Apparatus for nebulizing a liquid, comprises a chamber (214) for the liquid to be nebulized; an ultra­sonic generator (274) disposed within the chamber (214) to be submerged in a pool of the liquid to be nebulized and effective, when energized, to nebulize liquid in the pool; a sonic detector (200) located at a predetermined level of the chamber (214); and an electrical circuit con­trolled by the sonic detector (200) for energizing the ultrasonic generator (274), but automatically de-energizing the ultrasonic generator (274) when the liquid in the pool is at a level below that of the sonic detector (200).