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dengue fever

(Encyclopedia)dengue fever dĕngˈgē, –gā [key], acute infectious disease caused by four closely related viruses and transmitted by the bite of the female Aedes mosquito; it is also known as breakbone fever and...

crossing over

(Encyclopedia)crossing over, process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each ...

Challenger expedition

(Encyclopedia)Challenger expedition, British oceanographic expedition under the direction of the Scottish professor Charles Wyville Thompson and the British naturalist Sir John Murray. Taking place from 1872 to 187...

europium

(Encyclopedia)europium yo͝orōˈpēəm [key] [from Europe], metallic chemical element; symbol Eu; at. no. 63; at. wt. 151.964; m.p. about 820℃; b.p. about 1,600℃; sp. gr. 5.25 at 25℃; valence +2 or +3. Europ...

seeing

(Encyclopedia)seeing, in astronomy, the clarity with which stars and other celestial objects can be observed. It is primarily determined by the atmosphere of the earth. The most obvious phenomenon is twinkling, whe...

silica

(Encyclopedia)silica or silicon dioxide, chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. It occurs in sev...

plasma

(Encyclopedia)plasma, in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions (see electron and ion). It is electrically conductive and is affected by magn...

Sagan, Carl Edward

(Encyclopedia)Sagan, Carl Edward sāˈgən [key], 1934–96, American astronomer and popularizer of science, b. New York City. Early in his career he investigated radio emissions from Venus and concluded that the c...

Titicaca

(Encyclopedia)Titicaca tētēkäˈkä [key], lake, c.3,200 sq mi (8,290 sq km), 110 mi (177 km) long, and c.900 ft (270 m) deep at at its deepest point, in the Andes Mts., on the Bolivia-Peru border; second largest...

nursery

(Encyclopedia)nursery, in horticulture, an establishment or area for the propagation, breeding, and early cultivation of plants. In North America the term nursery originally specified a place where hardy woody plan...