UREY, HAROLD CLAYTON

(b. Walkerton, Indiana, 29 April 1893; d. La Jolla, California, 5 January 1981)

chemistry, geophysics, planetary science.

Harold Clayton Urey was born to Samuel Clayton and Cora (Reinoehl) Urey. His father a schoolteacher and minister of the Brethren church, died when Harold was six.and his mother later married another clergyman. Urey was heavily influenced by the pacifism of the Brethren church and refused to do defense research except during wartime. He studied zoology and chemistry at Montana State College, and later took his Ph.D. in physical chemistry under G. N. Lewis at the University of California at Berkeley. He was a pioneer in the application of quantum mechanics to chemistry. Urey was awarded the 1934 Nobel Prize in chemistry for the discovery of deuterium in 1931, and he and his group at Columbia University established isotopic chemistry and explored its application to biology and medicine. Urey applied his knowledge and experience to the war effort, directing the Manhattan Project’s separation of uranium for the atomic bomb. After the war Urey moved to the University of of Chicago and turned his efforts to problems of geophysics, cosmogony, and the origin of life, attempting to understand the evolution of the earth, moon, and planets from chemical and isotopic evidence, especially in meteorites. From 1958 until his death he carried on this work at the University of California at La Jolla. He was a friendly, if at times severely critical, adviser to the space program, especially the lunar exploration program. Urey often collaborated with colleagues and students, and although he could be fiercely independent and even volatile, he inspired deep admiration and affection. Harold Urey married Frieda Daum in 1926 while teaching at Johns Hopkins University, and treasured his stable family life, which came to include three daughters and one son.

After graduating from high school Urey taught in country schools in his native Indiana and later in Montana. “Money was tight for him as a college student” reported his colleague Ferdinand G. Brickwedde. “During the academic year he slept and studied in a tent. During his summers he worked on a road gang laying railroad track in the Northwest”. In his later life Urey raised parsimony and efficiency in the conduct of scientific work to a prime value. Urey graduated from Montana State University in 1917 with a B.Sc. in zoology and a minor in chemistry. His first research project was on protozoa of the Missoula River, and his interest in biology remained with him all his life.

When the United States entered World War I, Urey went to work in an industrial chemical laboratory in Philadelphia making munitions. He later said, when accepting the Nobel Prize, that exposure to industrial chemistry helped him realize that academia was the proper place for him. After the war Urey returned to Montana State, where he resumed teaching chemistry until 1921.

In 1921 Urey began graduate work in chemistry and thermodynamics with G. N. Lewis at Berkeley, receiving a Ph.D. in physical chemistry in 1923. His doctoral research on rotational contributions to the heat capacities and entropies of gases showed Urey’s talent in both theoretical calculations and empirical work. Urey was able to calculate entropies of diatomic gases that agreed with experimental data, and his method led directly to the current methods of calculating thermodynamic functions from spectroscopic data.

Urey spent the academic year 1923-1924 as an American-Scandinavian Fellow at the Institute for Theoretical Physics in Copenhagen studying atomic physics with Niels Bohr. He brought his considerable knowledge of quantum mechanics to Johns Hopkins University as an instructor in the chemistry department from 1925 to 1929. He joined Columbia University in 1929 as an associate professor of chemistry, and intensified his experimental and theoretical work in spectroscopy and quantum mechanics. He advanced to full professor in 1934.

At this time he collaborated with A. E. Ruark on the influential Atoms, Molecules, and Quanta an outgrowth of his teaching of quantum mechanics at Johns Hopkins. The book was the first comprehensive English language textbook on atomic structure and a major bridge between the new quantum physics and the field of chemistry.

In 1931 Urey discovered a heavy isotope of hydrogen, later named deuterium, with his colleagues Ferdinand Briekwedde and George Murphy, through spectroscopy of the product of fractional distillation of hydrogen near its triple point. The discovery had a significant influence on chemistry, physics, and medicine (first tracer used to study physiological changes in the human body). That he was led to the discovery through two previous independent errors helped reinforce a characteristic tentativeness concerning scientific hypotheses.

During the thirties Urey’s group separated isotopes of oxygen, carbon, nitrogen, and sulphur, Under Urey’s direction the group worked on medical and biological applications of isotopes of all the elements basic to biological processes. He served as first editor of the Journal of Chemical Physics (1933-1940), during which time it became the leading journal of the field.

Urey’s work was again interrupted by war in 1940 when he joined the Uranium Committee of the Manhattan Project. He became director of the Substitute Alloys Materials Laboratory established at Columbia for separation of uranium isotopes, one of three main branches of the Manhattan Project. His ability to be equally at home in theoretical research as well as practical laboratory activity showed itself again as he directed the transformation of laboratory apparatus into massive industrial plants built at Oak Ridge, Tennessee and elsewhere to separate uranium isotopes by gaseous diffusion.

In 1945 Urey joined the Fermi Institute for Nuclear Studies at the University of Chicago, where the reactor program had its headquarters. University of Chicago president Robert Hutchins later recalled that he had tried to recruit Urey before the war: “It seems to me, as I look back on it, that I spent most of my time at the University of Chicago trying to persuade Harold Urey to join the faculty. I think it’s too bad that I owe my eventual success in this high endeavor to General Groves”. Urey organized a series of nuclear institutes and helped to arrange for major corporate funding.

Having aided in the development of atomic weapons out of a sense of duty, Urey’s sense of social and scientific responsibility equally moved him to lead a group at Chicago that actively opposed dropping the atomic bomb on the Japanese. He worked with other scientists to defeat a bill that would have put control of future atomic research into the hands of the military, and in 1950 he resigned from the Atomic Energy Commission. He opposed Joseph McCarthy and made an appeal to save Julius and Ethel Rosenberg, accused of betraying atomic secrets, from execution. He devoted much time in his last years to the peaceful uses of atomic energy and to the exploration of alternative sources of energy. As a member of the Union of Concerned Scientists he joined in a 1975 petition to the White House to decrease the production of nuclear power plants.

In his postwar career Harold Urey turned almost fully to geochemistry and cosmic chemistry, which had already been a minor theme of his research of the 1930’s. Alfred Nier at Minnesota had developed an extraordinarily precise mass spectrometer. Urey increased the precision of the instrument and used it to determine temperatures of Cretaceous seas by measuring isotopic oxygen fractionation between ocean water and carbonate precipitated by belemnites that had inhabited the seas. With his student Cesare Emiliani he reported the research in “The Thermodynamic Properties of Isotopic Substances,” a paper that laid much of the foundation for the emerging discipline of isotopic geochemistry.

At Chicago, Urey, Willard Libby, Hans Seuss, Harrison Brown, Mark Inghram, Claire C. Patterson, Gerald Wasserburg, and others were forging the new discipline of cosmochemistry. The abundance tables that Urey and Seuss established at that time have since been revised, but are still in use. Urey’s concentration on isotopic and general chemical abundances as they occur and develop in nature led him to consider more deeply questions of the origin of the sun and solar system. This led him to study the composition of meteorites. Urey and Craig established that there were two distinct classes of meteorites, implying descent from parent bodies formed under very different conditions in the early solar system. Their analysis forms the basis of the present classification system.

During the summer of 1950 Urey read Ralph Baldwin’s The Face of the Moon, in which Baldwin argued that lava flows and cratering processes could explain the morphology of lunar features. Urey’s interests in the chemistry of the development of the earth, in meteorites as evidence-bearing relics of the early solar system, and in the moon all converged into a creative synthesis presented first during his Silliman lectures at Yale and later published in 1952 as The Planets: Their Origin and Development.

In this work Urey proposed bold hypotheses about the origin of the solar system, arguing that the planets had formed at relatively low temperatures where chemical and molecular processes rather than atomic processes would dominate. The postwar revival by C. F. von Weizsäcker and others of the nebular hypothesis was everyone’s starting point for theories of the origin of the solar system; but there was great latitude for discussing the specific sequence of events. The fission theory of the moon’s formation, popular since the nineteenth century, held that the moon split off from the earth. Urey believed that the moon accreted independently of the earth, perhaps even elsewhere in the solar system, after which it had been captured into earth orbit. The particulars of the debate, which continued throughout Urey’s career, concerned the precise chemical, physical, and dynamic nature of the population of intermediate objects from which the earth and moon had presumably formed, as well as the complicated sequence of events describing the physical and chemical processing of this material to produce the observed state of the earth and moon.

The Planets also drew others into the study of the solar system. As his student Carl Sagan asserted. “The mere fact that a scientist of Urey’s eminence considered a full-scale treatment of planetary cosmogony possible was a major contribution to the field, quite apart from his specific conclusions”, a sentiment echoed by many others.

Urey had a long-standing interest in astronomy and astrophysics. Just after the war he collaborated with astronomer Gerard P. Kuiper on lunar studies, in a relationship that began in admiration and fell into intense acrimony that lasted the rest of their lives. This uncharacteristic episode was played out in the Proceedings of the National Academy of Sciences, at meetings, and in correspondence. Yet Urey was able to subdue his intense feelings and collaborate with Kuiper as coinvestigator on the Ranger series of lunar impacting spacecraft in the early 1960’s.

Urey’s interests in biology, elemental abundances, and the origin of the solar system came together in a classic work published in 1951 with his student Stanley Miller. They sent electric discharges through an atmosphere of chemically pure methane, ammonia, and steam in a sealed container. After a week the container’s walls became red and turbid from amino acids produced therein. Urey’s starting point was the novel conclusion that cosmic abundances of elements implied an early reducing atmosphere for the earth. Carl Sagan reports that when Urey was asked what organic compounds he expected to be produced in such an environment, he replied “Beilstein”, alluding to the large published compendium on all organic compounds then known.

In 1958 Urey became professor-at-large at the University of California at La Jolla. From this base he embarked on yet another career, as a participant in and scientific counselor and adviser to the space program. Always vigilant, ever irascible with what he considered to be foolish decisions, Urey served as a scientific conscience, campaigning and arguing for the most economical and scientifically sound method of conducting space research. He firmly believed that every experiment carried into space at great expense should be designed to test a particular well-conceived hypothesis.

As a founding member of the prestigious National Academy of Sciences’ Space Science Board in the summer of 1958, Urey chaired the Committee on Chemistry of Space and Exploration of the Moon and Planets. When NASA was established later that year its director, Robert Jastrow, consulted with Urey about the research the space agency should undertake. Urey convinced Jastrow of the importance of the moon as the Rosetta stone of the solar system, preserving in its cratered face and unseen interior a record of the conditions under which the solar system formed. As Homer Newell, then NASA’s chief of space science, later recalled. “This was a powerful argument for including the Moon in the space science program. I, personally, did not need persuading, and Urey’s story provided good ammunition for moving the proposal on up the line. The persuasiveness of the argument carried the day at each stage, within NASA, in the Administration, and finally in Congress”. Urey, Jastrow, former colleagues from the Chicago Institute for Nuclear Studies, and others formed a planning committee out of which came the series of automated lunar exploration spacecraft that preceded the Apollo landings of the astronauts. Urey was involved in planning and selection of instruments to be carried, landing site selection, analysis of returned data, and a host of other issues. His influence culminated in the preparations made for the analysis of lunar samples returned to earth. The returned samples, ironically, bore evidence that contradicted Urey’s theory that the moon was a primordial object formed under relatively cold conditions and later captured by the earth.

Harold Urey received numerous awards and honors, including the Research Institute of America Silver Medal (1935 and 1960), Franklin Institute Medal (1943), Medal of Merit from President Truman for his Manhattan Project work, National Medal of Science (1964) from President Johnson for his space program work, and the Priestley Medal (1973).

BIBLIOGRAPHY

I. Original Works. Urey’s papers, comprising 61 cubic feet, are at the University of California. San Diego, Central University Library. They include a 44-page typescript autobiography, apparently dictated and transcribed, with a few corrections in Urey’s own hand. There is a transcript of an oral history interview by E. M. Emme and R. C. Hall, 18 October 1976, at the NASA History Office, Washington, D.C.

There is no available comprehensive bibliography of Urey’s publications. However, a letter. William Anders to Harold Urey, 14 October 1964, A. G. W. Cameron Papers, Harvard University, includes a tally of 956 papers then under consideration for reprinting in whole or in summary under four headings: Solar System (836); Abundance of Elements (61); Life (44); Tektites (15).

Urey’s first application of quantum mechanics to physical and chemical problems is in “The Distribution of Electrons in the Various Orbits of the Hydrogen Atom,” Astrophysical Journal, 59 , no. 1 (Jan. 1924), 1-10; a full treatment is found in A. Ruark and Harold Urey. Atoms, Molecules, and Quanta (New York, 1930).

The thinking that led Urey to the discovery of deuterium is presented in “Natural System of Atomic Nuclei,” Journal of the American Chemical Society, 53 (1931), 2872; and in H. C. Urey and Charles A. Bradley. Jr., “On the Relative Abundances of Isotopes,” Physical Review, 38 (1931), 718–724, The discovery was presented first in a letter to the editor, Physical Review, 39 (1932), 164, and then in a full article, “A Hydrogen Isotope of Mass 2 and its Concentration,” Physical Review, 40 (1932), 1–15, written with C. F. G. Brickwedde and G. M. Murphy. Urey’s Nobel Prize lecture is printed in “Some Thermodynamic Properties of Hydrogen Deuterium,” Angewandte Chemie, 48 (1935), 315–320. The application of deuterium is presented in “Deuterium and Its Compounds in Relation to Biology,” Cold Spring Harbor Symposium, 2 (1934), 47–56.

Urey’s thinking concerning the warming of the earth and its cosmogonical implications are in “A Hypothesis Regarding the Origin of the Movements of the Earth’s Crust,” Science, 110 (28 Oct. 1949), 445–446; “The origin and development of the earth and other terrestrial planets,” Geochimica et Cosmochimica Acta, 1 (1951), 207–277; “The origin and development of the earth and other terrestrial planets: A correction,” Geochimica et Cosmochimica Acta, 2 (1952), 263–268; “On the Origin of Continents and Mantles,” Proceedings of the National Academy of Sciences, 39 (1953), 933–946; “On the Concentration of Certain Elements at the Earth’s Surface,” Proceedings of the Royal Society of London, A 219 (1953), 281–292; “Chemical Evidence Regarding the Earth’s Origin,” International Union of Applied Chemistry, XII Congress, Plenary Lectures (Stockholm, 1953), 188–214, “The Origin of the Earth,” in Henry Faul, ed., Nuclear Geology (New York, 1954), 355–371; “Boundary Conditions for Theories of the Origin of the Solar System,” in L. H. Ahrens, et al., eds., Physics and Chemistry of the Earth, 2 (1957), 46–76; “Evidence Regarding the Origin of the Earth,” Geochimica et Cosmochimica Acta, 26 (1962), 1–13. Applications of the same thinking to other astronomical bodies is in H. C. Urey and Bertram Donn, “Chemical Heating for Meteorites,” Astrophysical Journal, 124 (1956), 307–310: and Bertram Donn and H. C. Urey, “Chemical Heating Processes in Astronomical Objects,” Memoires de la Société Royale des Sciences de Liege, 4, no. 18 (1957), 124–132.

Urey’s early attempts to derive the chemical evolution of the solar system can be traced in “The Abundance of the Elements,” Physical Review, 88 (1952), 248–252; “Chemical fractionation in the meteorites and the abundance of the elements,” Geochimica et Cosmochimica Acta, 2 (1952), 269–282; “On the Early Chemical History of the Earth and the Origin of Life,” Proceedings of the National Academy of Sciences, 38 (1952), 351–363. The arguments are synthesized in The Planets: Their Origin and Development (New Haven, 1952).

Urey’s later thinking on this subject and his reaction to the criticisms of others can be traced in “On the Dissipation of Gas and Volatized Elements from Protoplanets,” Astrophysical Journal Supplement, I, no. 6 (1954), 147–173; “The Cosmic Abundance of Potassium, Uranium, and Thorium and the Heat Balances of the Earth, the Moon, and Mars,” Proceedings of the National Academy of Sciences, 41 (1955), 127–144, and a correction note in ibid., 42 (1956), 889–891; “The Problem of Elemental Abundances,” in L. H. Ahrens, ed., Origin and Distribution of the Elements (Oxford, 1968), 247–254.

The significance of the earth’s moon and Urey’s thought concerning its origin and evolution are in “Some Criticisms of ‘On the Origin of the Lunar Surface Features’ by G. P. Kuiper,” Proceedings of the National Academy of Sciences, 41, no. 7 (1955), 423–428; “The Moon’s Surface Features,” Observatory, 76, no. 895 (1956), 232–235; “The Origin and Significance of the Moon’s Surface,” Vistas in Astronomy, 2 (1956), 1667–1680; “Composition of the Moon’s Surface,” Z. physiche Chem. Neue Folge, 16 (1958), 346–357; H. C. Urey. W. M. Elsasser, and M. G. Rochester, “Note on the Internal Structure of the Moon,” Astrophysical Journal, 129 (1959), 842–848; “Duration of the Intense Bombardment Processes on the Moon,” Astrophysical Journal, 132 (1960), 502–503; “Lines of Evidence in Regard to the Composition of the Moon,” Proceedings of the First International Space Science Symposium (Amsterdam, 1960); “The Origin and Nature of the Moon,” Endeavor, 19 (1960), 87–99; “The Moon,” in L. V. Berknerand H. Odishaw, eds., Science in Space (NY: 1961), 185–197; “Earth’s Daughter, Sister, or Uncle?” in W. Sullivan, ed., America’s Race for the Moon (New York, 1962), 97–102; “Origin and History of the Moon,” in Z. Kopal, ed., Physics and Astronomy of the Moon (New York, 1962), 481–523; “Meteorites and the Moon,” Science, 147 (1965), 1262–1265; “The Capture Hypothesis of the Origin of the Moon,” in B. G. Marsden and A. G. W. Cameron, eds., The Earth-Moon System (New York, 1966), 210–212); “Water on the Moon,” Nature, 216 (16 December 1967), 1094–1095; H. C. Urey and Kurt Marti, “Surveyor Results and the Composition of the Moon,” Science, 161 (6 September 1968), 1030–1032; “Origin and History of the Moon,” in E. Rabinowitch and R. S. Lewis, eds., Man on the Moon (New York, 1969); H. C. Urey, K. Marti. J. W. Hawkins, and M. K. Liu, “Model History of the Lunar Surface,” Proceedings of the Second Lunar Science Conference, vol. 2 (Cambridge, Mass., 1971), 987–998; “The Origin of the Moon and Solar System,” in K. Runcorn and H. Urey, eds.. The Moon, (Dordrecht: Reidel, 1972), 429–448; H. C. Urey and K. Marti, “Lunar Basalts,” Science, 164 (1972), 117–119; “Evidence for Objects of Lunar Mass in the Early Solar System and for Capture as a General Process for the Origin of Satellites,” Astrophysics and Space Science, 16 (1972), 311–323; “The Moon and Its Origin,” Science and Public Affairs (November 1973), 5–10; S. K. Runcorn and H. C. Urey, “A New Theory of Lunar Magnetism,” Science, 180 (11 May 1973), 636–638.

Urey’s work on meteorites and cosmic abundances can be traced in H. C. Urey and H. Craig, “The Composition of the Stone Meteorites and the Origin of the Meteorites,” Geochimica et Cosmochimica Acta, 4 (1953), 36–82; Bertram Donn and H. C. Urey, “On the Mechanism of Comet Outbursts and the Chemical Composition of Comets,” Astrophysical Journal, 123 (1956), 339–342; H. C. Urey and V. R. Murphy, “Isotopic Abundance Variations in Meteorites,” Science, 140 (1963), 385; “A Review of Atomic Abundances in Chondrites and the Origin of Meteorites,” Reviews of Geophysics, 2, no. 1 (Feb. 1964), 1–34; “Biological Material in Meteorites—A Review,” Science, 151 (1966), 157–166; G. Edwards and H. C. Urey, “Determination of Alkali Metals in Meteorites by a Distillation Process,” Geochimica et Cosmochimica Acta, 7 (1955), 154–168; “Diamonds, Meteorites, and the Origin of the Solar System,” Astrophysical Journal, 124 (1956), 523–637; H. C. Urey, A. Mele, and T. Mayeda, “Diamonds in Stone Meteorites,” Geochimica et Cosmochimica Acta, 13 (1957), 1–4; “Comments on Two Papers by John F. Lovering Concerning a Typical Parent Meteorite Body,” Geochimica et Cosmochimica Acta, 13 (1957), 335–338; “Meteorites and the Origin of the Solar System,” Yearbook of Physical Society (1957), 14–29; “The Early History of the Solar System as Indicated by the Meteorites,” Proceedings of the Chemical Society of London (1958), 7–67; H. Seuss and H. C. Urey, “Abundances of the Elements in Planets and Meteorites,” Handbuch der Physik, 52 (1958), 296–323; “Criticism of Dr. B. Mason’s Paper on ‘The Origin of Meteorites,’” Journal of Geophysical Research, 66, no. 6 (June 1961), 1988–1991; H. C. Urey et al., “Life-Forms in Meteorites,” Nature, 193 (1962), 1119–1133; “Lifelike Forms in Meteorites,” Science, 137 (1962), 623–628; “Parent Bodies of the Meteorites and the Origin of Chondrules,” Icarus, 7 (1967), 350–360.

Review articles and summaries of Urey’s thinking on the evolution of the solar system include “Primary and Secondary Objects,” Journal of Geophysical Research, 64 (November 1959), 1721–1737; “Atmospheres of the Planets,” Handbuch der Physik, 53 (1959); “On the Chemical Evolution and Densities of the Planets,” Geochimica et Cosmochimica Acta, 18 (1960), 151–153; “The Planets,” in L. V. Barkner and H. Odishaw, eds., Science in Space (New York, 1961), 119–217; “The Origin and Evolution of the Solar System,” in Donald P. LeGalley, ed., Space Sciences (New York, 1963), 123–168; “Chemical Evidence Relative to the Origin of the Solar System,” Monthly Notices of the Royal Astronomical Society, 131 (1966), 199–223.

Urey’s reflections on the space program and the direction it should take can be found in “Statement of Dr. Harold C. Urey. Professor-at-Large, University of California, San Diego, La Jolla, Calif.,” in Scientists’ Testimony on Space Goals, Hearings before the Committee on Aeronautical and Space Sciences, United States Senate, 88th. Congress, 1st Session, June 10–11, 1963 (Washington, DC : U.S. Government Printing Office, 1963), 50–62, General reflections on his own life and work can be found in “As I See It,” Forbes, 104, no. 2 (15 July 1969), 44–48; “Acceptance Speech by Dr. Harold C. Urey for the V. M. Goldschmidt Medal,” Geochimica et Cosmochimica Acta, 40 (1976), 570.

II. Secondary Literature. Urey’s collaborators describe the discovery of deuterium in George M. Murphy, “The Discovery of Deuterium,” in H. Craig et al., eds., Isotopic and Cosmic Chemistry (Amsterdam, 1964), 1–7, and Ferdinand G. Brickwedde, “Harold Urey and the Discovery of Deuterium,” in Physics Today, 35 (September 1982), 34–39. Robert E. Kohler, Jr., describes the application of isotopes to biochemical and biomedical research in “Rudolf Schoenheimer, Isotopic Tracers, and Biochemistry in the 1930’s,” in Historical Studies in the Physical Sciences, 8 (1977), 257–298, Other works of interest are Roger M. Stuewer, “The Naming of the Deuteron,” in Am. Jrnl. of Physics, 54 (1986), 206–218; and Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939/1946, vol. 1 of A History of the United States Atomic Energy Commission (University Park, Pennsylvania, 1962).

John G. Burke assesses the contributions of Urey and his collaborators in Cosmic Debris: Meteorites in History (Berkeley, 1986). Stephen G. Brush treats Urey’s interest in the moon and the evolution of the solar system in “Nickel for Your Thoughts: Urey and the Origin of the Moon,” in Science, 217 (1982), 891–898; “From Bump to Clump: Theories of the Origin of the Solar System, 1900–1960,” in Paul A. Hanle and Von Del Chamberlain, eds., Space Science Comes of Age: Perspectives in the History of the Space Sciences (Washington, D.C., 1981), 78–100; and “Harold Urey and the Origin of the Moon: The Interaction of Science and the Apollo Program,” in Proceedings of the Twentieth Goddard Memorial Symposium (17–19 March, 1982), 437–470. Urey’s role in the Apollo program is treated in passing in R. Cargill Hall, Lunar Impact: A History of Project Ranger (Washington, D.C., 1977); Henry S. F. Cooper, Moon Rocks (New York, 1970); and Courtney G. Brooks. James M. Grim-wood, and Loyd S. Swenson, Jr., Chariots for Apollo: A History of Manned Lunar Spacecraft (Washington, D.C.. 1979). A sociological perspective is provided by Ian I. Mitroff, The Subjective Side of Science: A Philosophical Inquiry into the Psychology of the Apollo Moon Scientists (Amsterdam, 1974).

Robert Jastrow provides a personal account of his and Urey’s work at NASA in “Exploring the Moon,” in Paul A. Hanle and Von Del Chamberlain, eds., Space Science Comes of Age: Perspectives in the History of the Space Sciences (Washington, D.C., 1981), 45–50. Homer Newell does the same in “Harold Urey and the Moon,” in Moon, 7 (1973), 1–5, and Beyond the Atmosphere: Early Years of Space Science (Washington, D.C.. 1980), especially 237–239.

Appreciations and obituaries include Richard Fifield, “A Doughty Chemist,” in New Scientist, 89 (15 January 1981), 167–168; Eugene Garfield, “A Tribute to Harold Urey,” in Current Comments, 49 (3 December 1979), 5–9; Cyril Ponnamperuma, “Harold Clayton Urey: Chemist of the Cosmos,” in Sky and Telescope, 61 (May 1981), 397; Carl Sagan, “Obituary, Harold Clayton Urey: 1893–1981,” in Icarus, 48 (1981), 348–352; J. Y. Smith. “Harold Urey Dies: Nobel Prize Chemist Helped Develop ABomb,” in Washington Post (7 Jan. 1981), C8; H. G. Thode and Hannes Alfvén, “Obituary, Harold C. Urey,” in Physics Today, 34 (April 1981), 82–84; G. J. Wasserburg, “Introduction of Harold Clayton Urey for the V. M. Goldschmidt Medal,” in Geochimica et cosmochimica acta, 40 (1976), 569–570.

Joseph N. Tatarewicz

Urey, Harold (1893-1981)

American biochemist

Already a scientist of great honor and achievement, Harold Urey's last great period of research brought together his interests and experiences in a number of fields of research to which he devoted his life. The subject of that research was the origin of life on Earth.

Urey hypothesized that the earth's primordial atmosphere consisted of reducing gases such as hydrogen, ammonia, and methane. The energy provided by electrical discharges in the atmosphere, he suggested, was sufficient to initiate chemical reactions among these gases, converting them to the simplest compounds of which living organisms are made, amino acids. In 1953, Urey's graduate student Stanley Lloyd Miller carried out a series of experiments to test this hypothesis. In these experiments, an electrical discharge passed through a glass tube containing only reducing gases resulted in the formation of amino acids.

The Miller-Urey experiment is a classic experiment in biology. The experiment established that the conditions that existed in Earth's primitive atmosphere were sufficient to produce amino acids, the subunits of proteins comprising and required by living organisms. In essence, the Miller-Urey experiment fundamentally established that Earth's primitive atmosphere was capable of producing the building blocks of life from inorganic materials.

The Miller-Urey experiment also remains the subject of scientific debate. Scientists continue to explore the nature and composition of Earth's primitive atmosphere and thus, continue to debate the relative closeness of the conditions of the experimental conditions to Earth's primitive atmosphere.

The Miller-Urey experiment was but one part of a distinguished scientific career for Urey. In 1934, Harold Urey was awarded the Nobel Prize in chemistry for his discovery of deuterium, an isotope, or species, of hydrogen in which the atoms weigh twice as much as those in ordinary hydrogen. Also known as heavy hydrogen, deuterium became profoundly important to future studies in many scientific fields, including chemistry, physics, and medicine. Urey continued his research on isotopes over the next three decades, and during World War II his experience with deuterium proved invaluable in efforts to separate isotopes of uranium from each other in the development of the first atomic bombs. Later, Urey's research on isotopes also led to a method for determining the earth's atmospheric temperature at various periods in past history. This experimentation has become especially relevant because of concerns about the possibility of global climate change.

Harold Clayton Urey was born in Walkerton, Indiana. His father, Samuel Clayton Urey, was a schoolteacher and lay minister in the Church of the Brethren. His mother was Cora Reinoehl Urey. After graduating from high school, Urey hoped to attend college but lacked the financial resources to do so. Instead, he accepted teaching jobs in country schools, first in Indiana (1911–1912) and then in Montana (1912–1914) before finally entering Montana State University in September of 1914 at the age of 21. Urey was initially interested in a career in biology, and the first original research he ever conducted involved a study of microorganisms in the Missoula River. In 1917, he was awarded his bachelor of science degree in zoology by Montana State.

The year Urey graduated also marked the entry of the United States into World War I. Although he had strong pacifist beliefs as a result of his early religious training, Urey acknowledged his obligation to participate in the nation's war effort. As a result, he accepted a job at the Barrett Chemical Company in Philadelphia and worked to develop high explosives. In his Nobel Prize acceptance speech, Urey said that this experience was instrumental in his move from industrial chemistry to academic life.

At the end of the war, Urey returned to Montana State University where he began teaching chemistry. In 1921 he decided to resume his college education and enrolled in the doctoral program in physical chemistry at the University of California at Berkeley. His faculty advisor at Berkeley was the great physical chemist Gilbert Newton Lewis. Urey received his doctorate in 1923 for research on the calculation of heat capacities and entropies (the degree of randomness in a system) of gases, based on information obtained through the use of a spectroscope. He then left for a year of postdoctoral study at the Institute for Theoretical Physics at the University of Copenhagen where Niels Bohr, a Danish physicist, was researching the structure of the atom. Urey's interest in Bohr's research had been cultivated while studying with Lewis, who had proposed many early theories on the nature of chemical bonding.

Upon his return to the United States in 1925, Urey accepted an appointment as an associate in chemistry at the Johns Hopkins University in Baltimore, a post he held until 1929. He interrupted his work at Johns Hopkins briefly to marry Frieda Daum in Lawrence, Kansas, in 1926. Daum was a bacteriologist and daughter of a prominent Lawrence educator. The Ureys later had four children.

In 1929, Urey left Johns Hopkins to become associate professor of chemistry at Columbia University, and in 1930, he published his first book, Atoms, Molecules, and Quanta, written with A. E. Ruark. Writing in the Dictionary of Scientific Biography, Joseph N. Tatarewicz called this work "the first comprehensive English language textbook on atomic structure and a major bridge between the new quantum physics and the field of chemistry." At this time he also began his search for an isotope of hydrogen. Since Frederick Soddy, an English chemist, discovered isotopes in 1913, scientists had been looking for isotopes of a number of elements. Urey believed that if an isotope of heavy hydrogen existed, one way to separate it from the ordinary hydrogen isotope would be through the vaporization of liquid hydrogen. Urey's subsequent isolation of deuterium made Urey famous in the scientific world, and only three years later he was awarded the Nobel Prize in chemistry for his discovery.

During the latter part of the 1930s, Urey extended his work on isotopes to other elements besides hydrogen. Urey found that the mass differences in isotopes can result in modest differences in their reaction rates

The practical consequences of this discovery became apparent during World War II. In 1939, word reached the United States about the discovery of nuclear fission by the German scientists Otto Hahn and Fritz Strassmann. The military consequences of the Hahn-Strassmann discovery were apparent to many scientists, including Urey. He was one of the first, therefore, to become involved in the U.S. effort to build a nuclear weapon, recognizing the threat posed by such a weapon in the hands of Nazi Germany. However, Urey was deeply concerned about the potential destructiveness of a fission weapon. Actively involved in political topics during the 1930s, Urey was a member of the Committee to Defend America by Aiding the Allies and worked vigorously against the fascist regimes in Germany, Italy, and Spain. He explained the importance of his political activism by saying that "no dictator knows enough to tell scientists what to do. Only in democratic nations can science flourish."

Urey worked on the Manhattan Project to build the nation's first atomic bomb. As a leading expert on the separation of isotopes, Urey made critical contributions to the solution of the Manhattan Project's single most difficult problem, the isolation of ²³⁵uranium.

At the conclusion of World War II, Urey left Columbia to join the Enrico Fermi Institute of Nuclear Studies at the University of Chicago where Urey continued to work on new applications of his isotope research. During the late 1940s and early 1950s, he explored the relationship between the isotopes of oxygen and past planetary climates. Since isotopes differ in the rate of chemical reactions, Urey said that the amount of each oxygen isotope in an organism is a result of atmospheric temperatures. During periods when the earth was warmer than normal, organisms would take in more of a lighter isotope of oxygen and less of a heavier isotope. During cool periods, the differences among isotopic concentrations would not be as great. Over a period of time, Urey was able to develop a scale, or an "oxygen thermometer," that related the relative concentrations of oxygen isotopes in the shells of sea animals with atmospheric temperatures. Some of those studies continue to be highly relevant in current research on the possibilities of global climate change.

In the early 1950s, Urey became interested in yet another subject: the chemistry of the universe and of the formation of the planets, including Earth. One of his first papers on this topic attempted to provide an estimate of the relative abundance of the elements in the universe. Although these estimates have now been improved, they were remarkably close to the values modern chemists now accept.

In 1958, Urey left the University of Chicago to become Professor at Large at the University of California in San Diego at La Jolla. At La Jolla, his interests shifted from original scientific research to national scientific policy. He became extremely involved in the U.S. space program, serving as the first chairman of the Committee on Chemistry of Space and Exploration of the Moon and Planets of the National Academy of Science's Space Sciences Board. Even late in life, Urey continued to receive honors and awards from a grateful nation and admiring colleagues.

See also Cell cycle and cell division; Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses

Urey, Harold (1893-1981)

American chemist

In 1934, Harold Urey was awarded the Nobel Prize in chemistry for his discovery of deuterium, an isotope, or species, of hydrogen in which the atoms weigh twice as much as those in ordinary hydrogen. Also known as heavy hydrogen, deuterium became profoundly important to future studies in many scientific fields, including chemistry, physics , and medicine. Urey continued his research on isotopes over the next three decades, and during World War II, his experience with deuterium proved invaluable in efforts to separate isotopes of uranium from each other in the development of the first atomic bombs. Later, Urey's research on isotopes also led to a method for determining the earth's atmospheric temperature at various periods in past history. Already a scientist of great honor and achievement, Urey's last great period of research brought together his interests and experiences to a study of the origin of life on Earth. Urey's experimentation has become especially relevant because of concerns about the possibility of global climate change.

Urey hypothesized that the earth's primordial atmosphere consisted of reducing gases such as hydrogen, ammonia, and methane. The energy provided by electrical discharges in the atmosphere, he suggested, was sufficient to initiate chemical reactions among these gases, converting them to the simplest compounds of which living organisms are made, amino acids. In 1953, Urey's graduate student Stanley Lloyd Miller carried out a series of experiments to test this hypothesis. In these experiments, an electrical discharge passed through a glass tube containing only reducing gases resulted in the formation of amino acids.

The Miller-Urey experiment is a classic experiment in molecular biology and genetics. The experiment established that the conditions that existed in Earth's primitive atmosphere were sufficient to produce amino acids, the subunits of proteins comprising and required by living organisms. In essence, the Miller-Urey experiment fundamentally established that Earth's primitive atmosphere was capable of producing the building blocks of life from inorganic materials.

The Miller-Urey experiment also remains the subject of scientific debate. Scientists continue to explore the nature and composition of Earth's primitive atmosphere and thus, continue to debate the relative closeness of the conditions of the experimental conditions to Earth's primitive atmosphere.

Urey was born in Walkerton, Indiana. His father, Samuel Clayton Urey, was a schoolteacher and lay minister in the Church of the Brethren. His mother was Cora Reinoehl Urey. After graduating from high school, Urey hoped to attend college but lacked the financial resources to do so. Instead, he accepted teaching jobs in country schools, first in Indiana (1911–1912) and then in Montana (1912–1914) before finally entering Montana State University in September of 1914, at the age of 21. Urey was initially interested in a career in biology, and the first original research he ever conducted involved a study of microorganisms in the Missoula River. In 1917, he was awarded his Bachelor of Science degree in zoology by Montana State.

The year Urey graduated also marked the entry of the United States into World War I. Although he had strong pacifist beliefs as a result of his early religious training, Urey acknowledged his obligation to participate in the nation's war effort. As a result, he accepted a job at the Barrett Chemical Company in Philadelphia and worked to develop high explosives. In his Nobel Prize acceptance speech, Urey said that this experience was instrumental in his move from industrial chemistry to academic life.

At the end of the war, Urey returned to Montana State University, where he began teaching chemistry. In 1921, he decided to resume his college education and enrolled in the doctoral program in physical chemistry at the University of California at Berkeley. His faculty advisor at Berkeley was the great physical chemist Gilbert Newton Lewis. Urey received his doctorate in 1923 for research on the calculation of heat capacities and entropies (the degree of randomness in a system) of gases, based on information obtained through the use of a spectroscope. He then left for a year of postdoctoral study at the Institute for Theoretical Physics at the University of Copenhagen where Niels Bohr , a Danish physicist, was researching the structure of the atom . Urey's interest in Bohr's research had been cultivated while studying with Lewis, who had proposed many early theories on the nature of chemical bonding.

Upon his return to the United States in 1925, Urey accepted an appointment as an associate in chemistry at the Johns Hopkins University in Baltimore, a post he held until 1929. He briefly interrupted his work at Johns Hopkins to marry Frieda Daum in Lawrence, Kansas, on June 12, 1926. Daum was a bacteriologist and daughter of a prominent Lawrence educator. The Ureys later had four children.

In 1929, Urey left Johns Hopkins to become associate professor of chemistry at Columbia University, and in 1930 he published his first book, Atoms, Molecules, and Quanta, written with A. E. Ruark. Writing in the Dictionary of Scientific Biography, Joseph N. Tatarewicz called this work "the first comprehensive English language textbook on atomic structure and a major bridge between the new quantum physics and the field of chemistry." At this time he also began his search for an isotope of hydrogen. Since Frederick Soddy, an English chemist, discovered isotopes in 1913, scientists had been looking for isotopes of a number of elements. Urey believed that if an isotope of heavy hydrogen existed, one way to separate it from the ordinary hydrogen isotope would be through the vaporization of liquid hydrogen. Urey's subsequent isolation of deuterium made Urey famous in the scientific world, and only three years later he was awarded the Nobel Prize in chemistry for his discovery.

During the latter part of the 1930s, Urey extended his work on isotopes to other elements besides hydrogen. Urey found that the mass differences in isotopes can result in modest differences in their reaction rates.

The practical consequences of this discovery became apparent during World War II. In 1939, word reached the United States about the discovery of nuclear fission by the German scientists Otto Hahn and Fritz Strassmann. The military consequences of the Hahn-Strassmann discovery were apparent to many scientists, including Urey. He was one of the first, therefore, to become involved in the U.S. effort to build a nuclear weapon, recognizing the threat posed by such a weapon in the hands of Nazi Germany. However, Urey was deeply concerned about the potential destructiveness of a fission weapon. Actively involved in political topics during the 1930s, Urey was a member of the Committee to Defend America by Aiding the Allies and worked vigorously against the fascist regimes in Germany, Italy, and Spain. He explained the importance of his political activism by saying "no dictator knows enough to tell scientists what to do. Only in democratic nations can science flourish."

Urey worked on the Manhattan Project to build the nation's first atomic bomb. As a leading expert on the separation of isotopes, Urey made critical contributions to the solution of the Manhattan Project's single most difficult problem, the isolation of uranium–235.

At the conclusion of World War II, Urey left Columbia to join the Enrico Fermi Institute of Nuclear Studies at the University of Chicago, where Urey continued to work on new applications of his isotope research. During the late 1940s and early 1950s, he explored the relationship between the isotopes of oxygen and past planetary climates. Since isotopes differ in the rate of chemical reactions, Urey said that the amount of each oxygen isotope in an organism is a result of atmospheric temperatures. During periods when the earth was warmer than normal, organisms would take in more of a lighter isotope of oxygen and less of a heavier isotope. During cool periods, the differences among isotopic concentrations would not be as great. Over a period of time, Urey was able to develop a scale, or an "oxygen thermometer," that related the relative concentrations of oxygen isotopes in the shells of sea animals with atmospheric temperatures. Some of those studies continue to be highly relevant in current research on the possibilities of global climate change.

In the early 1950s, Urey became interested in yet another subject: the chemistry of the universe and of the formation of the planets, including Earth. One of his first papers on this topic attempted to provide an estimate of the relative abundance of the elements in the universe. Although these estimates have now been improved, they were remarkably close to the values modern chemists now accept.

In 1958, Urey left the University of Chicago to become Professor at Large at the University of California in San Diego at La Jolla. At La Jolla, his interests shifted from original scientific research to national scientific policy. He became extremely involved in the U.S. space program, serving as the first chairman of the Committee on Chemistry of Space and Exploration of the Moon and Planets of the National Academy of Science's Space Sciences Board. Even late in life, Urey continued to receive honors and awards from a grateful nation and admiring colleagues.

See also Evolution, evidence of; Evolution; Evolutionary mechanisms; Radioactivity

Harold Clayton Urey

The American Scientist Harold Clayton Urey (1893-1981) received the Nobel Prize for chemistry in 1934 for his discovery of deuterium, the isotope of heavy hydrogen.

Harold Clayton Urey was born on April 29, 1893, in Walkerton, Ind., the son of Samuel Clayton Urey and Cora Rebecca Reinoehl Urey. After graduation from high school at 18, followed by some three months of education training at Earlham College, Harold taught in small country schools in Indiana and then Montana, where the family had moved. In 1914 he entered Montana State University (Bozeman) and graduated in three years with a baccalaureate in science.

The United States entered World War I in 1916 and Urey began work at the Barrett Chemical Company in Frankford near Philadelphia, preparing toluene for the production of TNT (trinitrotoluene) in 1917. In 1919 he returned to Montana to teach in the department of chemistry for 2 years. In 1921 he entered the graduate school at the University of California, Berkeley. Urey's interest focused upon molecular structure, a new field for scientists in theUnited States. His doctoral research on the conductivity of cesium vapor led him to the theory of thermal ionization in stellar atmospheres. Within two years he received the doctorate (1923) and, with a Scandinavian Foundation fellowship, later studied in Copenhagen at the Institute of Theoretical Physics, headed by Niels Bohr.

During his residence at Berkeley and Copenhagen, Urey began to work with some of the most prominent physicists and chemists of the 20th century, including Werner Heisenberg, Wolfgang Pauli and Georg von Hevsey. In Hamburg, Germany, he met he met Albert Einstein and James Franck, both of whom became his lifelong friends. In 1924 he returned to the United States to take a National Research Council fellowship at Harvard but instead accepted a faculty position as associate in chemistry at Johns Hopkins University, where he remained until 1929. There he and Russel Bichowsky hypothesized the idea of electron spin to explain the fine structure of the atomic spectral lines.

Between 1923 and 1929, Urey published 20 scientific papers or notes, almost all of them on aspects of atomic structure and the others on molecular band spectroscopy. In 1930 the book Atoms, Molecules, and Quanta, by A. E. Ruark and Urey, appeared. It became, and remained for a long time, one of the standard texts on the subject. Urey also published The Planets: Their Origins and Development in 1952.

While at Johns Hopkins, Urey married Frieda Daum in 1926. Her intelligence, good humor and warm hospitality made the Urey house a meeting place for scientists and intellectuals of all fields. As a tribute to her, the University of California at San Diego named its first large building the Harold and Frieda Urey Hall. The Ureys had four children: Gertrude, Elizabeth, Frieda Rebecca, Mary Alice and John Clayton.

In 1929 Urey became associate professor of chemistry at Columbia University in New York and professor in 1934. He refined his interest in sub-atomic and molecular structure and discovered a discrepancy in the atomic weight of hydrogen compared with that of oxygen, as measured by chemical means and by mass spectrometry. The discrepancy led to his identification of a heavy isotope of hydrogen (deuterium) found in concentrations of about 1 part per 5, 000. Calculating that the heavy isotope would boil less easily than the lighter, he had a friend at the Bureau of Standards, Ferinand G. Brickwedde, distill hydrogen. In the residue they and George M. Murphy discovered heavy water, molecules with one atom of oxygen and two atoms of hydrogen or deuterium. This led to Urey's award of the Nobel Prize for Chemistry in 1934.

Journal of Chemical Physics, published by the American Institute of Physics for the new breed of physical chemists interested in subatomic and molecular spectroscopy and structure. He remained editor until 1941, established the journal's preeminent reputation and the name of the field, chemical physics.

Urey's scientific work at Columbia became more and more concerned with the separation of isotopes of the lighter elements. The rarer isotopes of oxygen, nitrogen, carbon, and sulfur were concentrated. When full-scale work began at Columbia on the separation of the rare fissionable lighter isotope of uranium, U²³⁵, from the much more abundant U²³⁸, Urey became project head.

He then served as one of three program chiefs of the Manhattan Project. Even after the project's successful conclusion at Oak Ridge, Tennessee, he felt discouraged. The atmosphere of secrecy, the tight time schedule, and the limitations and conflicts of the work oppressed him.

After the war Urey, along with many others, was attracted to the plan of Chancellor Hutchins to build a group of research institutes at the University of Chicago. When the new institutes were founded, Urey was in the Institute for Nuclear Studies, later to become the Enrico Fermi Institute. For a while Urey, uncharacteristically, still suffering from the trauma of the war work, tended to drift, and he looked for new fields to conquer. He soon became interested in the past history of the earth and the planetary system. He initiated the use of analysis of the isotopic abundance in sea fossils to estimate the temperature of past oceans and helped prepare the most commonly accepted table of the elements. He also worked on meteoritic ages, composition, and classification. But his abiding interest then became earth's moon: Many of his later papers concerned the possible character of its formation and past history. He influenced astronomers and others to consider chemical evidence in the origin of the solar system and later became an active consultant to NASA, Lunar Sciences, and to the Space Science Board, National Academy of Sciences.

The trials of Ethel and Julius Rosenberg, who were accused of stealing secrets relating to the construction of the atomic bomb, attracted Urey's interest in 1952. The letters in their defense that he wrote to President Harry S. Truman, the New York Times and the presiding trial judge received mixed reviews from the public, but they further cemented his relationship with Albert Einstein, who wrote Urey a strong letter of support.

In 1958 Urey moved to the Scrimps Institution of Oceanography in La Jolla, where he continued to teach and to do active research in the general field of geochemistry and planetary science. He received numerous honors besides the Nobel Prize, including 23 honorary doctorates and membership or fellow of some 25 societies or academies. His bibliography of scientific publications exceeds 200 titles.

Urey later became a member of the Union of Concerned Scientists. In 1975 the organization petitioned then President Gerald Ford to limit the expansion of nuclear power plants. Urey's expressed alarm for the safety of nuclear power plants, nuclear waste disposal and the spread of nuclear weapons. He died of at heart attack at his home in La Jolla, California, near the University of California, San Diego, January 5, 1981.

Further Reading

Register of Harold Clayton Urey Papers, 1929-1981, Mandeville Special Collections Library, Geisel Library, University of California at San Diego. The collection contains his autobiography and all the available biographical sketches written during his career at University of California (San Diego). Shorter studies of his life and career are in Sarah R. Riedman, Men and Women behind the Atom (1958); Eduard Farber, Nobel Prize Winners in Chemistry, 1901-1961 (1953; rev. ed. 1963); Jay E. Greene, ed., 100 Great Scientists (1964); Nobel Foundation, Chemistry: Including Presentation Speeches and Laureates' Biographies, vol. 2 (1966); Henry A. Boorse and Lloyd Motz, eds., The World of the Atom (2 vols., 1966); and Frederic L. Holmes, editor, Dictionary of Scientific Biography (18 vols., 1990). Urey's cosmological ideas are discussed in Jagjit Singh, Great Ideas and Theories of Modern Cosmology (1961). □

Harold Clayton Urey

1893-1981

American Chemist

Harold Clayton Urey is best known for his work with isotopes. He received the Nobel Prize for Chemistry in 1934 for the discovery and isolation of deuterium, a heavy isotope of hydrogen. He also played a major role in the development of the atomic bomb and made important contributions to the study of isotopes of a number of elements and to the theory of the origin of the planets.

After receiving his doctorate at the University of California at Berkeley, Urey worked with Niels Bohr (1855-1962) in Denmark. Returning to the United States, he held positions at Johns Hopkins University, Columbia University, the Institute of Nuclear Study, the University of Chicago, and the University of California.

Isotopes of an element have the same number of protons (i.e., atomic number) but a different number of neutrons—and, therefore, different mass numbers. Much of Urey's research concerned the heavy isotope of hydrogen known as deuterium. The most common form of hydrogen is the simplest atom that can exist, having one proton in its nucleus and one associated electron. Its atomic number is 1, and its mass number is 1. The deuterium atom contains a neutron in its nucleus in addition to a proton. Its atomic number is 1, but its mass number is 2. Since both hydrogen isotopes have one electron and one proton, they are virtually identical in their chemical reactivity, but properties that are dependent on mass are significantly different. Urey noticed that the mass of hydrogen gas isolated from naturally occurring substances is minutely larger than would be expected if only hydrogen with mass number 1 was present. He proposed the existence of deuterium to explain this observation. In 1931 he announced the discovery of heavy water, which contains two atoms of deuterium instead of two "normal" hydrogen atoms. In 1931 Urey announced the production and isolation of pure deuterium, which he obtained by successive distillations of liquid hydrogen.

Urey was awarded the Nobel Prize for Chemistry in 1934 for his work on deuterium. He subsequently applied the methods that he had developed for the study of the isotopes of hydrogen to the separation and study of the isotopes of carbon, nitrogen, oxygen, and sulfur.

During World War II, Urey's expertise in the separation of isotopes resulted in his playing an important role in the Manhattan Project, the successful American effort to develop an atomic bomb. He directed the program at Columbia University that developed a method, employing the gaseous diffusion of uranium hexafluoride, for separating the radioactive isotope of uranium (U-235), which could be used in the bomb's fission reaction, from its more abundant isotope (U-238). After the war, he spoke out against the danger inherent in the use of nuclear power, especially nuclear war.

His work with isotopes led to a study of the abundance of naturally occurring isotopes on earth, resulting in a theory of the origin of the elements and their relative abundance in the sun and on other planets in the solar system. His research led him to propose that the earth's atmosphere was once made up of ammonia, methane, and hydrogen, and that reactions among these molecules could lead to the production of living entities. One of his students, Stanley Miller (1930- ), successfully demonstrated that when energy is supplied to a mixture of these gases, biological molecules are indeed produced. Theoretically, at least, these molecules could then interact to build living systems. Urey proposed further theories that explain the origin of the solar system as a condensation of gasses around the sun. He published the results of this theoretical work in The Planets: Their Origin and Development (1952).

J. WILLIAM MONCRIEF

ELECTROPHORESIS CRACKS THE CASE

Every person (except for identical twins) has a unique set of genetic material in the form of DNA. A technique called DNA fingerprinting takes advantage of this individuality. To create a DNA fingerprint, scientists must first acquire a sample of a person's cells, such as from blood, hair, or skin. After the DNA is removed from the cells, special proteins are added to it that cut the extremely long DNA molecules into fragments. The next step is to separate the fragments from one another based on their size. This is done by using elec trophoresis—the technique Arne Tiselius helped to develop. Elec trophoresis separates the fragments into bands. The pattern formed by the bands is different for each person.

The technique of DNA fingerprinting is sometimes used in criminal investigations. If a blood or skin sample is found at a crime scene, for example, a DNA fingerprint can be made from these cells. This fingerprint can then be compared to that of a suspect. If the patterns of the bands are identical, the results indicate that the suspect was present at the crime scene. If the patterns of the bands differ, the results prove that someone other than the suspect was the source of the crime-scene sample.

STACEY R. MURRAY