The Principles of Chemistry, Volume II - Dmitry Ivanovich Mendeleyev

H(OH), or H₂O, responds to (OH)(OH), or H₂O₂,

and so also—

H(HSO₄), or H₂SO₄, responds to (HSO₄)(HSO₄), or H₂S₂O₈.

Similar relations are seen everywhere, and they correspond to the principle of substitutions which I long since endeavoured to represent as one of the chemical generalisations called into life by the periodic law. So also sulphuric acid, if considered with reference to hydroxyl, and represented as follows:—

HO(SO₂OH),

has its corresponding compound in dithionic acid—

(SO₂OH)(SO₂OH), or H₂S₂O₆.

Therefore, also, phosphoric acid, HO(POH₂O₂), has, in the same sense, its corresponding compound in the subphosphoric acid of Saltzer:—

(POH₂O₂)(POH₂O₂), or H₄P₂O₆;

and we must suppose that the peroxide compound corresponding to phosphoric acid, if it be discovered, will have the following structure:—