Abstract: A device for permitting controlled emission of a volatile substance, in particular an air-treating agent. The device comprises a container having an annular ledge with two recesses on opposite sides and a frustro hemispherical hood open at the top, that fits on to the container. The hood has two studs protruding on opposite sides of its inner wall. Inside the hood are situated two shutters that are moveable in relation to each other on relative rotation between the container and the hood by means of slots, which are moved by the studs, to close or open the opening to an infinitely variable degree.