Helmholtz model See electrical
double layer.
heme See haem.
hemiacetals See acetals.
hemicellulose A *polysaccharide
found in the cell walls of plants. The
branched chains of this molecule
bind to cellulose microÜbrils, forming a network of cross-linked Übres.
hemihedral form The form of a
crystal in which only half the number of faces required for the symmetry are present. Compare holohedral
form.
hemihydrate A crystalline hydrate
containing two molecules of compound per molecule of water (e.g.
2CaSO 4 .H 2 O).
hemiketals See ketals.
henry Symbol H. The *SI unit of inductance equal to the inductance of a
closed circuit in which an e.m.f. of
one volt is produced when the electric current in the circuit varies uniformly 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 temperature 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 solvent.
heparin A glycosaminoglycan (mucopolysaccharide) 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 carbon
atoms, C 16 H 33 COOH; m.p. 59–61°C;
b.p. 227°C (100 mm Hg). It is present
in certain natural fats.
heptahydrate A crystalline hydrate that has seven moles of water
per mole of compound.
heptane A 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 cannabinoids See cannabinoids.
herbicide See pesticide.
Hermann–Mauguin system (international 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 spectroscopy), 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 C n . nmm is the same
group as C nv . There are two ms because of two distinct types of mirror
plane containing the n-fold axis. n22
is the same group as D n . The other
categories do not coincide with the
SchoenÛies system. is a group with
an n-fold rotation–inversion axis and
includes C 3h as 6
_
, S 4 as 4
_
, S 6 as 3
_
, and
S 2 as 1
_
. n/m is the same group as C nh
except that C 3h is regarded as 6
_
. n2m
is the same group as D nd , except that
D 3h is regarded as 62m. n/m 2/m 2/m,
abbreviated to n/mmm, is the same
group as D nh , except that D 3h is
regarded as 6
_
2m. (Unlike the
SchoenÛies system, the Hermann–
Mauguin system regards the threeHelmholtz model
268
h
www.AzShimi.ir www.AzShimi.com
double layer.
heme See haem.
hemiacetals See acetals.
hemicellulose A *polysaccharide
found in the cell walls of plants. The
branched chains of this molecule
bind to cellulose microÜbrils, forming a network of cross-linked Übres.
hemihedral form The form of a
crystal in which only half the number of faces required for the symmetry are present. Compare holohedral
form.
hemihydrate A crystalline hydrate
containing two molecules of compound per molecule of water (e.g.
2CaSO 4 .H 2 O).
hemiketals See ketals.
henry Symbol H. The *SI unit of inductance equal to the inductance of a
closed circuit in which an e.m.f. of
one volt is produced when the electric current in the circuit varies uniformly 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 temperature 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 solvent.
heparin A glycosaminoglycan (mucopolysaccharide) 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 carbon
atoms, C 16 H 33 COOH; m.p. 59–61°C;
b.p. 227°C (100 mm Hg). It is present
in certain natural fats.
heptahydrate A crystalline hydrate that has seven moles of water
per mole of compound.
heptane A 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 cannabinoids See cannabinoids.
herbicide See pesticide.
Hermann–Mauguin system (international 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 spectroscopy), 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 C n . nmm is the same
group as C nv . There are two ms because of two distinct types of mirror
plane containing the n-fold axis. n22
is the same group as D n . The other
categories do not coincide with the
SchoenÛies system. is a group with
an n-fold rotation–inversion axis and
includes C 3h as 6
_
, S 4 as 4
_
, S 6 as 3
_
, and
S 2 as 1
_
. n/m is the same group as C nh
except that C 3h is regarded as 6
_
. n2m
is the same group as D nd , except that
D 3h is regarded as 62m. n/m 2/m 2/m,
abbreviated to n/mmm, is the same
group as D nh , except that D 3h is
regarded as 6
_
2m. (Unlike the
SchoenÛies system, the Hermann–
Mauguin system regards the threeHelmholtz model
268
h
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