gʷher-₂
“to be warm, hot”Source of Latin formus, English warm, furnace, thermal
Root for warmth and heat, yielding English warm, Latin fornāx (furnace), Greek thermos.
Discussion
The PIE root *gʷher- (to be warm, to be hot) is the base from which the derivative *gʷher-mn̥- (warm thing, furnace — see id 1396) was formed. The root itself produced the fundamental vocabulary of heat across the major branches.
Greek thermós (θερμός, "warm, hot") is the most productive modern reflex: thermal, thermometer, thermos, thermostat, thermodynamics, and hypothermia all derive from the Greek continuation. The labiovelar *gʷh yielded Greek th- in this position.
Latin formus (warm) and fornāx (oven, furnace) continue the root in Italic with PIE *gʷh > Latin f. English furnace descends from this line.
English warm (OE wearm, from PGmc *warmaz) is the native Germanic reflex — PIE *gʷh > PGmc *w (the labial component survives, the velar is lost). This means English warm and Greek thermós are cognates from the same PIE root — a correspondence that demonstrates how dramatically Grimm's Law and the Greek labiovelar treatment can disguise originally identical words.
Sanskrit gharmá- (heat, warmth) confirms the Indo-Iranian reflex. The Vedic gharma names both physical heat and the ritual heat of sacrifice — fire as both natural phenomenon and sacred instrument.
The root connects to *péh₂wr̥ (fire — the substance) and *dʰeǵʰ- (to burn — the ignition) as part of the PIE thermal vocabulary: *péh₂wr̥ names fire, *dʰeǵʰ- names the act of kindling, and *gʷher- names the resulting warmth.