A Dictionary of Science SIXTH EDITION 1 1 Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. 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Printed in Great Britain by Clays Ltd, St Ives plc ISBN 978–0–19–956146–9 10 9 8 7 6 5 4 3 2 1 Appendices 893 SI units Base and dimensionless SI units Physical quantity Name Symbol length metre m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K luminous intensity candela cd amount of substance mole mol *plane angle radian rad *solid angle steradian sr *dimensionless units Derived SI units with special names Physical quantity Name of Symbol of SI unit SI unit frequency hertz Hz energy joule J force newton N power watt W pressure pascal Pa electric charge coulomb C electric potential difference volt V electric resistance ohm Ω electric conductance siemens S electric capacitance farad F magnetic flux weber Wb inductance henry H magnetic flux density tesla T (magnetic induction) luminous flux lumen lm illuminance lux lx absorbed dose gray Gy activity becquerel Bq dose equivalent sievert Sv SI Units 894 Decimal multiples and submultiples to be used with SI units Submultiple Prefix Symbol Multiple Prefix Symbol 10–1 deci d 10 deca da 10–2 centi c 102 hecto h 10–3 milli m 103 kilo k 10–6 micro µ 106 mega M 10–9 nano n 109 giga G 10–12 pico p 1012 tera T 10–15 femto f 1015 peta P 10–18 atto a 1018 exa E 10–21 zepto z 1021 zetta Z 10–24 yocto y 1024 yotta Y Conversion of units to SI units From To Multiply by in m 2.54 × 10–2 ft m 0.3048 sq. in m2 6.4516 × 10–4 sq. ft m2 9.2903 × 10–2 cu. in m3 1.638 71 × 10–5 cu. ft m3 2.831 68 × 10–2 l(itre) m3 10–3 gal(lon) l(itre) 4.546 09 miles/hr m s–1 0.477 04 km/hr m s–1 0.277 78 lb kg 0.453 592 g cm–3 kg m–3 103 lb/in3 kg m–3 2.767 99 × 104 dyne N 10–5 poundal N 0.138 255 lbf N 4.448 22 mmHg Pa 133.322 atmosphere Pa 1.013 25 × 105 hp W 745.7 erg J 10–7 eV J 1.602 10 × 10–19 kW h J 3.6 × 106 cal J 4.1868 Credits Editors John Daintith BSc, PhD Elizabeth Martin MA Advisers B. S. Beckett BSc, BPhil, MA(Ed) R. A. Hands BSc Michael Lewis MA Contributors Richard Batley PhD, MInstP Malcolm Hart BSc, MIBiol Tim Beardsley BA Robert S. Hine BSc, MSc Lionel Bender BSc Elaine Holmes BSc, PhD John Clark BSc Valerie Illingworth BSc, MPhil H. M. Clarke MA, MSc Anne Lockwood BSc W. M. Clarke BSc J. Valerie Neal BSc, PhD Derek Cooper PhD, FRIC R. A. Prince MA John Cullerne DPhil Richard Rennie BSc, MSc, PhD R. Cutler BSc Michael Ruse BSc, PhD E. K. Daintith BSc Jackie Smith BA D. E. Edwards BSc, MSc Brian Stratton BSc, MSc A. J. H. Goddard PhD, MIMechE, Elizabeth Tootill BSc, MSc FInstNucE, FInstP, MSRP David Eric Ward BSc, MSc, PhD William Gould BA Edmund Wright DPhil Preface This sixth edition of A Dictionary of Science, like its predecessors, aims to provide school and first-year university students with accurate explanations of any unfamiliar words they might come across in the course of their studies, in their own or adjacent disciplines. For example, students of the physical sciences will find all they are likely to need to know about the life sciences, and vice versa. The dictionary is also designed to provide non-scientists with a useful reference source to explain the scientific terms that they may encounter in their work or in their general reading. At this level the dictionary provides full coverage of terms, concepts, and laws relating to physics, chemistry, biology, biochemistry, palaeontology, and the earth sciences. There is also coverage of key terms in astronomy, cosmology, mathematics, biotechnology, and computer technology. In addition, the dictionary includes: • over 160 short biographical entries on the most important scientists in the history of the subject • ten features (each of one or two pages) on concepts of special significance in modern science • ten chronologies showing the development of selected concepts, fields of study, and industries • eight Appendices, including the periodic table, tables of SI units and conversion tables to and from other systems of units, summary classifications of the plant and animal kingdoms, and useful websites. For this sixth edition nearly 700 new entries have been added to the text, incorporating recent advances in all the major fields and increased coverage of astronomy, forensic chemistry, and computing. In compiling the dictionary, the contributors and editors have made every effort to make the entries as concise and comprehensible as possible, always bearing in mind the needs of the readers. Particular features of the book are its lack of unnecessary scientific jargon and its extensive network of cross-references. An asterisk placed before a word used in an entry indicates that this word can be looked up in the dictionary and will provide further explanation or clarification. However, not every word that is defined in the dictionary has an asterisk placed before it when it is used in an entry. Some entries simply refer the reader to another entry, indicating either that they are synonyms or abbreviations or that they are most conveniently explained in one of the dictionary’s longer articles. Synonyms and abbreviations are usually placed within brackets immediately after the headword. Terms that are explained within an entry are highlighted by being printed in boldface type. Where appropriate, the entries have been supplemented by fully labelled line-drawings or tables in situ. JD EM 2009 Contents Preface vii Credits viii A Dictionary of Science 1 Atomic Theory Chronology 63 The Big-Bang Theory (Feature) 86 Biochemistry Chronology 90 Cell Biology Chronology 144 Cosmology Chronology 200 Crystal Defects (Feature) 210 Electronics Chronology 275 El Niño (Feature) 284 Explosives Chronology 308 Genetically Modified Organisms (Feature) 352 Learning in Animals (Feature) 470 Microscopy Chronology 528 Moon Exploration Chronology 541 Magnetic Resonance Imaging (Feature) 569 Plastics Chronology 638 Polymers (Feature) 648 Refrigeration (Feature) 702 Solar System (Feature) 766 Optical Astronomical Telescopes (Feature) 809 Vitamins Chronology 863 Appendices SI units 893 Fundamental constants 895 The solar system 895 Geological time scale 896 Simplified classification of land plants 897 Simplified classification of the animal kingdom 898 The periodic table 899 Useful websites 900 A aa See . AAR See   . AAS See   . ab- A preÜx attached to the name of a prac- tical electrical unit to provide a name for a unit in the electromagnetic system of units (see  ), e.g. abampere, abcoulomb, abvolt. The preÜx is an abbrevi- ation of the word ‘absolute’ as this system is also known as the absolute system. Compare -. In modern practice both absolute and electrostatic units have been replaced by *SI units. abdomen The posterior region of the body trunk of animals. In vertebrates it contains the stomach and intestines and the organs of excretion and reproduction. It is particularly well deÜned in mammals, being separated from the *thorax by the *diaphragm. In many arthropods, such as insects and spi- ders, it may be segmented. Abelian group See . aberration 1. (in optics) A defect in the image formed by a lens or curved mirror. In chromatic aberration the image formed by a lens (but not a mirror) has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an *achromatic lens. In spherical aberration, the rays from the object come to a focus in slightly different positions as a result of the curvature of the lens or mirror. For a mirror receiving light strictly parallel with its axis, this can be cor- rected by using a parabolic surface rather than a spherical surface. Spherical aberra- tion in lenses is minimized by making both surfaces contribute equally to the ray devia- tions, and can be lessened (though with re- duced image brightness) by the use of diaphragms to let light pass only through the centre part of the lens. See also ; . 2. (in astronomy) The apparent dis- placement in the position of a star as a result of the earth’s motion round the sun. Light appears to come from a point that is slightly displaced in the direction of the earth’s mo- tion. The angular displacement α = v/c, where v is the earth’s orbital velocity and c is the speed of light. abiogenesis The origin of living from nonliving matter, as by *biopoiesis. See also  . abiotic factor Any of the nonliving factors that make up the abiotic environment in which living organisms occur. They include all the aspects of climate, geology, and at- mosphere that may affect the biotic environ- ment. Compare  . abomasum The fourth and Ünal chamber of the stomach of ruminants. It leads from the *omasum and empties into the small in- testine. The abomasum is referred to as the ‘true stomach’ as it is in this chamber that protein digestion occurs, in acidic condi- tions. See R. ABO system One of the most important human *blood group systems. The system is based on the presence or absence of *anti- gens A and B on the surface of red blood cells and of *antibodies against these in blood serum. A person whose blood contains ei- ther or both these antibodies cannot receive a transfusion of blood containing the corre- sponding antigens as this would cause the red cells to clump (see ). The table overleaf illustrates the basis of the sys- tem: people of blood group O are described as ‘universal donors’ as they can give blood to those of any of the other groups. See also  . abscisic acid A naturally occurring plant hormone that appears to be involved pri- marily in seed maturation, stress responses (e.g. to heat and waterlogging), and in regu- lating closure of leaf pores (stomata). In seeds, it promotes the synthesis of storage protein and prevents premature germina- tion. In leaves, abscisic acid is produced in large amounts when the plant lacks sufÜ- cient water, promoting closure of stomata and hence reducing further water losses. It was formerly believed to play a central role in *abscission, hence the name. abscissa See C . abscission The separation of a leaf, fruit, or other part from the body of a plant. It in- volves the formation of an abscission zone, at the base of the part, within which a layer of cells (abscission layer) breaks down. This process is suppressed so long as sufÜcient amounts of *auxin, a plant hormone, Ûow from the part through the abscission zone. However, if the auxin Ûow declines, for ex- ample due to injury or ageing, abscission is activated. Ethylene (*ethene) acts as the pri- mary trigger for abscission, inducing cells in the abscission zone to produce cellulase en- zymes that degrade cell walls. absolute 1. Not dependent on or relative to anything else, e.g. *absolute zero. 2. De- noting a temperature measured on an ab- solute scale, a scale of temperature based on absolute zero. The usual absolute scale now is that of thermodynamic *temperature; its unit, the kelvin, was formerly called the de- gree absolute (°A) and is the same size as the degree Celsius. In British engineering prac- tice an absolute scale with Fahrenheit-size degrees has been used: this is the Rankine scale. absolute alcohol See . absolute conÜguration A way of denot- ing the absolute structure of an optical iso- mer (see  ). Two conventions are in use: The D–L convention relates the structure of the molecule to some reference molecule. In the case of sugars and similar compounds, the dextrorotatory form of glyceraldehyde (HOCH2CH(OH)CHO), 2,3- dihydroxypropanal) was used. The rule is as follows. Write the structure of this molecule down with the asymmetric carbon in the centre, the –CHO group at the top, the –OH on the right, the –CH2OH at the bottom, and the –H on the left. Now imagine that the cen- tral carbon atom is at the centre of a tetrahe- dron with the four groups at the corners and that the –H and –OH come out of the paper and the –CHO and –CH2OH groups go into the paper. The resulting three-dimensional structure was taken to be that of d-glycer- aldehyde and called D-glyceraldehyde. Any compound that contains an asymmetric car- bon atom having this conÜguration belongs to the D-series. One having the opposite conÜguration belongs to the L-series. It is important to note that the preÜxes D- and L- do not stand for dextrorotatory and laevoro- tatory (they are not the same as d- and l-). In fact the arbitrary conÜguration assigned to D-glyceraldehyde is now known to be the correct one for the dextrorotatory form, al- though this was not known at the time. However, all D-compounds are not dextro- rotatory. For instance, the acid obtained by oxidizing the –CHO group of glyceraldehyde is glyceric acid (1,2-dihydroxypropanoic acid). By convention, this belongs to the D-series, but it is in fact laevorotatory; i.e. its name can be written as D-glyceric acid or l-glyceric acid. To avoid confusion it is better to use + (for dextrorotatory) and – (for laevorotatory), as in D-(+)-glyceraldehyde and D-(–)-glyceric acid. The D–L convention can also be used with alpha amino acids (compounds with the –NH2 group on the same carbon as the –COOH group). In this case the molecule is imagined as being viewed along the H–C bond between the hydrogen and the asym- metric carbon atom. If the clockwise order of the other three groups is –COOH, –R, –NH2, the amino acid belongs to the D-series; oth- erwise it belongs to the L-series. This is known as the CORN rule. The R–S convention is a convention based on priority of groups attached to the chiral carbon atom. The order of priority is I, Br, Cl, SO3H, OCOCH3, OCH3, OH, NO2, NH2, abscissa 2 a A B AB O A B A and B neither A nor B anti-B anti-A none anti-A and anti-B A, O B, O A, B, AB, O O A, AB B, AB AB A, B, AB, O Group Antigens on red cell surface Antibodies in serum Blood group of people donor can receive blood from Blood group of people donor can give blood to ABO system. COOCH3, CONH2, COCH3, CHO, CH2OH, C6H5, C2H5, CH3, H, with hydrogen lowest. The molecule is viewed with the group of lowest priority behind the chiral atom. If the clockwise arrangement of the other three groups is in descending priority, the com- pound belongs to the R-series; if the de- scending order is anticlockwise it is in the S-series. D-(+)-glyceraldehyde is R-(+)-glyc- eraldehyde. See illustration. A • Information about IUPAC nomenclature for the R–S system absolute expansivity See . absolute humidity See . absolute permittivity See . absolute pitch (perfect pitch) The ability of a person to identify and reproduce a note without reference to a tuned musical instru- ment. absolute space Space that exists as a background to events and processes and is not affected by objects or other entities in the universe. The idea underpins Newtonian physics, although many physicists have al- ways regarded absolute space as an undesir- able concept and suggested, as in *Mach’s principle, that fundamental physics should be described by *relational theories. absolute temperature See ; . absolute time Time that exists indepen- dently of any events or processes in the uni- verse. Like *absolute space, absolute time is a basic concept in Newtonian physics. absolute value (modulus) The square root of the sum of the squares of the real 3 absolute value a ��� �� ����� � � �������������� ��� � � �� ����� ��� ����� ���� �������������� ���������� ������������������ � ��� ��� ���� � ���� ��� � ��� � � � � � � � � ��������������� ��������������� �������������������������������������������� ��������������������������������������������������������������������������� ���������������������������������������������������������������������� Absolute configuration. numbers in a *complex number, i.e. the ab- solute value of the complex number z = x + iy is |z| = √(x2 + y2). absolute zero Zero of thermodynamic *temperature (0 kelvin) and the lowest tem- perature theoretically attainable. It is the temperature at which the kinetic energy of atoms and molecules is minimal. It is equiv- alent to –273.15°C or –459.67°F. See also - ; . absorbed dose See . absorptance Symbol α. The ratio of the radiant or luminous Ûux absorbed by a body to the Ûux falling on it. Formerly called ab- sorptivity, the absorptance of a *black body is by deÜnition 1. absorption 1. (in chemistry) The take up of a gas by a solid or liquid, or the take up of a liquid by a solid. Absorption differs from *adsorption in that the absorbed substance permeates the bulk of the absorbing sub- stance. 2. (in physics) The conversion of the energy of electromagnetic radiation, sound, streams of particles, etc., into other forms of energy on passing through a medium. A beam of light, for instance, passing through a medium, may lose intensity because of two effects: scattering of light out of the beam, and absorption of photons by atoms or mol- ecules in the medium. When a photon is ab- sorbed, there is a transition to an excited state. 3. (in biology) The movement of Ûuid or a dissolved substance across a plasma membrane. In many animals, for example, soluble food material is absorbed into cells lining the alimentary canal and thence into the blood. In plants, water and mineral salts are absorbed from the soil by the *roots. See ;  . absorption coefÜcient 1. (in physics) See L’ . 2. (in chemistry) The volume of a given gas, measured at standard temperature and pressure, that will dissolve in unit volume of a given liquid. absorption indicator See  - . absorption spectrum See . absorptivity See . ABS plastic Any of a class of plastics based on acrylonitrile–butadiene–styrene copoly- mers. abundance 1. The ratio of the total mass of a speciÜed element in the earth’s crust to the total mass of the earth’s crust, often ex- pressed as a percentage. For example, the abundance of aluminium in the earth’s crust is about 8%. 2. The ratio of the number of atoms of a particular isotope of an element to the total number of atoms of all the iso- topes present, often expressed as a percent- age. For example, the abundance of uranium–235 in natural uranium is 0.71%. This is the natural abundance, i.e. the abun- dance as found in nature before any enrich- ment has taken place. abyssal zone The lower depths of the ocean (below approximately 2000 metres), where there is effectively no light penetra- tion. Abyssal organisms are adapted for liv- ing under high pressures in cold dark conditions. See also  . a.c. See  . acac The symbol for the *acetylacetonato ligand, used in formulae. accelerant A Ûammable material used to start and spread a Üre in cases of arson. Petrol and parafÜn are the substances com- monly used. Traces of accelerant are de- tectable by gas chromatography in forensic work. acceleration Symbol a. The rate of in- crease of speed or velocity. It is measured in m s–2. For a body moving linearly with con- stant acceleration a from a speed u to a speed v, a = (v – u)/t = (v2 – u2)/2s where t is the time taken and s the distance covered. If the acceleration is not constant it is given by dv/dt = d2s/dt2. If the motion is not linear the vector character of displacement, velocity, and acceleration must be consid- ered. See also  . acceleration of free fall Symbol g. The acceleration experienced by any massive ob- ject falling freely in the earth’s gravitational Üeld. Experimentally this is almost constant for all positions near the earth’s surface, in- dependent of the nature of the falling body (provided air resistance is eliminated). This is taken to indicate the strict proportionality of *weight (the force causing the accelera- tion) and inertial *mass, on the basis of Newton’s second law of motion (see N’   ). There is some variation of g with latitude, because of the earth’s rotation and because the earth is absolute zero 4 a not completely spherical. The standard value is taken as 9.806 65 m s–2. The accel- eration of free fall is also called the accelera- tion due to gravity. accelerator 1. (in physics) An apparatus for increasing the kinetic energies of charged particles, used for research in nuclear and particle physics. See ;  - ; ; - . 2. (in chemistry) A substance that increases the rate of a chemical reaction, i.e. a catalyst. A • A list of world particle accelerators and accelera- tor laboratories, with links, from the ELSA web- site at the University of Bonn acceptor 1. (in chemistry and biochem- istry) A compound, molecule, ion, etc., to which electrons are donated in the forma- tion of a coordinate bond. 2. (in biochem- istry) A *receptor that binds a hormone without any apparent biological response. 3. (in physics) A substance that is added as an impurity to a *semiconductor because of its ability to accept electrons from the va- lence bands, causing p-type conduction by the mobile positive holes left. Compare . acceptor levels Energy levels of an accep- tor atom in a *semiconductor, such as alu- minium, in silicon. These energy levels are very near the top of the valence band, and therefore cause p-type conduction. See also  . access point (wireless access point) A device that acts as the core of a wireless net- work. It communicates with wireless nodes within its range and provides the necessary facilities for them to network successfully. Access points commonly also manage a link to a wired network, allowing their nodes to link with corporate networks, the Internet, etc. acclimation The physiological changes occurring in an organism in response to a change in a particular environmental factor (e.g. temperature), especially under labora- tory conditions. Thermal acclimation studies reveal how such properties as metabolic rate, muscle contractility, nerve conduction, and heart rate differ between cold- and warm-acclimated members of the same species. These changes occur naturally dur- ing *acclimatization and equip the organism for living in, say, cold or warm conditions. acclimatization 1. The progressive adap- tation of an organism to any change in its natural environment that subjects it to phys- iological stress. 2. The overall sum of processes by which an organism attempts to compensate for conditions that would sub- stantially reduce the amount of oxygen de- livered to its cells. Compare . accommodation 1. (in animal physiol- ogy) Focusing: the process by which the focal length of the *lens of the eye is changed so that clear images of objects at a range of distances are displayed on the retina. In hu- mans and some other mammals accommo- dation is achieved by reÛex adjustments in the shape of the lens brought about by relax- ation and contraction of muscles within the *ciliary body. 2. (in animal behaviour) Ad- justments made by an animal’s nervous or sensory systems in response to continuously changing environmental conditions. accretion The way in which collisions with relatively slow-moving smaller objects add to the mass of a larger celestial object. The process accelerates as the increased mass strengthens the gravitational Üeld of the larger object. For example, the planets are thought to have formed by the accretion of dust particles onto *planetesimals. Other ac- creting objects probably include black holes and protostars. accretion disc A disc-shaped rotating mass formed by gravitational attraction. See  ;  ;  . accumulator (secondary cell; storage battery) A type of *voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the op- posite direction to that in which the cell supplies current, reverses the chemical reac- tions in the cell. The common types are the *lead–acid accumulator and the *nickel–iron and nickel–cadmium accumulators. See also – . acellular Describing tissues or organisms that are not made up of separate cells but often have more than one nucleus (see - ). Examples of acellular structures are muscle Übres. Compare . acentric Describing an aberrant chromo- some fragment that lacks a centromere. Such fragments are normally lost because they are unable to orientate properly during cell division. 5 acentric a acetaldehyde See . acetaldol See  . acetals Organic compounds formed by ad- dition of alcohol molecules to aldehyde mol- ecules. If one molecule of aldehyde (RCHO) reacts with one molecule of alcohol (R′OH) a hemiacetal is formed (RCH(OH)OR′). The rings of aldose sugars are hemiacetals. Fur- ther reaction with a second alcohol mol- ecule produces a full acetal (RCH(OR′)2). It is common to refer to both types of com- pounds simply as ‘acetals’. The formation of acetals is reversible; acetals can be hydrol- ysed back to aldehydes in acidic solutions. In synthetic organic chemistry aldehyde groups are often converted into acetal groups to protect them before performing other reac- tions on different groups in the molecule. See also . A • Information about IUPAC nomenclature acetaldehyde 6 a OR R H OH R O H OR R H OH OR R H OR R1OH aldehyde alcohol hemiacetal 1 + hemiacetal 1 R1OH alcohol 1 + hemiacetal 1 Formation of acetals. acetamide See . acetanilide A white crystalline primary amide of ethanoic acid, CH3CONHC6H5; r.d. 1.2; m.p. 114.3°C; b.p. 304°C. It is made by reacting phenylamine (aniline) with excess ethanoic acid or ethanoic anhydride and is used in the manufacture of dyestuffs and rubber. The full systematic name is N- phenylethanamide. acetate See . acetate process See . acetic acid See  . acetoacetic acid See - . acetoacetic ester See  -- . acetone See ;  . acetone–chlor–haemin test (Wagenaar test) A *presumptive test for blood in which a small amount of acetone (propenal) is added to the bloodstain, followed by a drop of hydrochloric acid. Haemoglobin produces derivatives such as haematin and haemin, forming small characteristic crystals that can be identiÜed under a microscope. acetylacetonato The ion (CH3COCHCOCH3)–, functioning as a biden- tate ligand coordinating through the two oxygen atoms. In formulae, the symbol acac is used. acetylation See . acetyl chloride See  . acetylcholine (ACh) One of the main *neurotransmitters of the vertebrate nervous system. It is released at some (cholinergic) nerve endings and may be excitatory or in- hibitory; it initiates muscular contraction at *neuromuscular junctions. Once acetyl- choline has been released it has only a tran- sitory effect because it is rapidly broken down by the enzyme *cholinesterase. acetylcholinesterase See . acetyl coenzyme A (acetyl CoA) A com- pound formed in the mitochondria when an acetyl group (CH3CO–), derived from the breakdown of fats, proteins, or carbohy- drates (via *glycolysis), combines with the thiol group (–SH) of *coenzyme A. Acetyl CoA feeds into the energy generating *Krebs cycle and also plays a role in the synthesis and oxidation of fatty acids. acetylene See . acetylenes See . acetyl group See  . acetylide See . achene A dry indehiscent fruit formed from a single carpel and containing a single seed. An example is the feathery achene of clematis. Variants of the achene include the *caryopsis, *cypsela, *nut, and *samara. See also . Acheson process An industrial process for the manufacture of graphite by heating coke mixed with clay. The reaction involves the production of silicon carbide, which loses silicon at 4150°C to leave graphite. The process was patented in 1896 by the US inventor Edward Goodrich Acheson (1856–1931). achiral Describing a molecule that does not contain a *chirality element. achondrite A stony meteorite that has no spherical silicate particles (chondrules) found in the meteorites called chondrites. Achondrites do not contain iron or nickel and have a coarser crystal structure than chondrites. achromatic lens A lens that corrects for chromatic *aberration by using a combina- tion of two lenses, made of different kinds of glass, such that their *dispersions neutralize each other although their *refractions do not. The aberration can be reduced further by using an apochromatic lens, which con- sists of three or more different kinds of glass. aciclovir (acyclovir; acycloguanosine) A drug used to treat cold sores, shingles, geni- tal blisters, or other lesions caused by her- pesvirus infection. It is an analogue of the base guanine and acts by interfering with DNA replication of the virus. acid 1. A type of compound that contains hydrogen and dissociates in water to pro- duce positive hydrogen ions. The reaction, for an acid HX, is commonly written: HX ˆ H+ + X– In fact, the hydrogen ion (the proton) is sol- vated, and the complete reaction is: HX + H2O ˆ H3O+ + X– The ion H3O+ is the oxonium ion (or hy- droxonium ion or hydronium ion). This deÜnition of acids comes from the Arrhe- nius theory. Such acids tend to be corrosive substances with a sharp taste, which turn lit- mus red and give colour changes with other *indicators. They are referred to as protonic acids and are classiÜed into strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only par- tially dissociated (e.g. ethanoic acid and hy- drogen sulphide). The strength of an acid depends on the extent to which it dissoci- ates, and is measured by its *dissociation constant. See also . 2. In the Lowry–Brønsted theory of acids and bases (1923), the deÜnition was ex- tended to one in which an acid is a proton donor, and a base is a proton acceptor. For example, in HCN + H2O ˆ H3O+ + CN– the HCN is an acid, in that it donates a pro- ton to H2O. The H2O is acting as a base in ac- cepting a proton. Similarly, in the reverse re- action H3O+ is an acid and CN– a base. In such reactions, two species related by loss or gain of a proton are said to be conjugate. Thus, in the reaction above HCN is the con- jugate acid of the base CN–, and CN– is the conjugate base of the acid HCN. Similarly, H3O+ is the conjugate acid of the base H2O. An equilibrium, such as that above, is a com- petition for protons between an acid and its conjugate base. A strong acid has a weak conjugate base, and vice versa. Under this deÜnition water can act as both acid and base. Thus in NH3 + H2O ˆ NH4+ + OH– the H2O is the conjugate acid of OH–. The deÜnition also extends the idea of acid–base reaction to solvents other than water. For in- stance, liquid ammonia, like water, has a high dielectric constant and is a good ioniz- ing solvent. Equilibria of the type NH3 + Na+Cl– ˆ Na+NH2– + HCl can be studied, in which NH3 and HCl are acids and NH2– and Cl– are their conjugate bases. 3. A further extension of the idea of acids and bases was made in the Lewis theory (G. N. Lewis, 1923). In this, a Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This deÜnition en- compasses ‘traditional’ acid–base reactions. In HCl + NaOH → NaCl + H2O the reaction is essentially H+ + :OH– → H:OH i.e. donation of an electron pair by OH–. But it also includes reactions that do not involve ions, e.g. H3N: + BCl3 → H3NBCl3 in which NH3 is the base (donor) and BCl3 the acid (acceptor). The Lewis theory estab- lishes a relationship between acid–base re- actions and *oxidation–reduction reactions. See HSAB . See also  ; - ; . acid anhydrides (acyl anhydrides) Com- pounds that react with water to form an acid. For example, carbon dioxide reacts with water to give carbonic acid: CO2(g) + H2O(aq) ˆ H2CO3(aq) A particular group of acid anhydrides are an- hydrides of carboxylic acids. They have a general formula of the type R.CO.O.CO.R′, 7 acid anhydrides a