result is indicated by a cream precipi-
tate.
McLeod gaugeA vacuum pressure
gauge, devised by Herbert McLeod
(1841–1923), in which a relatively
large volume of a low-pressure gas is
compressed to a small volume in a
glass apparatus. The volume is re-
duced to an extent that causes the
pressure to rise sufÜciently to sup-
port a column ofÛuid high enough
to read. This simple device, which re-
lies on *Boyle’s law, is suitable for
measuring pressures in the range 10^3
to 10–3pascal.
McMillan–Mayer theoryA
theory of solutions of nonelectrolytes
developed by the US scientists W. G.
McMillan and J. E. Mayer in 1945.
The theory shows that there is a one-
to-one correspondence between the
equations describing a nonideal gas
and those describing dilute solutions
of nonelectrolytes. In particular, they
showed that there is a correspon-
dence between the pressure of the
gas and the osmotic pressure of the
solution. This enables an expansion
for solutions to be written, which is
analogous to the virial expansion of
nonideal gases with analogues of the
virial coefÜcients. These coefÜcients
can be calculated with the analogue
of potential being the potential of
mean force of N solute molecules in
the pure solvent. The McMillan–
Mayer theory can also be extended to
distribution functions.MDA(methylenedioxyampheta-
mine)A hallucinogenic drug,
C 10 H 13 CO 2 , originally designed for
medical use but now extensively
used as a club drug. Its effects are
similar to those of MDMA (See ec-
stasy).MDMAMethylenedioxymetham-
phetamine. See ecstasy.mean free path The average dis-tance travelled between collisions by
the molecules in a gas, the electrons
in a metallic crystal, the neutrons in
a moderator, etc. According to the
*kinetic theory the mean free path
between elastic collisions of gas mol-
ecules of diameter d (assuming they
are rigid spheres) is 1/√ 2 nπd^2 , where
n is the number of molecules per
unit volume in the gas. As n is pro-
portional to the pressure of the gas,
the mean free path is inversely pro-
portional to the pressure.
mean free time The average time
that elapses between the collisions of
the molecules in a gas, the electrons
in a crystal, the neutrons in a moder-
ator, etc. See mean free path.
mechanical bondingBonding
that involves a mechanical constraint
preventing two parts of a molecule
separating, rather than a chemical
linkage based on transfer or sharing
of electrons. It is found in *rotax-
anes, *catenanes, and *molecular
knots.
mechanismThe way in which a
particular chemical reaction occurs,
described in terms of the steps in-
volved. For example, the hydrolysis
of an alkyl chloride proceeds by the
SN1 mechanism (see nucleophilic
substitution).Mecke’s test A *presumptive test
for amphetamines, methampheta-
mines, and heroin. Mecke’s reagent
consists of 1 gram of selenious acid
in 100 ml of concentrated sulphuric
acid. Different substances give differ-
ent results. Ecstasy, for example,
gives a light blue colour, turning to
turquoise, and then dark blue.
Heroin gives a yellow colour chang-
ing to green. LSD gives an olive-green
colour, changing to black. Mescaline
gives a brownish-orange colour.
medium frequency (MF)A radio
frequency in the range 0.3–3 mega-McLeod gauge 346m