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Aston, Francis William

Aston, Francis William

(b. Harbonne, Birmingham, England, 1 September 1877; d; Cambridge, England, 20 November 1945)

experimental chemistry, physics.

Aston was the second son of William Aston, a metal merchant and farmer, and Fanny Charlotte Hollis, the daughter of a Birmingham gunmaker. After primary education at Harbonne vicarage school, Aston spent four years at Malvern College. In 1893 he entered Mason’s College, Birmingham, where he studied for the London intermediate science examination with the chemists W. A. Tilden and P. F. Frankland and the physicist J. H. Poynting. In 1898 he obtained a Forster Scholarship to work with Frankland on the stereochemistry of dipyromucyltartaric acid esters. Simultaneously he took a course in fermentation chemistry, and from 1900 to 1903 he earned a living as a brewery chemist at Wolverhampton. He returned to Birmingham University (formed from Mason’s College in 1900) from 1903 to 1908 as a physics research student with Poynting, and after a world tour in 1909 he spent a term at Birmingham as an assistant lecturer. From 1910 to 1919 Aston worked with J. J. Thomson at the Cavendish Laboratory, Cambridge, and the Royal Institution, London; first as a personal assistant, then from 1913 as a Clerk Maxwell Scholar. This period was interrupted by the war, during which Aston returned to chemistry as a technical assistant at the Royal Aircraft Establishment, Farnborough. In 1919 he was elected a fellow of Trinity College, Cambridge, where he spent the remainder of his life.

Conservative in politics and of no decided religious views, Aston was an animal lover, a keen traveler, a varied and skilled sportsman, a technically brilliant photographer, and an accomplished amateur musician. Like most of J. J. Thomson’s associates, he acquired an interest in finance, and in consequence of skilled investment he was able to leave a large estate to Trinity College and several scientific beneficiaries. Aston was a bachelor, a poor teacher and lecturer, and a lone worker who detested the thought of experimental collaboration. (Only six out of 143 papers were collaborative.) He recognized his own fallibility as a theorist, and frequently sought the aid of such mathematical physicists as F. A. Lindemann (Lord Cherwell), R. H. Fowler, and W. W. Sawyer. Aston received the Nobel Prize for chemistry in 1922. He held several honorary degrees, and was elected to, and received medals from, many scientific institutions. From 1936 to 1945 he was chairman of the Committee on Atoms of the International Union of Chemistry.

Although Aston liked to recall that his first two publications were on organic chemistry, these two papers broke no new ground although they did exhibit his talent for devising ingenious apparatus. The appearance of Thomson’s Conduction of Electricity Through Gases in 1903 opened up, for Aston the chemist, the physicist’s world of cathode rays, positive rays, and X rays. Already an expert glassblower, and trained by Frankland in “extreme care and meticulous accuracy,” he began to work under Poynting on the variable structure of the phenomena observed during gaseous conduction at low pressures. He was particularly interested in the variation, with pressure and current, of the length of the dark space between the cathode and the negative glow named after W. C. Crookes. By making special Geissler discharge tubes with movable aluminum cathodes, Aston was able to obtain a sufficiently well-bounded Crookes space to demonstrate that its length was proportional to where P is the pressure and C is the current. In 1908, while using hydrogen and helium, he detected a new “primary cathode dark space,” about a millimeter thick and directly adjoining the cathode. This phenomenon now bears Aston’s name. Research on the relationship between the Crookes dark space and current, voltage, pressure, and electrode nature and design continued intermittently until 1923. Aston then abandoned it in order to devote all his attention to isotopes.

When Aston became Thomson’s assistant in 1910, he was given the task of improving the apparatus in which a beam of positively charged particles (positive rays), which emerged through a perforated cathode in a discharge tube, were deflected by perpendicularly arranged electric and magnetic fields into sharp, visible parabolas of constant e/m (charge over mass). Aston produced an improved spherical discharge tube, finely engineered cathode slits, an improved pump, a coil for detecting vacuum leaks, and an ingenious camera for photographing the parabolas. In 1912 he thought this apparatus for positive ray analysis gave a rigorous proof that all the individual molecules of any given substance had the same mass. This Daltonian belief was rudely shattered in the same year when Thomson obtained two parabolas, of mass 20 and 22, for neon. There were two obvious possibilities: if neon had a true atomic weight of 20 (instead of 20.2), then either mass 22 was an unknown hydride, NeH2, or a new element, meta-neon. Thomson investigated the first possibility and left Aston to check the unlikely alternative.

Aston, who was sympathetic toward F. Soddy’s contemporary ideas on radioactive isotopes, tried to separate the meta-neon by fractional distillation, and later by diffusion. He invented a quartz microbalance, which was sensitive to 10-9 gram, to measure the density of the minute heavier fraction. The partial separation of a new element, with the same properties as neon, was announced in 1913; Thomson, however, remained doubtful. During the war Aston had time to think over the problem and to debate the possibility of the existence of natural isotopes with the skeptical F. A. Lindemann.

In 1919, to test the neon isotope hypothesis, Aston built a positive ray spectrograph, or mass spectrograph, with a resolving power of I in 100 and an accuracy of I part in 103. The design was based upon an optical analogy. Just as white light can be analyzed into an optical spectrum by a prism, so an electric field will disperse a beam of heterogeneous positive rays. By arranging a magnetic field to deflect the dispersed rays in the opposite direction, but in the same plane, rays of uniform mass can be focused into a mass spectrum on a photographic plate, irrespective of their velocities. This was a great advance on Thomson’s apparatus, where the arrangement of the fields produced parabolas that were dependent on the velocities of the positive rays. Aston adopted several methods to calculate the masses of the particles, including comparison with a calibration curve of reference lines of known masses. In the case of neon, the intensities of the 20 and 22 mass lines implied a relative abundance of about 10: 1, enough to produce an average mass of 20.2, the known atomic weight of neon. Neon was isotopic.

Two larger mass spectrographs were built. The second (1927) had five times more resolving power and an accuracy of 1 in 104; the third (1935) had a resolving power of 1 in 2000 and a claimed accuracy of 1 in 105. The latter instrument proved difficult to adjust, and World War II intervened before any significant work could be done with it. By then, however, Aston’s instruments had been surpassed by the mass spectrometers developed by A. J. Denmpster (1918), K. T. Bainbridge (1932), and A. O. Nier (1937).

Aston’s personal motto, “Make more, more, and yet more measurements,” led him to analyze successfully all but three of the nonradioactive elements in the periodic table. But since the mass spectrograph was unsuitable for detecting minute amounts of isotopes, he missed finding those of oxygen and hydrogen. In 1930 Aston showed how his instrument could be used photometrically to determine and correct chemical atomic weights. Here much depended on his brilliant development of photographic plates that were highly sensitive to positive ions.

In December 1919 Aston announced the “wholenumber rule” that atomic masses were integral on the scale O16 (a notation introduced by Aston in 1920). Fractional atomic weights were merely “fortuitous statistical effects due to the relative quantities of the isotopic constituents,” and the elements were to be defined physically by their atomic numbers, rather than in terms of isotopic mixtures. Prout’s hypothesis (1816), that all elements were built up from atoms of a common substance, appeared to be vindicated at last.

Aston’s work, therefore, provided important insights into the structure of the atom and the evolution of the elements. At first, only hydrogen appeared to violate the whole-number rule. Aston explained this seeming violation as due to the “loss” of mass within this atom by binding energy; mass was additive only when nuclear charges were relatively distant from one another. This concept of “packing” had been proposed on theoretical grounds by W. D. Harkins (1915), and derived ultimately from J. C. G. Marignac (1860). However, it soon became clear that all elements deviated slightly from whole numbers. In 1927, with his second machine, Aston measured and codified the deviations in terms of the “packing fraction” (the positive or negative deviation of an atomic mass from an integer divided by its mass number). By plotting these fractions against mass numbers, Aston obtained a simple curve which gave valuable information on nuclear abundance and stability.

Aston’s achievements were kept continually before the scientific public by revised editions of his excellent book Isotopes (1922). This included observations on the abundance and distribution of isotopes and a clear forecast of the power and dangers of harnessed atomic energy.

BIBLIOGRAPHY

I. Original Works. A full list of Aston’s papers can be found in Hevesy (see below); to it should be added “The Mass-spectra of the Elements (Part II),” in The London. Edinburgh and Dublin Philosophical Magazine, 40 (1920), 628–636; reports of the Committee on Atoms of the International Union of Chemistry (Paris, 1936–1941); and the obituary of J. J. Thomson, in The Times (London. 4 Sept. 1940).

The most important papers arc “Experiments on the Length of the Cathode Dark Space.” in Proceedings of the Royal Societe of London, 79A (1907), 80–95; “Experiments on a New Cathode Dark Space,” ibid., 80A (1908), 45–49; “Sir J. J. Thomson’s New Method of Chemical Analysis,” in Science Progress, 7 (1912), 48–65; “A New Elementary Constituent of the Atmosphere,” in British Association for the Advancement of Science Reports, 82 (1913). 403; “A Micro-balance for Determining Densities,” in Proceedings of the Royal Society of London, 89A (1914), 439–446; “A Positive Ray Spectrograph,” in The London, Edinburgh and Dublin Philosophical Magazine, 38 (1919). 707–714; “The Possibility of Separating Isotopes,” ibid., 37 (1919). 523–534, written with F. A. Lindemann; “Problems of the Massspectrograph,” ibid., 43 (1922). 514–528, written with R. H. Fowler; “Photographic Plates for the Detection of Mass Rays,” in Proceedings of the Cambridge Philosophical Society, 22 (1923–1925), 548–554; “A New Mass-spectrograph,” in Proceedings of the Royal Society of London, 115A (1927), 487–514, a Bakerian lecture; “The Photometry of Mass-spectra,” ibid, 126A (1930). 511–525; and “A Secondorder Focusing Mass-spectrograph,” ibid., 163A (1937), 391–404.

Also of interest are his Nobel lecture. “Mass-spectra and Isotopes.” in Nobel Lectures in Chemistry (1922–1941) (Amsterdam-London-New York, 1966); and “Forty Years of the Atomic Theory,” in J. Needham and W. Pagel, eds., Background to Modern Science (Cambridge, 1938).

Aston’s books are Isotopes (London, 1922; 2nd ed., 1924); and Mass-spectra and Isotopes (London, 1933; 2nd ed., 1942), on which he was assisted by C. P. Snow.

A few of Aston’s original instruments are displayed in various museums; a quartz microbalance. 1913 (Science Museum. London); fragments from the neon diffusion apparatus, 1913 (Cavendish Laboratory Museum. Cambridge); the first mass spectrograph, 1919 (Science Museum, London); the third mass spectrograph, 1935 (Cavendish Laboratory Museum). The mass spectrograph of 1927 appears to have been broken up.

II. Secondary Literature. Works concerning Aston include N. Feather, “F. W. Aston,” in Dictionary of National Biography (1941–1950) (Oxford, 1959), pp. 24–26; G. C. de Hevesy, “F. W. Aston,” in Obituary Notices of Fellows of the Royal Society, 5 (1945–1948), 635–651, which includes a photograph and a bibliography; F. M. Green, “The Chudleigh Mess,” in R. A. E. News (Jan. 1958), pp. 4–7, available only at Farnborough; G. P. Thomson, “F. W. Aston,” in Nature, 157 (1946), 290–292, and J. J. Thomson and the Cavendish Laboratory (London, 1964), pp. 93, 136; J. J. Thomson, Rays of Positive Electricity (London, 1913). passim; and Who Was Who (1941–1950) (London, 1952), p. 38. which includes a complete list of honors.

W. H. Brock

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Francis William Aston

Francis William Aston

The British chemist and physicist Francis William Aston (1877-1945) invented the mass spectrograph and discovered the isotopic complexity of the elements.

Francis Aston was born on Sept. 1, 1877, at Harborne, Birmingham, where his father was a metal merchant and ran a small farm. He studied chemistry at Mason College (which later became the University of Birmingham). In his spare time he trained himself in the various arts of apparatus construction, especially glassblowing. When his scholarship expired, he took a post with a brewery firm. After 3 years he returned to the University of Birmingham. In 1910 an invitation arrived from J. J. Thomson to join him at the Cavendish Laboratory, Cambridge.

Separation of the Isotopes

Thomson was examining positive "rays" produced in electric-discharge tubes at low pressure. These were in fact atoms stripped of some or all of their outer electrons and thus carried an overall positive charge. Thomson had obtained parabolic tracks by submitting the rays to the simultaneous application of magnetic and electrostatic fields. With Aston's help he discovered that neon gas had a small component that gave a separate parabolic track. Since each parabola was characterized by a unique mass-charge ratio, deductions concerning particle masses could be made, and it was concluded that the atomic masses of the major and minor components of neon were 20 and 22 respectively.

Aston then attempted to separate the two components by physical means and to measure their densities on a quartz microbalance of his own devising. In 1913 he achieved a partial separation by submitting the gas to repeated diffusions through pipe clay; small, but significant, differences in gas density were found for the two samples obtained.

Mass Spectrograph

Gradually, the concept of isotopes was becoming clearer and more generally accepted, and in 1919 Aston worked out some ideas on a new instrument which could give results indicative of mass alone. Unlike Thomson's apparatus, Aston's invention employed magnetic and electrostatic fields producing opposite deflections in the same plane. By focusing the beams through fine slits, Aston obtained a series of lines, each of which corresponded to a definite particle mass. The series of lines was a mass spectrum, and the original instrument was the first mass spectrograph.

With this equipment Aston began an examination of the isotopic composition of more than 50 elements. In those cases where neither the element nor any of its available compounds were volatile, he utilized a solid product containing the element as the anode of his discharge tube. In almost every case the isotopic mass was a whole number within the limits of experimental accuracy (1 in 1000). The only notable exception was hydrogen, 1.008. Thus isotopy was not a rare phenomenon, as some workers had supposed, but widespread and affecting most elements. Aston was led by these integral isotopic weights to conclude that all nuclei are composed of protons (of unit weight) and of negligibly light electrons.

The importance of precise values for the isotopic masses led Aston to design an improved mass spectrograph in 1925, with an accuracy of 1 in 10,000. A later instrument (1927) gave an accuracy improved by a factor of 10. With this refined apparatus he discovered a great many new isotopes, often present in only very small amounts in the natural element.

Assessment of His Work

Many important consequences flowed from Aston's work on the mass spectrograph. As he himself recognized, the fractional isotopic weight of hydrogen implied that if it were converted to helium substantial amounts of the mass would be converted into energy. Using Albert Einstein's relativity relationship, Aston predicted that the energy liberated in a nuclear reaction of this kind would be enormous. His opinions were justified when the first atomic bomb was exploded a few months before his death.

The more immediate importance of the mass spectrograph was its ability to give data on nuclear masses with great precision, thus laying the foundations of the atomic energy industry. More recently, the mass spectrometer has proved an indispensable tool for structural investigations in organic chemistry.

The importance of Aston's work was quickly recognized, and in 1921 he was elected a fellow of the Royal Society. The following year he received the Nobel Prize in chemistry. His authoritative book Isotopes first appeared in 1922 and was followed by many other editions up to 1941. His other book, Mass Spectra and Isotopes, appeared in 1933.

Other Interests

In addition to his work at the Cavendish Laboratory, Aston made some valuable scientific contributions to the study of astronomical eclipses. In 1925 he photographed the sun's corona from Sumatra. He made expeditions to study the solar eclipses of 1932 and 1936 in Canada and Japan respectively, though clouds prevented direct observations. However, Aston was able to study the polarization of the light in the neighborhood of the eclipsed sun.

Aston never married. He was often accompanied on his travels by his sister, Helen, to whom he was deeply devoted. He died at Trinity College, Cambridge, on Nov. 20, 1945.

Further Reading

There is no full-length biography of Aston. Edvard Farber, ed., Great Chemists (1961), contains a short biography, and an old but still useful source, Bernard Jaffe, Crucibles: The Lives and Achievements of the Great Chemists (1932), offers an adequate account of Aston's life and work. The Nobel Foundation's Nobel Lectures, Including Presentation Speeches and Laureates' Biographies: Chemistry, 1922-1941 (1967) contains a brief biography and a résumé of the work for which Aston received the prize. F. J. Moore, A History of Chemistry, revision prepared by William T. Hall (1939), includes a brief sketch of Aston and places his work in historical perspective. □

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Aston, Francis William

Francis William Aston, 1877–1945, English physicist and chemist. He was affiliated with the Cavendish Laboratory, Cambridge, from 1910. In 1922 he received the Nobel Prize in Chemistry mainly for his discovery of a number of isotopes in nonradioactive elements by means of a mass spectrograph of his own invention. His writings include Isotopes (1922) and Mass-Spectra and Isotopes (1933).

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Aston, Francis William

Aston, Francis William (1877–1945) British physicist awarded the 1922 Nobel Prize in chemistry for his work on isotopes. He developed the mass spectrograph, which he used it to identify 212 naturally occurring isotopes.

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