NOVELTY - Bottom heater, for a chamber furnace with bundle(s) of reaction tubes, comprises a heating stone forming the bottom; and in sequence, an insulating stone with vertical ducts for the tubes; and housings with air and fuel gas supplies respectively.    USE - The furnace is used for carrying out endothermic gas reactions, especially for the production of hydrogen cyanide (HCN) by the 'BMA' process (all claimed) (from methane and ammonia).    ADVANTAGE - Existing heaters for endothermic gas reactions give uneven heating, require costly plant, are potentially hazardous and unsuitable for large scale operation at high temperature or cannot be used with reaction tubes passing through the bottom. The present heater can be used with tubes extending through the bottom and gives flame parallel to the tuber. It is safer and more economical to operate. It gives uniform heating on all sides and a more uniform horizontal temperature profile.    DETAILED DESCRIPTION - Bottom heater, for a chamber furnace with not less than 1, especially 1 bundle of extended reaction tubes, comprises a heating stone forming the bottom of the chamber; and below this, in sequence, an insulating stone with vertical ducts for the reaction tubes; an air housing with air supply; and a fuel gas housing with gas supply. It also has fuel gas jet(s), preferably exceeding the number of reaction tubes, with a jet and lance. The gas lances extend through the insulating stone, with an annular gap between the lance and surrounding stone, and through the air housing, which is connected to the annular gap at the top. The reaction tubes extend through both stones and both housings and there is a seal in the ducts between the air and gas housings.    An INDEPENDENT CLAIM is also included for a chamber furnace with a bottom heated of the above type and also freely extensible reaction tubes passing through the top and bottom of the combustion chamber, reaction gas feed(s) to the reaction tubes, a cooling head and reaction gas and combustion gas discharges.    DESCRIPTION OF DRAWING(S) - The drawing shows a section through the bottom heater, which also formed the bottom of a chamber furnace.    Bottom heater (100)    Combustion chamber (101)    Bottom of combustion chamber (102)    Wall of combustion chamber (103)    Upper support for furnace (104)    Lower support for furnace (105)    Heating stone (106)    Insulating stone (107)    Air housing (108)    Fuel gas housing (109)    Insulating mat (seal) (110)    Duct for reaction tube (111)    Reaction tube (112)    Duct for fuel gas jet (113)    Fuel gas jet (114)    Fuel gas lance (115)    Turbulence jet (116)    Supporting collar (117)    Opening for gas lance (118)    Annular gap (119)    Flame (120)    Air housing cover (121)    Air housing side wall (122)    Air housing bottom (123)    Fuel gas housing cover (124)    Fuel gas housing bottom (125)    Fuel gas housing side wall (126)    Bolt (127)    Nut (128)    Air supply port (129)    Fuel gas supply port (130)    Seal (131)    Cooling head (132)    Reaction gas housing (133)    Reaction gas supply (134)    Reaction gas discharge (135)    Air supply (136)    Fuel gas supply (137)    Recuperator (138)    Heat exchange tube (139)    Flue gas outlet (140)    Reaction tube duct at bottom (air housing) (141)    Reaction tube duct at top (air housing) (142)    Fuel gas lance duct at bottom (143)    Fuel gas lance duct at top (144)    Reaction tube duct at bottom (fuel gas housing) (145)    Reaction tube duct at top (fuel gas housing) (146)