The Infancy of Atomic Physics

Hercules in His Cradle

Alex Keller

• Publisher: Dover Publications
• Year: 2013
• ISBN:
  9780486149950
• Topic: Physical Sciences
• Subtopic: Nuclear Physics

Atomic physics is a mighty Hercules that dominates modern civilization, promising immense reserves of power but threatening catastrophic war and radioactive pollution. The story of the atom's discovery and the development of techniques to harness its energy offers fascinating insights into the forces behind twenty-first-century technology. This compelling history portrays the human faces and lives behind the beginnings of atomic science.
The Infancy of Atomic Physics ranges from experiments in the 1880s by William Crookes and others to the era just after the First World War, when Rutherford's first speculations on the structures of the atomic nucleus led to the discovery of the neutron -- and thus to nuclear weapons and nuclear power. It describes the dramatic researches as they were made, and it shows how they were interpreted in the scientific language of their time. This survey not only depicts the impressions of leading scientists like Thomson, Rutherford, Einstein, and Bohr, but it also reflects the views of ordinary laboratory scientists as well as the ways in which innovations were introduced to the wider public.

1

MYSTERIOUS ATOM IN MYSTERIOUS ETHER

‘I was brought up to look at the atom as a nice, hard fellow, red or grey in colour according to taste’ said Rutherford once. In colonial New Zealand in the 1880s the atom of the schoolroom had not changed all that much since the days when Newton wrote that matter had probably been formed in the beginning ‘in solid, massy, hard, impenetrable, movable particles’, cannoning off one another in an endless cosmic snooker game. For they were ‘incomparably harder than any porous bodies compounded of them; even so very hard as never to wear or break in pieces; no ordinary Power being able to divide what God himself made one in the first Creation’. These indivisible indestructible atoms were summoned in the seventeenth century from the oblivion in which they had lain since the decline of the Greek philosophy which had first posited them, and they were recalled for much the same purposes as those ancient Greeks had originally conceived them. They were to provide a model of the structure of matter which would enable us to reduce the multifarious qualitative changes of the world to the movements of bodies devoid of all qualities whatsoever save those which can be expressed geometrically. So all the variations and alterations of colour, temperature, texture and shape, growth and decay, action and reaction can be reduced to spatial ones. Behind this lies a problem that has troubled minds since the first Ionian philosophers began to query nature, if not before; what Plato called the opposition between the Same and the Different. What is it that survives unperturbed through all the manifold fluctuations and flickerings of nature? Is there some substrate that does not change, whose movements can explain the variety and instability that we see? The arguments of early atomic physics mark one stage in this ancient debate—but it is only a stage upon the road. Einstein’s vain search for a unified field theory, which occupied much of the latter part of his life (to his friends’ regret) is also part of this quest for an underlying unity, so too is the hunt for the elusive quark or some other sub-sub-particle, which will make sense and Sameness out of the excessive Difference of all our present particles.

Newton made old atomism more realistic by introducing the concepts of mass and force, in addition to size and shape which were all that his ancient predecessors had at their disposal. A hundred years after Newton, John Dalton gave fresh meaning to the atom: he showed how physical sense could be made of the new chemical elements, if it be supposed that these elements, by definition forms of matter that defeat analysis, are composed of Newton’s indestructible atoms, each element having its own particular kind of atom. Then it should be possible to calculate atomic weights by relating the different weights of equal volumes of those elements. By the end of the nineteenth century an immense amount of successful work in chemistry seemed to rest upon this view of atoms, differing in weight alone, yet resisting absolutely any attempt to break them up.

Consider the Periodic System, which helped so wonderfully to impose a straightforward pattern on the confusion of elements. When Dalton’s idea was first accepted, there were about three dozen elements. By 1860 the number had nearly doubled, and as the century drew on more continued to be discovered. Many chemists hoped to find some principle of classification that would reduce the variety. During the early 1860s, laws of octaves, zigzags, and spirals were proposed, to express the repetition of certain characters in groups of elements. Perhaps all had evolved from seven or eight original types, which would match in the organic world that evolution of organisms, whose laws were even then being demonstrated by Darwin and his friends. Among those influenced by such objectives was Dmitri Mendeleev, appointed Professor of Chemistry at Saint Petersburg in 1867. In his new post, he was impressed with the need for a logical system to teach his students about elements. In March 1869 he suddenly found his key, arranging them in order of atomic weight; at every eighth place there was a periodic return to the same chemical properties, particularly those forming compounds with other elements. When Mendeleev was able to predict three elements to fill gaps in his table, which were duly discovered, and possessed the weights and properties he said they should, the utility of his scheme was made obvious. Its reputation was confirmed by his insistence that the weights of some of the elements already known would have to be adjusted in order to fit; for fresh measurements were to prove his estimates right. But was this system merely useful—an aid to discovery, and an aid to memory? Since it depended on weights—and so on mass—it might make more sense if some of the heavier elements were compounds. Yet how could a certain increase of mass bring an element back into line with one much lighter? If chemistry depends on the indivisibility of elements, it must equally depend on the indivisibility of atoms, a ne plus ultra beyond which no man can pass.

Still, however convenient, the atom remained something of an ad hoc proposition. Its very existence was debated throughout the nineteenth century. By the standards of the day no one could produce direct evidence that there were any such things. They might help us picture the basic unities of nature to the mind’s eye—but were they really there? A few were bold enough to argue that all such talk was but a name to cover our ignorance. Relative weights and proportions we know, so they claimed, but not the nature of these atoms, which as individuals we can never perceive. Some still hankered after a prime matter, and thought the larger atoms would turn out one day to be compounds of the smaller. At the outset of the century a London physician, William Prout, had proposed that all were aggregates of hydrogen, the lightest element, but his hopes were disappointed when more precise analysis showed that atomic weights could not be whole number multiples of hydrogen. Whatever might be chosen as the unit, every atomic weight involved a fraction. There might be a remote chance that some lighter form of matter would be discovered, of which hydrogen itself, and all the other elements, would prove to be constructed. So models of the atom were put forward which were more sophisticated than the kind of red or grey (i.e. black and white), billiard ball picture called up by Rutherford’s recollection. The traditional view was certainly still accepted by most scientists, and taken for granted by ordinary educated people who thought they understood the world as science taught it. In their eyes atoms were ‘infinitesimally small, but still finite units of matter impenetrable, indivisible and endowed with enormous energies . . . floating like buoys in an ocean of ether’.¹ So Samuel Laing, ex-MP, ex-railway magnate described the atom, when in old age, in 1889, he sat down to expound his rationalist and agnostic philosophy of man and the universe. For such men, this atom was the sure foundation of material things in an uncertain world. True, he knew of alternatives, and in the end the real nature of the atom remained for him a ‘problem for the future’, still somewhat hard to comprehend.

But the great Maxwell, asked to explain the basic principles of physics for the Encyclopaedia Britannica in 1875, declared that in the atom ‘we have something which has existed either from eternity or at least from times anterior to the existing order of nature’. Indeed the constant masses of each particular kind of atom, and their constant relationships with others, to form molecules, suggested to him the molecules too were permanent. No atoms are now being formed; none now break down. They neither come into being nor change their shape or size; ‘till not only these worlds and systems, but the very order of nature itself is dissolved, we have no reason to expect’ that fresh ones will be manufactured. No less constant were they in their vibrations, and their effects upon others. How could they be made up from random agglomerations of smaller parts, if all were so completely identical, ‘like the nuts and screws of some locomotive or gun factory’, as Laing put it. All the same, could atoms somehow have an internal structure which might explain the details of physics and chemistry? A handful of eccentrics thought so. By now, the number of elements had grown to over eighty. If each had its own atom, how were they distinguished? If an atom of gold is almost four times the weight of an atom of iron, and differs in that alone, is it not four times the size? Why then is it impossible to break down the gold, which can easily be imagined as split into four? Or, of course, assemble a gold atom out of those of iron . . . No, said the great majority, let us dismiss such teasing notions as the fond dreams of alchemy, unsuited to this age of rational common sense.

On Thursday 2 September 1886, the Chemical section of the British Association for the Advancement of Science sat down to hear their President deliver his address. They could expect an exciting display of imagination and agility from William Crookes—editor of Chemical News, director of the Electric Light and Power Company, analyst of London’s water supply, gold-mine owner, fertile inventor, speculative businessman, ardent Theosophist and leading member of the Society for Psychic Research. Always ready with ingenious ways to exploit theoretical discoveries, he was by far the most flamboyant character in British science. With the waxed points of his long moustachios perpendicular over his square grey beard, his very appearance was more showman or magician than sober scientist. His enterprises, like his hypotheses, often failed. Enough succeeded to give him a handsome livelihood, and even left him free to dash in where academics feared to tread. From such a president the audience could hope for fireworks; he always spoke in a blaze of florid metaphors and literary quotations from high romantic verse. They were not disappointed. As the orderly array of atomic elements had grown so very complicated, he began, might not most of them be modifications of some simpler nature? Otherwise confusion would continue to grow worse—or rather, as such a prosy statement was not in Crookes’s style, ‘they extend before us as stretched the wide Atlantic before the gaze of Columbus, mocking, taunting and murmuring strange riddles, which no man has yet been able to solve’. Lamentably he had no evidence that any of them had yet been transmuted to another, but still he felt sure that they were complex. Perhaps if no known elements could be added together to make up the remainder, there was some element of negative weight, possibly the ‘etherial fluid’, which could be subtracted when necessary. Or it may be helium, then known only in the solar spectrum, would turn out to have a weight only a fraction of that of hydrogen. Crookes looked with a kindly eye on these and similar fancies. But the nub, as he saw it, was to conceive this aggregation as an evolution on best Darwinian lines.

In the beginning, so his vision ran, some basic stuff existed—the ‘protyle’ or first matter—in an ultra-gaseous state. ‘This vast sea of incandescent mist’ would be indescribably hot; but then some process akin to cooling makes the scattered particles of the ‘fire mist’ come together. As their coherence requires energy, they must drain it from their neighbourhood, and so chill it down still further. So the protyle over long ages has gradually hardened, first into the lighter elements, successively into the heavier ones. Where they cooled slowly, these elements would be quite distinct. But when they did so rapidly, a group would be born with a close family resemblance, such as the iron-nickel-cobalt group, a little like Darwin’s animal genera, for those that have numerous species resemble one another more than do the species of genera with few.

Fig. 2. William Crookes with Crookes tube.

Crookes’s prime example was the cluster of rare earths. Over the years since Mendeleev had published his table, several had been discovered. No other elements so upset its tidiness, for they just did not fit his columns properly: all were too alike; were to be found only in samples of a handful of rare minerals dug out of a few sites; and no one could guess how many there might be. Crookes himself had devoted much of his time to frustrated efforts to sort them out. Almost every year somebody claimed to have discovered a new one. Usually, he was wrong. Now Crookes could explain this as the curious effect of particularly rapid refrigeration, so these earths had not had time to become true species. The ores which contain them are ‘the cosmical lumber room where elements in a state of arrested development . . . are aggregated’—he means, like chemical Neanderthal men, surviving in odd corners like some Guyanese plateau of Conan Doyle’s Lost World.

As well as the loss of heat, the formation of matter would also bear the impress of electric force, if you imagine that as you descend (in the cooling) you also move from side to side (as in a spiral staircase), swinging across a mean and so varying in magnetic and electric characteristics (more or less electropositive or electronegative in sequence along the horizontal line); having also chemical valency, or combining power of one, two, or three. One complete swing of the pendulum, out to each extremity and back again, would give you ‘the elements of water, ammonia, carbonic acid, the atmosphere, plant and animal life, phosphorus for the brain, salt for the seas, clay for the solid earth . . . nitrates, fluorides, sulphates . . . sufficient for a world and inhabitants not so very different from what we enjoy at the present time. True, the human inhabitants would have to live in a state of more than Arcadian simplicity and the absence of calcic phosphate would be awkward as far as bone is concerned. But what a happy world it would be! No silver or gold coinage, no iron for machinery, no platinum for chemists, no copper wire for telegraphy, no zinc for batteries, no mercury for pumps, and alas! no rare earths to be separated.’ These heavier elements were most prominent in recent invention, perhaps because relatively rare. A world could be made of the first sixteen elements—so perhaps the rest came after them, and derived from them? His evolutionary picture implied there might be ‘pseudoelements’ like Darwinian varieties—that have not achieved true elementary status. In the process of development, those which were chemically and electrically viable would absorb those immediately lighter or heavier than themselves. Then the atoms of each would not all be of the one weight, but an average of the whole class of particles whose weights were similar, but not identical. For this, he believed he had some very positive evidence. This lecture was Crookes at his most sparkling, and in its paragraphs are hidden many of the prepossessions of the years to come, as yet still in embryo. Not that Crookes ever abandoned his offspring: he kept on at his great idea. Thirteen years later, he was still explaining how the ‘vis generatrix when full of primal vigour, as it swept to and fro in a universe of protyle and descended the irreversible spiral, segregated at suitable positions’ along its way: first came hydrogen and all the lighter elements, until in the end ‘at its lowest round with cooling temperature and failing energy, it could put forth nothing better than thorium and uranium’. He very quickly had to change his tune about the enfeebled character of the heavy elements—they packed in far more than the light ones. But the idea of evolution from the simplest form remained in the background, only waiting its turn to step forward again.

To the student of physics another concept that took shape during the 1840s and 1850s was much more important than these atomic riddles, for it obliged us to adopt a basic change in the way we look at the universe and all that is in it. The conservation and transformation of energy and the increase of entropy—the laws of thermodynamics—have a long history, and their formation in acceptable terms involves a complicated story, one worse complicated by the bitter priority disputes in which several leading personalities engaged. Let others unravel what Helmholtz, Clausius, Mayer, Joule, Kelvin, Rankine, Carnot, Clapeyron contributed. But—whoever said so first—by 1870 it could be agreed that energy is conserved through all its transformations, so that in the processes of nature it is neither created nor passes away; mechanical energy may be translated into heat energy; heat has an exact mechanical equivalent; thus is made firm ‘the connexion between the science of heat and the theorems of dynamics’. No less certain was the one-way movement of available energy. Heat cannot flow from a colder body into a hotter one, although there does not seem to be an obvious kinetic parallel to this irreversibility. While the second law of thermodynamics was acknowledged to be as true and as consequential as the first, there was more hesitation over its meaning. Electrical motion and chemical reactions were but other forms of energy, which must be equivalent too. Perhaps vital force was just another manifestation, subject to the same laws but different in its mode of operation, as the electromotive differs from the chemical.

Planck describes how the principle of the conservation of energy hit him like a revelation. The power of an absolute and universal law enthralled him. His teacher at school explained the relation between kinetic and potential energy by a little joke about a bricklayer, who strains to raise his hod of bricks to the roof of some high building, where the kinetic energy ‘does not get lost; it remains stored perhaps for many years, undiminished and latent’ in the structure, until in the course of time it is loosened, potential energy becomes kinetic again—it slips out and falls on the head of a passer-by. Some people have complained about the antique sound of ‘potential energy’—it smacks too much, they say, of Aristotle’s view of motion as the realization of a potentiality. J.J. Thomson found the concept troublesome when he learnt about the conservation of energy at Manchester in the early 1870s: ‘I found the idea of kinetic energy being transformed into something of quite a different nature very perplexing’. Surely all energy is one, its transformation simply ‘the transference of kinetic energy from one home to another, the effects it produces depending on the nature of its home’. As his friend Poynting put it, ‘we see a pattern of many colours—one thread disappears, then one of a different hue under the surface of the cloth appears in its place—but is it really one thread which now exposes to us a different face?’ Perhaps the energy laws have now become something to be learnt and utilized, something which literati ought to be made to know about. A hundred years ago they were novel, exciting, triumphant, a vision of those principles which underlie the multiformity of the world. As so often in science they also provided a fresh set of problems for the next generation to tackle.

If physics now had energy and chemists the periodic system to unify their respective disciplines, another achievement of those mid-Victorian years stretched out like a panhandle to join the territory of physics to the land of chemistry . . . or at least to its gaseous provinces: the new kinetic model of gases, resting upon the application of statistics. Where it would be quite impossible to measure or observe the behaviour of any individual, general but precise ideas can be formulated about the average conduct of large numbers, which thus establish the probability of any particular set of events. By introducing this kind of statement in the 1860s, Maxwell opened a route to a new approach to physics. The kinetic theory of gases—then often called the dynamic or molecular theory—made it possible to subsume the gas laws in a general explanatory picture. Over the two centuries since Robert Boyle and his experiments on the springiness of the air, these laws had evolved so as to link in simple equations the volume of a gas to its pressure and temperature. If we now suppose gases to be composed of an immense crowd of molecules, buzzing like a swarm of bees in a box, ‘in a state of intensely rapid motion in all directions . . . very far apart in proportion to their size and continually coming into contact with each other’, they will rebound without loss of motion, because perfectly elastic, explains A.R. Wallace, who saw this as one of the prime discoveries to be related in his book on The wonderful century. As a gas becomes hotter, its molecules move the faster, which increases the pressure of the bombardment. Through this doctrine it should be possible to understand the elasticity of gases and the process of their diffusion; to show how they will expand into as much space as they are permitted, spattering the walls of any body that encloses them with a continuous shower of molecular missiles; and see why two gases will mix so perfectly, even though they are of different density. From such a theory it should prove feasible to calculate the size of the molecules, their ‘mean free path’ between collisions (Wallace gives a value of two hundred times the diameter of the molecules, with eight thousand million collisions a second at room temperature, in every cubic inch), the number of them in a given volume, and above all their average velocity at a given temperature. Then the idea should also apply to liquids and solids. For if the evaporation of gases from liquids is effected by heating the...

Table of contents

• DOVER SCIENCE BOOKS
• Title Page
• Copyright Page
• PREFACE
• Table of Contents
• INTRODUCTION
• 1 - MYSTERIOUS ATOM IN MYSTERIOUS ETHER
• 2 - WHY AT THAT TIME? WHY IN THOSE PLACES?
• 3 - CONTROVERSY OVER THE CATHODE
• 4 - LIGHT INVISIBLE
• 5 - LIGHT EVERLASTING
• 6 - ‘THESE ARE THE DAYS OF RAYS’
• 7 - ENERGY FRAGMENTED, MATTER DISSOLVED
• 8 - INTO THE CORE OF THINGS
• 9 - NIELS BOHR
• 10 - ATOM AND RADIATION RECONCILED, IN DISCONTINUITY
• 11 - VERY PRETTY—BUT WILL IT WORK?
• 12 - HOW MANY ELECTRONS?
• 13 - EQUAL NUMBERS—BUT UNEQUAL WEIGHTS?
• 14 - WAR AND DISINTEGRATION
• BIBLIOGRAPHY
• INDEX