Etymology
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Pyrex (n.)

1915, proprietary name (Corning Glass Works, Corning, N.Y.) of a type of hard, heat-resistant glass, an arbitrary coinage, in which advertisement writers and eager etymologists see implications of Greek pyr "fire" and perhaps Latin rex "king;" but the prosaic inventors say it was based on pie (n.1), because pie dishes were among the first products made from it. The -r-, in that case, is purely euphonious.

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pyrexia (n.)

"fever, a higher bodily temperature than is normal," 1769, medical Latin, from Greek pyrexis "feverishness," from pyressein "to be feverish, to be ill of fever," from pyretos "fever, burning heat" (related to pyr "fire," from PIE root *paewr- "fire") + abstract noun ending -ia. Formerly sometimes nativized as pyrexy. Related: Pyrexial; pyrexic; pyrexical.

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pyrgologist (n.)

"one versed in the structure and history of towers," 1877, from Greek pyrgos "a tower, wall-tower, siege-tower; highest point of a building" (a technical term in architecture, of uncertain origin, according to Beekes "clearly Pre-Greek") + -ologist. It seems to have been used once, in The Athenaeum of Aug. 18, 1877, and then forgotten except in dictionaries.

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pyrite (n.)

"metallic iron disulfide," occurring naturally in cubes and crystals, "fool's gold," 1550s, from Old French pyrite (12c.), from Latin pyrites, from Greek pyritēs lithos "stone of fire, flint" (so called because it glitters), from pyritēs "of or in fire," from pyr (genitive pyros) "fire" (from PIE root *paewr- "fire"). Related: Pyritic.

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pyro- 

before vowels pyr-, word-forming element form meaning "fire," from Greek pyr (genitive pyros) "fire, funeral fire," also symbolic of terrible things, rages, "rarely as an image of warmth and comfort" [Liddell & Scott], from PIE root *paewr- "fire." Pyriphlegethon, literally "fire-blazing," was one of the rivers of Hell.

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pyroclastic (adj.)

in geology, "formed by volcanic agencies," especially in reference to fast-moving, dense, superheated surges of ash, gas and rock in a volcanic eruption; by 1862 in reference to the rocks that result; see pyro- "fire" + clastic, indicating "broken in pieces, fragments."

The word "ash" is not a very good one to include all the mechanical accompaniments of a subaerial or subaqueous eruption, since ash seems to be restricted to a fine powder, the residuum of combustion. A word is wanting to express all such accompaniments, no matter what their size and condition may be, when they are accumulated in such mass as to form beds of "rock." We might call them perhaps "pyroclastic materials," but I have endeavoured in vain to think of an English word which should express this meaning, and believe, therefore, that the only plan will be to retain the word "ash," giving it an enlarged technical meaning, so as to include all the fragments accumulated during an igneous eruption, no matter what size or what shape they may be. [J. Beete Jukes, "The Student's Manual of Geology," Edinburgh, 1862]
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pyrogen (n.)

1858, as a proposed word for "electricity considered as a material substance possessing weight," from pyro- + -gen. Meaning "fever-producer, substance which, introduced into the blood, induces fever" is from 1896. Related: Pyrogenic. Greek pyrogenes meant "born in fire, wrought by fire" (compare pyrogenesis).

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pyrogenesis (n.)

"the production of fire or heat," 1858; see pyro- + genesis. Pyrogenetic is attested from 1855 as "having the property of producing heat;" by 1838 in geology as "produced by heat."

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pyrolatry (n.)
"fire-worship," 1660s, from pyro- + -latry "worship of." Related: Pyrolater.
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pyrolusite (n.)

"manganese dioxide," a common ore, 1828, from Greek elements pyro- "by heat, by fire" (from PIE root *paewr- "fire") + lysis "a loosening" (from PIE root *leu- "to loosen, divide, cut apart"). The name was given in Roman times, when the substance was used, in a heated state, to de-colorize glass.

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