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tar and pitch

(Encyclopedia)tar and pitch, viscous, dark-brown to black substances obtained by the destructive distillation of coal, wood, petroleum, peat, and certain other organic materials. The heating or partial burning of w...

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...

Gay-Lussac, Joseph Louis

(Encyclopedia)Gay-Lussac, Joseph Louis zhôzĕfˈ lwē gā-lüsäkˈ [key], 1778–1850, French chemist and physicist. He was professor in Paris at the Sorbonne, at the Polytechnic School, and at the Jardin des Pla...

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...

diagnosis

(Encyclopedia)diagnosis, determination of the nature of a disease or ailment. A clinical diagnosis is based on the medical history and physical examination of the patient: it may be confirmed with X-Rays, CAT Scans...

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...

gas

(Encyclopedia)gas, in physics, one of the three commonly recognized states of matter, the other two being solid and liquid. A substance in the gaseous state has neither definite shape nor definite volume. Like liqu...

dysprosium

(Encyclopedia)dysprosium dĭsprōˈzēəm [key] [Gr.,=hard to get at], metallic chemical element; symbol Dy; at. no. 66; at. wt. 162.500; m.p. 1,412℃; b.p. 2,562℃; sp. gr. 8.54 at 25℃; valence+3. Dysprosium i...

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...