Dictionary of Chemistry [6th Ed.]

according to the International Com-

mission on Radiological Protection.

dosimeterAny 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 pres-

ence of ionizing radiation.

double bondSee chemical bond.

double decompositionSee

metathesis.

double layerSee electrical dou-

ble layer.

double refraction The property,

possessed by certain crystals (notably

calcite), of forming two refracted rays

from a single incident ray. The ordi-

nary ray obeys the normal laws of re-

fraction. The other refracted ray,

called the extraordinary ray, follows

different laws. The light in the ordi-

nary 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 se-

lenite, have two optic axes; they are

biaxial crystals. The phenomenon is

also known as birefringence and the

double-refracting crystal as a birefrin-

gent crystal. See also polarization.

double saltA 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 arrange-

ment of Ca2+and Mg2+ions in its

crystal lattice. *Alums are double sul-

phates. Double salts only exist in the

solid; when dissolved they act as a

mixture of the two separate salts.

Double oxides are similar.

doubletA pair of associated lines

in certain spectra, e.g. the two lines

that make up the sodium D-lines.

Downs process A process for ex-

tracting sodium by the electrolysis of

molten sodium chloride. The Downs

cellhas a central graphite anode sur-

rounded by a cylindrical steel cath-

ode. Chlorine released is led away

through a hood over the anode.

Molten sodium is formed at the cath-

ode 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 extract-

ing magnesium from sea water by

adding calcium hydroxide to precipi-

tate magnesium hydroxide.

Dragendorff test A *presumptive

test for alkaloids. The Dragendorff

reagent has two solutions. One is bis-

muth nitrate in acetic acid. This is

followed by a sodium nitrate solu-

tion. Alkaloids are indicated by a red-

dish-brown deposit.

dropping-mercury electrode See

polarography.

dry cellA primary or secondary cell

in which the electrolytes are re-

strained fromÛowing in some way.

Many torch, radio, and calculator bat-

teries are *Leclanché cells in which

the electrolyte is an ammonium chlo-

ride 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 chlo-

ride cell, the electrolyte is a paste of

dosimeter 186

d