CHAPTER XVIII

SCIENTIFIC HYPOTHESIS--RADIOACTIVE SUBSTANCES

The untrained mind, reliant on so-called facts and distrustful of mere theory, inclines to think of truth as fixed rather than progressive, static rather than dynamic. It longs for certainty and repose, and has little patience for any authority that does not claim absolute infallibility. Many a man of the world is bewildered to find Newton's disciples building upon or refuting the teachings of the master, or to learn that Darwin's doctrine is itself subject to the universal law of change and development. Though in ethics and religion the older order changes yielding place to new, and the dispensation of an eye for an eye and a tooth for a tooth finds its fulfilment and culmination in a dispensation of forbearance and non-resistance of evil, still many look upon the overthrow of any scientific theory not as a sign of vitality and advance, but as a symptom of the early dissolution or at least of the bankruptcy of science. It is not surprising, therefore, that the public regard the scientific hypothesis with a kind of contempt; for a hypothesis (ὑπόθεσις, foundation, supposition) is necessarily ephemeral. When disproved, it is shown to have been a false supposition; when proved, it is no longer hypothetic.

Yet a page from the history of science should indicate that hypotheses play a rôle in experimental science and lead to results that no devotee of facts and scorner of mere theory can well ignore.

In 1895 Sir William Ramsay, who in the previous year had discovered an inert gas, argon, in the atmosphere, identified a second inert gas (obtained from minerals containing uranium and thorium) as helium (ἥλιος, sun), an element previously revealed by spectrum analysis as a constituent of the sun. In the same year Röntgen, while experimenting with the rays that stream from the cathode in a vacuum tube, discovered new rays (which he called X-rays) possessed of wonderful photographic power. At the beginning of 1896 Henri Becquerel, experimenting on the supposition, or hypothesis, that the emission of rays was associated with phosphorescence, tested the photographic effects of a number of phosphorescent substances. He exposed, among other compounds, crystals of the double sulphate of uranium and potassium to sunlight and then placed upon the crystals a photographic plate wrapped in two thicknesses of heavy black paper. The outline of the phosphorescent substance was developed on the plate. An image of a coin was obtained by placing it between uranic salts and a photographic plate. Two or three days after reporting this result Becquerel chanced (the sunlight at the time seeming to him too intermittent for experimentation) to put away in the same drawer, and in juxtaposition, a photographic plate and these phosphorescent salts. To his surprise he obtained a clear image when the plate was developed. He now assumed the existence of invisible rays similar to X-rays. They proved capable of passing through sheets of aluminum and of copper, and of discharging electrified bodies. Days elapsed without any apparent diminution of the radiation. On the supposition that the rays might resemble light he tried to refract, reflect, and polarize them; but this hypothesis was by the experiments of Rutherford, and of Becquerel himself, ultimately overthrown. In the mean time the French scientist obtained radiations from metallic uranium and from uranous salts. These, in contrast with the uranic salts, are non-phosphorescent. Becquerel's original hypothesis was thus overthrown. Radiation is a property inherent in uranium and independent both of light and of phosphorescence.

On April 13 and April 23 (1898) respectively Mme. Sklodowska Curie and G. C. Schmidt published the results of their studies of the radiations of the salts of thorium. Each of these studies was based on the work of Becquerel. Mme. Curie examined at the same time the salts of uranium and a number of uranium ores. Among the latter she made use of the composite mineral pitchblende from the mines of Joachimsthal and elsewhere, and found that the radiations from the natural ores are more active than those from pure uranium. This discovery naturally led to further investigation, on the assumption that pitchblende contains more than one radioactive substance. Polonium, named by Mme. Curie in honor of her native country, was the third radioactive element to be discovered. In the chemical analysis of pitchblende made by Mme. Curie (assisted by M. Curie) polonium was found associated with bismuth. Radium, also discovered in this analysis of 1898, was associated with barium. Mme. Curie succeeded in obtaining the pure chloride of radium and in determining the atomic weight of the new element. There is (according to Soddy) about one part of radium in five million parts of the best pitchblende, but the new element is about one million times more radioactive than uranium. It was calculated by M. Curie that the energy of one gram of radium would suffice to lift a weight of five hundred tons to a height of one mile. After discussing the bearing of the discovery of radioactivity on the threatened exhaustion of the coal supply Soddy writes enthusiastically: "But the recognition of the boundless and inexhaustible energy of Nature (and the intellectual gratification it affords) brightens the whole outlook of the twentieth century." The element yields spontaneously radium emanation without any apparent diminution of its own mass. In 1899 Debierne discovered, also in the highly complex pitchblende, actinium, which has proved considerably less radioactive than radium. During these investigations M. and Mme. Curie, M. Becquerel, and those associated with them were influenced by the hypothesis that radioactivity is an _atomic property_ of radioactive substances. This hypothesis came to definite expression in 1899 and again in 1902 through Mme. Curie.

In the latter year the physicist E. Rutherford and the chemist F. Soddy, while investigating the radioactivity of thorium in the laboratories of McGill University, Montreal, were forced to recognize that thorium continuously gives rise to new kinds of radioactive matter differing from itself in chemical properties, in stability, and in radiant energy. They concurred in the view held by all the most prominent workers in this subject, namely, that radioactivity is an atomic phenomenon. It is not molecular decomposition. They declared that the radioactive substances must be undergoing a spontaneous transformation. The daring nature of this hypothesis and its likelihood to revolutionize physical science is brought home to one by recalling that three decades previously an eminent physicist had said that "though in the course of ages catastrophes have occurred and may yet occur in the heavens, though ancient systems may be dissolved and new systems evolved out of their ruins, the molecules [atoms] out of which these systems are built--the foundation stones of the material universe--remain unbroken and unworn."

In 1903 Rutherford and Soddy stated definitely their hypothesis, generally known as the "Transformation Theory," that the atoms of radioactive substances suffer spontaneous disintegration, a process unaffected by great changes of temperature (or by physical or chemical changes of any kind at the disposal of the experimenter) and giving rise to new radioactive substances differing in chemical (and physical) properties from the parent elements. The radiations consist of α particles (atoms of helium minus two negative electrons), β particles, or electrons (charges of negative electricity), and γ rays, of the nature of Röntgen rays and light but of very much shorter wave length and of very great penetrating power. It is by the energy inherent in the atom of the radioactive substance that the radiations are ejected, sometimes, in the case of the γ rays, with velocity sufficient to penetrate two feet of lead. It is through these radiations that spontaneous transformation takes place. After ten years of further investigation Rutherford stated that this hypothesis affords a satisfactory explanation of all radioactive phenomena, and gives unity to what without it would seem disconnected facts. Besides accounting for old experimental results it suggests new lines of work and even enables one to predict the outcome of further investigation. It does not really contradict, as some thought might be the case, the principle of the conservation of energy. The atom, to be sure, can no longer be considered the smallest unit of matter, as the mass of a β particle is approximately one seventeen-hundredths that of an atom of hydrogen. Still the new hypothesis is a modification and not a contradiction of the atomic theory.

The assumption that the series of radioactive substances is due, not to such molecular changes as chemistry had made familiar, but to a breakdown of the atom seemed to Rutherford in 1913 at least justified by the results of the investigators whose procedure had been dictated by that hypothesis. He set forth in tables these results (since somewhat modified), indicating after the name of each radioactive substance the nature of the radiation through the emission of which the element is transformed into the next-succeeding member of its series.

_List of Radioactive Substances_

URANIUM α particles Uranium X β + γ Uranium Y β IONIUM α

RADIUM α + slow β Emanation α Radium A α Radium B β + γ Radium C { C{1} α + β + γ { C{2} β RADIUM D } RADIO-LEAD } slow β Radium E β + γ Radium F } Polonium } α

THORIUM α MESOTHORIUM 1 no rays Mesothorium 2 β + γ RADIOTHORIUM α Thorium X α + β Emanation α Thorium A α Thorium B slow β Thorium C { C{1} α { C{2} α Thorium D β + γ

ACTINIUM no rays Radio-actinium α + β Actinium X α Emanation α Actinium A α Actinium B slow β Actinium C α Actinium D α + γ

Even a glance at this long list of new elements reveals certain analogies between one series of transformations and another. Each series contains an emanation, or gas, which through the loss of α particles is transformed into the next following member of the series. Continuing the comparison in either direction, up or down the lists, one could readily detect other analogies.

There is some ground for thinking that lead is the end product of the Uranium series. To reverse the process of the transformation and produce radium from the base metal lead would be an achievement greater than the vaunted transmutations of the alchemists. Although that seems beyond the reach of possibility, the idea has stirred the imagination of more than one scientist. "The philosopher's stone," writes Soddy, "was accredited the power not only of transmuting the metals, but of acting _as the elixir of life_. Now, whatever the origin of this apparently meaningless jumble of ideas may have been, it is really a perfect and but very slightly allegorical expression of the actual present views we hold to-day." Again, it is conjectured that bismuth is the end-product of the thorium series. The presence of the results of atomic disintegration (like lead and helium) has proved of interest to geology and other sciences as affording a clue to the age of the rocks in which they are found deposited.

Before Rutherford, Mme. Curie, and others especially interested in radioactive substances, assumed that atoms are far different from the massy, hard, impenetrable particles that Newton took for granted, Sir J. J. Thomson and his school were studying the constitution of the atom from another standpoint but with somewhat similar results. This great physicist had proved that cathode rays are composed not of negatively charged molecules, as had been supposed, but of much smaller particles or corpuscles. Wherever, as in the vacuum tube, these electrons appear, the presence of positively charged particles can also be demonstrated. It is manifest that the atom, instead of being the ultimate unit of matter, is a system of positively and negatively charged particles. Rutherford in the main concurred in this view, though differing from Sir J. J. Thomson as to the arrangement of corpuscles within the atom. Let it suffice here to state that Rutherford assumes that the greater mass of the atom consists of negatively charged particles rotating about a positive nucleus. The surrounding electrons render the atom electrically neutral.

This corpuscular theory of matter may throw light on the laws of chemical combination. The so-called chemical affinity between two atoms of such and such valencies, which Davy and others since his time had regarded as essentially an electrical phenomenon, seems now to admit of more definite interpretation. Each atom is negatively or positively charged according to the addition or subtraction of electrons. Chemical composition takes place between atoms the charges of which are of opposite sign, and valency depends on the number of unit charges of electricity. Moreover, the electrical theory of matter lends support to the hypothesis that there is a fundamental unitary element underlying all the so-called elements. The fact that elements fall into groups and that their chemical properties vary with their atomic weights long ago suggested this assumption of a primitive matter, _protyl_, from which all other substances were derived. In the light of the corpuscular theory as well as of the transformation theory it seems possible that the helium atom and the negative corpuscle will offer a clue to the genesis of the elements.

What is to be learned from this rapid sketch, of the discovery of the radioactive substances, concerning the nature and value of scientific hypothesis? For one thing, the scientific hypothesis is necessary to the experimenter. The mind runs ahead of and guides the experiment. Again, the hypothesis suggests new lines of research, enables one in some cases to anticipate the outcome of experiment, and may be abundantly justified by results. "It is safe to say," writes Rutherford, "that the rapidity of growth of accurate knowledge of radioactive phenomena has been largely due to the influence of the disintegration theory." The valid hypothesis serves to explain facts, leads to discovery, and does not conflict with known facts or with verified generalizations, though, as we have seen, it may modify other hypotheses. Those who support a hypothesis should bring it to the test of rigid verification, avoiding skepticism, shunning credulity. Even a false assumption, as we have seen, may prove valuable when carefully put to the proof.

The layman's distrust of the unverified hypothesis is in the main wholesome. It is a duty not to believe it, not to disbelieve it, but to weigh judicially the evidence for and against. The fact that assumption plays a large part in our mental attitude toward practical affairs should make us wary of contesting the legitimacy of scientific hypotheses.

No one would deny the right of forming a provisional assumption to the intelligence officer interpreting a cipher, or to the detective unravelling the mystery of a crime. The first assumes that the message is in a certain language, and, perhaps, that each symbol employed is the equivalent of a letter, his assumption is put to the proof of getting a reasonable and consistent meaning from the cipher. The detective assumes a motive for the crime, or the employment of certain means of escape; even if his assumption does not clear up the mystery, it may have value as leading to a new and more adequate assumption.

Henri Poincaré has pointed out that one of the most dangerous forms of hypothesis is the unconscious hypothesis. It is difficult to prove or disprove because it does not come to clear statement. The alleged devotee of facts and of things as they are, in opposing the assumptions of an up-to-date science, is often, unknown to himself, standing on a platform of outworn theory, or of mere vulgar assumption. For example, when Napoleon was trying to destroy the commercial wealth of England at the beginning of the nineteenth century, he unconsciously based his procedure on an antiquated doctrine of political economy. For him the teachings of Adam Smith and Turgot were idle sophistries. "I seek," he said to his Minister of Finance, "the good that is practical, not the ideal best: the world is very old, we must profit by its experience; it teaches that old practices are worth more than new theories: you are not the only one who knows trade secrets." We are not here especially concerned with the question of whether Napoleon was or was not pursuing the best means of breaking down English credit. He did try to prevent the English from exchanging exports for European gold, while permitting imports in the hope of depleting England of gold. But in pursuing this policy he thought he was proceeding on the ground of immemorial practice, while he was merely pitting the seventeenth-century doctrine of Locke against the doctrine of Adam Smith which had superseded it.

According to one scientific hypothesis, "Species originated by means of natural selection, or, through the preservation of favored races in the struggle for life." This assumption was rightly subjected to close scrutiny in 1859 and the years following. The ephemeral nature of the vast majority of hypotheses and the danger to progress of accepting an unverified assumption justify the demand for demonstrative evidence. The testimony having been examined, it is our privilege to state and to support the opposing hypothesis. It was thus that the hypothesis that the planets move in circular orbits, recommended by its simplicity and æsthetic quality, was forced to give way to the hypothesis of elliptical orbits. Newton's hypothesis that light is due to particles emitted by all luminous bodies yielded, at least for the time, to the theory of light vibrations in an ether pervading all space. The path of scientific progress is strewn with the ruins of overthrown hypotheses. Many of the defeated assumptions have been merely implicit errors of the man in the street, and they are overthrown not by facts alone, but by new hypotheses verified by facts and leading to fresh discoveries.

According to John Stuart Mill, "It appears ... to be a condition of a genuinely scientific hypothesis, that it be not destined always to remain an hypothesis, but be of such a nature as to be either proved or disproved by that comparison with observed facts which is termed Verification." This statement is of value in confirming the general distrust of _mere_ hypothesis, and in distinguishing between the unverified and unverifiable presupposition and the legitimate assumption which through verification may become established doctrine.

REFERENCES

J. Cox, _Beyond the Atom_, 1913 (Cambridge Manuals of Science and Literature).

R. K. Duncan, _The New Knowledge_, 1905.

H. Poincaré, _Science and Hypothesis_.

E. Rutherford, _Radioactive Substances and their Radiations_.

F. Soddy, _The Interpretation of Radium_.

F. Soddy, _Matter and Energy_ (Home University Library).

Sir William A. Tilden, _Progress of Scientific Chemistry in our Own Time_, 1913.