D
2,4-D 2,4-dichlorophenoxyacetic
acid (2,4-dichlorophenoxyethanoic
acid): a synthetic auxin frequently
used as a weedkiller of broad-leaved
weeds. See also pesticide.
Dakin reaction See baeyer–
villiger reaction.
daltonSee atomic mass unit.
Dalton, John(1766–1844) British
chemist and physicist. In 1801 he for-
mulated his law of partial pressures
(see dalton’s law), but he is best re-
membered for *Dalton’s atomic
theory, which he announced in
- Dalton also studied colour
blindness (a condition, once called
Daltonism, that he shared with his
brother).
Dalton’s atomic theory A theory
of chemical combination,Ürst
stated by John Dalton in 1803. It in-
volves the following postulates:
(1) Elements consist of indivisible
small particles (atoms).
(2) All atoms of the same element are
identical; different elements have dif-
ferent types of atom.
(3) Atoms can neither be created nor
destroyed.
(4) ‘Compound elements’ (i.e. com-
pounds) are formed when atoms of
different elements join in simple ra-
tios to form ‘compound atoms’ (i.e.
molecules).
Dalton also proposed symbols for
atoms of different elements (later re-
placed by the present notation using
letters).
Dalton’s law The total pressure of
a mixture of gases or vapours is
equal to the sum of the partial pres-
sures of its components, i.e. the sum
of the pressures that each compo-
nent would exert if it were present
alone and occupied the same volume
as the mixture of gases. Strictly
speaking, the principle is true only
for ideal gases. The law was discov-
ered by John Dalton.Daniell cellA type of primary
*voltaic cell with a copper positive
electrode and a negative electrode of
a zinc amalgam. The zinc-amalgam
electrode is placed in an electrolyte
of dilute sulphuric acid or zinc sul-
phate solution in a porous pot,
which stands in a solution of copper
sulphate in which the copper elec-
trode is immersed. While the reac-
tion takes place ions move through
the porous pot, but when it is not in
use the cell should be dismantled to
prevent the diffusion of one elec-
trolyte into the other. The e.m.f. of
the cell is 1.08 volts with sulphuric
acid and 1.10 volts with zinc sul-
phate. It was invented in 1836 by the
British chemist John Daniell (1790–
1845).dark reaction See photosynthesis.darmstadtiumSymbol Ds. A radio-
active transactinide; a.n. 110. It has
several isotopes; the most stable
being^281 Ds, with a half-life of about
1.6 minutes. It can be produced by
bombarding a plutonium target with
sulphur nuclei or by bombarding a
lead target with nickel nuclei. Its
chemical properties probably resem-
ble those of platinum. Darmstadtium
was named after the German city of
Darmstadt, the location of the Insti-