the combustion of phosphorus in air is transformed into P_{2}O_{5} in an excess of oxygen. When moist phosphorus slowly oxidises in the air, it not only forms phosphorous and phosphoric acids, but also _hypophosphoric acid_, H_{4}P_{2}O_{6}, which when in a dry state easily splits up at 60° into phosphorous and metaphosphoric acids (H_{4}P_{2}O_{6} = H_{3}PO_{3} + HPO_{3}), but differs from a mixture of these acids in that it forms well-characterised salts, of which the sodium salt, H_{2}Na_{2}P_{2}O_{6}, is but slightly soluble in water (the sodium salts of phosphoric and phosphorous acids are easily soluble), and that it does not act as a reducing agent, like mixtures containing phosphorous acid.[11] [11] Salzer proved the existence of hypophosphoric acid (it is also called subphosphoric acid), in which many chemists did not believe. Drawe (1888) and Rammelsberg (1892) investigated its salts. It may be obtained in a free state by the following method. The solution of acid produced by the slow oxidation of moist phosphorus is mixed with a solution (25 p.c.) of sodium acetate. A salt, Na_{2}H_{2}P_{2}O_{6},6H_{2}O, crystallises out on cooling; it is soluble in 45 parts of water, and gives a precipitate of Pb_{2}P_{2}O_{6} with lead salts (Ag_{4}P_{2}O_{6} with salts of silver). The lead salt is decomposed by a current of hydrogen sulphide, when lead sulphide is precipitated, while the solution, evaporated under the receiver of an air-pump, gives crystals of H_{4}P_{2}O_{6},2H_{2}O, which easily lose water and give H_{4}P_{2}O_{6}. The salts in which the H_{4} is replaced by Ni_{2}, or NiNa_{2}, or CdNa_{2}, &c., are insoluble in water.