Liquid phosphuretted hydrogen (and free hydrogen) is also formed, together with the phosphine, so that the gaseous product, on escaping from the water into the air, takes fire spontaneously, forming beautiful white rings of phosphoric acid. In this experiment, as in that with calcium phosphide, it is the liquid, P_{2}H_{4}, that takes fire; but the phosphine set light to by it also burns, PH_{3} + O_{4} = PH_{3}O_{4}. The same phosphuretted hydrogen, PH_{3}, may be obtained pure, and not spontaneously combustible, by igniting the hydrates of phosphorous acid (4PH_{3}O_{3} = PH_{3} + 3PH_{3}O_{4}) and hypophosphorous acid (2PH_{3}O_{2} = PH_{3} + PH_{3}O_{4}); or, more simply, by the decomposition of calcium phosphide by hydrochloric acid, because then all the liquid phosphide, P_{2}H_{4}, is decomposed into non-volatile P_{2}H and gaseous PH_{3}. Pure phosphine liquefies when cooled to -90°, boils at -85°, and solidifies at -135° (Olszewski). When phosphorus burns in an excess[10 bis] of _dry_ oxygen, then only _phosphoric anhydride_, P_{2}O_{5} is formed. It is prepared by dropping pieces of phosphorus through a wide tube, fixed into the upper neck of a large glass globe, on to a cup suspended in the centre of the globe. These lumps are set alight by touching them with a hot wire, and the phosphorus burns into P_{2}O_{5}. The dry air necessary for its combustion is forced into the globe through a lateral neck, and the white flakes of phosphoric anhydride formed are carried by the current of air through a second lateral neck into a series of Woulfe's bottles, where they settle as friable white flakes. Phosphoric anhydride may also be formed by passing dry air through a solution of phosphorus in carbon bisulphide. All the materials for the preparation of this substance must be carefully dried, because it _combines_ with great eagerness _with water_, at the same time developing a large amount of heat and forming metaphosphoric acid, HPO_{3}, from which the water cannot be separated by heat. Phosphoric anhydride is a colourless snow-like substance, which attracts moisture from the air with the utmost avidity. It fuses at a red heat, and then _volatilises_. Its affinity for water is so great that it takes it up from many substances. Thus it converts sulphuric acid into sulphuric anhydride, and carbohydrates (wood, paper) are carbonised, and give up the elements of water when brought into contact with it. [10] The air must first be expelled from the flask by hydrogen, or some other gas which will not support combustion, as otherwise an explosion might take place owing to the spontaneous inflammability of the phosphuretted hydrogen.