=Properties.=--Pure hydrate of potassium is a compact, white solid, usually met with in the form of sticks. When heated to a temperature a little under redness, it melts to a nearly colourless liquid; in this state it is intensely corrosive. It rapidly absorbs moisture from the air, and moist potash also absorbs with great avidity carbon dioxide; it is powerfully alkaline, changing red litmus to blue. It is soluble in half its weight of cold water, great heat being evolved during solution; it forms two definite hydrates--one, KHO + H₂O; the other, KHO + 2H₂O. It is sparingly soluble in ether, but is dissolved by alcohol, wood-spirit, fusel oil, and glycerin. § 103. =Pharmaceutical Preparations.=--Potassium hydrate, as well as the solution of potash, is officinal in all pharmacopœias. The _liquor potassæ_, or solution of potash, of the British Pharmacopœia, is a strongly alkaline, caustic liquid, of 1·058 specific gravity, and containing 5·84 per cent. by weight of KHO. It should, theoretically, not effervesce, when treated with an acid, but its affinity for CO₂ is so great that all solutions of potash, which have been in any way exposed to air, contain a little carbonate. Caustic sticks of potash and lime used to be officinal in the British Pharmacopœia. Filho’s caustic is still in commerce, and is made by melting together two parts of potassium hydrate and one part of lime in an iron ladle or vessel; the melted mass is now moulded by pouring it into leaden moulds. Vienna paste is composed of equal weights of potash and lime made into a paste with rectified spirit or glycerin. § 104. =Carbonate of Potash= (K₂CO₃ + 1½H₂O), when pure, is in the form of small white crystalline grains, alkaline in taste and reaction, and rapidly deliquescing when exposed to moist air; it gives all the chemical reactions of potassium oxide, and carbon dioxide. Carbonate of potash, under the name of _salt of tartar_, or potashes, is sold by oilmen for cleansing purposes. They supply it either in a fairly pure state, or as a darkish moist mass containing many impurities. § 105. =Bicarbonate of Potash= (KHCO₃) is in the form of large transparent rhombic prisms, and is not deliquescent. The effervescing solution of potash (_liquor potassæ effervescens_) consists of 30 grains of KHCO₃ in a pint of water (3·45 grms. per litre), and as much CO₂ as the water will take up under a pressure of seven atmospheres.