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.