phosphoryl nitride, PON--that is, the nitrile of metaphosphoric acid. The amide corresponding with the pyrophosphate P_{2}O_{3}(NH_{4}O)_{4} should be P_{2}O_{3}(NH_{2})_{4}, and the nitriles corresponding to the latter would be P_{2}O_{2}N(NH_{2})_{3}, P_{2}ON_{2}(NH_{2})_{2}, and P_{2}N_{3}(NH_{2}). The composition of the first is the same as that of the above diphosphamide. The third pyrophosphoric nitrile has a formula P_{2}N_{4}H_{2}, and this is the composition of the body known as _phospham_, PHN_{2} (in a certain sense this is the analogue of N_{3}H polymerised, Chapter VI.) Indeed, phospham has been obtained by heating the products of the action of ammonia on phosphoric chloride, as an insoluble and alkaline powder, which gives ammonia and phosphoric acid when subjected to the action of water. The same substance is obtained by the action of ammonium chloride on phosphoric chloride (PNCl_{2} is first formed, and reacts further with ammonia, forming phospham), and by igniting the mass which is formed by the action of ammonia on phosphorus trichloride. Formerly the composition of phospham was supposed to be PHN_{2}, now there is reason to think that its molecular weight is P_{3}H_{3}N_{6}. The above compounds correspond with normal salts, but nitriles and amides corresponding to acid salts are also possible, and they will be acids. For example, the amide PO(HO)_{2}(NH_{2}), and its nitrile, will be either PN(HO)_{2} or PO(HO)(NH), but at all events of the composition PNH_{2}O_{2}, and having acid properties. The ammonium salt of this _phosphonitrilic acid_ (it is called phosphamic acid), PNH(NH_{4})O_{2}, is obtained by the action of ammonia on phosphoric anhydride, P_{2}O_{5} + 4NH_3 = H_{2}O + 2PNH(NH_{4})O_{2}. A non-crystalline soluble mass is thus formed, which is dissolved in a dilute solution of ammonia and precipitated with barium chloride, and the resultant barium salt is then decomposed with sulphuric acid, and thus a solution of the acid of the above composition is obtained.