In general, fluorine and phosphorus give stable compounds: PF_{3}, POF_{3}, and PF_{5}, as would be expected from the fact that in passing from Cl to I (_i.e._ as the atomic weight of the halogen increases) the stability of the compounds with P and the tendency to give PX_{5} (Note 24) decreases. _Phosphorus trifluoride_ is obtained by heating a mixture of ZnF_{2} and PBr_{3}, by the action of AsF_{3} upon PCl_{3}, by heating phosphide of copper with PbF_{2}, &c. It is a strong-smelling gas, which liquefies at -10° under a pressure of 40 atmospheres, giving a colourless liquid. It dissolves easily in (is absorbed by, reacts with) water, and acts upon glass; when mixed with Cl_{2} it combines with it (Poulenc, 1891), forming PCl_{2}F_{3}, a colourless gas of normal density, which is transformed into a liquid at 8°, decomposes into PF_{3} + Cl_{2} at 250°, and, with a small amount of water, gives _oxy-fluoride_ of phosphorus, POF_{3} (with a large amount of water it gives PH_{3}O_{4}), which Moissan (1891) obtained by the action of dry HF upon P_{2}O_{5}, and Thorpe and Tutton (1890) by heating a mixture of cryolite and P_{2}O_{5}. It is a gas of normal density, like PF_{3}, and was obtained by Moissan by the action of fluorine upon PF_{3} (PSF_{3}, _see_ Chapter XX., Note 20). Thus the forms PX_{3} and PX_{5} not only exist in many solid and non-volatile substances, but also as vapours.