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-galactosidase,

which breaks down lactose; the operon

that regulates lactose metabolism is

called the * lac operon.

jade A hard semiprecious stone consist-

ing either of jadeite or nephrite. Jadeite ,

the most valued of the two, is a sodium

aluminium pyroxene, NaAlSi 2 O 6 . It is

prized for its intense translucent green

colour but white, green and white,

brown, and orange varieties also occur.

The only important source of jadeite is in

the Mogaung region of upper Burma.

Nephrite is one of the amphibole group of

rock-forming minerals. It occurs in a vari-

ety of colours, including green, yellow,

white, and black. Important sources in-

clude Siberia, Turkistan, New Zealand,

Alaska, China, and W USA.

jadeite See jade.

Jahn–Teller effect If a likely structure

of a nonlinear molecule or ion would

have degenerate orbitals (i.e. two molecu-

lar orbitals with the same energy levels)

the actual structure of the molecule or

ion is distorted so as to split the energy

levels (‘raise’ the degeneracy). The effect

is observed in inorganic complexes. For

example, the ion [Cu(H 2 O) 6 ] 2+ is octahe-

dral and the six ligands might be ex-

pected to occupy equidistant positions at

the corners of a regular octahedron. In

fact, the octahedron is distorted, with

four ligands in a square and two opposite

ligands further away. If the ‘original’

structure has a centre of symmetry, the

distorted structure must also have a cen-

tre of symmetry. The effect was predicted

theoretically by H. A. Jahn and Edward

Teller in 1937.

jasper An impure variety of * chal-

cedony. It is associated with iron ores and

as a result contains iron oxide impurities

that give the mineral its characteristic

red or reddish-brown colour. Jasper is

used as a gemstone.

jaw The part of the vertebrate skeleton

that provides a support for the mouth

and holds the teeth. It consists of two

bones, the upper jaw (maxilla) and the

lower jaw (mandible). Members of the Ag-

natha lack jaws.

Jeans instability Instability in a cloud

of gas in space due to

uctuations in the

density of the gas, causing the matter in

the cloud to clump together and lead to

gravitational collapse. The conditions

under which this occurs were worked out

by Sir James Hopwood Jeans (1877–1946)

in terms of Newtonian gravity. The analo-

gous analysis of this problem using gen-

eral relativity theory is the basis of the

theory of * structure formation.

jejunum The portion of the mam-

malian * small intestine that follows the

* duodenum and precedes the * ileum.

The surface area of the lining of the je-

junum is greatly increased by numerous

small outgrowths ( see villus). This facili-

tates the absorption of digested material,

which is the prime function of the je-

junum.

jelly

sh See cnidaria.

Jenner, Edward (1749–1823) British

physician, who is best known for intro-

ducing smallpox vaccination to Britain in

1796 (announced two years later), using a

vaccine made from cowpox.

jet A variety of * coal that can be cut and

polished and is used for jewellery, orna-

ments, etc.

jet propulsion (reaction propulsion)

The propulsion of a body by means of a

force produced by discharging a Û uid in

the form of a jet. The backward-moving

Jacob–Monod hypothesis The theory

postulated by the French biologists

François Jacob (1920– ) and Jacques

Monod (1910–76) in 1961 to explain the

control of * gene expression in bacteria

( see operon). Jacob and Monod investi-

gated the expression of the gene that

codes for the enzyme

jet stream

442

uid reacts on the body in which it

was produced, in accordance with New-

ton’s third law of motion, to create a reac-

tive force that drives the body forward. Jet

propulsion occurs in nature, the squid

using a form of it to propel itself through

water. Although jet-propelled boats and

cars have been developed, the main use of

jet propulsion is in aircraft and spacecraft.

Jet propulsion is the only known method

of propulsion in space. In the atmosphere,

jet propulsion becomes more ef

repulsion. * Multiplets of many-electron

atoms having a large atomic number are

characterized by j-j coupling. Multiplets in

the * shell model of nuclei characterized

by j-j coupling are invoked to explain the

* magic numbers of nuclei. The multiplets

of many atoms and nuclei are intermedi-

ate between j-j coupling and * Russell–

Saunders coupling ( intermediate coupling ).

joint The point of contact between two

(or more) bones, together with the tissues

that surround it. Joints fall into three

classes that differ in the degree of free-

dom of movement they allow: (1) immov-

able joints , e.g. the * sutures between the

bones that form the cranium; (2) slightly

movable joints , e.g. the * symphyses be-

tween the vertebrae of the spinal column;

and (3) freely movable or synovial joints ,

e.g. those that occur between the limb

bones. Synovial joints include the ball-

and-socket joints (between the limbs and

the hip and shoulder girdles), which allow

movement in all directions; and the hinge

joints (e.g. at the knee and elbow), which

allow movement in one plane only. A

synovial joint is bound by ligaments and

lined with * synovial membrane.

cient at

higher altitudes, as ef Ü ciency is inversely

proportional to the density of the me-

dium through which a body is Û ying. The

three principal means of providing jet

propulsion are the turbojet, the ramjet,

and the rocket. The turbojet is an air-

breathing * heat engine based on the * gas

turbine, used to power jet aircraft. The

ramjet is also an air-breathing engine, but

compression of the oxidant is achieved by

the forward motion of the device through

the atmosphere. This enables the com-

pressor and turbine of the gas turbine to

be dispensed with and the remaining sys-

tem consists simply of an inlet diffuser, a

combustion chamber in which fuel is

burnt, and a jet nozzle through which the

products of combustion are discharged.

Used in guided missiles, the ramjet must

be accelerated to its operating velocity be-

fore it can

y ( see also pulse jet). These

two forms of jet propulsion, being air-

breathing engines, can only be used in the

earth’s atmosphere. The * rocket, how-

ever, carries its own oxidant and can thus

be used in space. See also ion engine.

jet stream A narrow wind current that

occurs in the earth’s atmosphere above

the lower troposphere ( see earth’s atmos-

phere). The wind Û ows towards the east at

speeds of between 60 km/h (summer) and

125 km/h (winter).

jeweller’s rouge Red powdered

haematite, iron(III) oxide, Fe 2 O 3 . It is a

mild abrasive used in metal cleaners and

polishes.

j-j coupling A type of * coupling in

many-fermion systems, such as electrons

in atoms and nucleons in nuclei, in which

the energies associated with the spin–

orbit interactions are much higher than

the energies associated with electrostatic

humerus

ligament

synovial membrane

synovial fluid

cartilage

ulna

a hinge joint (the elbow)

pelvis (socket)

cartilage

synovial membrane

ligament

head of femur (ball)

synovial

fluid

a ball-and-socket joint (the hip)

Types of freely movable joint

jet of

443

Joule’s laws

Joliot-Curie, Irène (1897–1956) French

physicist, daughter of Marie and Pierre

* Curie, who was educated by her mother

and her scientist associates. In 1921 she

began work at the Radium Institute, be-

coming director in 1946. In 1926 she mar-

ried Frédéric Joliot (1900–58). They shared

the 1935 Nobel Prize for chemistry for

their discovery of arti Ü cial radioactivity

the previous year.

joliotium See transactinide elements.

Joly’s steam calorimeter An appara-

tus invented by John Joly (1857–1933) to

measure the speci

ows through the

junction. The frequency of this current

depends on the size of the potential dif-

ference.

A junction of this type is called a

Josephson junction ; two or more junctions

joined by superconducting paths form a

Josephson interferometer . Such junctions

can be used in measuring fundamental

constants, in de

ning a voltage standard,

and in the highly accurate measurement

of magnetic Ü elds. An important potential

use is in logic components in high-speed

computers. Josephson junctions can

switch states very quickly (as low as 6

picoseconds). Moreover they have very

low power consumption and can be

packed closely without generating too

much heat. It is possible that computers

based on such devices could operate 50

times faster than the best existing ma-

chines. The effects are named after Brian

Josephson (1940– ), who predicted them

theoretically in 1962.

joule Symbol J. The * SI unit of work and

energy equal to the work done when the

point of application of a force of one new-

ton moves, in the direction of the force, a

distance of one metre. 1 joule = 10 7 ergs =

0.2388 calorie. It is named after James

Joule.

Joule, James Prescott (1818–89)

British physicist. In 1840 he discovered

the relationship between electric current,

resistance, and the amount of heat pro-

duced. In 1849 he gave an account of the

* kinetic theory of gases, and a year later

announced his best-known Ü nding, the

* mechanical equivalent of heat. Later,

with William Thomson (Lord * Kelvin), he

discovered the * Joule–Thomson effect.

Joule heating The production of heat

in a conductor as a result of the passage of

an electric current through the conductor.

The quantity of heat produced is given by

* Joule’s law (def. 1).

Joule’s laws 1. The heat ( Q ) produced

when an electric current ( I ) Û ows through

a resistance ( R ) for a time ( t ) is given by Q

= I 2 Rt . 2. The * internal energy of a given

c heat capacity of a gas

at constant volume. Two equal spherical

containers are suspended from the oppo-

site ends of a balance arm. One sphere is

evacuated and the other contains the sam-

ple gas. The whole apparatus is enclosed

in a steam bath, the speci Ü c heat capacity

of the sample gas being calculated from

the difference between the masses of the

water that condenses on each sphere.

Josephson effects Electrical effects ob-

served when two superconducting ma-

terials (at low temperature) are separated

by a thin layer of insulating material (typi-

cally a layer of oxide less than 10 –8 m

thick). If normal metallic conductors are

separated by such a barrier it is possible

for a small current to Û ow between the

conductors by the * tunnel effect. If the

materials are superconductors ( see super-

conductivity), several unusual phenom-

ena occur:

(1) A supercurrent can

ow through the

barrier; i.e. it has zero resistance.

(2) If this current exceeds a critical value,

this conductivity is lost; the barrier then

only passes the ‘normal’ low tunnelling

current and a voltage develops across the

junction.

(3) If a magnetic

eld is applied below the

critical current value, the current density

changes regularly with distance across the

junction. The net current through the bar-

rier depends on the magnetic

eld ap-

plied. As the Ü eld is increased the net

current increases from zero to a maxi-

mum, decreases to zero, increases again

to a (lower) maximum, decreases, and so

on. If the Ü eld exceeds a critical value the

superconductivity in the barrier vanishes

and a potential difference develops across

the junction.

(4) If a potential difference is applied

across the junction, a high-frequency

alternating current

Joule–Thomson effect

444

ni-

tion of thermodynamic temperature) as in

a real gas intermolecular forces would

cause changes in the internal energy

should a change of volume occur. See also

joule–thomson effect.

Joule–Thomson effect (Joule–Kelvin

effect) The change in temperature that

occurs when a gas expands through a

porous plug into a region of lower pres-

sure. For most real gases the temperature

falls under these circumstances as the gas

has to do internal work in overcoming the

intermolecular forces to enable the expan-

sion to take place. This is a deviation from

* Joule’s law (def. 2). There is usually also a

deviation from * Boyle’s law, which can

cause either a rise or a fall in temperature

since any increase in the product of pres-

sure and volume is a measure of external

work done. At a given pressure, there is a

particular temperature, called the inver-

sion temperature of the gas, at which the

rise in temperature from the Boyle’s law

deviation is balanced by the fall from the

Joule’s law deviation. There is then no

temperature change. Above the inversion

temperature the gas is heated by expan-

sion; below it, it is cooled. The effect was

discovered by James Joule working in col-

laboration with William Thomson (later

Lord Kelvin).

Jovian Relating to the planet Jupiter.

JUGFET See transistor.

jugular vein A paired vein in the neck

of mammals that returns blood from the

head to the heart. It joins the subclavian

vein at the base of the neck.

jumping gene See transposon.

junction detector (solid-state detector)

A sensitive detector of * ionizing radiation

in which the output is a current pulse

proportional to the energy falling in or

near the depletion region of a reverse-

biased * semiconductor junction. The Ü rst

types were made by evaporating a thin

layer of gold on to a polished wafer of n -

type germanium; however, gold–silicon

devices can be operated at room tempera-

ture and these have superseded the ger-

manium type, which have to be operated

at the temperature of liquid nitrogen to

reduce noise. When the gold–silicon junc-

tion is reverse-biased a depletion region,

devoid of charge carriers (electrons and

holes), forms in the silicon. Incoming ion-

izing radiation falling in this depletion re-

gion creates pairs of electrons and holes,

which both have to be collected in order

to give an output pulse proportional to

the energy of the detected particle.

Junction detectors are used in medicine

and biology as well as in space systems.

junction transistor See transistor.

Jupiter The largest planet in the * solar

system and the Ü fth in order from the sun

(778.41

10 6 km distant). Its equatorial di-

ameter is 142 985 km and its * sidereal pe-

riod is 11.86 years. Viewed through an

optical telescope it appears as an elliptical

disc crossed by a number of bands. With a

* radio telescope thermal radiation can be

detected from the Jovian stratosphere in-

dicating a temperature between 100 and

130 K. Microwave nonthermal emission

can also be detected from the Jovian * Van

Allen belts. The planet has at least 63

satellites, the largest of which, Io, has a

diameter of 3242 km ( see galilean satel-

lites). A planetary ring of rocks was dis-

covered in 1979 by the US Galileo probe.

Jurassic The second geological period of

the Mesozoic era. It followed the Triassic,

which ended about 213 million years ago,

and extended until the beginning of the

Cretaceous period, about 144 million

years ago. It was named in 1829 by A.

Brongniart after the Jura Mountains on

the borders of France and Switzerland.

Jurassic rocks include clays and lime-

stones in which fossil

ora and fauna are

abundant. Plants included ferns, cycads,

ginkgos, rushes, and conifers. Important

invertebrates included * ammonites (on

which the Jurassic is zoned), corals, bra-

chiopods, bivalves, and echinoids. Reptiles

dominated the vertebrates; the

ying

reptiles – the pterosaurs – and the Ü rst

primitive bird, Archaeopteryx , appeared.

juvenile hormone A hormone secreted

by insects from a pair of endocrine glands

( corpora allata ) close to the brain. It in-

hibits metamorphosis and maintains the

presence of larval features.

rst

mass of gas is independent of its volume

and pressure, being a function of temper-

ature alone. This law applies only to

* ideal gases (for which it provides a de

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