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plutonium

(Encyclopedia)plutonium plo͞otōˈnēəm [key], radioactive chemical element; symbol Pu; at. no. 94; mass no. of most stable isotope 244; m.p. 641℃; b.p. 3,232℃; sp. gr. 19.84 at 20℃; valence +3, +4, +5, or ...

crater

(Encyclopedia)crater, circular, bowl-shaped depression on the earth's surface. (For a discussion of lunar craters, see moon.) Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised ce...

city

(Encyclopedia)city, densely populated urban center, larger than a village or a town, whose inhabitants are engaged primarily in commerce and industry. In the United States a city is legally an incorporated municipa...

exobiology

(Encyclopedia)exobiology or astrobiology, search for extraterrestrial life within the solar system and throughout the universe. Philosophical speculation that there might be other worlds similar to ours dates back ...

Shiites

(Encyclopedia)Shiites shēˈītz [key] [Arab., shiat Ali,=the party of Ali], the second largest branch of Islam, Shiites currently account for 10%–15% of all Muslims. Shiite Islam originated as a political moveme...

helium

(Encyclopedia)helium hēˈlēəm [key], gaseous chemical element; symbol He; at. no. 2; at. wt. 4.0026; m.p. below −272℃ at 26 atmospheres pressure; b.p. −268.934℃ at 1 atmosphere pressure; density 0.1785 g...

sulfur

(Encyclopedia)CE5 Sulfur crystals sulfur or sulphur sŭlˈfər [key], nonmetallic chemical element; symbol S; at. no. 16; interval in which at. wt. ranges 32.059–32.076; m.p. 112.8℃ (rhombic), 119.0℃ (mon...

tide

(Encyclopedia)tide, alternate and regular rise and fall of sea level in oceans and other large bodies of water. These changes are caused by the gravitational attraction of the moon and, to a lesser extent, of the s...

infrared astronomy

(Encyclopedia)infrared astronomy, study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and building...

Holocaust

(Encyclopedia)Holocaust hŏlˈəkôstˌ, hōˈlə– [key], name given to the period of persecution and extermination of European Jews by Nazi Germany. Romani (Gypsies), homosexuals, Jehovah's Witnesses, the disabl...