Helmholtz modelSee electrical
double layer.hemeSee haem.hemiacetalsSee acetals.
hemicelluloseA *polysaccharide
found in the cell walls of plants. The
branched chains of this molecule
bind to cellulose microÜbrils, form-
ing a network of cross-linkedÜbres.hemihedral formThe form of a
crystal in which only half the num-
ber of faces required for the symme-
try are present. Compare holohedral
form.hemihydrateA crystalline hydrate
containing two molecules of com-
pound per molecule of water (e.g.
2CaSO 4 .H 2 O).hemiketalsSee ketals.henrySymbol H. The *SI unit of in-
ductance equal to the inductance of a
closed circuit in which an e.m.f. of
one volt is produced when the elec-
tric current in the circuit varies uni-
formly at a rate of one ampere per
second. It is named after the U.S.
physicist Joseph Henry (1797–1878).Henry’s law At a constant tempera-
ture the mass of gas dissolved in a
liquid at equilibrium is proportional
to the partial pressure of the gas. The
law, discovered in 1801 by the British
chemist and physician William
Henry (1775–1836), is a special case
of the partition law. It applies only to
gases that do not react with the sol-
vent.heparinA glycosaminoglycan (mu-
copolysaccharide) with anticoagulant
properties, occurring in vertebrate
tissues, especially the lungs and
blood vessels.heptadecanoic acid (margaric
acid)A white crystalline carboxylic
acid with a linear chain of carbonatoms, C 16 H 33 COOH; m.p. 59–61°C;
b.p. 227°C (100 mm Hg). It is present
in certain natural fats.heptahydrateA crystalline hy-
drate that has seven moles of water
per mole of compound.heptaneA liquid straight-chain
alkane obtained from petroleum,
C 7 H 16 ; r.d. 0.684; m.p. –90.6°C; b.p.
98.4°C. In standardizing *octane
numbers, heptane is given a value
zero.heptaoxodiphosphoric(V) acid
See phosphoric(v) acid.heptavalent(septivalent)Having a
valency of seven.herbal cannabinoidsSee cannabi-
noids.
herbicideSee pesticide.Hermann–Mauguin system (in-
ternational system) A notation
used to describe the symmetry of
point groups. In contrast to the
*SchoenÛies system, which is used
for isolated molecules (e.g. in spec-
troscopy), the Hermann–Mauguin
system is used in *crystallography.
Some of the categories are the same
as the SchoenÛies system. n is the
same group as Cn. nmmis the same
group as Cnv. There are two ms be-
cause of two distinct types of mirror
plane containing the n-fold axis. n 22
is the same group as Dn. The other
categories do not coincide with the
SchoenÛies system. is a group with
an n-fold rotation–inversion axis and
includes C 3 has 6_
, S 4 as 4_
, S 6 as 3_
, and
S 2 as 1_
. n/m is the same group as Cnh
except that C 3 his regarded as 6
_
. n 2 m
is the same group as Dnd, except that
D 3 his regarded as 62m. n/m 2/m 2/m,
abbreviated to n/mmm, is the same
group as Dnh, except that D 3 his
regarded as 6
_
2 m. (Unlike the
SchoenÛies system, the Hermann–
Mauguin system regards the three-Helmholtz model 268h