collected and the volume measured,
from which the mass of nitrogen in a
known mass of sample can be found.
2.A method ofÜnding the relative
molecular masses of volatile liquids
by weighing. A thin-glass bulb with a
long narrow neck is used. This is
weighed full of air at known temper-
ature, then a small amount of sample
is introduced and the bulb heated (in
a bath) so that the liquid is vaporized
and the air is driven out. The tip of
the neck is sealed and the bulb
cooled and weighed at known (room)
temperature. The volume of the bulb
is found byÜlling it with water and
weighing again. If the density of air
is known, the mass of vapour in a
known volume can be calculated.
The techniques are named after
Jean Baptiste André *Dumas.dupletA pair of electrons in a cova-
lent chemical bond.
Duquenois–Levine testA widely
used presumptive test for tetrahydro-
cannabinol and other cannabinoids.
The Duquenois–Levine reagent has a
solution containing 2% vanillin and
1% ethanal. Concentrated hydrochlo-
ric acid is added and then chloro-
form. A purple colour in the
chloroform layer indicates a positive
result.
DuraluminTradename for a class
of strong lightweight aluminium al-
loys containing copper, magnesium,
manganese, and sometimes silicon.
Duralumin alloys combine strength
with lightness and are extensively
used in aircraft, racing cars, etc.Dutch metalAn alloy of copper
and zinc, which can be produced in
very thin sheets and used as imita-
tion gold leaf. It spontaneously in-
Ûames in chlorine.
dye laserA type of laser in which
the active material is a dye dissolved
in a suitable solvent (e.g. RhodanineG in methanol). The dye is excited by
an external source. The solvent
broadens the states into bands and
consequently laser action can be ob-
tained over a range of wavelengths.
This allows one to select a speciÜc
wavelength (using a grating) and to
change the wavelength of the laser.
Such a device is called a tuneable
laser. Dye lasers are also used in pro-
ducing very short pulses of radiation.
The technique is to use a dye that
stops absorbing radiation when a
high proportion of its molecules be-
come excited. The cavity then be-
comes resonant and a pulse of
radiation is produced. This technique
can give pulses of about 10 nanosec-
onds duration and is used in *femto-
chemistry.dyesSubstances used to impart col-
our to textiles, leather, paper, etc.
Compounds used for dyeing
(dyestuffs) are generally organic com-
pounds containing conjugated dou-
ble bonds. The group producing the
colour is the *chromophore; other
noncoloured groups that inÛuence or
intensify the colour are called *aux-
ochromes. Dyes can be classiÜed ac-
cording to the chemical structure of
the dye molecule. For example, azo
dyescontain the –N=N– group (see
azo compounds). In practice, they
are classiÜed according to the way in
which the dye is applied or is held on
the substrate.
Acid dyes are compounds in
which the chromophore is part of
a negative ion (usually an organic
sulphonate RSO 2 O–). They can be
used for proteinÜbres (e.g. wool and
silk) and for polyamide and acrylic
Übres. Originally, they were applied
from an acidic bath. Metallized dyes
are forms of acid dyes in which the
negative ion contains a chelated
metal atom. Basic dyes have chro-
mophores that are part of a positiveduplet 188d