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
principles of logwood, cochineal, Persian berries, etc. Hence these
substances will be combined and carried down with such precipitated
basic salts. The complex compounds thus produced are coloured substances
known as lakes. For example, if I take a solution containing basic
sulphate of alumina, prepared as I have already described, and add to
some Alizarin, and then heat the mixture, I shall get a red lake of
Alizarin and alumina precipitated. If I had taken sulphate of iron
instead of sulphate of alumina, and proceeded in a similar manner, and
added Alizarin, I should have obtained a dark purple lake. Now if you
imagine these reactions going on in a single fibre of a textile
material, you have grasped the theory and purpose of mordanting. The
textile fabric is drawn through the alumina solution to fill the pores
and tubes of the fabric; it is then passed through a weak alkaline bath
to basify or render basic the aluminium salt in the pores; and then it
is finally carried into the dye-bath and heated there, in order to
precipitate the colour lake in the fibre. The method usually employed to
mordant woollen fabrics consists in boiling them with weak solutions of
the metallic salts used as mordants, often with the addition of acid
salts, cream of tartar, and the like. A partial decomposition of the
metallic salts ensues, and it is induced by several conditions: (1) The
dilution of the liquid; (2) the heating of the solution; (3) t