I bought this while I was working in France. It's an oil nebuliser that uses low volume compressed air from an aquarium style pump to atomise essential oils and turn them into an airborne vapour.
It uses a glass venturi arrangement to draw the oil up from the base of the unit and then atomise it with a fine jet of air blowing across the top of a glass tip. The majority of the oil is sprayed like an aerosol against the side of the glass vessel, where it drains back down into the reservoir. The smallest droplets get lifted up as a fine mist by the low airflow and exit the top of the unit as a white cloud of oil vapour. A bit like an ultrasonic atomiser, but with pure oil.
This device seemed to be sold as an air steriliser for medical establishments. It is designed to take natural essential oils, bot also atomises synthetic aroma oils and other liquids.