the integer which controls the X-ray spectrum is the same as the number of electrical units in the nucleus, and these experiments therefore give the strongest possible support to the hypothesis of van den Broek. Soddy has pointed out that the chemical properites of the radio-elements are strong evidence that this hypothesis is true for the elements from thallium to uranium, so that its general validity would now seem to be established.

RADIO-ACTIVITY AND THE STRUCTURE OF THE ATOM

RADIO-ACTIVITY was discovered by Becquerel, who found that uranium and its compounds gave out a hitherto unknown kind of radiation. M. and Madame Curie, by separating the most active part of certain uranium minerals in which this activity was present to a degree greater than in uranium, isolated the salt of a new and intensely active element which they called radium.

The explanation of the new phenomena was given by Rutherford and Soddy, who pointed out that radio-activity was always accompanied by the formation of new chemical substances, that it depended on the elements present and not on their state of combination, and that the amounts of energy evolved were immensely greater than those associated with any known chemical change. They concluded that radio-activity was due to the spontaneous explosion of atoms into atoms of less atomic weight, and particles shot forth with great velocity. Of these particles some, the so-called a-rays, are helium atoms, and others, the B rays, are identical in nature with the electrons of cathode rays. The line of research thus opened up has led to theories concerning the structure of the atom. Of these the most generally accepted is that chiefly due to Sir Ernest Rutherford, now Cavendish Professor of Physics at Cambridge.

X-rays, light, and the electromagnetic waves used in wireless telegraphy are the same in kind, differing only in the length of the waves and the frequency of the vibrations. The spectra of light and of X-rays are characteristic of the elements from which they arise. Hence they are atomic phenomena, and the parts of the atoms which send them forth must be electrical. We thus reach by another road, opened up by Lorentz and Larmor, the electron discovered by Thomson in cathode rays, and are led to an electrical theory of matter.

Thomson's electron is a negative unit of electricity, and, if it is to be imagined as part of a neutral atom, that atom must contain equivalent positive electricity. The deflexion or scattering of a particles from radio-active substances by collision with atoms indicates that this positive electricity exists as a very minute central nucleus, able to exert very intense forces on the a particles. By these and many other laborious experiments, the innermost structure of the atom has been investigated. An a particle, flying with a speed approaching that of light, is the most intense concentration of energy known to us. If anything can smash an atom into fragments, it would be the impact of an a particle. And Rutherford has now found evidence that some atoms are thus broken up.

The dream of the mediaeval alchemist is realised. Not only do radio-active elements disintegrate spontaneously-that seems beyond human control-but, by a bombardment of a-rays, infinitely little, and yet infinitely more intense in its own minute field than an artillery or high explosive, man can produce these changes at will, and gain at last his age-long desire, the transmutation of the elements.

THE STABILITY OF ATOMS

By SIR ERNEST Rutherford

(Abridged Report of a Lecture delivered before the Physical Society on June 10, 1921.)

DURING the latter half of the nineteenth century it was generally accepted that the atoms of the chemist and physicist were permanent and indestructible, and were uninfluenced by the most drastic physical and chemical agencies available. The existence of elements in the earth that appeared to have suffered no change within periods of time measured by the geological epochs gave a strong support to the prevailing view of the inherent stability of the elements. The discovery at the beginning of the twentieth century that the radio-active elements uranium and thorium were undergoing a veritable transformation, spontaneous and quite uncontrollable by the agencies at our disposal, was the first serious shock to our belief in the permanency of the elements. The essential phenomena which accompanied the series of transformations soon became clear. The disintegration of an atom was accompanied either by the emission of a swift atom of

helium carrying a positive charge, or of a swift electron. With the exception possibly of potassium and rubidium, only the heavy radio-active elements showed this lack of stability. The great majority of the chemical elements appeared, as before, to be inherently stable and to be unaffected by the most intense forces at our disposal.

A number of attempts have been made from time to time to test whether the atoms of the elements can be broken up by artificial methods. Some thought they had obtained evidence of the production of hydrogen and helium in the electric discharge tube. It is, however, a matter of such great difficulty to prove the absence of these elements as a contamination in the materials used that the evidence of transformation has not carried conviction to the minds of the majority of scientific men.

In this lecture an account will be given of some preliminary experiments which indicate the possibility of artificial disintegration of some of the ordinary elements by a new method. Before discussing the results, it is desirable to say a few words on the modern conception of the structure of the atom. The results have been interpreted on the nuclear theory of atomic constitution. According to this view, the atom is to be regarded as consisting of a minute positively charged nucleus, in which most of the mass of the atom is concentrated, surrounded at a distance by a distribution of negative electrons which make the atom electrically neutral. We know that one or more of these outer electrons can be easily removed from the atom. The atom thus undergoes a kind of transformation, but only a temporary one, for the missing electrons are readily recaptured from neighbouring atoms. It seems not unlikely that the whole of the exterior electrons might be removed from an atom without interfering sensibly with the stability of its nucleus. Under suitable conditions, the atom would promptly regain its lost electrons and be indistinguishable from the original atom. In order to produce a permanent transformation of the atom, it is necessary to disintegrate the nucleus. Such a disruption of the nucleus occurs spontaneously in the radio-active atoms, and the processes appear to be irreversible under ordinary conditions.

The nucleus, however, is very small, and its constituent parts are probably held together by strong forces; and only a few

agencies are available for an attack on its structure. The most concentrated source of energy at our command is a swift a-particle, and it is to be expected that an a-particle would occasionally approach so close to the nucleus as to disintegrate its structure. It is, indeed, from a study of the deflexion of swift a-particles in passing through matter that we have obtained the strongest evidence in support of the theory of the nuclear constitution of atoms. In the region surrounding a heavy nucleus, the inverse square law holds for the forces of repulsion between the charged a-particle and positively charged nucleus. The particle describes a hyperbolic orbit round the nucleus, and the amount of its deflection depends on the closeness of its approach. It is from a study of this scattering of a-particles, combined with Moseley's interpretation of the X-ray spectra of the elements, that we know the magnitude of the resultant positive charge on the nucleus. This charge, in fundamental units, is equal to the atomic or ordinal number of the element, and varies between 1 for hydrogen and 92 for uranium. Recently Chadwick has shewn by direct measurements of the scattering of a-particles that the charge on the nucleus is in close accord with Moseley's deduction, and has thus verified the correctness of this fundamental conclusion.

Some information about the dimensions of the nucleus can be obtained by studying the amount of scattering of a-particles at large angles by different atoms. The general results indicate that the nucleus of a heavy atom, if assumed spherical, cannot have a radius greater than 6 x 10-12 cm. It is not unlikely that the dimensions may be smaller than this. No doubt the size of a nucleus decreases with its atomic mass, and it is to be expected that the nucleus of the light elements should be smaller than for the heavy atoms. It is thus clear that the volume occupied by the nucleus is exceedingly small compared with that occupied by the atom as a whole.

A direct collision of an a-particle with this minute nucleus is thus a rare occurrence. It can be estimated that even in the case of heavy elements only one a-particle in about 10,000 makes a close collision with the nucleus. On account of the powerful repulsive field of the latter, the a-particle may either be turned back before reaching the nucleus, or be so diminished in energy that it is unable to effect its disruption. The case of

the lighter elements, however, is much more favourable; for the repulsive forces are so much weaker that we may expect the a-particle to enter the nucleus structure without much loss of energy, and thus to be an effective agent in promoting the disintegration of the atom.

One of the most interesting cases to consider is that in which an a-particle (helium nucleus) collides with the nucleus of a hydrogen atom. Marsden showed that in some cases the H-atom is set in such swift motion that it can be detected by the scintillation produced on a zinc sulphide screen. The maximum speed obtainable is 1.6 times that of the incident particle, and such a swift H-atom is able to travel four times as far as the a-particle before being stopped. For example, the maximum range of a H-atom set in motion by an a-particle from radium C-range 7 cm. in air-corresponds to 29 cm. of air.

A close examination of the production of swift H-atoms by this method showed that the number was about 30 times greater than that to be expected if the colliding nuclei behaved as point charges repelling each other according to the inverse square law. This, and other observations, show that the law of the inverse square ceases to hold in such intense collisions, where the closest distance of approach is of the order 3 x 10-13 cm. It is probable that this distance is comparable with the actual dimensions of the structure of the a-particle itself. Some recent experiments by Chadwick and Bieler indicate that there is an abrupt change in the law of force at distances of about 5 × 10-13 cm. So far, no definite evidence has been obtained as to the nature of these forces which arise in the close collisions between nuclei. Attention should be directed to the enormous intensity of the electrical forces that come into play in such close collisions-forces much greater than can be brought to bear on an atom by ordinary laboratory methods. Unless the nucleus is a very stable structure, it is to be anticipated that it should be greatly disturbed, if not disintegrated, under the influence of such intense forces.

We must now consider the experiments which indicate that some of the lighter elements can be disintegrated by the action of a-particles. When a stream of a-particles is passed through dry air or nitrogen, a number of scintillations are observed far beyond the range of the a-particle. These scintillations are due