NOVELTY - The conical internal surface (8) of the envelope (6) converges downstream. A central conical rod runs along the longitudinal axis from a perforated plate (12) isolated from the envelope and earthed and is surrounded by a perforated cylinder connected to the envelope by radial dividers (15) forming at least four resonance cavities (16). The envelope is supplied with an alternating high voltage    USE - Treatment of polluting gases, in particular containing CO2 or to obtain molecular breakdown of the pollutants in a gas. This can be used in a vehicle engine exhaust or in the outlet from an industrial chimney, in particular at a cement works.    ADVANTAGE - This device can treat more than one type of pollutant at a time, where previous designs were specific to a particular pollutant.    DETAILED DESCRIPTION - The device has a first treatment chamber (2) of length L1 of which all the parts are made of an electrically conducting material. The chamber has a gas inlet (9a) of diameter D1 and a gas outlet (10a) of diameter D2. At least four radial dividers (15) diametrally opposed in pairs form at least four resonance cavities (16). These dividers are in electrical contact by their outside edges with the internal surface of the first chamber and support a first perforated cylinder (19) of length l1 by fixing to their inside edges. A first electrical phase delivering an alternating high voltage supplies the envelope of the first treatment chamber and the first perforated plate is connected to earth. An acceleration chamber (3) for the molecules and particles of length L2 is made of electrically isolating material with a cylindrical envelope (26) of internal diameter D3 coaxial with the first treatment chamber and fixed by its upstream end (3a) to the downstream end of the first treatment chamber. The acceleration chamber contains a transverse plate (27) with center O made in an electrically conducting material with at least one perforation placed in a circular central zone centered on O to allow gas flow through the plate. A second treatment chamber (4) of length L2 made of an electrically conducting material has a diverging internal conical surface (31) with an inlet (32a) of diameter D4 and an outlet (33a) of diameter D5. The second chamber is coaxial with the first and fixed at its upstream end to the downstream end of the acceleration chamber. A second central conical rod (34) extends from the transverse plate of the acceleration chamber. Second and third perforated coaxial cylinders (36, 41) of diameter D6 and D7 respectively (with D7 greater than D6) are fixed by its upstream edge to the internal surface of the second treatment chamber with the downstream end free, each with at least one tooth protruding longitudinally into the second chamber. A third central conical rod (45) extends from a second perforated plate fixed axially at the downstream end of the second chamber, electrically isolated from the second chamber's envelope. A fourth perforated cylinder extends longitudinally in the second chamber from the second perforated plate around the third rod. A second electrical phase delivering an alternating high voltage is fed to the envelope of the second chamber, with the transverse plates of the acceleration chamber and the second treatment chamber connected to earth. The device can also include an ionic regulator.    INDEPENDENT CLAIMS are made for a method of treating polluting gases using the device described above, and application of the device to treatment of CO2 from a vehicle engine or from industrial chimneys, in particular cement works.    DESCRIPTION OF DRAWING(S) - The figure shows a cross section along the X-X' axis of the device in one form    First treatment chamber (2)    Acceleration chamber (3)    Upstream end (3a)    Second treatment chamber (4)    Envelope (6)    Conical internal surface (8)    Inlet (9a)    Outlet (10a)    Central conical rod (11)    Perforated plate (12)    Radial divider (15)    Resonance cavity (16)    First perforated cylinder (19)    Cylindrical envelope (26)    Transverse plate (27)    Internal conical surface (31)    Inlet (32a)    Outlet (33a)    Second central conical rod (34)    Second perforated coaxial cylinder (36)    Third perforated coaxial cylinder (41)    Third central conical rod (45)