1942: Physics

Nearly all of the outstanding physicists of this country, and nearly all of their capable students, have been engaged in secret work throughout 1942. Much the same is the case in the other belligerent countries with which we are still in communication, and in the few remaining neutral countries research is scanty and handicapped. Under these tragic circumstances it seems best to omit the section on Physics until the return of happier times.

1941: Physics

Effects of the War.

The history of physics during 1941 is a startling and a fascinating one. Sir Edward Grey at the onset of the first World War created one of the classic sayings of history, by speaking of the lights going out all over Europe. The lights of physics have mostly gone out so far as the general public can see, though actually they are now burning with great luster in laboratories to which the world at large is denied all access. The process of apparent extinction began even before the outbreak of formal war: the present writer, being in England in July of 1939, heard the comment 'If this state of affairs continues, the only people in England left to work in pure physics in another few months will be the refugees.' In the United States the process began within a month after the fall of France and a year and a half before Pearl Harbor: as early as that did the 'mobilization of physics' commence. By October 1941, more than half of the outstanding physicists of the United States were engaged in what then was still called 'defense work.'

Of the character of this busy and hidden work, nothing may yet be said. Hereafter there will be much to say; in the meantime, one must be content with such statements as that of President Conant of Harvard, 'This is a physicists' war,' and that of Sir William Bragg addressing the Royal Society of London in November 1940: 'The RAF could not carry out its operations without the knowledge resulting from the studies of cathode rays and electrons made by our physicists, which is equivalent to saying that by this time we might well have lost the war.'

This is not to imply that the journals of physics suddenly ceased to appear, as though cut off with a knife, in June of 1940 or at any subsequent date, although something nearly as drastic befell France. But in the English-speaking countries the journals have continued to come out, with a gradual shrinking in content. If number of published pages were a measure of the number of novel ideas, one would certainly have to say that 1941 was a year of greater vitality in 'uncensored' physics than any of the years before the first world war. This, however, is very far from being the case; and valuable as the recent work has been, one must characterize a large part of it as being a checking-in-detail of the remoter and more intricate consequences of theories already long established, while another large part consists in gropings after theories for phenomena which have not yet yielded up their secrets. The lack of sensational novelties is certainly due in the main to the withdrawal of so many of the leaders of physics from their familiar fields, but one should also realize that periods of rapid advance have a way of alternating with periods of quiet consolidation, and a 'quiet period' of the latter kind was already overdue in 1941.

Nuclear Physics.

The experimental side of nuclear physics had evidently reached in 1940 a stage of temporary completeness. Every element had been transmuted into at least one other, every element had been obtained by transmutation of at least one other, every element had been produced in one or more radioactive forms. There were very plausible reasons for affirming that the 'atom-smashers,' defying the name with which the journalists had dubbed them, had already managed to synthesize practically every type of atom-nucleus capable of existence either permanent or extending over a detectable period of time. A great deal of delicate work remained to be done in weighing the atoms both old and new, in measuring the lifetimes of the unstable or radioactive ones, and in measuring the energies of the particles and the wavelengths of the waves which these unstable ones emit: also, in choosing the best ways for producing the particularly interesting ones among these, and magnifying the necessary apparatus. Work of these latter types accounted for most of the recent publications in the field; and one may say that experimental nuclear physics, when it passed behind the veil in 1941, had happily closed its first period of great discovery and entered prosperously upon its first period of routine measurement. Meanwhile the next period of great discovery was being hopefully anticipated from the greatest of all cyclotrons, commenced at Berkeley late in 1940 and to be finished (circumstances permitting) in 1943, able presumptively to deliver the equivalent of more than 100,000,000 volts. The hitherto-unconquered problem of producing electrons of comparable energies has just been mastered by Kerst, whose 'rheotron' involves a variable magnetic field (not a constant one, like that of the cyclotron): the electrons revolve in this field with a steadily-rising velocity, a particular relation being maintained between the magnetic flux through their orbit and the fieldstrength at the orbit itself in order to ensure that they shall remain in the same circular path throughout. Recent newspaper reports indicate the attainment of energies equivalent to what 20,000,000 volts of direct potential would produce. X-rays of unprecedented power may be expected from such an apparatus, and the medical and biological applications of these and of the new radioactive substances produced by transmutation will probably be less hampered by the war than most fields of 'pure' physics.

Theoretical nuclear physics remains in an inchoate and perplexing state, awaiting perhaps some totally new idea which may come suddenly or never. The nucleus continues to be regarded, as for the past nine years, as a clump of particles of two kinds, neutron and proton. These are held together by very strong short-range forces analogous to cohesion, but also they appear to have a nearly-irreducible volume: thus the contemporary model of the atom-nucleus is quaintly similar to Newton's image of a solid body, to wit, a clump of hard incompressible particles cohering together by a specific attraction. It is well known that corpuscles of light originate in atoms and are absorbed in other atoms, by virtue of the electrical forces which operate within the atoms and between them. It is similarly conjectured that to this intra-nuclear cohesion correspond other corpuscles which are emitted and absorbed by the nuclear particles. This, the so-called 'meson theory of nuclear forces,' is still under development and incessant revision, and the time for judging of its value has not come.

Investigation of Cosmic Rays.

Investigation of cosmic rays has survived better into the war period than has many another field of physics, but without showing as yet any sign of completing its task of interpretation. It is now generally agreed that the great majority of the particles which are actually observed, be they at sea-level or at great heights in the stratosphere, are not the 'primary' particles which rain down on the earth from interstellar space. The electrons, the mesons or mesotrons, the photons or light-corpuscles of high energy, and many more massive charged particles, which produce phenomena of such bewildering variety, are born in the atmosphere itself; one, two, three or countless stages of transformation may intervene between them and the primaries. The primaries themselves, which a score of years ago were thought to be photons and then were considered to be electrons, are now held to be neither: the most-widely held idea is, that they are protons. The mesons (generally believed to be the very particles which figure in the theory of nuclear forces aforesaid) are held to originate in the upper atmosphere; they are radioactive, and most of them expire by self-conversion into an electron and a neutral particle before they reach sea-level. The fast electrons are presumed to be in part the offspring of mesons, in part they are believed to have existed as bound electrons in the molecules of the air and to have been driven out of these by impacts of mesons. The magnificent phenomenon of the 'cosmic-ray shower' in which literally thousands of electrons (and sometimes mesons and heavier particles also) spring from a small volume of dense matter, has been abundantly studied. Assuming that the energy of all the particles in such a shower was originally borne by a single primary particle, one arrives at quite fantastic figures for the energies of these last, a billion times as great as that of the most energetic particle which has ever yet issued from a cyclotron.

Chemical Physics.

In the field of chemical physics, the present stage may be described as a stage of replacement: that of the replacing of the older concepts of the forces holding atoms together (formerly called by such names as 'cohesion' and 'chemical attraction') and the forces keeping atoms apart (formerly described sometimes as 'repulsive forces,' and sometimes merely by speaking of incompressible or slightly-compressible atoms) by the newer concepts based upon quantum mechanics. In these the electrons form, as it were, the cement which holds the atoms together, whether in simple diatomic molecules such as those of oxygen, or in massive pieces of sodium or silver. This is, of course, what would be expected, but it must be remembered that classical mechanics would forbid any such conclusion, would in fact require that any system of electrons and nuclei be intrinsically unstable. Not only does quantum-mechanics permit molecules and blocks of metals to be stable by virtue of the electrical forces acting among their component particles — it actually prescribes the amount of this stability, i.e., the amount of energy required to dissociate the molecule or vaporize the metal. For many molecules and a few metals the calculated values agree admirably with the observed ones. For many molecules and most metals the calculations are impossibly hard to make, but nevertheless the new way of visualizing the structure is superseding the old way, and the 'valence-bond' so familiar to students of chemistry as a line drawn between the symbols of two atoms, is changing into a group of two or more electrons rapidly moving about in the region between the atom-nuclei. Progress in this field may still continue, as chemists have so far been less diverted from their normal occupations than have physicists.

1940: Physics

Nuclear Physics.

Throughout 1940 the type of physics called 'nuclear' continued to be the most conspicuous; and one of the most portentous events of the year was the Conference on Applied Nuclear Physics held in the autumn in Cambridge. Many a physicist who had thought himself versed in the field was none the less surprised at being confronted with no fewer than one hundred papers, taking an entire week with three simultaneous sessions on most of the days. Very few of the hundred were devoted either to nuclear theory or to description of apparatus, and not a large proportion to the medical uses of radioactivity. Six half-day sessions were absorbed by the study of animal and plant metabolism with radioactive tracers: that is to say, feeding the radioactive isotopes of various elements to animals and plants, and following the wanderings of each through the structure of the organism. As lately as six years ago this was feasible with three elements only, none being of great biological interest (lead, polonium, radium). At this conference were reported researches made with carbon, phosphorus, sulphur, sodium, chlorine, potassium, calcium, manganese, iron, arsenic, bromine, rubidium and strontium, all of these radioactive bodies being recent products of the art of transmutation. Three sessions were devoted to applications of radioactive isotopes in following the trends and measuring the rates of chemical reactions, particularly the 'exchange reactions' which take place when compounds involving chemically-identical atoms are brought into contact with one another in or out of solution, and the atoms of one compound exchange their places with the identical atoms of the other. Radioactive isotopes of magnesium, copper, iodine and mercury were used in these researches, as well as some of the others previously listed. Two sessions were filled with metallurgical applications, especially the study of diffusion of like through like—e.g., the diffusion which occurs when a thin film of radioactive copper is laid down upon the surface of a block of ordinary copper, and the radioactivity spreads gradually through the entire mass. Medical applications though not predominant were not neglected, and the dangers of unwise exposure to radioactive bodies or X-rays or neutrons were the subject of long and grave debate.

Radioactive Isotopes.

Since radioactive isotopes are coming to be so variously useful, deserving mention are two of the most outstanding which were discovered during 1939-1940 and will soon be coming into use: 'hydrogen 3' and 'carbon 14.' The former had been known as a product of transmutation for several years, but was supposed to be stable: in proving it radioactive, Alvarez and Cornog enabled physicists to say now and hereafter that there are radioactive isotopes of every known element. Hydrogen 3 is the lightest and simplest of all radioactive nuclei: it transforms itself into helium 3 by emission of a negative electron. Carbon 14 likewise emits a negative electron, transforming itself into nitrogen. The half-lives of both are remarkably long for artificial radioactive substances (many decades) and the emitted electrons are of feeble energy; but both should be very useful in the study of metabolism and of chemical reactions. Another interesting radioactive nucleus is the first well-attested isotope of 'Element 85,' which requires thirty-million-volt alpha-particles (available only from the newest and greatest cyclotron) to produce it from bismuth. Another is the first well-attested isotope of 'Element 93,' which is prepared by projecting slow neutrons into uranium 238; a fraction of these are caught by uranium nuclei, some of which subsequently emit negative electrons, becoming thus the element in question; McMillan has lately proved that this also is radioactive. The great multitude of radioactive substances engendered by fission of uranium or thorium has still not been fully explored; there are probably a hundred or more, among which certainly no fewer than sixteen different elements are represented. Fission has been produced (at the Westinghouse research laboratory) by gamma-ray photons of great energy. The stable isotope 198 of mercury has been made from gold in sufficient quantity so that its spectrum-lines can be detected, an achievement which may lead to the establishment of a new standard of wavelength.

Neutrons.

The usefulness of neutrons continues to increase. The powerful effects of rapid neutrons upon living tissue are now the object of research in many an institute where physics is combined with medicine or biology. During late 1939 and early 1940 it was announced (by Beyer, Whitaker, Rasetti) that single crystals are much more penetrable to slow neutrons than are polycrystalline masses of the identical substance and equal thickness: in calcite the percentage of transmitted neutrons may be threefold as great. Late in 1940 it was announced (Nix, Beyer, Dunning) that annealed iron-nickel alloys are more penetrable by several per cent than are the same alloys when quenched. The latter difference is ascribed to the fact that the annealed alloys are 'ordered,' the quenched alloys 'disordered,' in the sense in which metallurgists use these words; both differences are traced to the undulatory quality which neutrons, in common with all other forms of matter and of light, possess. The magnetic moment of the neutron has been measured by Alvarez and Bloch. When it is subtracted from the well-known magnetic moment of the proton, the remainder agrees closely with the well-known magnetic moment of the deuteron. Now the deuteron is formed of a proton and a neutron adhering together, so the result agrees beautifully with the simple conception that the two particles adhere with their magnetic moments pointing in opposite directions. Nevertheless the theorists are dissatisfied, for such a simple conception appears untenable to them.

Mesons.

In the cosmic-ray field, it is the mesotron or meson which at present holds the spotlight. The existence of this surprising particle, intermediate in mass between electron and proton (around 150-200 times heavier than the former and 10 times lighter than the latter) is now no longer questioned. Its radioactivity (for like the nuclei just mentioned, it is unstable and resolves its instability by emitting an electron) is now very nearly if not quite beyond question, though as lately as July of 1939 a conference of cosmic-ray specialists at Chicago held it by no means proved. Were it not for this radioactivity, the percentage of mesons absorbed by (say) the three kilometers of air extending between altitudes of 0.5 and 3.5 kilometers in the atmosphere would be no greater than that absorbed by 40 centimeters of lead. Actually the former is considerably greater than the latter, this being proved by comparing the number of mesons underneath a lead plate at the greater altitude with the number in the open air at the lesser altitude (Rossi, Hilberry, Neher, Pomerantz and many others). From the comparisons it follows that the mesons disappear in the air at a rate compatible with an average lifetime of a few microseconds. 'Disappearance' means, in this connection, the separation of the meson into a neutral particle and a free electron; the neutral particles have not been detected, the electrons have been observed but have much less penetrating power than the mesons. Since the lifetime of the meson is so short, one is practically obliged to infer that these particles are created in the upper atmosphere. This has in fact been demonstrated by Jesse, Schein and Wollan of A. H. Compton's school, who sent up sounding-balloons equipped with automatic apparatus for the registration of mesons, and found the number of these particles rising rapidly at first and then reaching a maximum at an altitude where the pressure of the air is less than a tenth what it is at sea-level. Other apparatus demonstrated the productions of mesons at these altitudes by chargeless particles, presumed to be photons. Expansion-chambers taken in airplanes to heights of 30,000 feet exhibit many tracks of indubitable mesons and even of protons, these last now being regarded by some as the true primary cosmic radiation which enters the atmosphere from extra-terrestrial space and engenders all the other kinds of rays.

Pressures.

It would be unfair to yield the whole of this brief notice to nuclear physics. As a counterpoise I quote some of the work of Bridgman with very high pressures, a field in which for many years he has rarely been rivaled and never surpassed. Having three years before extended the range of attainable pressures to 50,000 (kg./cm²) by making the pressure-chamber of tapering form and forcing it like a stopper into a tapering cavity in a thick metal plate, he now in 1940 presents the first large group of data obtained at such pressures, viz., the shrinkages of volume suffered by 6 of the more compressible elements and by 38 compounds. He established new 'fixed points' of the pressure-scale, comparable with the fixed points of thermometry, by measuring the pressure at which mercury freezes at 30°C., and the pressure at which there occurs a 'polymorphic transition' of bismuth marked by a sudden appreciable shrinkage: these are 13,715 and 25,420 respectively. The presence of a piece of bismuth in a compressor acts as the presence of a piece of ice in a system undergoing a gradual heating or cooling: that is to say, when the compressing piston is forced down the pressure ceases to rise as soon as the transition commences, remaining constant at the critical value until all of the lump is transformed. Bridgman then found himself unexpectedly able to extend his pressure-range to and even beyond 200,000, owing to the remarkable increase in the strengths of metals when these are exposed to confining pressure exerted hydrostatically from all sides instead of one side only; this occurs in his newest apparatus, in which advantage is taken of the high confining pressures made available by the construction described above. Sodium chloride experiences a shrinkage of over 20 per cent and sulphur one of 30 per cent, but the long-sought-for transformation of graphite into diamond is still not achieved.

1939: Physics

Nuclear Fission.

It is always perilous to venture what future generations will accept as the weightiest discovery of any particular year, but for physics of 1939 there seems less risk than usual in making a guess: it is, the discovery of 'nuclear fission.' Several hundreds of transmutations had been produced and understood in the twenty previous years, but in no one of them did a massive atomic nucleus divide itself into two nearly equal parts. We now are acquainted with at least four examples of this novel type, for which the name was borrowed from biology where it is used to refer to the division of cells. In these four cases, the 'fissuring' nuclei are formed by the entry and absorption of neutrons into nuclei of the last three elements of the Periodic Table. viz., thorium (atomic number 90, symbol ₉₀Th), Protactinium (₉₁Pa) and uranium (₉₂U). With thorium and protactinium neutrons of high energy are required, and it is thought that the commonest isotope of each element is responsible for the whole effect. With uranium the fission is caused both by fast and by very slow neutrons, and it is thought that two different isotopes are affected, making up the total of four cases.

What was first observed was the formation of many radioactive bodies, which through their chemical properties have been identified as belonging to elements in the middle range of the Periodic Table: among them are isotopes of bromine (₃₅Br), krypton (₃₆r), rubidium (₃₇Rb), strontium (₃₈Sr), yttrium (₃₉Y), tellurium (₅₂Te), iodine (₅₃I), xenon (₅₄Xe), caesium (₅₅Cs), barium (₅₆Ba) and lanthanum (₅₇La). Some of these originate from others of the list, and it is still not clear which pair is directly formed in the initial partition, nor even whether it is always the same pair which is initially formed. Whichever the initial pair, the atomic numbers of its members must add up to the value characteristic of the neutron-bombarded element, e.g., 92 for uranium; for atomic number is proportional to nuclear charge, and the charge of the exploding nucleus must be divided between the fragments. We may as well suppose that uranium splits into barium (56) and krypton (36); for two important consequences can be derived from this assumption which would be equally true for any other fragment-pair, consisting of two elements lying near the middle of the Periodic Table, and having atomic numbers adding up to 92.

The first of these is, that the particles of the pair must jump apart with energies of tremendous motion (on the nuclear scale). This is because the masses of nuclei at rest depend upon atomic number in such a way that the sum of the masses of the uranium nucleus and the absorbed neutron is definitely and markedly greater than the sum of the masses of our hypothetical fragment-pair at rest. This superfluity of mass would according to older ideas be obliged to vanish; but according to the doctrine of relativity, the mass of a body increases with its speed, and we can assume that the fragments are moving just rapidly enough to raise their masses to the degree needed to take care of that apparent superfluity. The requisite speeds correspond to a kinetic energy now estimated at about 180 Mev for the two particles jointly. (One Mev = one million electron-volts = 1.5910^-6 erg.) Particles of such energy can make unprecedented amounts of sudden ionization in an ionization-chamber, and by virtue of this property they were observed in January 1939. They can also leap clear out of the superficial layers of a neutron-bombarded layer of uranium or thorium, and be collected by themselves on a suitably-located plate, which then shows the radioactivity of themselves and their descendants. Estimates of the kinetic energy of the particles can be made from their ionizing power, or from the thickness of air which they are able to traverse, or by thermal measurements upon the bombarded layer; they agree well with the theoretical value.

The second consequence is altogether different and yet flows also from the weights of the hypothetical fragments. Consider the fission of the uranium isotope of mass nearly 238. (Unit of mass: one-sixteenth of the mass of the commonest type of oxygen atom.) The explosive nucleus is formed by the entry into this of a neutron of mass nearly 1, and accordingly has a mass of nearly 239. The fragment-masses must then add up to a value nearer to 239 than to any other integer. The radioactive barium which is observed in the experiments is almost certainly the barium isotope of mass nearly 139, or Ba¹³⁹. If this is one of the fragments the other must be Kr¹⁰⁰. But the heaviest stable isotope of krypton is Kr⁸⁶, and to find an element having a stable isotope of mass nearly 100 we have to go along the Periodic Table all the way from krypton to molybdenum, which with its atomic number of 42 is six elements beyond krypton.

To put this result in general terms: whatever the initial fragment-pair, one at least (and probably each) of its members is bound to be a nucleus 'much too heavy for its atomic number.' But remembering that atomic number is a measure of nuclear positive charge, one sees that by ejecting a sufficient number of negative electrons (six, in the case of Kr¹⁰⁰) the nucleus could elevate its charge to such an extent that the actual mass would be stable for the new value of atomic number. The electrons might come off seriatim with intervals between; in such a case there would be a chain of radioactive substances, each an emitter of negative electrons. Again, the fragment might conceivably shed neutrons until its weight was reduced to a value compatible with its charge; or it might shed neutrons and electrons alternately. Consider also that different fissions may result in different initial fragment-pairs — and one sees that fission may be expected to result in an almost countless multitude of radioactive bodies emitting negative electrons, and also in the release of quantities of neutrons. Both of these expectations are verified by the facts.

The releasing of neutrons might conceivably have a dramatic, even a terrific consequence — the new-made neutrons might produce fresh fissions of nearby nuclei, these in turn fresh neutrons and so onward by cumulation, so that a block of thorium or uranium would be a high explosive liable to be detonated not only by any deliberate bombardment but even by any wandering neutron. If for this it were a sufficient condition that more than one fresh neutron should be produced at every fission, such an explosion would already have happened; for that condition is almost certainly realized (a recent estimate is that 3 to 4 neutrons are released in consequence of every fission). Luckily it is not a sufficient condition; partly because the likelihood of fission depends very much on the energy of the neutrons, and those released in these operations do not initially have the suitable energy; mainly because free neutrons can be captured and made harmless in ways which do not entail anything so violent as fission. It is even possible (and frequent) for a neutron to be captured by a nucleus of uranium 238 in such a way that the resulting nucleus of uranium 239 does not undergo fission at all, but remains in existence for hours or days, then emits a negative electron, and thenceforward is apparently stable for good and all. If the target consists of a mixture of other elements with the uranium, the possibilities of harmless capture are much increased by the others. It is still debatable whether the explosion might occur in an ideally pure block of uranium, or perhaps a block consisting of only one uranium isotope. The consensus of opinion at present is that it would not.*

* The names of some of those important in these discoveries are: Hahn, Strassmann (Berlin); Bohr, Frisch, Meitner (Copenhagen); Joliot, Curie, Savitch (Paris); Dunning, Tuve, McMillan, Abelson (U. S. A.). Some of the radioactive bodies resulting from fission were discovered years ago, but the elements of which they are isotopes were not properly identified and the mode of their formation was quite differently (and quite wrongly, as it now appears) conceived. The discovery of the initial fragments through their ionizing-power was made independently in many laboratories, first at Copenhagen; the isolation of the radioactive bodies by use of the ability of the fragments to leap out of the target was first achieved by Joliot.

Atom Nuclei.

While the phenomena of fission have diverted attention considerably from the more familiar types of transmutation, work on these continues apace with the aid of the ever-growing number of electrostatic generators and cyclotrons. So great a proportion of these is in America, that one may reasonably hope that nuclear physics is not destined to suffer a serious set-back from the war. The latest and greatest of cyclotrons, the new 60-inch built by Lawrence, and his colleagues at Berkeley, has proved itself competent of imparting energies of 25,000,000 electron-volts to deuterons and 50,000,000 to alpha particles. The scattering of beams of fast-moving protons and neutrons by hydrogen — which is to say, by assemblages of stationary protons — has been intensively studied for the information which it is capable of giving as to the short-range forces of attraction between protons and protons on the one hand, neutrons and protons on the other. These are the forces which hold together the constituents of atom-nuclei, and so make possible the existence of atoms.

Many atom-nuclei display the connected properties of magnetic moment (M) and angular momentum (P). There are several methods of ascertaining P, and two of determining M, but none is applicable to all nuclei and there are some nuclei which resist them all, so that knowledge and ignorance in this field have been strangely and unpleasantly mixed. Lately Rabi, with his colleagues Millman and Kusch, has developed a remarkable new method of measuring M/P, applicable to the nuclei of all atoms which can be built into chemical compounds satisfying certain fairly easy requirements as to vapor-pressure and stability. It depends on two facts, first that a nucleus traversing a steady magnetic field of strength H precesses about the direction of the field with a frequency (M/P)H and second that an oscillating field oriented at right angles to the steady field will have a pronounced effect on the nuclei when and only when its frequency of oscillation n is very close to the precession-frequency. The effect in question is a reorientation of the nuclei, and the experiment is so set up that the reoriented nuclei quit the beam in which they were traveling so that a detector set up in the prolongation of that beam reports a sharp falling-off in intensity when n passes through the value MH/P or H passes through the value of Pn/M. Very exact values of M/P have been obtained in this way for both isotopes of hydrogen, lithium, boron, potassium, chlorine and rubidium and for the principal isotope of nitrogen, fluorine, sodium, aluminium and caesium. In many of these cases P was already known, so that a good value of M could at once be computed.

Liquid Helium II.

Recent progress in the study of 'liquid helium II' makes 1939 a good year for describing the qualities of this remarkable fluid. (Another and a sadder reason is, that the description stands a chance of remaining up-to-date for years to come, since the laboratories where nearly all of the work has been done are mostly diverted by now to the uses of war.) When gaseous helium liquefies at 4.2° K. or thereabouts the liquid at first presents no very singular properties, but when it is further cooled it undergoes at 2.19° K. a sudden transformation and becomes a substance which, though dense and mobile as a liquid, behaves as though it were less viscous than any gas and yet a better conductor of heat than solid copper itself. One experimenter (Kapitza) planned to separate two optically-flat surfaces by a narrow gap and let the liquid flow slowly between them, but found that even when the surfaces were actually resting one upon the other the helium slid between them at so rapid a rate that no measurement could be made.

When descending through narrow capillaries (from a few tenths to a few thousandths of a mm. in diameter) under the pull of gravity, this fluid violates all of the laws made familiar by a century of study of ordinary liquids and ordinary gases. For a tube of constant length, the velocity of the fluid increases less rapidly than the pressure-head; for a constant pressure-head the velocity decreases less rapidly than the reciprocal of the length; when tubes of different bores are compared, the velocity passes through a minimum for a certain value of diameter. For the narrowest available tubes (which are really slits between wires squeezed tightly together) the velocity is almost independent of the pressure and the gradient thereof.

Measurements of the transport of heat are also made with columns of liquid helium II contained in narrow capillaries. It is found that the rate of flow of heat along the column is not proportional to the temperature-difference between the end thereof; the ratio of these two quantities is highest when the two have their lowest measurable values, so that people speak of helium II as having a thermal conductivity which decreases with increasing heat-flow. Curious phenomena involving motion occur in these experiments; the liquid helium tends to creep bodily along the capillary in the opposite direction to the heat-flow, and if instead of being in a tube the liquid is perfusing the crevices between the particles of a tightly-packed powder, it may be made to spring in a jet several inches high by transfer of heat to the powder (by shining the light of an ordinary flashlight against it).

The strange facility of motion suggested by some of these observations is displayed very clearly in experiments on beakers or jars with open tops. If such a beaker, empty, is lowered partway into a bowl of liquid helium II, the liquid promptly fills it up until the menisci within and without stand at just the same height; if the partly-filled beaker is now lifted out, the liquid comes out of it rapidly though invisibly, collects in droplets on the bottom and drips back into the bowl. The influx or efflux is at a rate strictly proportional to the periphery of the beaker; it is therefore in a flowing surface-film that the liquid travels. To evaluate the 'speed' of the film in cc. of drained-off liquid per second per cm. of periphery is relatively easy; to evaluate the literal speed in cm/sec requires however a knowledge of the surface-density of the film, and this is obtained by allowing a broad expanse of coiled metal to cover itself with the film and then (when heated) to shed the film into a very narrow calibrated capillary closed at the bottom, in which the liquid derived from the film can be seen and its amount can be estimated. The figure given for the speed is 20 cm/sec at 1.1° K.; it is not dependent on the difference in height between the menisci within and without. It seems quite probable that the curiosities of the passage of the liquid through narrow channels and the passage of heat through narrow columns of the liquid are partly at least attributable to this remarkable type of motion.* See also NAVAL SCIENCE, AMERICAN.

* Some of the outstanding names in this field of research are Onnes, Keesom, Miss Keesom (Leyden); Mendelssohn, Daunt, Simon (Oxford); Allen, Jones, Misener (Cambridge); Burton (Toronto).

1938: Physics

Study of the Atom.

The year 1938 continued to be a close follower of its predecessors in the investigation of the structure of the atom. The chemistry textbooks of 35 to 40 years ago proclaimed the atom to be a solid, indivisible particle. In fact its very name, atom, came from the Greek word meaning indivisible.

When Sir J. J. Thomson and his school began the study of the discharge of electricity through gases, and it began to appear that such conduction could not occur without a dismemberment of the atoms and molecules composing the gases, there was much shaking of heads on the part of those who knew the atom was not divisible; but when the late Lord Rutherford, then Professor Ernest Rutherford and a former student of Sir J. J. Thomson, put forth his theory that it was possible to transmute one element into another, the imagination of man had gotten quite beyond his control — or that was the firm conviction of those who still believed in the immutability of the atom.

Throughout all the years in which extensive investigations have been going on concerning the architectural design of the atom, there has been one common procedure; and that is to disrupt the atom and study its component parts amid the particles composing the wreckage. The atom is divisible into a number of smaller particles, and 1938 has witnessed a real advance in our knowledge of what these particles are which constitute the atom.

In order to make clear just what our procedure in studying atomic structure has been, we could refer to the study of an orange. As this fruit comes to us from the store, it appears a solid, spherical-shaped mass with a yellowish color. We could measure its diameter, we could obtain its mass in grams; but not until we removed its skin, pulled apart its sections and dissected them under the microscope, would we begin to understand something about the nature and structure of an orange. Only as we destroy the orange do we begin to know something about it. It is just this procedure which has been pursued for the most part in studying the atom. Bombard it with high speed projectiles until we have blown it into bits, and then see if these parts possess characteristics distinct from the original and are not just minute fragments like the original.

Belief in the value of research on the design and structure of the atom has led not only commercial organizations but large educational institutions into spending huge sums in the installation of various devices for disrupting the atom.

Atom Smashing.

Essentially these outfits for bombarding and thereby disrupting atoms consist, first, in producing unusually large potential differences or, what amounts to the same thing, intense electric fields by which electrically charged particles may be hurled with enormous speeds.

These high-speed projectiles are then directed toward groups of certain definite atoms, and by means of special devices the products of the dismemberment of the atoms may be studied.

There have been at least three main lines along which investigators have moved in developing means for producing high speeds in electrically charged particles and thereby obtaining high-speed projectiles for bombarding the various atoms of the universe:

(1) Development of the old-fashioned electrostatic generators, such as the Toepler-Holtz and the Wimshurst machines. This development has been largely due to the work of Van de Graaff and his associates. There is just being finished at the Massachusetts Institute of Technology such a machine under the personal supervision of Van de Graaff, from which it is hoped that a potential difference of 15,000,000 volts may be obtained.

(2) High potential differences have also been established by means of step-up transformers connected in series. The General Electric Company at Pittsfield, Mass. and the California Institute of Technology in Pasadena have experimented successfully along this line and have produced potential differences of over 1,000,000 volts.

(3) Perhaps the most popular form of device for speeding up charged particles of matter to serve as projectiles for disrupting atoms is the cyclotron, developed by E. O. Lawrence, of the University of California in Berkeley. In this atom-smashing instrument of Lawrence's the charged particles are started on their paths in a magnetic field which is normal to the path of the projectiles. The magnetic field causes the charged particles to move in an ever-widening spiral, and in cyclic steps the particles are given a boost in their speed by the application of a strong electric field. The final speed will depend upon the number of times the charged particles are accelerated, just as the height to which a child goes in a swing will depend upon how many times and how hard a bystander can give the swinging child a push at just the proper time. This form of electric projectile thrower has been developed in over a dozen educational institutions in the United States.

The Van de Graaff machine commends itself for the steadily acting potential difference and also for the magnitude of the energy available in such machines. The advantage of Lawrence's cyclotron is the ability of the apparatus to impart high speed to charged particles without having to use such high potential differences as do the other devices.

In all of these atom-smashing devices one is impressed quite as much by the technical difficulties which had to be overcome to make them successful atomic catapults as by the brilliant results which have come from their use.

There is a fallacy in following too closely the analogy of destroying the orange in order to find out of what it is composed. It does mean its complete destruction so far as the orange is concerned. In the disintegration of the atom (atom smashing), however, this process is accompanied by one of creation. It is a real case of the transmutation of one element into another. In the death of one atom is the life of another.

Projectiles Used.

Thus far little has been said about the nature of the projectiles used in bombarding the atoms in order to disrupt them. There are five projectiles commonly used:

(1) Gamma rays, or x-rays of high frequency and therefore of short wave-lengths. These rays are the least effective of all those used in atomic bombardment.

(2) Protons, or the nuclei of hydrogen atoms (Protuim). These may be obtained from high-voltage discharge tubes operated by a Van de Graaff machine or from a cyclotron. The Proton is a positively-charged particle.

(3) Deuterons, or the nuclei of heavy hydrogen atoms (Deuteron). They may be obtained in a fashion similar to the protons.

(4) Alpha particles, or nuclei of helium atoms, may be obtained in the same way as protons and deuterons.

(5) Neutrons, or the particles having the mass of a proton but not carrying an electric charge. These are obtained from nuclear reactions when atoms are bombarded with the other projectiles just mentioned.

The negatron and the positron, particles which possess 1/1835 part of the mass of the proton, may also be given high speed in an electric field because the first carries a negative electric charge, and the second an equal positive charge. In both cases the charges are equal to the positive charge carried by the proton. Although able to acquire high speed, neither the positron nor the negatron seems to carry sufficient energy to batter the atoms into their component parts and is, therefore, not an effective agent in the disintegration processes of the atom. The negatron, or negatively charged particle, when joined to the proton forms a hydrogen atom, and together they possess a total mass of M₁₁=1.662 x 10^-24 grams.

The heavy atom of hydrogen is produced by the addition of a neutron to the ordinary hydrogen atom. Consequently, the nucleus of the heavy atom of hydrogen, the deuteron, consists of one proton and one neutron. It is particularly effective as a projectile for disrupting atoms.

Acquaintance with the alpha particle has extended over a comparatively long period of time, being one of the products of radioactive disintegration with which Rutherford worked. The helium nucleus consists of two neutrons and two protons. In the neutral atom of helium, two negatrons are added.

Component Elements of the Atoms.

Having described some of the high-speed projectiles of the physics laboratory, we may now ask ourselves what are the elements into which we decompose an atom? Are there components common to all atoms? The description of the two hydrogen atoms and of the helium atom at once throws light on atomic structure as possessing common factors.

In the very structure of the helium atom, for instance, we see that the projectiles used are themselves components of the atom. How far have we progressed in a complete analysis of the atoms? All we can say at present is that we have confidence in the existence of some of the constituents of the atoms and possess doubts about others. Very early in the studies of discharges of electricity through gases and of radioactivity, we became acquainted with the negatron and the proton as elements of the atom. We felt as our work progressed that all atoms could be reduced to these two building blocks, and the elements seemed then to be simply different aggregations of these two particles.

In the summer of 1932, however, this simple picture of the atoms was rudely shattered by an experiment performed at the California Institute of Technology by C. D. Anderson. He found unmistakable evidence of a third element within the atom, to which he gave the name positron, a particle possessing the mass of the negatron, 1/1835 part of the mass of the proton, and carrying an equal positive charge.

Then came a series of brilliant discoveries in Germany, France, and England, wherein it was found that when beryllium atoms were bombarded by alpha particles from polonium there were, amid the wreckage, particles which possessed the mass of the proton but were devoid of electrical charges. These particles were called neutrons.

Quite recently new evidence has been produced in cosmic-ray studies tending to show that another particle may exist in the atom; in fact, evidence is deduced that there are two of the same mass, but of equal and opposite charges of electricity. Anderson and Neddermeyer of the California Institute of Technology have given photographic evidence for these particles. At first they were called the X particle with a preponderating tendency in the United States to call this unknown particle the baryton from the Greek meaning a heavy electron. The Danish physicist Bohr would call it the Yukon in honor of Yukawa, the Japanese physicist who first postulated the existence of such a particle. The X particle has a mass about 240 times that of the negatron. Furthermore, Heitler of Bristol University, Bristol, England, has postulated another particle whose mass is the same as the X particle but also devoid of an electrical charge.

Some years ago Bainbridge, of Harvard University, claimed experimental evidence for a particle with a mass equal to that of the negatron or the positron, but also without a charge. This discovery appears now without substantiation, but nevertheless from a theoretical point of view was helpful in explaining the effects found in the disintegration of atoms. At the present time there appear to be three classes of particles composing the atom.

From this list it appears that we have 6 definite particles as elements in the atoms which compose our universe. Just how they are joined together in the atom itself is quite beyond the most imaginative picture we can draw today, but surely progress is being made as we find out more and more about the parts which form the whole.

How the Atom Particles May Be Visualized.

How does the physicist know when he finds a new particle? No naked eye, no microscope however powerful can see these particles just described, and so special means must be devised whereby one may experimentally visualize what is going on.

These high-speed particles are said to be ionizers, that is, when such swift projectiles pass through a gas their impacts on the atoms or molecules of the gas break those atoms up into charged particles called ions. This ionization of the gas makes it electrically conducting, because there are free electrical charges in the space through which the high speed particles have moved.

Furthermore, these ions or charged particles in a conducting gas become the nuclei on which droplets of water will form when the point of saturation is reached and condensation occurs.

If now one focuses his attention on one single high-speed particle as it is fired through such a gas, it will leave a trail of these ions in its wake, on which, if proper conditions are imposed, visible droplets of water will form and indicate, when illuminated, the path along which the high speed particle has been shot.

Illumination of this series of droplets makes the trail of the particle readily visible to the naked eye and also possible of being photographed. In this way the paths of the high-speed particles may be mapped and, if such a particle hits an atom head on, various particles will be ejected from the atom at high speed which, in turn, will make their traces and which are peculiar to the particles making them. In this fashion the investigators have learned a great deal about the constituents of the atoms by their behavior in a so-called cloud chamber. From the direction, magnitude and character of these visible paths of the high-speed particles it is possible to infer a great deal regarding the size and quality of the parts which compose an atom. Such in large part has been the program of research in Pure Physics for 1938.

Applied Physics.

The past five years has seen another important development in the field of physics, viz., Applied Physics.

Heretofore, the moment a physicist applied the principles of physics to some particular problems, he ceased to be a physicist and became an engineer.

No better illustration of this new field can be found than in that branch of work called the physics of solids, particularly of metals. The inquiring metallurgist and the physicist have found that the methods of quantum mechanics are able to throw much light on the properties of solids, such as the interatomic, intermolecular forces, which give us a measure of adhesive and cohesive forces.

Problems of Hardness.

When a scientific association numbering over ten thousand members devotes a large part of the program of its annual meeting to the one subject, hardness, that particular subject must take on unusual significance. At the twentieth annual meeting of the American Society for Metals held in Detroit, Oct. 17 to 21, 1938, a large part of its program was devoted to the conditions which influence the hardness of metals, to the methods for measuring hardness and to allied topics on hardness. In fact, starting on the evening of Oct. 19 shortly after 7 P.M., a symposium was held on the general subject of hardness and was continued until after midnight. This program was of interest to physicists as attested by the number present at this meeting of metallurgists primarily. It is a subject in which physicists are interested because hardness is primarily a function of cohesive and adhesive forces.

What is hardness? Everyone knows when a body is hard and when another is soft. Fundamentally, a body is hard when it offers resistance to penetration, say by one's thumb or some stylus agreed upon. Actually, we know very little about hardness if we are thinking in terms of absolute units of measure. We have all sorts of devices and gadgets for measuring hardness. They give us a relative measure of resistance to penetration, but so many factors come into the measurements, that even these relative values have little significance.

In the final analysis we can at least say that these various so-called hardness testers really measure the suitability of a substance for a particular purpose. Thus, in the manufacture of automobiles, hardness criteria become important means for telling whether the quality of the various parts are suitable for the different purposes to which they are to be assigned. The hardness of the chocolate coating on candies tells us whether it will be suitable for shipping without too much breaking down and forming a soft conglomeration at its journey's end.

During 1938 considerable work has been done investigating the physical processes which go on when the indenter of a hardness tester penetrates the surface of the material whose hardness is to be measured.

Microphotographs of the material immediately around the point of contact of the indenter show that a plastic flow or slippage along crystal planes has occurred. The lines of slip are easily discernible in the photographs. The greater the slippage, the greater the indentation and, therefore, the softer the material. One may define the hardness of a solid as its resistance to slippage along slip planes. This involves cohesive and adhesive forces, as previously mentioned. The same processes come into the picture of the phenomenon of creep, and one must distinguish between creep which seems to last over indefinite periods such as one finds in metals at high temperatures and the temporary creep which one finds at ordinary temperature. Marble, glass, and the modern plastics like Bakelite and Leucite are examples of an indefinite period of slippage at slip planes. As the physicist studies these various physical phenomena will he come to know what hardness means? It is a problem of applied physics in which the physicist of today is profoundly interested.

There has been presented in these pages the two broad lines along which Physics is expanding today — the trails of pure and applied physics with illustrations of each.