as soda, potash, iron,
alumina, chrome, tin, nickel, cobalt, etc., forming a series of salts.
Those of soda and potash are usually soluble in water, while those of
the other metals are insoluble, and are usually of strong colour. It
is on this property of forming these insoluble coloured bodies, colour
lakes, as they are called, that the value of the adjective dye-stuffs
in dyeing depends.

The group of adjective colouring matters may be subdivided into two
divisions, not depending upon any differences in the mode of
application, but upon certain differences in the results they give.
Perhaps the best example of an adjective dye-stuff is Alizarine. This
body has a faint red colour, but of itself possesses absolutely no
colouring power. When, however, it is brought into combination with
such metallic oxide as alumina, iron and chrome, then it forms
coloured bodies, the colour of which varies with the metal with which
it is in union, thus with alumina, it is a bright red; with iron, a
dark violet, almost black; with chrome, a deep red; with tin, a (p. 070)
scarlet; and so on. This is a representative of the true adjective
dyes, which comprise most of the so-called Alizarine dye-stuffs, and
logwood, fustic, and most of the natural dye-stuffs. Another division
of the group includes a few colouring matters of recent introduction,
like Azo green, Alizarine yellow, Galloflavine, Anthracene yellow,
Flavazol, etc., which, while forming insoluble colour lakes with
metallic oxides, do not give different colours with different metals.

This class of dye-stuffs, owing to their forming these insoluble
colours, gives really fast colours, capable of resisting lengthened
exposure to light and air, and resisting washing, acids and alkalies.
Of course there are differences between the various members of the
group in this respect, and even the resisting power of an individual
member depends a good deal on the metal with which it is combined, and
the care with which the process of dyeing has been carried out.

In the dyeing of these adjective dye-stuffs, upon the various fibres,
and on wool in any particular, the object is to bring about in any
convenient way the formation on the fibre of the metallic combination
of the colouring principle and the mordant, and it is obvious that if
a satisfactory result is to be obtained, then this must be done in a
very thorough manner. There are three ways in which this combination
of colouring prin