When sulphuric acid is added to tricalcic phosphate, the following reaction takes place:-- (1.) 3CaO, P_{2}O_{5} + 2(H_{2}O, SO_{3}) (Tricalcic phosphate), (Sulphuric acid), = 2(CaO, SO_3) + CaO, 2H_{2}O, P_{2}O_{5} (Gypsum), (Monocalcic phosphate). (2.) 3CaO, P_{2}O_{5} + 3(H_{2}O, SO_{3}) = 3CaO, SO_{3} + 3H_{2}O, P_{2}O_{5}, or 2H_{3}PO_{4}. NOTE III. (p. 390). This equation gives the chemical reaction taking place when soluble phosphate is reverted, owing to the presence of undissolved phosphate:---- 3CaO, P_{2}O_{5} + CaO, 2H_{2}O, P_{2}O_{5} (Tricalcic phosphate), Monocalcic phosphate, = 2CaO, H_{2}O, P_{2}O_{5} + 2CaO, H_{2}O, P_{2}O_{5} (Dicalcic phosphate), (Dicalcic phosphate). NOTE IV. (p. 390). "Just what the reactions are which are produced by the iron and alumina compounds has never been made out very clearly. But some idea of them may be gained from the following suggestions, which were thrown out by the English chemist Patterson. Suppose the sulphuric acid has dissolved a quantity of iron or alumina, then we may have the reaction:---- Fe_{2}O_{3}, 3SO_{3} + CaO, 2H_{2}O, P_{2}O_{5} = Fe_{2}O_{3}, P_{2}O_{5} + CaO, SO_{3} + 2(H_{2}O, SO_{3}), and the free acid thus formed would proceed to dissolve more iron or alumina from the rock that had previously escaped decomposition, and the reaction here formulated would occur again and again. Here we have a cumulative process continually increasing the quantity of insoluble Fe_{2}O_{3}, P_{2}O_{5}, and diminishing in the same proportion the soluble P_{2}O_{5}. Again, we may have simply---- 2Fe_{2}O_{3} + 3(CaO, 2H_{2}O, P_{2}O_{5}) = 2(Fe_{2}O_{3}, P_{2}O_{5}) + 3CaO, P_{2}O_{5}; where three molecules of the soluble phosphoric acid are made to revert to the insoluble state at one blow. "In case the iron in the original rock were in the state of ferrous oxide, perhaps the following reaction might occur:---- 4(FeO, SO_{3}) + 2O + CaO, 2H_{2}O, P_{2}O_{5} + 3CaO, P_{2}O_{5} = 2(Fe_{2}O_{3}, P_{2}O_{5}) + 4(CaO, SO_{3}). In all these equations, except the last, alumina would serve as well as oxide of iron."--(_Vide_ Storer's 'Agricultural Chemistry,' vol. i. pp. 276, 277.) NOTE V. (p. 396). The following table shows the relative trade values of phosphoric acid in different manures:-- I.--WOLFF, 1893. Phosphate soluble in water (as in super) 100 Precipitated phosphate, Peruvian guano 92 Reverted phosphate, finest steamed bone-dust fish-guano, poudrette 83 Phosphatic guanos (Baker Island), wood-ashes 75 Coarser bone-dust, powdered animal charcoal, bone-ash 67 Coarse fragments of bone, powdered phosphorite and coprolite, Thomas-slag, farmyard manure 33 II.--AMERICAN, 1892.