Potash or Potassa, is the protoxide of potassium. It is called the vegetable alkali, because it is obtained in an impure state by the incineration of vegetables. Potash is always formed when potassium is put into water, or when it is exposed at common temperatures to dry air or oxygen gas. By the former method the protoxide is obtained in combination with water; and in the latter it is anhydrous. It consists of 39.15 parts, or 1 equivalent of potassium, and 8 parts, or 1 equivalent of oxygen. Hydrate of potash is solid at common temperatures; it fuses at a heat rather below redness, and assumes a somewhat crystalline texture in cooling. It is highly deliquescent, and requires about half its weight of water for solution. It is also soluble in alcohol. It destroys all animal textures, and, on this account, is employed in surgery as a caustic. It changes the blue colour of violets and cabbage to green; reddened litmus to purple; and yellow turmeric to a reddish brown. It has been called lapis causticus, but is now termed potassa and fused potassa. It is prepared by evaporating the aqueous solution of potash, in a silver or clean iron capsule, to the consistence of oil, and then pouring it into moulds.

It may be purified by solution in alcohol and evaporation to the same extent as before, in a silver vessel. The operation should be performed as expeditiously as possible, to prevent the absorption of carbonic acid. A perfectly pure solution of potash will remain transparent on the addition of lime water; will not effervesce with dilute sulphuric acid, nor give any precipitate on blowing air from the lungs through it by means of a tube.

Pure potash, for experimental purposes, may most easily be obtained by igniting cream of tartar in a crucible, dissolving the residue in water, filtering, boiling with a quantity of quicklime, and, after subsidence, decanting the clear liquid and evaporating in a loosely covered silver capsule till it flows like oil, and then pouring it out on a clean iron plate. A solid white cake of pure hydrate of potash is thus obtained without the agency of alcohol; it must be immediately broken into fragments and kept in a well-stoppered phial. Potash is employed as a reagent in detecting the presence of bodies, and in separating them from each other. The solid hydrate, owing to its strong affinity for water, is used for depriving gases of hygrometric moisture, and is admirably fitted for forming frigorific mixtures. Potash may be distinguished from soda by a test recommended by M. Harkort. Oxide of nickel when fused by the blow-pipe flame with borax, gives a brown glass; and this glass, if melted with a mineral containing potash, becomes blue, - an effect which is not produced by the presence of soda.