NOTE I. (p. 388).

The formulÆ, and molecular and percentage composition, of the different phosphates, are given in the following table:—

NOTE II. (p. 388).

When sulphuric acid is added to tricalcic phosphate, the following reaction takes place:—

(2.) 3CaO, P₂O₅ + 3(H₂O, SO₃) = 3CaO, SO₃ + 3H₂O, P₂O₅, or 2H₃PO₄.

NOTE III. (p. 390).

This equation gives the chemical reaction taking place when soluble phosphate is reverted, owing to the presence of undissolved 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:——

Fe₂O₃, 3SO₃ + CaO, 2H₂O, P₂O₅ = Fe₂O₃, P₂O₅ + CaO, SO₃ + 2(H₂O, SO₃),

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 Fe₂O₃, P₂O₅, and diminishing in the same proportion the soluble P₂O₅. Again, we may have simply——

2Fe₂O₃ + 3(CaO, 2H₂O, P₂O₅) = 2(Fe₂O₃, P₂O₅) + 3CaO, P₂O₅;

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, SO₃) + 2O + CaO, 2H₂O, P₂O₅ + 3CaO, P₂O₅ = 2(Fe₂O₃, P₂O₅) + 4(CaO, SO₃).

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:—