chamber 2 the material 5 which forms obstacles for the passage of the gas through the apparatus is preferably constituted by a metallic sponge this material 5 may not only be used to fill the space between the wall of the outer shell and the inner enclosure but such material may also be applied to the inner surface of the wall of the enclosure as shown in the drawing but in this case of course care must be taken that the materal 5 does not contact the resistance wires 3 passing through the chamber 2 the material 5 in the space between the walls of the outer shell and the inner enclosure not only forms a tortuous path for the exhaust gas but serves also as sound absorber so that the gas when finally ejected into the atmosphere will not produce any noise as shown in the drawing the exhaust gas from an internal combustion engine is fed into the space between the outer shell and the inner enclosure by means of a tube 6 so that the gas passing through the space filled with the sponge material is heated since as mentioned before the wall of the inner enclosure is formed from a material having a high heat conductive capacity the gas passes then from the annular space between the outer shell and the inner enclosure through passage means 8 into the interior of the inner enclosure to be further heated therein by the heat developed by the resistance wires 3 and.