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.