[227] For formulae of the different phosphates, see Appendix, Note I., p.
398.

[228] For chemical formulae, showing reaction, see Appendix, Note II., p.
398.

[229] Of course it is well known that free phosphoric acid is obtained
by acting upon phosphate of lime with an excess of sulphuric acid; but
the point above referred to as having been recently discovered is, that
when phosphate of lime is acted upon, even by a small quantity of
sulphuric acid, free phosphoric acid is formed.

[230] For chemical formulae showing this reversion, see Appendix, Note
III., p. 399.

[231] For chemical theories on reversion of soluble phosphate by iron
and alumina, see Appendix, Note IV., p. 399.

[232] See Appendix, Note V., p. 400.

APPENDIX TO CHAPTER XIII.

NOTE I. (p. 388).

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

----------------------------------------------------------------------------
| Composition in terms of--
+--------------------------+-------------------
| Molecular weight. | Per cent.
---------------+-------------+-----+------+------+------+-----+------+------
| | | |Phos- | | | |Phos-
Name. | Symbol. |Lime.|Water.|phoric|Total.|Lime.|Water.|phoric
| | | |acid. | | | |acid.
---------------+-------------+-----+------+------+------+-----+------+------
Tri- or bone- |3CaO, | | | | | | |
phosphate. | P_{2}O_{5} | 168 | 0 | 142 | 310 |54.19| 0.00| 45.81
Bi- or di- |2CaO, H_{2}O,| | | | | | |
phosphate. | P_{2}O_{5} | 112 | 18 | 142 | 272 |41.18| 6.61| 52.21
Mono- or super-|CaO, 2H_{2}O,| | | | | | |
phosphate. | P_{2}O_5 | 56 | 36 | 142 | 234 |23.93| 15.39| 60.68
---------------+-------------+-----+------+------+------+-----+------+------

NOTE II. (p. 388).

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_