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 got a clue to the principle of
mordants and also to the importance of a sound chemical knowledge in
dealing most effectively with them, and I may tell you that the man who
did most to elucidate the theory of mordanting is not a practical man in
the general sense of the term, but a man of the highest scientific
attainments and standing, namely, Professor Liechti, who, with his
colleague Professor Suida, did probably more than any other man to clear
up much that heretofore was cloudy in this region. We have seen that
with aluminium sulphate, basic salts are precipitated, _i.e._ salts with
such a predominance of appetite for acids, or such _quasi_-acids as
phenolic substances, that if such bodies were present they would combine
with the basic parts of those precipitated salts as soon as the latter
were formed, and all would be precipitated together as one complex
compound. Just such peculiar _quasi_-acid, or phenolic substances are
Alizarin, and most of the natural adjective dyestuffs, the colouring