~Boric oxide~ (B_{2}O_{3}). Boron forms one well-known oxide, B_{2}O_{3},
called boric anhydride. It is formed as a glassy mass by heating boric
acid to a high temperature. It absorbs water very readily, uniting with
it to form boric acid again:

B_{2}O_{3} + 3H_{2}O = 2H_{3}BO_{3}.

In this respect it differs from silicon dioxide, which will not combine
directly with water.

~Boric acid~ (H_{3}BO_{3}). This is found in nature in considerable
quantities and forms one of the chief sources of boron compounds. It is
found dissolved in the water of hot springs in some localities,
particularly in Italy. Being volatile with steam, the vapor which
escapes from these springs has some boric acid in it. It is easily
obtained from these sources by condensation and evaporation, the
necessary heat being supplied by other hot springs.

Boric acid crystallizes in pearly flakes, which are greasy to the touch.
In the laboratory it is easily prepared by treating a strong, hot
solution of borax with sulphuric acid. Boric acid being sparingly
soluble in water crystallizes out on cooling:

Na_{2}B_{4}O_{7} + 5H_{2}O + H_{2}SO_{4} = Na_{2}SO_{4} + 4H_{3}BO_{3}.

The substance is a mild antiseptic, and on this account is often used in
medicine and as a preservative for canned foods and milk.

~Metaboric and polyboric acids.~ When boric acid is gently heated it is
converted into metaboric acid (HBO_{2}):

H_{3}BO_{3} = HBO_{2} + H_{2}O.

On heating metaboric acid to a somewhat higher temperature tetraboric
acid (H_{2}B_{4}O_{7}) is formed:

4HBO_{2} = H_{2}B_{4}O_{7} + H_{2}O.

Many other complex acids of boron are known.

~Borax.~ Borax is the sodium salt of tetraboric acid, having the formula
Na_{2}B_{4}O_{7}.10 H_{2}O. It is found in some arid countries, as
southern California and Tibet, but is now made commercially from the
mineral colemanite, which is the calcium salt of a complex boric acid.
When this is treated with a solution of sodium carbonate, calcium
carbonate is precipitated and borax crystallizes from the solution.

When heated borax at first swells up greatly, owing to the expulsion of
the water of crystallization, and then melts to a clear glass. This
glass has the property of easily dissolving many metallic oxides, and on
this account borax is used as a flux in soldering, for the purpose of
removing from the metallic surfaces to be soldered the film of oxide
with which they are likely to be co