Dictionary of Chemistry [6th Ed.]

ide solution is added to a solution of
mercury(II) nitrate. Both forms de-
compose to the elements at high
temperature. The black precipitate
formed when sodium hydroxide is
added to mercury(I) nitrate solution
is sometimes referred to as
mercury(I) oxide (Hg 2 O) but is proba-
bly a mixture of HgO and free mer-
cury.

mercury(II) sulphide A red or
black compound, HgS, occurring nat-
urally as the minerals cinnabar (red)
and metacinnabar (black). It can be
obtained as a black precipitate by
bubbling hydrogen sulphide through
a solution of mercury(II) nitrate. The
red form is obtained by sublimation.
The compound is also called vermil-
ion(used as a pigment).

mer isomer See isomerism.

mescalineA powerful hallucino-
genic compound obtained from pey-
ote– theÛowering head of a type of
Mexican cactus. Mescaline is a class A
drug in the UK. It can be detected by
the Mecke test.

349 metal

m

O

O

O

CH 3

CH 3

CH 3

C

H 2

C

H 2

NH 2

Mescaline

meso-isomerSee optical activity.

mesomerism(mesomeric effect) A
former name for *resonance in mol-
ecules. See also electronic effects.

mesomorphSee lyotropic meso-
morph.

mesoscopicDesignating a size
scale intermediate between those of

the *microscopic and the *macro-

scopic states. Mesoscopic objects and

systems require quantum mechanics

to describe them.

meta-1.PreÜx designating a ben-

zene compound in which two sub-

stituents are in the 1,3 positions on

the benzene ring. The abbreviation

m- is used; for example, m-xylene is

1,3-dimethylbenzene. Compare

ortho-; para-. 2.PreÜx designating a

lower oxo acid, e.g. metaphosphoric

acid. Compare ortho-.

metabolic pathwaySee metabo-

lism.

metabolismThe sum of the chem-

ical reactions that occur within living

organisms. The various compounds

that take part in or are formed by

these reactions are called metabo-

lites. In animals many metabolites

are obtained by the digestion of food,

whereas in plants only the basic

starting materials (carbon dioxide,

water, and minerals) are externally

derived. The synthesis (*anabolism)

and breakdown (*catabolism) of most

compounds occurs by a number of

reaction steps, the reaction sequence

being termed a metabolic pathway.

Some pathways (e.g. *glycolysis) are

linear; others (e.g. the *Krebs cycle)

are cyclic.

metaboliteSee metabolism.

metaboric acidSee boric acid.

metalAny of a class of chemical el-

ements that are typically lustrous

solids that are good conductors of

heat and electricity. Not all metals

have all these properties (e.g. mer-

cury is a liquid). In chemistry, metals

fall into two distinct types. Those of

the s- and p-blocks (e.g. sodium and

aluminium) are generally soft silvery

reactive elements. They tend to form

positive ions and so are described as

electropositive. This is contrasted