ion (usually an amine salt or ionized
imino group). They are used for
acrylicÜbres and also for wool and
silk, although they have only moder-
ate fastness with these materials.
Direct dyes are dyes that have a
high afÜnity for cotton, rayon, and
other celluloseÜbres. They are ap-
plied directly from a neutral bath
containing sodium chloride or
sodium sulphate. Like acid dyes, they
are usually sulphonic acid salts but
are distinguished by their greater
substantivity (afÜnity for the sub-
strate), hence the alternative name
substantive dyes.
Vat dyes are insoluble substances
used for cotton dyeing. They usually
contain keto groups, C=O, which are
reduced to C–OH groups, rendering
the dye soluble (the leuco form of the
dye). The dye is applied in this form,
then oxidized by air or oxidizing
agents to precipitate the pigment in
theÜbres. Indigo and anthroquinone
dyes are examples of vat dyes. Sul-
phur dyes are dyes applied by this
technique using sodium sulphide so-
lution to reduce and dissolve the dye.
Sulphur dyes are used for cellulose
Übres.
Disperse dyes are insoluble dyes
applied in the form of aÜne disper-
sion in water. They are used for cellu-
lose acetate and other synthetic
Übres.
Reactive dyes are compounds that
contain groups capable of reacting
with the substrate to form covalent
bonds. They have high substantivity
and are used particularly for cellu-
loseÜbres.
dynamical systemA system gov-
erned by dynamics (either classical
mechanics or quantum mechanics).
The evolution of dynamical systems
can be very complex, even for sys-
tems with only a few degrees of free-
dom, sometimes involving such
considerations as ergodicity, which
originated in *statistical mechanics.
The evolution of dynamical systems
can be studied using the phase space
for the system. *Chaos is an example
of the complex behaviour that can
occur in a dynamical system.dynamic equilibriumSee equilib-
rium.dynamiteAny of a class of high ex-
plosives based on nitroglycerin. The
original form, invented in 1867 by
Alfred Nobel, consisted of nitroglyc-
erin absorbed in kieselguhr. Modern
dynamites, which are used for blast-
ing, contain sodium or ammonium
nitrate sensitized with nitroglycerin
and use other absorbers (e.g. wood
pulp).
dysprosium Symbol Dy. A soft sil-
very metallic element belonging to
the *lanthanoids; a.n. 66; r.a.m.
162.50; r.d. 8.551 (20°C); m.p. 1412°C;
b.p. 2562°C. It occurs in apatite,
gadolinite, and xenotime, from
which it is extracted by an ion-
exchange process. There are seven
natural isotopes and twelve artiÜcial
isotopes have been identiÜed. ItÜnds
limited use in some alloys as a neu-
tron absorber, particularly in nuclear
technology. It was discovered by Paul
Lecoq de Boisbaudran (1838–1912) in
1886.
A- Information from the WebElements site
dystectic mixture A mixture of
substances that has a constant maxi-
mum melting point.
189 dystectic mixture
d