from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. will o' the wisp, a strange light that attracts travellers
- n. A vegetable, usually a pumpkin, but alternatively a turnip, carved into the form of a face and lighted within by a candle. Associated chiefly with the holiday Halloween.
Jack ("male given name") + o' + lantern. Attested as a term for ignis fatuus in English folklore in the 1660s, mostly in East Anglia but also in southwestern England. The sense of carved pumpkin in American English is attested in 1834. (Wiktionary)
If you want the vapor to come out of a pumpkin, carve the jack o'lantern, then place a tall container inside with some dry ice.