zinc chloride rather than ammonium
chloride. The electrical characteris-
tics are similar to those of the
Leclanché cell but the cell works bet-
ter at low temperatures and has
more efÜcient depolarization charac-
teristics. A number of alkaline sec-
ondary cells can be designed for use
as dry cells. In these, the electrolyte
is a liquid (sodium or potassium hy-
droxide) held in a porous material or
in a gel. Alkaline dry cells typically
have zinc–manganese dioxide, silver
oxide–zinc, nickel–cadmium, or
nickel–iron electrode systems (see
nickel–iron accumulator). For spe-
cialized purposes, dry cells and bat-
teries have been produced with solid
electrolytes. These may contain a
solid crystalline salt, such as silver io-
dide, an ion-exchange membrane, or
an organic wax with a small amount
of dissolved ionic material. Such cells
deliver low currents. They are used
in miniature cells for use in elec-
tronic equipment.
dry iceSolid carbon dioxide used as
a refrigerant. It is convenient because
it sublimes at –78°C (195 K) at stan-
dard pressure rather than melting.
drying oilA natural oil, such as lin-
seed oil, that hardens on exposure to
the air. Drying oils contain unsatu-
rated fatty acids, such as linoleic and
linolenic acids, which polymerize on
oxidation. They are used in paints,
varnishes, etc.
DSC Differential scanning calorime-
try. See thermal analysis.
D-seriesSee absolute configura-
tion.
DTA Differential thermal analysis.
See thermal analysis.
dubniumSymbol Db. A radioactive
*transactinide element; a.n. 105. It
wasÜrst reported in 1967 by a group
at Dubna near Moscow and was
conÜrmed in 1970 at Dubna and at
Berkeley, California. It can be made
by bombarding californium–249 nu-
clei with nitrogen–15 nuclei. Only a
few atoms have ever been made.
A- Information from the WebElements site
Dulong and Petit’s law For a
solid element the product of the rela-
tive atomic mass and the speciÜc
heat capacity is a constant equal to
about 25 J mol–1K–1. Formulated in
these terms in 1819 by the French
scientists Pierre Dulong (1785–1838)
and Alexis Petit (1791–1820), the law
in modern terms states: the molar
heat capacity of a solid element is ap-
proximately equal to 3R, where R is
the *gas constant. The law is only ap-
proximate but applies with fair accu-
racy at normal temperatures to
elements with a simple crystal struc-
ture.
A- Original paper
Dumas, Jean Baptiste André
(1800–84) French chemist, who be-
came an apothecary in Geneva,
where in 1818 he investigated the
use of iodine to treat goitre. He then
took up chemistry and moved to
Paris. In 1826 he devised a method of
measuring *vapour density. He went
on to discover various organic com-
pounds, including anthracene (1832),
urethane (1833), and methanol
(1834), which led him in 1840 to pro-
pose the theory of types (functional
groups).Dumas’ method1.A method of
Ünding the amount of nitrogen in an
organic compound. The sample is
weighed, mixed with copper(II)
oxide, and heated in a tube. Any ni-
trogen present in the compound is
converted into oxides of nitrogen,
which are led over hot copper to re-
duce them to nitrogen gas. This is187 Dumas’ method
d