Harold Urey and thediscovery of deuteriumChemistry, nuclear physics, spectroscopy andthermodynamics came together to predict and detect heavy hydrogenbefore the neutron was known.

Ferdinand G. Brickwedde

It was on Thanksgiving day in 1931that Harold Clayton Urey found defini-tive evidence of a heavy isotope ofhydrogen. Urey's discovery of deuter-ium is a story of the fruitful use ofprimitive nuclear and thermodynamicmodels. But it is also a story of missedopportunity and errors—errors thatare particularly interesting because ofthe crucial positive role that some ofthem played in the discovery. A look atthe nature of the theoretical and ex-perimental work that led to the detec-tion of hydrogen of mass 2 revealsmuch about the way physics and chem-istry were done half a century ago.

Although George M. Murphy and Icoauthored with Urey the papers1"3

reporting the discovery, it was Ureywho proposed, planned and directedthe investigation. Appropriately, theNobel Prize for finding a heavy isotopeof hydrogen went to Urey.

In this article we will look first at theresearch that led to the discovery, asthat work was understood at the time.Then we will look at some of the sameactivity with the understanding thatonly hindsight can give. Throughoutthe discussion I will include fragmentsfrom my memory—illustrative epi-sodes connected with the discovery.

Urey's careerUrey died last year at 87 years of age,

after a remarkably productive and in-teresting life. He was a chemist withvery broad interests in science, remi-niscent of the natural philosophers ofthe eighteenth and nineteenth centur-ies. Murphy4, who went on to becomeprofessor and head of the departmentof chemistry at New York University,died in 1968.

Urey was born on a farm in Indianain 1893, and in childhood moved with

Ferdinand G. Brickwedde is Evan Pugh Re-search Professor of Physics emeritus at Penn-sylvania State University, in University Park,Pennsylvania.

his family to a homestead in Montana.After graduating from high school, hetaught for three years in public schools,and then entered Montana State Uni-versity as a zoology major and chemis-try minor. Money was tight for him asa college student. During the academicyear he slept and studied in a tent.During his summers he worked on aroad gang laying railroad track in theNorthwest.

Urey graduated with a BS degree in1917, when there was a need for chem-ists in the war effort. He worked forthe Barrett Chemical Company inPhiladelphia on war materials. Afterthe war, Urey taught chemistry for twoyears at Montana State University,and in 1921 entered the University ofCalifornia at Berkeley as a graduatestudent in chemistry, working underthe guidance of the renowned chemicalthermodynamicist Gilbert N. Lewis.As a graduate student, Urey was apioneer in the calculation of thermo-dynamic properties from spectroscopicdata. He received a PhD in 1923 andspent the next academic year as anAmerican-Scandinavian FoundationFellow in the Physical Institute ofNiels Bohr in Copenhagen.

After Copenhagen, Urey joined thefaculty at Johns Hopkins University.Although in the chemistry department,he attended the physics department'sregular weekly "journal" meetings forfaculty and graduate students, and heparticipated in the discussions. It wasat these meetings that I, as a graduatestudent in physics, became acquaintedwith Urey. While Urey was at Hopkins,he and Arthur E. Ruark coauthored theclassic textbook, Atoms, Molecules, andQuanta, which was the first comprehen-sive text on atomic structure written inEnglish. I proofread the entire book ingalley for the authors.

Urey's work bridged chemistry andphysics. In 1929 he was appointedassociate professor of chemistry at Co-lumbia University, and from 1933 to

1940 he was the founding editor of theAmerican Institute of Physics publica-tion, Journal of Chemical Physics.When the biographical publication"American Men of Science" took noteof scientists selected for recognition bytheir peers, Urey was elected in phy-sics. In 1934—only three years afterthe discovery of deuterium—Urey wasawarded the Nobel Prize in chemistry.

Before the searchIn 1913, Arthur B. Lamb and Richard

Edwin Lee, working at New York Uni-versity, reported5 a very precise mea-surement of the density of pure water.Their measurements were sensitive to2xlO~7 g/cm3. Various samples ofwater, which were carefully preparedusing the best purification techniquesand temperature controls, varied indensity by as much as 8xlO~7 g/cm3.They concluded that pure water doesnot possess a unique density.

Today we know that water varies inisotopic composition, and that samplesof water with different isotopic compo-sitions have different vapor pressures,making distillation a fractionating pro-cess. The Lamb-Lee investigation isinteresting because it was the firstreported experiment in which an isoto-pic difference in properties was clearlyin evidence. It is the earliest recogniz-able experimental evidence for iso-topes. (The existence of isotopes wasproposed independently by FrederickSoddy, in England, and by KasimirFajans, in Germany, in 1913.) Thinkwhat the result might have been hadLamb and Lee pursued a progressivefractionation of water by distillationand separated natural water into frac-tions with different molecular weights.

Less than two decades later, by thetime of the discovery of deuterium,isotopes were an active field of re-search. The rapid development of nu-clear physics after 1930 was initiatedby isotope research. It was a time ofsearch for as-yet undiscovered isotopes,

34 PHYSICS TODAY / SEPTEMBER 1982 0031-9228/ 8 2 / 0900 34-06/ $01.00 (cl 1982 American Institute Of Physics

especially of the light elements, hydro-gen included, and Urey was very mucha participant.

I remember a conversation in 1929with Urey and Joel Hildebrand, a fam-ous professor of chemistry at Berkeley.It took place during a taxi ride betweentheir hotel and the conference centerfor a scientific meeting we were attend-ing in Washington. When Urey askedHildebrand what was new in researchat Berkeley, Hildebrand replied thatWilliam Giauque and Herrick John-ston had just discovered that oxygenhas isotopes with atomic weights 17and 18, the isotope of weight 18 beingthe more abundant. Their paper6

would appear shortly in the Journal ofthe American Chemical Society. ThenHildebrand added, "They could nothave found isotopes in a more impor-tant element." Urey responded: "No,not unless it was hydrogen." This wastwo years before the discovery of deu-terium. Urey did not remember thisremark, but I did.

At the time, answers were beingsought to questions such as: Why do

isotopes exist, and what determinestheir number, relative abundances andmasses (packing fractions)?

Urey, along with others, constructedcharts of the known isotopes to showrelationships bearing on their exis-tence. The figure on page 36 is one ofUrey's charts. At the time, the neutronhad not been discovered—it was disco-vered in 1932, the year after deuter-ium. The chart was based on the the-ory that atomic nuclei were composedof protons, plotted here as ordinates,and nuclear electrons, plotted as abscis-sae—the number of protons was thenuclear mass number, and the numberof nuclear electrons was the number ofprotons minus the atomic number ofthe element. In Urey's chart, the filledcircles represent the nuclei from H' toSi30 that were known to exist before1931. The open circles represent nucleiunknown before 1931. The chart's pat-tern of staggered lines, when extendeddown to H1, suggested to Urey that theatoms H2, H3 and He5 might existbecause they are needed to completethe pattern.

Urey had a copy of this chart hang-ing on a wall of his laboratory. Theisotope helium-5 does not exist, and thestaggered line does not provide a placefor the isotope helium-3, which wasdiscovered later. The diagram is onlyof historical interest now, but it was anincentive to Urey to look for a heavyisotope of hydrogen.

Prediction and evidenceIn 1931—the year of the discovery of

deuterium—Raymond T. Birge, a pro-fessor of physics at the University ofCalifornia, Berkeley, and Donald H.Menzel, professor of astrophysics atLick Observatory, published7 a letter tothe editor in Physical Review on therelative abundances of the oxygen iso-topes in relation to the two systems ofatomic weights that were then in use—the physical system and the chemicalsystem. Atomic weights in the physicalsystem were determined with the massspectrograph and were based on settingthe atomic weight of the isotope O16 atexactly 16. In the chemical system,atomic weights were determined by

Harold Clayton Urey and a country schoolhouse in Indiana where he taughtafter graduating from high school. Urey taught for three years in public schools

in Indiana and Montana before he entered Montana State University.(Schoolhouse photo from the Urey collection, AIP Niels Bohr Library.)

PHYSICS TODAY / SEPTEMBER 1982 35

bulk techniques, and the values werebased on setting at 16 the atomicweight of the naturally occurring mix-ture of oxygen isotopes, O16, O17 andO18. Thus the atomic weights of asingle isotope or element on the twoscales should differ. The weightnumbers should be greater on the phys-ical scale.

However, in 1931 the atomic weightsof hydrogen on the two scales were thesame within the claimed experimentalerrors. The chemical value was1.00777 + 0.00002. The mass-spectro-graphic value, determined by FrancisW. Aston of the Cavendish Laboratory,was 1.00778 + 0.00015. Birge andMenzel pointed out that the near coin-cidence of these two atomic weightsleads to the conclusion that normalhydrogen is a mixture of isotopes—H1

in high concentration and a heavy iso-tope in low concentration. The atomicweight was not higher on the physicalscale because the mass-spectroscopictechniques saw only the light isotope.

To the heavy isotope they gave thesymbol H2, perhaps the first time thissymbol occurred in the literature. As-suming the atomic weight of heavyhydrogen to be two, Birge and Menzelcalculated its relative abundance fromthe supposed equivalence of the atomicweights of hydrogen-1 on the physicalscale and the normal mixture of hydro-gen isotopes on the chemical scale.They obtained 1/4500 for the abun-dance of H2 relative to H1.

Within a day or two at most afterreceiving the 1 July 1931 issue of thePhysical Review, Urey proposed andplanned an investigation to determineif a heavy isotope of hydrogen did reallyexist.

Urey and Murphy, working at Co-lumbia, identified hydrogen and its iso-tope spectroscopically, using theBalmer series lines. The atomic spec-trum was produced with a Wood's elec-tric discharge tube operated in the so-called black stage—the configurationof current and pressure that moststrongly excites hydrogen's atomicspectrum relative to its molecular spec-trum. They observed the spectra witha 21-foot grating, in the second order.The dispersion was 1.3 A per mm. Theexpected shifts, then, were of the orderof 1 mm, as the numbers in the tableindicate. The vacuum wavelengths ofdeuterium's lines were calculated us-ing the Balmer series formula

l/AH=RH(l/22-l/n2) (1)n = 3, 4, 5 , . . .

RH = (2irIe4/h3c)memH/(mf! + mH)and the "best" values for the atomicconstants. The Balmer a-lines of hy-drogen and deuterium are separated by1.8 A, the /?-lines by 1.3 A, and the y-lines by 1.2 A. The concentrations ofdeuterium relative to hydrogen are de-

10 15

NUCLEAR ELECTRONS

Protons versus "nuclear electrons" foratomic nuclei from H1 to Si30. The plot shows apattern that led Urey to look for a heavyisotope of hydrogen. Open circles representnuclei that were unknown in 1931, when thechart was produced.

termined by comparing the measuredtimes required to produce plate lines ofH and D of equal photographic densi-ties. The exposure times for H/? and Hywere about 1 second.

Using cylinder hydrogen, Urey andMurphy found very faint lines at thecalculated positions for D/?, T)y and D<5.The lines were faint because of the lowconcentration of deuterium in normalhydrogen. There was a possibility thatthe new lines arose from impurities, orwere grating ghost lines arising fromthe relatively intense hydrogen Balmerspectrum.

Clinching evidenceUrey decided not to rush into print to

stake a claim to priority in this impor-tant discovery; he decided to postponepublication until he had conclusive evi-dence that the "new" spectral linesattributed to the heavy isotope wereauthentic and not impurity or ghostlines. This evidence could be obtainedby increasing the deuterium concentra-tion in the hydrogen filling the Wood'stube and looking for an increase inintensity of the deuterium Balmer linesrelative to the hydrogen Balmer lines.

After careful consideration of differ-ent methods for increasing the deuter-ium concentration, Urey decided on adistillation that would make use of ananticipated difference in the vaporpressures of liquid H2 and liquid HD.He made a statistical, thermodynamiccalculation of the vapor pressures ofsolid H2 and solid HD at the triple pointof H2, 14 kelvins, where the liquid andcrystal phases of H2 are in equilibriumand have the same vapor pressure. Thecalculation was based on the Debyetheory of solids and the zero-point vi-brational energy of the solid, 9R6/8 inthe Debye notation. At 14 K, the calcu-lated ratio of vapor pressures, P(HD)/P(H2), is 0.4, indicating a large differ-

ence in the vapor pressures of solid H2and HD. On this basis Urey expected asizeable difference in the vapor pres-sures of liquid H2 and HD at 20.4 K, theboiling point of H2.

Urey approached me at the NationalBureau of Standards in Washington,inviting me to join the search for aheavy isotope of hydrogen by evaporat-ing 5- to 6-liter quantities of liquidhydrogen to a residue of 2 cm3 of liquid,which would be evaporated into glassflasks and sent by Railway Express toColumbia University for spectroscopicexamination. At the time, 1931, therewere only two laboratories in the Unit-ed States where liquid hydrogen wasavailable in 5- or 6-liter quantities.One was the National Bureau of Stan-dards in Washington and the other wasGiauque's laboratory at the Universityof California, Berkeley. I was happy tocooperate, and I prepared—by distill-ing liquid hydrogen at the Bureau ofStandards—the samples of gas inwhich the heavy isotope was identified.

The first sample I sent to Urey wasevaporated at 20 K and a pressure ofone atmosphere. It showed no appre-ciable increase in intensity of the spec-tral lines attributed to heavy hydrogen.This was unexpected.

The next samples were evaporated ata lower temperature—14 K at 53 mm ofmercury pressure, the triple point ofH2—where the relative difference inthe vapor pressures of H2 and HD wasexpected to be larger than at 20 K, andthe rate at which heavy hydrogen isconcentrated was expected to be morerapid.

These samples showed 6- or 7-foldincreases in the intensities of theBalmer lines of deuterium. On thisbasis, it could be concluded that thelines in the normal hydrogen spectrumattributed to deuterium were reallydeuterium lines, but the clinching evi-dence was finding that the photograph-ic image of the Da line—the mostintense D-Balmer line—was a partiallysplit doublet as predicted by theory forthe Balmer series spectrum.

From measurements of the relativeintensities of the H and D Balmerseries lines, Urey estimated that therewas one heavy atom per 4500 lightatoms in normal hydrogen. Later mea-surements showed it to be nearer one in6500.

Unraveling a comedy of errorsIt is now clear why the first distilled

hydrogen sent to Urey did not show theexpected increase in the deuteriumconcentration, and maybe even showed

News story on the awarding of the 1934Nobel Prize in chemistry. Article appeared 16

November 1934. (Copyright The New YorkTimes. Reprinted by permission.)

36 PHYSICS TODAY / SEPTEMBER 1982

a small decrease. The explanationcame with the discovery of the electro-lytic method for separating H and D,suggested by Edward W. Washburn,chief chemist at the National Bureau ofStandards, and verified8 experimental-ly by Washburn and Urey just after thepublication of our April 1932 paper.3

When Urey considered differentmethods for concentrating deuterium,he included the electrolytic method,and discussed it with Victor LaMer, acolleague at Columbia, and a worldauthority on electrochemistry. LaMerwas so discouraging about success inseparating hydrogen isotopes by elec-trolysis that Urey abandoned the elec-trolytic method and adopted the distil-lation method. LaMer reasoned thatthe differences in equilibrium concen-trations of isotopes at the electrodes ofa cell at room temperature would bevery small and hence a fractionation ofthe isotopes would be negligible.

Washburn viewed the situation dif-ferently. He pointed to the large rela-tive difference in atomic weights of thehydrogen isotopes—a relative differ-ence that is much larger for the hydro-gen isotopes than for the isotopes of anyother element. Hence, thought Wash-

Calculated Balmer series wavelengths

Line

a0r5

(A)6564.6864862.7304341.7234102.929

W)

(A)6562.8994861.4074340.5414101.812

calculated(A)

1.7871.3231.1821.117

-D)observed

(A)1.791.331.191.12

These values were calculated using equation 1 with MH = 1.007775 g Mo = 2 01363 gm0 = 5.491 MO * g and flH = 109677.759 cm '

burn, the hydrogen isotopes might be-have differently from the isotopes ofother elements.

Washburn, the empiricist, was right;the isotopes of hydrogen are separatedrelatively easily by electrolysis, butthis was not realized until after thediscovery of deuterium.

The hydrogen we liquefied and dis-tilled for Urey was generated electroly-tically. Before preparing the first sam-ple for Urey, the electrolytic generatorwas completely dismantled, cleanedand filled with a freshly prepared solu-tion of sodium hydroxide. Because deu-terium becomes concentrated in theelectrolyte in the generator, the firstgaseous hydrogen to be discharged was

NOBEL AWARD GOES

Columbia Scientist Gets the1934 Chemistry Prize forDiscovering 'Heavy Water.'

ACHIEVEMENT WAS HAILED

Seen as of Especial Value inCancer Study—Has Proved

Great Spur to Research.

Wireless to THE NEW YORK TIMES.

STOCKHOLM, Nov. 15.—TheNobel Prize in Chemistry for 1934was awarded today to ProfessorHarold C. Urey of Columbia Uni-versity because of his discovery of"heavy water."

The chemistry prize for 1933 willnot be awarded. It was also an-nounced that there would be noprize in physics for this year.

Achievement Was Hailed.Dr. Harold Clayton Urey won a

position in the forefront amongacientists by his discovery of "heavywater," which has been hailed byscientists the world over as rankingamong the great achievements ofjModern science.

The Willard Gil- Med?'Chicago sectionChemicp'Dr. *

WINS NOBEL PRIZE.Professor Harold Urey.

DEFENSE TO SUBPOENALINDBERGH FOR TRIAL

Betty Gow Also to Be Summonedas Witness—Fight Planned toRelease Haaptmann Funds.

Special to THE NEW YORK TIMES.FLEMINGTON, N. J., Nov. 15.—

Colonel Charles A. Lindbergh andBetty Gow, w>- ••> fir^t son's•urse, will ' • • de

i s e ••

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ConSi

DIVOI

Mrs.Th

SpeBUFJ

ment isions oby Ogd

, Treasuin addwelf?busination

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deficient in deuterium. The concentra-tion of deuterium in the hydrogenevolved was about one sixth the concen-tration of deuterium in the electrolyte,and hence about one sixth the concen-tration of deuterium in normal hydro-gen. Distillation of the deuterium-defi-cient liquid hydrogen increased theconcentration of D relative to H andrestored in the first sample approxi-mately the original concentration ofdeuterium in normal hydrogen.

As electrolysis progressed, water wasadded to replace that which was con-sumed. The concentration of deuter-ium in the electrolyte increased to thepoint where the rate at which deuter-ium left the generator balanced therate at which it arrived in the addedwater. Hence, after the electrolyticgenerator had been in use for sometime, there was a dynamic equilibrium;so the hydrogen evolved from the gen-erator for our second and third samplesfor Urey had approximately the nor-mal concentration of deuterium.When we liquefied this hydrogen andevaporated 5 or 6 liters down to 2 cm3,the concentration of deuterium in theresidue was increased by a factor ofabout six.

Here we lower the curtain on a "com-edy of errors"—LaMer's error of notunderstanding better the principlesthat govern isotopic fractionation dur-ing electrolysis, and my error of attrib-uting to sloppy technique our failure toeffect an increase of deuterium concen-tration in the first sample we sent toUrey. Had I analyzed our part of theprocess, I think we might have disco-vered the electrolytic concentration ofdeuterium. Had LaMer been moreknowledgeable, Urey would have madehis own concentration of deuteriumelectrolytically and I should have hadno part in the discovery of deuterium.

Reporting the resultAfter the discovery of deuterium,

Urey faced a very practical problem inreporting it—a problem characteristicof the status of research before WorldWar II. Urey's research at Columbia,and ours at the National Bureau ofStandards, where I was chief of the lowtemperature laboratory, was carriedout without the support of any govern-ment research grant. It was said that

PHYSICS TODAY / SEPTEMBER 1982 37

research in that period was done withstring and sealing wax; it was in factdone mostly with homemade appara-tus. The US government policy ofgrants in support of research datesfrom a later time—from World War II.

Before the War it was a problem tofind funds for travel to scientific meet-ings. I received a telephone call fromUrey, telling me that it appeared hewas not going to get funds to travel tothe December 1931 American PhysicalSociety meeting at Tulane University,where he planned to present a paperreporting the discovery of deuterium.He asked me if I could get travel fundsand present the paper. For this I had tosee Lyman J. Briggs, assistant directorof research and testing at the Bureau ofStandards. Briggs, soon to be namedNBS director, was an understandingand considerate physicist who, onlearning of the work to be reported,made funds available for my travel. Inthe meantime, Bergen Davis, a promi-nent physicist at Columbia, heard ofUrey's problem and went to see Colum-bia president Nicholas Murray Butler,who made funds available for Urey'stravel. So we both went to Tulane forthe APS meeting, and Urey presentedthe ten-minute paper.1 Over the nextfew months we published more detailin a letter2 to the editor and a full-length paper3 in Physical Review.

I remember asking Birge at a laterAPS meeting why he and Giauque hadnot followed up on his prediction7 of theexistence of heavy hydrogen. Theymight have demonstrated the existenceof deuterium by concentrating theheavy isotope through distillation of alarge quantity of liquid hydrogen asUrey and I had done. Giauque had avery fine, large-capacity hydrogen liq-uefier suitable for this. Birge's replywas that he was busily engaged onother important work that demandedhis attention. When I told Urey of thisdiscussion, his comment was: "What inthe world could Birge have been work-ing on that was so important?"

Apropos of the above, I quote herefrom a letter of 6 May 1981 from RobertW. Birge, son of Raymond T. Birge, andalso a physicist:

After reading some more about myfather's life, I think I know why hedidn't try to concentrate deuter-ium. I believe he was an analystmore than a hardware builder andit probably never occurred to himto do it that way. He said that atthe time several people were try-ing to see the deuterium lines inspectra, but they [Urey, Brick-wedde and Murphy] did it first.But as you know, the importantpoint was that Urey realized that[the concentration of] deuteriumcould be enhanced.

The two men remained friends

Mass spectrometer with Urey at the controls,courtesy of King Features Syndicate.)

throughout their lifetime.Frederick Soddy, the English che-

mist who received the 1921 Nobel Prizein Chemistry for discovering the pheno-menon of isotopy, did not accept thenotion that deuterium was an isotope ofhydrogen. Soddy worked with isotopesof the naturally radioactive elements,whose atomic weights are large andwhose isotopic relative mass differ-ences are small. These isotopes showedno observable differences in chemicalproperties and were inseparable chemi-cally. When Soddy coined the wordisotope he gave it a definition thatincluded chemical inseparability of iso-topic species of the same element. Thiswas generally accepted before the dis-covery of the neutron in 1932.

After the discovery of the neutron,isotopes were denned as atomic specieshaving the same number of protons intheir nuclei but different numbers ofneutrons. But Soddy stuck to chemicalinseparability as a criterion for iso-topes and therefore refused to recog-nize deuterium as an isotope of hydro-gen. For Soddy, deuterium was aspecies of hydrogen, with differentatomic weight, but not an isotope ofhydrogen.

A fortunate mistakeFour years after the discovery of

deuterium, Aston reported9 an error in

after the discovery of deuterium. (Photograph

his earlier mass-spectrographic valueof 1.00778 for the atomic weight ofhydrogen-1 on the physical scale—thevalue used by Birge and Menzel in their1931 letter.7 The revised value on thephysical scale was 1.00813, which cor-responds to 1.0078 on the chemicalscale, in agreement with the then cur-rent value for the atomic weight ofhydrogen (1.00777) on the chemicalscale. There was then no need or placefor a heavy isotope of hydrogen. Theconclusion of Birge and Menzel wasthus rendered invalid. Indeed, on thebasis of Aston's revised value, Birgeand Menzel would have been obliged toconclude that, if anything, there was alighter—not a heavier—isotope of hy-drogen.

The prediction of Birge and Menzelof a heavy isotope of hydrogen wasbased on two incorrect values for theatomic weight of hydrogen, namely As-ton's mass-spectrographic value andthe chemical value, which also shouldhave been greater. We are obliged toconclude that the experimental errorin the determination of the atomicweights exceeded the difference in theatomic weights on the two scales.

Urey was not aware of this when heplanned his experiment. It was notuntil 1935 when Urey's Nobel lecturewas in proof that Aston published hisrevised value. Urey added the follow-

38 PHYSICS TODAY / SEPTEMBER 1982

ing to the printed Nobel lecture:Addendum

Since this [Nobel lecture10] waswritten, Aston has revised hismass-spectrographic atomicweight of hydrogen (H) to 1.0081instead of 1.0078. With this massfor hydrogen, the argument byBirge and Menzel is invalid. How-ever, I prefer to allow the argu-ment of this paragraph [the thirdparagraph of Urey's Nobel lecture]to stand, even though it now ap-pears incorrect, because this pre-diction was of importance in thediscovery of deuterium. Withoutit, it is probable we would not havemade a search for it and the discov-ery of deuterium might have beendelayed for some time.

Needless to say, Urey and his collea-gues were very glad that an error ofthis kind had been made. Aston saidthat he did not know what the moral ofit all was. He would hardly advisepeople to make mistakes intentionally,and he thought perhaps the only thingto do was to keep on working.

Impact of the discoveryIt has been said that Nobel prizes in

physics and chemistry are awarded forwork, experimental or theoretical, thathas made a significant change in ongo-ing work and thinking in science. Theannouncement that Urey was chosenas the 1934 laureate in chemistry cameless than three years after that ten-minute paper in New Orleans announc-ing the discovery of deuterium. Thisuncommonly early award followed aspectacular display in deuterium-relat-ed research. In the first two-year peri-od following the discovery, more than100 research papers were published onor related to deuterium and its chemi-cal compounds, including heavy water.And there were more than a hundredmore11 in the next year, 1934.

The use of deuterium as a tracermade it possible to follow the course ofchemical reactions involving hydrogen.This was especially fruitful in investi-gations of complex physiological pro-cesses and in medical chemistry, as inthe breakdown of fatty tissue and incholesterol metabolism.

Also, the discovery of heavy hydro-gen provided a new projectile, the deu-teron, for nuclear bombardment ex-periments. The deuteron provedmarkedly efficient in disintegrating anumber of light nuclei in novel ways.As the deuteron, with one proton andone neutron, is the simplest compoundnucleus, studies of its structure and ofits proton-neutron interaction took onfundamental importance for nuclearphysics.

Many of the early research papersdealt with isotopic differences in phys-ical and chemical properties. Theories

developed for the atomic mass depen-dence of physical and chemical proper-ties were tested experimentally. Theseinvestigations were especially interest-ing because, before the discovery ofdeuterium, chemical properties weregenerally supposed to be determined bythe number and configuration of theextranuclear electrons, quantities thatare identical for isotopes of the sameelement. It had not been realized thatchemical properties are also affected—but to a lesser degree—by the mass ofthe nucleus.

In thinking about Urey's search fordeuterium, beginning with his earlydiagram of the isotopes, I am remindedof the Greek inscription on the facadeof the National Academy of Sciencesbuilding in Washington, taken fromAristotle:

The search for truth is in one wayhard and in another easy, for it isevident that no one can master itfully or miss it wholly. But eachadds a little to our knowledge ofnature, and from all the facts as-sembled there arises a certaingrandeur.

* * */ wish to acknowledge the valuable assis-tance of my wife, Langhorne Howard Brick-wedde, especially for her help in recallingincidents of the early thirties connected withthe discovery of deuterium. This article isbased on a paper I presented 22 April 1981 inBaltimore, Maryland, at the inaugural ses-sion of the American Physical Society's Divi-sion of History of Physics.

References

1. The thirty-third annual meeting of theAmerican Physical Society at TulaneUniversity, 29-30 December 1931. Ab-stracts of papers presented: Phys. Rev.39, 854. Urey, Brickwedde and Murphyabstract #34.

2. H. C. Urey, F. G. Brickwedde, G. M.Murphy, Phys. Rev. 39, 164 (1932).

3. H. C. Urey, F. G. Brickwedde, G. M.Murphy, Phys. Rev. 40, 1 (April 1932).

4. For an interesting account of the discov-ery of deuterium, see G. M. Murphy,"The discovery of deuterium," in Isoto-pic and Cosmic Chemistry, H. Craig, S. L.Miller, G. J. Wasserburg, eds., North-Holland, Amsterdam (1964). (Dedicatedto Urey on his seventieth birthday.)

5. A. B. Lamb, R. E. Lee, J. Am. Chem. Soc.35, part 2, 1666 (1913).

6. W. F. Giauque, H. L. Johnston, J. Am.Chem. Soc. 51, 1436, 3528 (1929).

7. R. T. Birge, D. H. Menzel, Phys. Rev. 37,1669 (1931).

8. E. W. Washburn, H. C. Urey, Proc. Nat.Acad. Sci. US 18, 496 (1932).

9. F. W. Aston, Nature 135, 541 (1935);Science 82, 235 (1935).

10. H. Urey in Nobel Lectures in Chemistry,1922-1941, published for the NobelFoundation by Elsevier, Amsterdam(1966).

11. Industrial and Engineering Chemistry,News Edition 12, 11 (1934). •

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