CAUSTIC SODA (Sodium hydroxide, NaOH) is a solid, white, deliquescent chemical compound much used as an inter mediate material in the production of many chemicals. It is gen erally made by one of two methods, either through exchange of base by the causticizing of soda-ash with hydrated lime or directly, through the action of an electric current upon a solu tion of common salt (NaC1). The latter newer process is tending to supersede the former, especially where cheap electric power is available or where a ready market for chlorine and hydrogen, by products of the electrolytic process, exists. Because of its large production, caustic soda is classed as a "heavy chemical." It is used as an alkaline reagent, in the manufacture of soap, sodium salts, rayon, paper, medicines, textile products, in petroleum and vegetable oil refining, in tanning and in the synthesis of many organic compounds such as indigo, alizarin, resorcin, formates and oxalates.