The combustion of phosphuretted hydrogen in oxygen also takes place under water when the bubbles of both gases meet, and it is very brilliant. The phosphuretted hydrogen obtained by the action of phosphorus on caustic potash always contains free hydrogen, and often even the greater part of the gas evolved consists of hydrogen. _Pure phosphuretted hydrogen_ (not containing hydrogen or liquid or solid phosphides) is obtained by the action of a solution of potash on phosphonium iodide: PH_{4}I + KHO = PH_{3} + KI + H_{2}O (in just the same way as ammonia is liberated from ammonium chloride). The reaction proceeds easily, and the purity of the gas is seen from the fact that it is entirely absorbed by bleaching powder and is not spontaneously inflammable. Its mixture with oxygen explodes when the pressure is diminished (Chapter XVIII., Note 8). The vapours of bromine, nitric acid, &c., cause it to again acquire the property of inflaming in the air; that is, they partially decompose it, forming the liquid hydride, P_{2}H_{4}. Oppenheim showed that when red phosphorus is heated at 200° with hydrochloric acid in a closed tube it forms the compound PCl_{3}(H_{3}PO_{3}), together with phosphine. [10 bis] If there be a deficiency of oxygen, _phosphorous anhydride_ P_{2}O_{3} is formed. It was obtained by Thorpe and Tutton (1890) and is easily volatilised, melts at 22°·5, boils without change (in an atmosphere of N_{2} or CO_{2}) at 173°, and is therefore easily separated from P_{2}O_{3}, which volatilises with difficulty. The vapour density shows that the molecular weight is double, _i.e._ P_{4}O_{6} (like As_{2}O_{3}). Although colourless, phosphorous anhydride (its density in a state of fusion at 24° = 1·936) turns yellow and reddens in sun-light (possibly red phosphorus separates out ?), and decomposes at 400° forming hypophosphorous anhydride P_{2}O_{4} (Note 11) and phosphorus. It passes into P_{2}O_{5} in air and oxygen, and when slightly heated in oxygen becomes luminous, and ultimately takes fire. Cold water slowly transforms P_{2}O_{3} into phosphoric acid, but hot water gives an explosion and leads to the formation of PH_{3}, (P_{4}O_{6} + 6H_{2}O = PH_{3} + 3PH_{3}O_{4}). Alkalis act in the same manner. It takes fire in chlorine and forms POCl_{3} and PO_{2}Cl, and combines with sulphur at 160°, forming P_{2}S_{2}O_{3} (the molecular formula is double this) a substance which volatilises in vacuo and is decomposed by water into H_{2}S and phosphoric acid, _i.e._ it may be regarded as P_{2}O_{5}, in which O_{2} has been replaced by two atoms of sulphur. Judging from the above, the mixture of P_{2}O_{3} and P_{2}O_{5} formed in