ardor (n.)

"heat of passion or desire," mid-15c., from Old French ardure "heat, glow; inflammation; passion" (12c., Modern French ardeur), from Latin ardorem (nominative ardor) "a flame, fire, burning, heat;" also of feelings, etc., "eagerness, zeal," from ardere "to burn," from PIE root *as- "to burn, glow" (source also of Old English æsce "ashes;" see ash (n.1)). In Middle English used of base passions; since Milton's time of noble ones.