WILLARD GIBBS MEDAL Awarded to Harold C. Urey for his work on the isotopes of hydrogen
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AMERICAN CHEMICAL SOCIETY, spoke of the Willard Gibbs Medal, and W. D. Harkins discussed Dr. Urey’s work and presented the medal to him. I n accepting the award, the medalist talked on “Significance of the Hydrogen Isotopes.” On April 30, a t the University of Chicago, Dr. Urey delivered a scientific address on “The Vapor Pressures of the Hydrogens.’’
H E twenty-third presentation of the Willard Gibbs Medal, founded by Killiam A. Converse, was made to Dr. Urey, professor of chemistry at Columbia University, on April 27, 1934, a t the Stevens Hotel in Chicago. The award is made annually by a jury of twelve chemists selected from different parts of the United States. L. F. Supple, chairman of the Chicago Section of the 4
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Significance of the Hydrogen Isotopes HAROLD C. UREY,Columbia University, New York, N. Y. I’l” very sober thoughts indeed I of the isotope of hydrogen of atomic weight 2. accept the Willard Gibbs Medal. Without the help of Dr. B r i c k w e d d e , it This medal stands for such high would have been difficult to be certain of accomplishment in our science that one can the e x i s t e n c e of this i s o t o p e , and Dr. accept it only with great introspection and Murphy worked by my side a t all hours of humility. I wish to express to you my apthe day and night until its e x i s t e n c e was preciation of this honor. certainly established. Professor Harkins has asked what features Medals should be regarded by those who in my education have contributed most to present them and by those who receive them more as marking definite milestones passed my success. I have received great benefit from all the various institutions where I in our science than as rewards for personal secured my e d u c a t i o n and with which I accomplishment. Some of these are sharp have subsequently been associated. Parand distinct; others are less so. None of us could possibly do the things which we find ticularly, I should mention my years a t the University of Montana where I received my interesting if i t were not for the careful work Photo bu Ossip Garber Studio first inspiration for scientific work, in the of many others who preceded us or are workHAROLDC. UREY Departments of both Zoology and Chemising simultaneously on our problems. The great a d v a n c e s - 0 f s c i e n c e d u r i n g the try. A. W. Bray, a t present head of the Department of Zoology a t the Rensselaer Polytechnic Institute, past century and particularly in recent years are largely due was a close personal friend and a wonderful teacher. R. H. to the cooperative efforts of many individuals. That this Jesse, J. W. Howard, W.G. Bateman, and W.N. Jones, now is true is illustrated beautifully by the events which led to of the Carnegie Institute of Technology, were my early pro- the discovery of the hydrogen isotope. The situation in fessors of chemistry. The whole direction of my scientific 1931 was such that some one was sure to look for and find career was largely determined by these men. Like so many this interesting atom. I am pleased indeed that it should professors in small colleges who devote their time largely to have been my privilege to carry the torch of science as this teaching, they made personal friends of their students and milestone was passed, but the essential thing is that i t was gave an intimate association which proves so valuable to passed; the particular individual by whom it was done is their students. Also, I should mention my years a t the unimportant. University of California where I came under the inspiring WORKPOINTING TO EXISTESCE OF HYDROGEN ISOTOPE influence of Gilbert N. Lewis, and my experience a t the University of Copenhagen. Here I received valuable knowledge That the heavier hydrogen should exist was indicated by from Nils Bohr and H. A. Kramers, now a t the University of regularities in the numbers and composition of the nuclei of Utrecht. But I must also recall a most valuable thing which known atoms and it was the study of these regularities which I learned as a small boy from my mother. It was she who first led me to suspect the existence of this isotope. Of taught me that “man does not live by bread alone but by course this study did not indicate whether an expected but every word that proceedeth out of the mouth of God.” unknown isotope should be sufficiently abundant to make Of all of the lessons that I have learned in my life, this one its detection possible. About this time Professor Allison has been most valuable. also reported the existence of two minima in his magnetoI wish to express my appreciation of the generous support optic method of analysis which he interpreted as being due which Johns Hopkins University first and Columbia Uni- to two isotopes of hydrogen. Later R. T. Birge and D. H. versity later have given to my researches during the past ten Menzel gave reasons for believing that deuterium should be years, and the encouragement they have provided in their present to the extent of 1 part in 4500 of the more abundant continued belief in my work. variety of hydrogen. Their arguments were based on the Also, I wish to acknowledge the aid of my students and preceding work of Aston of Cambridge on the one hand, and research assistants during that time, particularly of my W. -4.Noyes and Morley particularly on the other. These young friends F. G. Brickwedde of the Bureau of Standards men determined the atomic weights of hydrogen by two and G. M. Murphy of Columbia University in the discovery different methods: Aston by his mass spectrograph secured
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the mass of protium or light hydrogen, while Soyes and solid hydrogen in detail and it took some time to straighten Morley secured the average atomic weights of the two hy- out all of the details, particularly the zero-point energy of a drogens. The results disagreed, and the disagreement as solid. This is a residual vibrational energy of a solid which pointed out by Birge and Menzel could be understood if a remains even a t the absolute zero of temperature. Finally, heavier isotope of hydrogen existed. This work was neces- we were sure that we had the problem correctly solved. The sary or, a t least, very advantageous to the subsequent search liquid hydrogen plant a t the Bureau of Standards was the for this rare atom. nearest to us and I had known Dr. Brickwedde as a graduate Up to that time, no isotope so rare as that of hydrogen student a t Johns Hopkins University. He checked our calhad been detected. Some method of concentration was culations and agreed to distill the liquid hydrogen. The necessary in order to prove its existence. Over twenty years liquid hydrogen machine was undergoing extensive repairs, ago Debye worked out a detailed theory of the heat capacity and it was about two months before he was able to carry out of solid substances, based upon the quantum theory of Planck the distillations. We were all on pins and needles with eagerand its development by Einstein. This theory gave us an ness to carry out this crucial experiment. The hydrogen isounderstanding of the energy possessed by solids which was tope had been with us all the time, and the methods of sepaso illuminating that i t made possible an approximate esti- ration and identification could have been foreseen for about mate of the properties of an unknown solid from our k n o d - seventeen years, but no one used them. Yet, after we had edge of the properties of one already known. The properties chanced upon these ideas, the methods were so simple and of solid natural hydrogen are well known, and, from these, obvious that almost anyone might have stumbled upon them certain properties of the heavier hydrogen could be deduced a t any moment. using Debye’s theory. Among others, the pressure of hyFinally a flask of gaseous hydrogen came from the Bureau drogen gas over the solid hydrogen could be estimated, and of Standards. Dr. Murphy and I went to work immediately the theory showed that the boiling points of the hydrogens, and in one month did about four months’ work. We did protium and deuterium, should be markedly different. This fully two ordinary days’ work each day, and labored Sundays made possible their separation by fractional distillation. and Thanksgiving Day as well. Mrs. Urey was a scientific That theory and experiment do not agree exactly, as we know widow for that month. We expected to find about one per now, may lead to other interesting discoveries. However, cent of H2 in the sample. It certainly was not present in this gave us the method of concentrating the rare isotope of any such concentration. We checked our calculations and hydrogen, and Dr. Brickwedde prepared the concentrated realized that the evaporation should have been made a t the samples of hydrogen by this means. triple point of hydrogen instead of the boiling point in order In order to discover an isotope, i t is necessary to determine to be more effective. We wrote to Dr. Brickwedde and very its atomic weight by some method. Two have been used shortly more hydrogen came. I n the meantime, we tried to detect the H2 in the hydrogen in the past: One is the deflection of charged atoms in magnetic and electric fields which separate the atoms according we already had. I n our work on the spectrum we had to to mass and deposit them on photographic plates or other use a grating, which consists of a concave mirror of speculum detecting devices, and the other is by means of the spectra metal with many fine rulings on it. These parallel lines are of molecules containing two atoms. h-either of these meth- ruled by a diamond point and on our grating there are 14,000 ods could be used in the case of the hydrogen isotopes be- lines to the inch. A machine moves the metal under the cause their sensitivity is not sufficiently great. In 1913 ruling point which draws a line on the metal, raises, moves Bohr showed that the wave lengths of light emitted by hy- forward, drops on the metal, and makes another scratch. drogen atoms should depend on the masses of these atoms For the grating to be perfect, these lines would have to be and his theory gave a quantitative relation between these absolutely uniformly spaced on the metal, which is impossimasses and these wave lengths so that this method could be ble. Our grating was ruled by Henry Rowland of Johns Hopkins University in 1899 and is excellent though it gives used in the case of the hydrogen isotopes. Many other things could be mentioned which led up to false lines on the photographic plates. A line appears for and made possible our own work on the hydrogen isotopes, each wave length, but, in addition, other lines appear which such as the experimental apparatus, but the foregoing illus- are known as ghosts. Some of these lines do not interfere trate my principal theme-namely, that all scientific work with the detection of the deuterium lines. But did our gratdepends on the careful work of our predecessors and co- ing happen to give false lines just a t the position expected workers and that our rapid advance in the sciences is due for the deuterium lines? A mistake in the detection of such largely to the freedom with which we publish the results of an important atom as deuterium would be unforgivable. our own work. A realization of these facts makes it difficult Were these faint wave lengths which we found in the specto take any really profound credit to ourselves for indivi- trum of ordinary hydrogen real or ghosts? That problem increased our consumption of cigarets about tenfold and made dual accomplishments. The exact method of proving the existence of deuterium us quite unsuitable for human society. In and out of a dark is now obvious. Dr. Brickwedde distilled the liquid hydro- room in the basement of the Physics Building, developing gen, and Dr. Murphy and I found that the atomic spectrum about 100 photographic plates, was what occupied us t h a t of hydrogen did show the presence of the wave lengths of month. We would develop a plate and look for the lines as the plate lay in the fixing bath. They persisted but could lighk calculated for a hydrogen atom of mass 2 . not be made more intense. Then we used our best sample EXPERI~IENTS ON THE PROBLEM of hydrogen, which was the residue from the evaporation of The actual work on this problem was the most exciting 4000 cc. of hydrogen near the melting point, and the plate thing I have ever done, for from the first we knew that a hy- showed those wave lengths greatly bcreased in intensity. drogen isotope would be by far the most interesting of any They could not be ghosts. They were broad like unresolved of the isotopes. The possibility of the existence of a hydro- doublets, as they should be. The red line was clearly a gen isotope had occurred to me years before. The method doublet. The separation of the two doublets was close to of concentration-distillation of liquid hydrogen-came to the calculated separation. Still we were not sure. What me a t lunch one day early in August, 1931: I immediately possible errors were there? The lines were surely there. discussed it with Dr. Murphy who was my research assistant. They mere not among those reported for the molecule of hyI had never considered the thermodynamic properties of drogen. Finally we concluded that they must be due to the
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heavier hydrogen, We wrote an abstract for the Christmas meeting of the American Physical Society and reported our results a t S e w Orleans. The method of discovery was physical rather than chemical, though the boundary between the two sciences has become very nebulous in recent years. This discovery was quickly confirmed by Walker Bleakney of Princeton Cniversity using a mass spectograph, and thus the hydrogen isotope was saved from the harron-ing experience of doubts which beset so many important discoveries. The subsequent work has been so rapid that we almost need to report in a daily paper; in fact, some of us have been in the papers so much recently that we might take over a couple of the large dailies for this purpose with advantage to all. A committee might well be appointed to study the feasibility of such a plan! I calculated the difference in electrode potentials of the hydrogens with a view to the separation of the isotopes by a n electrolytic method in December, 1931. Yo separation seemed possible. E. W. Washburn did not calculate but experimented instead, and found that a very appreciable separation did occur in electrolytic processes. I was associated with Dr. Washburn in this work, but the idea was his and all credit for the discovery belongs to him. The recent extensive work on deuterium was made possible by this effective and rapid method of separation. His expectation of a favorable result was based, so he told me, on his experience with other electrolytic processes where the elements are not always discharged in the order of their normal electrode potentials. Under certain conditions, oxygen is produced where chlorine should be expected. Early in 1932 Dr. Rittenberg and Imade some calculations on the equilibrium constants for certain chemical reactions involving hydrogen, chlorine, and iodine. These showed that appreciable differences in these constants were to be expected. The differences are so great that, if deuterium had been present in a larger proportion in natural hydrogen, its effects could not have been overlooked. Marly of our fundamental laws of chemistry could not have been established. It is difficult to estimate what the effects on the history of chemistry would have been. The development of chemistry as a n exact science might have been greatly retarded. The atomic weight of hydrogen would not have been constant and perhaps the general acceptance of the atomic theory would hare been delayed. On the other hand, the discovery of the isotopes of hydrogen might have been made much sooner, though what we could have done with them in the middle of the nineteenth century, I do not know.
INFLUENCE ox FCTURE SCIEXTIFIC DEVELOPMEKTS The discovery of the hydrogen isotope and its successful preparation in highly concentrated form by G. S . Lewis has given rise to much excitement in scientific circles. Our bodies, clothes, food, wooden objects, all contain hydrogen as an abundant constituent. Of the three or four hundred thousand compounds known, fully 80 or 90 per cent contain hydrogen. Water is the most important liquid in our daily lives and also in science. Moreover, hydrogen is the simplest atom known. We believe that it contains only two particles -a heavy, positively charged nucleus, and a light, negatively charged electron. Thus one can readily understand why a new hydrogen, together with all its compounds, proves to be so interesting. It is difficult to say much about future developments but the use of the heavy hydrogen atom in chemistry, physics, and biology during the past year givea a good notion of what we may expect. CHEMISTRY.I n science we are constantly attempting to rationalize our observations and experiments by means of theories, the theories being really only exact methods of de-
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scribing the observed phenomena. Throughout all our chemical theories, the masses of the atoms play an important part. Our kinetic theory of gases, for example, states that the number of collisions between the molecules of a gas is inversely proportional to the square root of their masses; and the velocities with which chemical reactions proceed also depend upon the masses of the atoms concerned. Illany other considerations enter into such theories, but this is always one important constant. Up to this year we have never been able to determine directly whether the theory and experiment agree, for, if we worked with atoms of different elements, their characteristics changed in other ways in addition to the mass. Only with the separation of the hydrogen isotopes were we able to attack this problem directly. We can now test the theories dealing with the velocities of chemical reactions-the so-called thermodynamic properties of substances-and other similar phenomena by using the two varieties of hydrogen. The ratio of masses is conveniently large, so that comparatively large effects are to be expected. Examples which have been studied so far are the equilibrium constants of chemical reactions-for example, the equilibrium between hydrogen gas and iodine to form hydrogen iodide or the equilibrium between the deuteroacetone and protowater, and the protoacetone and the deuterowater. In the first of these cases, exact agreement is secured between theory and experiment; in the latter case, the theory cannot be made sufficiently exact, but approximate agreement is secured. I n the case of the vapor pressures of the hydrogens which we used in the original separation, we find that theory and experiment do not agree except as to order of magnitude. This disagreement is interesting and will undoubtedly give us much more information in regard to the solid state of hydrogen. Many questions of this type will certainly be made clearer by studies using the hydrogen isotopes. For example, one wonders why i t is that deuterium melts 4.6' C. above protium and that deuterowater melts 3.8" C. above protowater. We do not understand this phenomena of melting points adequately, and there are many points about the properties of solids and liquids which will certainly be made clearer as a result of this study. During recent years we have made extensive studies on the velocities of chemical reactions in water solutions particularly, and also in the gaseous state. The theory of these reactions is in a fair state of development, but the effect of mass on such phenomena will certainly be clarified by a study of reactions in which deuterium and its compounds replace protium and its compounds. For example, we have found that the velocity of reaction between the protowater and aluminum carbide to form methane is ahout twenty-three times as great as the reaction between deuterowater and aluminum carbide to give the deuteromethane. This is really an enormous difference and very easily measured. Such studies give us a grasp on reaction kinetics quite beyond our dreams of a few years ago. The two isotopes of hydrogen also permit us to follow, more or less, individual atoms as they are attached to our various chemical compounds. For example, in a collision between two hydrogen molecules, each containing two atoms of hydrogen, do the molecules trade partners? If all the hydrogen atoms are exactly the same, the question is foolish so far as physical measurement is concerned, but if a molecule of protium collides with a molecule of deuterium with the exchange of partners, molecules will be formed containing one atom of protium and one of deuterium. Tests will then determine whether such molecules are formed when the gases protium and deuterium are mixed; if they are, this can occur only as the result of such an exchange of partners. At ordinary temperatures this does not occur. In similar cases where protium water and deuterium are mixed, we find that, except
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in the presence of a catalyst, the hydrogen atoms in the water do not trade places with the hydrogen atoms of the molecular gas. I n the case of sugar, i t appears that about 40 or 50 per cent of the hydrogens do exchange with the hydrogen of water, and that the remainder do not. I n many cases it should be possible to introduce a deuterium atom in some part of a complicated organic compound and then follow this atom in subsequent chemical reactions. It is, in fact, a ‘(tagged” atom which can be introduced into chemical compounds a t one point and later identified after rather extensive changes have taken place. BIOLOGY. The biological interest of the deuterowater can hardly be overemphasized, since all living things live essentially in a water solution. Up to the present time experiments, particularly by G. N. Lewis and H. S. Taylor and their co-workers, indicate that animals die when placed in heavy water of high concentration, though they are able to live in the 30 per cent water. The evidence in regard to plants is more contradictory. Professor Lewis finds that tobacco seeds do not sprout in heavy water, while Dr. Chessley and Dr. Suguira find that wheat seeds do sprout in such water. In other cases that have been investigated, certain fluorescent bacteria do not give out their fluorescent light in the presence of heavy water, while other varieties continue to emit light. Some investigations have been made on the effects of heavy water a t only slightly increased concentrations over natural water. Such experiments should indicate the true biological usefulness of heavy water, if it has any, to a much greater degree than the ones in more concentrated water. Experiments made by Dr. Barnes indicate that deuterium may have a stimulating effect on living organisms if it is present in low concentrations. It is my own expectation that both animals and plants can be acclimatized to high concentrations of heavy water, but that probably their living processes will be much slower. We may lengthen the life of a mouse, but he will live more slowly a t the same time. MEDICINE.The medicinal effects have often been mentioned, but mostly without adequate foundation. Experiments made on the effects of heavy water on cancer seem to indicate that there is little difference in the behavior of such tissue in the presence of either proto- or deuterowater. It may be, however, that medicinals which will have valuable properties can be prepared using deuterium. PHYSICS.The nucleus of the deuterium atom is a t present one of the most delightful playthings for physicists. I n recent years we have learned to transmute the elements one into another. This is accomplished by bombarding these atoms with very high-speed particles, using the high-voltage
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machines which have been developed particularly by Lawrence, Lauritsen, Tuve, and van der Graaff in this country, and by Rutherford and his associates in England. The particles which have been used in the past are the proton, which is the nucleus of the protium atom, and the alphaparticle which is the nucleus of the helium atom. Recently, we have discovered the neutron, which is a n uncharged particle of small dimensions having a mass nearly equal to that of the proton, and the deuton or nucleus of the deuterium atom. The most intense source so far known for the neutrons is secured by bombarding beryllium with deutons. The charged particles, the proton and the deuton, fall through large electric fields and are allowed to fall upon solid bodies. The alpha-particles are secured from radioactive substances and the neutrons from the bombardment of substances either with alpha-particles or deutons. As a result of such bombardment, lithium is converted to helium and beryllium, and boron, carbon, and nitrogen are converted into other elements. En these processes large amounts of energy are liberated in the individual process. The over-all consumption of energy is much greater than that produced, however. After all, it is difficult to hit a nucleus of an atom having a diameter of a million millionth of a centimeter; only a few direct hits are secured in an experiment, and, unless a fairly direct hit is secured, no reaction takes place. A few years ago, we did not know that we would have the additional particles, neutron, deutons, and positrons. But one problem that will probably be solved in considerable detail in the course of the next ten years is the structure of the atomic nucleus. During the past twenty-five years we have succeeded in obtaining a broad understanding of the structure of the electron atmosphere of atoms. I n the next ten years we will probably have a similar understanding of the nuclei of atoms. I n unraveling this structure of the nucleus, the deuterium atom will certainly play a n important part. Next to the proton, the deuton is the simplest of all known nuclei. Our present idea is that i t consists of two particles, a proton and a neutron. All other nuclei must be more complex than this, and, if we are to obtain an adequate theory of their structure, i t will probably be necessary first to secure a theory of the structure of the deuterium nucleus. It occupies perhaps the same position relative to an understanding of the structure of nuclei that the protium atom did in relation to the structure of the external electron atmosphere of atoms, and, as all chemists and physicists realize, the unraveling of this complicated problem would have been much more difficult if it had not been for this simplest of all atoms being placed in our hands.
THE DORRCLASSIFIER SHOWNIN THE FOREGROUND OPERATESI N CLOSEDCIRCUIT WITH BALLMILLS,PREPARING 48-MESH FLOTATION FEED
