans).
hydrogen
Lightest chemical element, chemical symbol H, atomic number 1. A colorless, odorless, tasteless, flammable gas, it occurs as the diatomic molecule H2. Its atom consists of one proton (the nucleus) and one electron; the isotopes deuterium and tritium have an additional one and two neutrons, respectively. Though only the ninth most abundant element on earth, it represents about 75% of all matter in the universe. Hydrogen was formerly used to fill airships; nonflammable helium has replaced it. It is used to synthesize ammonia, ethanol, aniline, and methanol; to treat petroleum fuels; as a reducing agent (see reduction) and to supply a reducing atmosphere; to make hydrogen chloride (see hydrochloric acid) and hydrogen bromide; and in hydrogenation. Liquid hydrogen (boiling point -487° F, or -252.8° C) is used in the laboratory to produce extremely low temperatures, in bubble chambers, and as a rocket fuel. Combustion of hydrogen with oxygen produces water. The properties of most acids, especially in water solutions, arise from the hydrogen ion (H+, also referred to as the hydronium ion, H3O+, the form in which H+ is found in a water environment) See also hydride, hydrocarbon.
Hadrian
Roman emperor (117-38), Trajan's nephew and successor. After years of intrigue, he was adopted and named successor just before Trajan's death. He executed his senatorial opponents, abandoned Trajan's conquests in Armenia and Mesopotamia, and coped with unrest in Mauretania and Parthia. He traveled widely, and many of his accomplishments were related to his visits abroad. He began construction of Hadrian's Wall, and he visited and disciplined troops in Algeria and elsewhere. An admirer of Greek civilization, he completed the temple of Zeus in Athens and created a federation of Greek cities. He launched a building program at Delphi and was initiated into the Eleusinian mysteries. After his young companion Antinoü s drowned in the Nile (130), he grieved openly; he erected statues of the boy throughout the realm, and cults sprang up widely. He named Antoninus Pius his successor, to be followed by Marcus Aurelius.
hadron
Any of the subatomic particles that are built from quarks and thus interact via the strong force. The hadrons fall into two groups: mesons and baryons. Except for protons and neutrons, which are bound in nuclei, all hadrons have short lives and are produced in high-energy ...
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