Radium-F ? 143 days alpha
...
Lead 207 ? no rays
As soon as the transmutation theory of radio-activity was accepted,
it became natural to speculate about the intimate structure of the
radio-active atoms, and the mode in which they broke up with the
liberation of some of their store of internal energy. How could we
imagine an atomic structure which would persist unchanged for long
periods of time, and yet eventually spontaneously explode, as here an
atom and there an atom reached a condition of instability?
The atomic theory of corpuscles or electrons fortunately was ready to
be applied to this new problem. Of the resulting speculations the most
detailed and suggestive is that of J.J. Thomson. ("Phil. Mag." March,
1904.) Thomson regards the atom as composed of a number of mutually
repelling negative corpuscles or electrons held together by some central
attractive force which he represents by supposing them immersed in a
uniform sphere of positive electricity. Under the action of the two
forces, the electrons space themselves in symmetrical patterns, which
depend on the number of electrons. Three place themselves at the corner
of an equilateral triangle, four at those of a square, and five form
a pentagon. With six, however, the single ring becomes unstable, one
corpuscle moves to the middle and five lie round it. But if we imagine
the system rapidly to rotate, the centrifugal force would enable the
six corpuscles to remain in a single ring. Thus internal kinetic energy
would maintain a configuration which would become unstable as the energy
drained away. Now in a system of electrons, electromagnetic radiation
would result in a loss of energy, and at one point of instability we
might well have a sudden spontaneous redistribution of the constituents,
taking place with an explosive violence, and accompanied by the ejection
of a corpuscle as a beta-ray, or of a large fragment of the atom as an
alpha-ray.
The discovery of the new property of radio-activity in a small number of
chemical elements led physicists to ask whether the property might not
be found in other elements, though in a much less striking form. Are
ordinary materials slightly radio-active? Does the feeble electric
conductivity always observed in the air contained within the walls of
an electroscope depend on ionizing radiations from the material of the
walls themselves? The question is very difficult, owing to the wide
distribution of slight traces of radium. Contact with radium emanation
results in a deposit of the fatal radium-D, which in 40 years is but
half removed. Is the "natural" leak of a brass electroscope due to
an intrinsic radio-activity of brass, or to traces of a radio-active
impurity on its surface? Long and laborious researches have succeeded in
establishing the existence of slight intrinsic radio-activity in a
few metals such as potassium, and have left the wider problem still
unsolved.
It should be noted, however, that, even if ordinary elements are not
radio-active, they may still be undergoing spontaneous disintegration.
The detection of ray-less changes by Rutherford, when those changes
are interposed between two radio-active transformations which can
be followed, show that spontaneous transmutation is possible without
measureable radio-activity. And, indeed, any theory of disintegration,
such as Thomson's corpuscular hypothesis, would suggest that atomic
rearrangements are of much more general occurrence than would be
apparent to one who could observe them only by the effect of the
projectiles, which, in special cases, owing to some peculiarity of
atomic configuration, happened to be shot out with the enormous velocity
needed to ionize the surrounding gas. No evidence for such ray-less
changes in ordinary elements is yet known, perhaps none may ever be
obtained; but the possibility should not be forgotten.
In the strict sense of the word, the process of atomic disintegration
revealed to us by the new science of radio-activity can hardly be called
evolution. In each case radio-active change involves the breaking up of
a heavier, more complex atom into lighter and simpler fragments. Are
we to regard this process as characteristic of the tendencies in accord
with which the universe has reached its present state, and is passing
to its unknown future? Or have we chanced upon an eddy in a backwater,
opposed to the main stream of advance? In the chaos from which the
present universe developed, was matter composed of large highly complex
atoms, which have formed the simpler elements by radio-active or
ray-less disintegration? Or did the primaeval substance consist
of isolated electrons, which have slowly come together to form the
elements, and yet have left here and there an anomaly such as that
illustrated by the unstable family of uranium and radium, or by some
such course are returning to their state of primaeval simplicity?