nSaint Elmo's firean electrical discharge accompanied by ionization of surrounding atmosphere
Webster's Revised Unabridged Dictionary
Saint Elmo's firea luminous, flamelike appearance, sometimes seen in dark, tempestuous nights, at some prominent point on a ship, particularly at the masthead and the yardarms. It has also been observed on land, and is due to the discharge of electricity from elevated or pointed objects. A single flame is called a Helena, or a Corposant; a double, or twin, flame is called a Castor and Pollux, or a double Corposant. It takes its name from St. Elmo, the patron saint of sailors.
He looked round the room, but his niece had vanished "like Saint-Elmo's fires," to use his favorite expression.