In order to see the relation between phosphoric acid and hypophosphoric acid which does not contain the elements of phosphorous acid (because it does not reduce either gold or mercury from their solutions), but which nevertheless is capable of being oxidised (for example, by potassium permanganate) into phosphoric acid, it is simplest to apply the law of substitution. This clearly indicates the relation between oxalic acid, (COOH)_{2}, and carbonic acid, OH(COOH). The relation between the above acids is exactly the same if we express phosphoric acid as OH(POO_{2}H_{2}), because in this case P_{2}H_{4}O_{6}, or (POO_{2}H_{2})_{3}, will correspond with it just as oxalic does with carbonic acid. A similar relationship exists between hyposulphuric or dithionic acid, (SO_{2}OH)_{2}, and sulphuric acid, OH(SO_{2}OH), as we shall find in the following chapter. Dithionic acid corresponds with the anhydride S_{2}O_{5}, intermediate between SO_{2} and SO_{3}; oxalic acid with C_{2}O_{3}, intermediate between CO and CO_{2}; hypophosphoric acid corresponds with the anhydride P_{2}O_{4}, intermediate between P_{2}O_{3} and P_{2}O_{5}, and the analogue of N_{2}O_{4}. Judging by the general law of the formation of acids (Chapter XV.), the series of phosphorus compounds should include the following _ortho-acids_ and their corresponding anhydrides, answering to phosphuretted hydrogen, H_{3}P:-- H_{3}PO_{4}, phosphoric acid, and P_{2}O_{5}, anhydride, H_{3}PO_{3}, phosphorous acid, and P_{2}O_{3}, anhydride, H_{3}PO_{2}, hypophosphorous acid, and P_{2}O, anhydride.[12] The last of these (the analogue of N_{2}O) is almost unknown. Phosphoric anhydride (P_{2}O_{5}) with a small quantity of water does not at first give orthophosphoric acid, PH_{3}O_{4}, but a compound P_{2}O_{5},H_{2}O, or PHO_{3}, whose composition corresponds with that of nitric acid; this is _metaphosphoric acid_. Even with an excess of water, combining with phosphoric anhydride, this metaphosphoric acid, and not the ortho-, passes at first into solution. Metaphosphoric acid in solution only passes into orthophosphoric acid when the solution is heated or after a lapse of time. [12] Besides the hydrates enumerated, a compound, PH_{3}O, should correspond with PH_{3}. This hydrate, which is analogous to hydroxylamine, is not known in a free state, but it is known as triethylphosphine oxide, P(C_{2}H_{5})_{3}O, which is obtained by the oxidation of triethylphosphine, P(C_{2}H_{5})_{3}. It must be observed that there may also be lower oxides of phosphorus corresponding with PH_{3}, like N_{2}O and NO, and there are even indications of the formation of such compounds, but the data concerning them cannot be considered as firmly established.