Note 27 — Chapter VI · TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

Phosphorus (Chapter XIX.) gives the hydride PH3, corresponding with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, PH3O4, is able to lose water and give pyro-and meta-phosphoric acids. The latter is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really meta-nitric acid. So also nitrous acid, HNO2, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is NH3O3 = N(OH)3. Hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, HNO3 (= ½N2O3,H2O), and ortho-nitric acid, H3NO4 (= ½N2O3,3H2O), an intermediate pyro-nitric acid, N2H4O7, corresponding to pyrophosphoric acid, P2H4O7. We shall see (for instance, in Chapter XVI., Note 21) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), MNO3, to combine with bases M2O, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (MNO3 + M2O = M3NO4).