its upper portion for removal of chlorine gas the electrolysis compartment 4 is enclosed at the top by a refractory lined closure 11 so as to form a gas tight seal therebetween a multiplicity of anodes 7 and cathodes 6 form a part of the electrolysis compartment 4 a plurality of heavy plate like graphite anodes 7 are mounted in the top closure 11 so as to project downward into the electrolysis compartment 4 with their lower edges near the bottom of the latter position of each anode is such that its longitudinal dimension extends from the front to the rear of the compartment 4 as best illustrated in fig2 longitudinally the anodes 7 extend between the partition wall 3 and the rear wall 29 of the sell a suitable electrical connecting means 8 is provided at the upper ends of the anodes 7 in addition cooling means is formed for extracting heat from the anodes the electrolysis compartment 4 also includes a plurality of cathodes 6 which may consist of steel plates the cathodes 6 are arranged at localities between successive anodes so that the electrodes alternate in mutually parallel arrays along the electrolysis compartment the cathodes 6 also extend longitudinally within the electrolysis compartment the cathodes that are disposed between pairs of anodes are arranged in spaced pairs and carried by suitable mounting and electrical connecting structure which extends through the wall and has a suitable connection means the cathodes of each described pair are disposed suitably close to the respective adjacent anodes the walls of the cell are made of heavy refractory construction and can be conveniently built of refractory blocks the entire structure may have an outer insulating layer and an outer steel casing 1 is provided for.