saturated with acid, are termed basic salts. Thus we have
Al_{2}(OH)_{2}(SO_{4})_{2}, Al_{2}(OH)_{4}SO_{4}, as well as
Al_{2}(SO_{4})_{3}, and we can get these basic salts by treating the
normal sulphate [Al_{2}(SO_{4})_{3}] with sufficient caustic soda to
remove the necessary quantities of sulphuric acid. Now it is a curious
thing that of these aluminium sulphates the fully saturated one,
Al_{2}(SO_{4})_{3}, is the most stable, for even on long boiling of its
solution in water it suffers no change, but the more basic is the
sulphate the less stable it becomes, and so the more easily it
decomposes on heating or boiling its solution, giving a deposit or
precipitate of a still more basic sulphate, or of hydrated alumina
itself, Al_{2}(OH)_{6}, until we arrive at the salt
Al_{2}(SO_{4})_{2}(OH)_{2}, which is quite unstable on boiling;
Al_{2}(SO_{4})(OH)_{4} would be more unstable still. This behaviour may
be easily shown experimentally. We will dissolve some "cake alum" or
normal sulphate of alumina, Al_{2}(SO_{4})_{3}, in water, and boil some
of the solution. 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. Hence, generally, the sources or root substances of the
best and most efficient mordants are the metals of high specific
appetite or valency. I think we have now