according to the International Commission on Radiological Protection.
dosimeter Any device used to
measure absorbed *dose of ionizing
radiation. Methods used include the
ionization chamber, photographic
Ülm, or the rate at which certain
chemical reactions occur in the presence of ionizing radiation.
double bond See chemical bond.
double decomposition See
metathesis.
double layer See electrical double layer.
double refraction The property,
possessed by certain crystals (notably
calcite), of forming two refracted rays
from a single incident ray. The ordinary ray obeys the normal laws of refraction. The other refracted ray,
called the extraordinary ray, follows
different laws. The light in the ordinary ray is polarized at right angles
to the light in the extraordinary ray.
Along an optic axis the ordinary and
extraordinary rays travel with the
same speed. Some crystals, such as
calcite, quartz, and tourmaline, have
only one optic axis; they are uniaxial
crystals. Others, such as mica and selenite, have two optic axes; they are
biaxial crystals. The phenomenon is
also known as birefringence and the
double-refracting crystal as a birefringent crystal. See also polarization.
double salt A crystalline salt in
which there are two different anions
and/or cations. An example is the
mineral dolomite, CaCO 3 .MgCO 3 ,
which contains a regular arrangement of Ca
2+ and Mg
2+ ions in its
crystal lattice. *Alums are double sulphates. Double salts only exist in the
solid; when dissolved they act as a
mixture of the two separate salts.
Double oxides are similar.
doublet A pair of associated lines
in certain spectra, e.g. the two lines
that make up the sodium D-lines.
Downs process A process for extracting sodium by the electrolysis of
molten sodium chloride. The Downs
cell has a central graphite anode surrounded by a cylindrical steel cathode. Chlorine released is led away
through a hood over the anode.
Molten sodium is formed at the cathode and collected through another
hood around the top of the cathode
cylinder (it is less dense than the
sodium chloride). The two hoods and
electrodes are separated by a coaxial
cylindrical steel gauze. A small
amount of calcium chloride is added
to the sodium chloride to lower its
melting point. The sodium chloride
is melted electrically and kept
molten by the current through the
cell. More sodium chloride is added
as the electrolysis proceeds.
Dow process A method of extracting magnesium from sea water by
adding calcium hydroxide to precipitate magnesium hydroxide.
Dragendorff test A *presumptive
test for alkaloids. The Dragendorff
reagent has two solutions. One is bismuth nitrate in acetic acid. This is
followed by a sodium nitrate solution. Alkaloids are indicated by a reddish-brown deposit.
dropping-mercury electrode See
polarography.
dry cell A primary or secondary cell
in which the electrolytes are restrained from Ûowing in some way.
Many torch, radio, and calculator batteries are *Leclanché cells in which
the electrolyte is an ammonium chloride paste and the container is the
negative zinc electrode (with an
outer plastic wrapping). Various
modiÜcations of the Leclanché cell
are used in dry cells. In the zinc chloride cell, the electrolyte is a paste of
dosimeter
186
d
www.AzShimi.ir www.AzShimi.com
dosimeter Any device used to
measure absorbed *dose of ionizing
radiation. Methods used include the
ionization chamber, photographic
Ülm, or the rate at which certain
chemical reactions occur in the presence of ionizing radiation.
double bond See chemical bond.
double decomposition See
metathesis.
double layer See electrical double layer.
double refraction The property,
possessed by certain crystals (notably
calcite), of forming two refracted rays
from a single incident ray. The ordinary ray obeys the normal laws of refraction. The other refracted ray,
called the extraordinary ray, follows
different laws. The light in the ordinary ray is polarized at right angles
to the light in the extraordinary ray.
Along an optic axis the ordinary and
extraordinary rays travel with the
same speed. Some crystals, such as
calcite, quartz, and tourmaline, have
only one optic axis; they are uniaxial
crystals. Others, such as mica and selenite, have two optic axes; they are
biaxial crystals. The phenomenon is
also known as birefringence and the
double-refracting crystal as a birefringent crystal. See also polarization.
double salt A crystalline salt in
which there are two different anions
and/or cations. An example is the
mineral dolomite, CaCO 3 .MgCO 3 ,
which contains a regular arrangement of Ca
2+ and Mg
2+ ions in its
crystal lattice. *Alums are double sulphates. Double salts only exist in the
solid; when dissolved they act as a
mixture of the two separate salts.
Double oxides are similar.
doublet A pair of associated lines
in certain spectra, e.g. the two lines
that make up the sodium D-lines.
Downs process A process for extracting sodium by the electrolysis of
molten sodium chloride. The Downs
cell has a central graphite anode surrounded by a cylindrical steel cathode. Chlorine released is led away
through a hood over the anode.
Molten sodium is formed at the cathode and collected through another
hood around the top of the cathode
cylinder (it is less dense than the
sodium chloride). The two hoods and
electrodes are separated by a coaxial
cylindrical steel gauze. A small
amount of calcium chloride is added
to the sodium chloride to lower its
melting point. The sodium chloride
is melted electrically and kept
molten by the current through the
cell. More sodium chloride is added
as the electrolysis proceeds.
Dow process A method of extracting magnesium from sea water by
adding calcium hydroxide to precipitate magnesium hydroxide.
Dragendorff test A *presumptive
test for alkaloids. The Dragendorff
reagent has two solutions. One is bismuth nitrate in acetic acid. This is
followed by a sodium nitrate solution. Alkaloids are indicated by a reddish-brown deposit.
dropping-mercury electrode See
polarography.
dry cell A primary or secondary cell
in which the electrolytes are restrained from Ûowing in some way.
Many torch, radio, and calculator batteries are *Leclanché cells in which
the electrolyte is an ammonium chloride paste and the container is the
negative zinc electrode (with an
outer plastic wrapping). Various
modiÜcations of the Leclanché cell
are used in dry cells. In the zinc chloride cell, the electrolyte is a paste of
dosimeter
186
d
www.AzShimi.ir www.AzShimi.com