in salts, due to the varying appetites or combining powers of bases.
Sodium only forms simple monoacid salts, as sodium chloride (NaCl),
sodium sulphate (Na_{2}SO_{4}); calcium forms diacid salts, _e.g._
calcium chloride (CaCl_{2}); and aluminium and iron, triacid salts, for
example, aluminium sulphate [Al_{2}(SO_{4})_{3}] and iron (ferric)
sulphate [Fe_{2}(SO_{4})_{3}]. Now in these triacid salts we can remove
some of the acid groups and substitute the elements of water, OH, or
hydroxyl, as it is called, for them. Such salts, then, only partly
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