Potash. Potash, or caustic potash, consists chemically of potassium hydroxide, KOH, and may be regarded as derived from water by the replacement of one-half of the hydrogen in the latter by potassium. It is formed when this element acts upon water, and is usually prepared by boiling a solution of potassium carbonate with slaked lime, the solution being evaporated down in silver basins and cast in sticks. If required pure, it is recrystallised from hot alcohol. It is a. very powerful alkali, and absorbs carbonic acid and water very readily. It is a powerful caustic, acting strongly on organic matter. It is an invaluable reagent in the chemical laboratory, and is employed in pharmacy and in very many chemical industries.