balance in an improved fashion of course small amounts of mercury vapor will pass through the condenser or heat exchanger and be lost therefore make up mercury will be required but the amount of mercury needed to be returned to the unit is reduced to a factor of less than 1 100 and generally less than 1 200 of that which would otherwise be required in addition the returning liquid mercury has been cooled to a temperature below the boiling point of water and generally considerably therebelow the cooled mercury will tend to pick up mercury vapors as it flows back into the decomposer in countercurrent relation therewith thus improving the efficiency of the mercury separation in its process form the invention provides a method for making alkali hydroxide and chlorine in a plurality of electrolytic cell units of the amalgam type wherein a decomposer is provided for each cell and wherein the hydrogen exiting from each decomposer is passed through an overhead outlet into a condenser the condensate condensed in the condenser is collected in a liquid separator the mercury is returned to the decomposer back through the overhead outlet and the water condensate is recycled to the decomposer at the normal water feed location thus the process steps are automatically carried out by the apparatus disclosed in general the vapors leaving the decomposer are at a temperature below the.