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.