No deposit or precipitate is produced; the salt is
stable. To another portion of the solution we will add some caustic
soda, NaOH, in order to rob the normal sulphate of alumina of some of
its sulphuric acid. This makes the sulphate of alumina basic, and the
more basic, the more caustic soda is added, the sodium (Na) of the
caustic soda combining with the SO_{4} of the sulphate of alumina to
form sulphate of soda (Na_{2}SO_{4}), whilst the hydroxyl (OH) of the
caustic soda takes the position previously occupied by the SO_{4}. But
this increase of basicity also means decrease of stability, for on
boiling the solution, which now contains a basic sulphate of alumina, a
precipitate is formed, a result which also follows if more caustic soda
is added, production of still more basic salts or of hydrated alumina,
Al_{2}(OH)_{6}, taking place in either case.
_Mordanting or Fixing Acid (Phenolic) Colours._--But what has all this
to do with mordanting? is possibly now the inquiry. So much as this,
that only such unstable salts as I have just described, which decompose
and yield precipitates by the action on them of alkalis, heat, the
textile fibres themselves, or other agencies, are suitable to act as
true mordants.
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