process expels moisture, chemically
combined water, and volatile matter;
in the case of sulphides, the sulphur
is expelled as sulphur dioxide and
the ore is converted into an oxide.
Rochelle saltPotassium sodium
tartrate tetrahydrate, KNaC 4 H 4 O 6.
4H 2 O. A colourless crystalline salt
used for its piezoelectric properties.
Rochon prism An optical device
consisting of two quartz prisms; the
Ürst, cut parallel to the optic axis, re-
ceives the light; the second, with the
optic axis at right angles, transmits
the ordinary ray without deviation
but the extraordinary ray is deÛected
and can be absorbed by a screen. The
device can be used to produce plane-
polarized light and it can also be
used with ultraviolet radiation.
rock An aggregate of mineral parti-
cles that makes up part of the earth’s
crust. It may be consolidated or un-
consolidated (e.g. sand, gravel, mud,
shells, coral, and clay).
rock crystal See quartz.
rocking-chair cell See intercala-
tion cell.
rock salt See halite.
rock salt structure (sodium chlo-
ride structure) A type of ionic crystal
structure in which the cations have a
face-centred cubic arrangement, with
anions occupying all the octahedral
holes. It can equally be described as a
fcc array of anions with cations in
the octahedral holes. Each type of
ion has a coordination number of 6.
Examples of compounds that have
the structure are NaCl, KBr, AgCl,
AgBr, HgO, CaO, FeO, NiO, and SnAs.
A
- An interactive version of the structure
roentgen The former unit of dose
equivalent (see radiation units). It is
named after the discoverer of X-rays,
W. K. Roentgen (1845–1923).roentgenium Symbol Rg. A radio-
active transactinide; a.n. 111. It was
made by fusion of^209 Bi with^64 Ni.
Only a few atoms have been de-
tected.
A- Information from the WebElements
site
RohypnolSee flunitrazepam.root-mean-square value (RMS
value)1.(in statistics) A typical value
of a number (n) of values of a quan-
tity (x 1 ,x 2 ,x 3 ...) equal to the square
root of the sum of the squares of the
values divided by n, i.e.
RMS value = √[(x 12 + x 22 + x 32 ...)/n]
2.(in physics) A typical value of a
continuously varying quantity, such
as an alternating electric current, ob-
tained similarly from many samples
taken at regular time intervals dur-
ing a cycle. Theoretically this can be
shown to be the effective value, i.e.
the value of the equivalent direct cur-
rent that would produce the same
power dissipation. For a sinusoidal
current this is equal to Im/√2, where
Imis the maximum value of the cur-
rent.
ROSDALA type of *line notation
used in databases. It is based on a
*connection table for the molecule
written in a single text line. ROSDAL
is an acronym for Representation of
Organic Structure Descriptions
Arranged Linearly.
Rose’s metal An alloy of low melt-
ing point (about 100°C) consisting of
50% bismuth, 25–28% lead, and
22–25% tin.
rosin A hard natural resin obtained
from pine tree oil and the wastes
from processing wood pulp. It may
be colourless, yellow, brown, or465 rosin
r