The corporation shall set apart as an earned surplus seventy-five percent of its net earnings each year until such earned surplus equals the total of the paid-in capital and paid-in surplus then outstanding. The earned surplus shall be held in cash or invested in obligations of the United States government, and shall be kept and used to meet losses and contingencies of the corporation, and, whenever the amount of the earned surplus shall become impaired, it shall be built up again to the required amount in the manner provided for its original accumulation. Earned surplus not required to be accumulated shall be available for the payment of such dividends as the board of directors shall deem expedient.