ride. When phosphorus burns in an insufficient supply of
air the product is partially the trioxide; in oxygen or an excess of air
the pentoxide is formed. The pentoxide is much the better known of the
two. It is a snow-white, voluminous powder whose most marked property is
its great attraction for water. It has no chemical action upon most
gases, so that they can be very thoroughly dried by allowing them to
pass through properly arranged vessels containing phosphorus pentoxide.

~Acids of phosphorus.~ The important acids of phosphorus are the
following:

H_{3}PO_{3} phosphorous acid.
H_{3}PO_{4} phosphoric acid.
H_{4}P_{2}O_{7} pyrophosphoric acid.
HPO_{3} metaphosphoric acid.

These may be regarded as combinations of the oxides of phosphorus with
water according to the equations given in the discussion of the
characteristics of the family.

1. _Phosphorous acid_ (H_{3}PO_{3}). Neither the acid nor its salts are
at all frequently met with in chemical operations. It can be easily
obtained, however, in the form of transparent crystals when phosphorus
trichloride is treated with water and the resulting solution is
evaporated:

PCl_{3} + 3H_{2}O = H_{3}PO_{3} + 3HCl.

Its most interesting property is its tendency to take up oxygen and pass
over into phosphoric acid.

2. _Orthophosphoric acid (phosphoric acid)_ (H_{3}PO_{4}). This acid can
be obtained by dissolving phosphorus pentoxide in boiling water, as
represented in the equation

P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4}.

It is usually made by treating calcium phosphate with concentrated
sulphuric acid. The calcium sulphate produced in the reaction is nearly
insoluble, and can be filtered off, leaving the phosphoric acid in
solution. Very pure acid is made by oxidizing phosphorus with nitric
acid. It forms large colorless crystals which are exceedingly soluble in
water. Being a tribasic acid, it forms acid as well as normal salts.
Thus the following compounds of sodium are known:

NaH_{2}PO_{4} monosodium hydrogen phosphate.
Na_{2}HPO_{4} disodium hydrogen phosphate.
Na_{3}PO_{4} normal sodium phosphate.

These salts are sometimes called respectively primary, secondary, and
tertiary phosphates. They may be prepared by bringing together
phosphoric acid and appropriate quantities of sodium hydroxide.
Phosphoric acid also forms mixed salts, that is, salts containing two
different metals. The most