{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, powdere