familiar compound of this kind is microcosmic
salt, which has the formula Na(NH_{4})HPO_{4}.

_Orthophosphates._ The orthophosphates form an important class of salts.
The normal salts are nearly all insoluble and many of them occur in
nature. The secondary phosphates are as a rule insoluble, while most of
the primary salts are soluble.

3. _Pyrophosphoric acid_ (H_{4}P_{2}O_{7}). On heating orthophosphoric
acid to about 225 deg. pyrophosphoric acid is formed in accordance with the
following equation:

2H_{3}PO_{4} = H_{4}P_{2}O_{7} + H_{2}O.

It is a white crystalline solid. Its salts can be prepared by heating a
secondary phosphate:

2Na_{2}HPO_{4} = Na_{4}P_{2}O_{7} + H_{2}O.

4. _Metaphosphoric acid (glacial phosphoric acid)_ (HPO_{3}). This acid
is formed when orthophosphoric acid is heated above 400 deg.:

H_{3}PO_{4} = HPO_{3} + H_{2}O.

It is also formed when phosphorus pentoxide is treated with cold water:

P_{2}O_{5} + H_{2}O = 2HPO_{3}.

It is a white crystalline solid, and is so stable towards heat that it
can be fused and even volatilized without decomposition. On cooling from
the fused state it forms a glassy solid, and on this account is often
called glacial phosphoric acid. It possesses the property of dissolving
small quantities of metallic oxides, with the formation of compounds
which, in the case of certain metals, have characteristic colors. It is
therefore used in the detection of these metals.

While the secondary phosphates, on heating, give salts of pyrophosphoric
acid, the primary phosphates yield salts of metaphosphoric acid. The
equations representing these reactions are as follows:

2Na_{2}HPO_{4} = Na_{4}P_{3}O_{7} + H_{2}O,

NaH_{2}PO_{4} = NaPO_{3} + H_{2}O.

~Fertilizers.~ When crops are produced year after year on the same field
certain constituents of the soil essential to plant growth are removed,
and the soil becomes impoverished and unproductive. To make the land
once more fertile these constituents must be replaced. The calcium
phosphate of the mineral deposits or of bone ash serves well as a
material for restoring phosphorus to soils exhausted of that essential
element; but a more soluble substance, which the plants can more readily
assimilate, is desirable. It is better, therefore, to convert the
insoluble calcium phosphate into the soluble primary phosphate before it
is applied as fertilizer. It will be seen by reference to the formulas