between the levels 2p^10 and 5d^5 of
the nuclide krypton–86. This deÜni-
tion (in 1958) replaced the older deÜ-
nition of a metre based on a
platinum–iridium bar of standard
length. When the *metric system
was introduced in 1791 in France,
the metre was intended to be one
ten-millionth of the earth’s meridian
quadrant passing through Paris. How-
ever, the original geodetic surveys
proved the impractibility of such a
standard and the original platinum
metre bar, the mètre des archives, was
constructed in 1793.metric systemA decimal system
of units originally devised by a com-
mittee of the French Academy,
which included J. L. Lagrange and
P. S. Laplace, in 1791. It was based on
the *metre, the gram deÜned in
terms of the mass of a cubic centime-
tre of water, and the second. This
centimetre-gram-second system (see
c.g.s. units) later gave way for
scientiÜc work to the metre-kilo-
gram-second system (see m.k.s. units)
on which *SI units are based.Meyer, Viktor (1848–97) German
chemist who worked at Zürich and
later at Heidelberg on a wide range
of chemical topics. He was theÜrst to
prepare oximes and the sulphur com-
pound theophene. He is known for
his method of measuring relative
molecular mass by determining
vapour density. Meyer also worked
on stereochemistry and was theÜrst
to identify steric hindrance in chemi-
cal reactions.micaAny of a group of silicate min-
erals with a layered structure. Micas
are composed of linked SiO 4 tetrahe-
dra with cations and hydroxyl group-
ings between the layers. The general
formula is X 2 Y4–6Z 8 O 20 (OH,F) 4 , where
X = K,Na,Ca; Y = Al,Mg,Fe,Li; andZ = Si,Al. The three main mica miner-
als are:
*muscovite, K 2 Al 4 (Si 6 Al 2 O 20 )(OH,F) 4 ;
*biotite, K 2 (Mg,Fe2+)6–4(Fe3+,Al,Ti)0–2-
(Si6–5Al2–3O 20 )(OH,F) 4 ;
lepidolite, K 2 (Li,Al)5–6(Si6–7Al2–1O 20 )-
(OH,F) 4.
Micas have perfect basal cleavage and
the thin cleavageÛakes areÛexible
and elastic. Flakes of mica are used as
electrical insulators and as the dielec-
tric in capacitors.micelleAn aggregate of molecules
in a *colloid. For example, when
soap or other *detergents dissolve in
water they do so as micelles – small
clusters of molecules in which the
nonpolar hydrocarbon groups are in
the centre and the hydrophilic polar
groups are on the outside solvated by
the water molecules.Michaelis–Menten curveA graph
that shows the relationship between
the concentration of a substrate and
the rate of the corresponding en-
zyme-controlled reaction. The curve
only applies to enzyme reactions in-
volving a single substrate. It was de-
vised by Leonor Michaelis (1875–
1949) and Maud Menten (1879–1960).
The graph can be used to calculate
the Michaelis constant (Km), which is
the concentration of a substrate re-
quired in order for an enzyme to act
at half of its maximum velocity
(Vmax). The Michaelis constant is a
measure of the afÜnity of an enzyme
for a substrate. A low value corre-
sponds to a high afÜnity, and vice
versa. See also enzyme kinetics.
A- Original paper on Michaelis–Menten
kinetics
micro- Symbol μ. A preÜx used in
the metric system to denote one mil-
lionth. For example, 10–6metre = 1
micrometre (μm).
microbalance A sensitive *balancemetric system 354m