Ernest Rutherford

“ I think we may conclude that the present rate of loss of heat of the earth might have continued unchanged for long periods of time in consequence of the supply of heat from radio-active matter in the earth. It thus seems probable that the earth may have remained for very long intervals of time at a temperature not very different from that observed to-day, and that, in consequence, the time during which the earth has been at a temperature capable of supporting the presence of animal and vegetable life may be very much longer than the estimate made by Lord Kelvin from other data. ”

“ This minute but massive nucleus is, in a sense, a world of its own which is little, if at all, influenced by the ordinary physical and chemical forces at our command. In many respects, the problem of nuclear structure is much more difficult than the corresponding problem of the arrangement and motions of the planetary electrons, where we have a wealth of available information, both physical and chemical, to test the adequacy of our theories. The facts known about the nucleus are few in number and the methods of attack to throw light on its structure are limited in scope. ”

“ It seems likely therefore that the cause of the disruption of the atoms of the radio-elements and their products resides in the atoms themselves. According to the modern views of the constitution of the atom, it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be. In accordance with the hypothesis of J. J. Thomson, it may be supposed that the atoms consist of a number of small positively and negatively charged particles in rapid internal movement, and held in equilibrium by their mutual forces. ”

“ There is good reason to believe that in the majority of cases, a single helium atom is expelled during the atomic explosion. This is certainly the case for radium itself and its series of products. On the other hand, Bronson has drawn attention to certain cases, viz. the emanations of actinium and of thorium, where apparently two and three atoms of helium respectively are expelled at one time. No doubt these exceptions will receive careful investigation in the future. It is of interest to note that uranium itself appears to expel two α-particles for one from each of its products. ”

“ From a study of the scattering of α particles by the atoms of matter and from the classical researches of Moseley on X-ray spectra, we know that the resultant positive charge on the nucleus of any atom, in terms of the fundamental unit of electronic charge, is given numerically by the atomic or ordinal number of the element, due allowance being made for missing elements. We know that with few exceptions all nuclear charges, from 1 for the lightest atom, hydrogen, to 92 for the heaviest element, uranium, are represented by elements found in the earth. ”

“ No radio-active substance, left to itself, can continue to radiate indefinitely; it must ultimately lose its activity. In the case of bodies like uranium and radium, the loss of activity is so slow that no sensible alteration has been observed over a period of several years, but it can be deduced theoretically that the activity of radium will eventually decrease to half value in a period of about 1000 years, while in the case of a feebly radio-active substance like uranium, more than a 100 million years must elapse before the diminution of the activity becomes appreciable. ”

“ We have so far dealt with collisions of an α particle with a heavy atom. We know, however, from the results of Rutherford, Chadwick and Bieler that in a collision of an α particle with the lightest atom, hydrogen, the law of the inverse square breaks down entirely when swift particles are used. Not only are the numbers of H nuclei set in swift motion much greater than is to be expected in the simple-point nucleus theory, but the change of number with the velocity of the α particle varies in the opposite way from the simple theory. ”

“ No such effect, however, has yet been observed in any case of radio-active change, where none of the active products produced are allowed to escape from the system. The rate of decay is unaltered by any chemical or physical agency, and in this respect the changes in radio-active matter are sharply distinguished from ordinary chemical changes. For example, the rate of decay of activity from any product takes place at the same rate when the substance is exposed to light as when it is kept in the dark, and at the same rate in a vacuum as in air or any other gas at atmospheric pressure. ”

“ It is, however, at first sight rather surprising that no evidence of the individual existence of protons in a nucleus is obtained from a study of the transformations of the radioactive elements, where the processes occurring must be supposed to be of a very fundamental character. As far as our observations have gone, electrons and helium nuclei, but no protons, are ejected during the long series of transformations of uranium, thorium and actinium. One of the most obvious methods for determining the structure of a nucleus is to find a method of disintegrating it into its component parts. ”

“ The ions which are produced in gases at ordinary pressure have an apparent size, as determined from their rates of diffusion, large compared with the molecule of the gas in which they are produced. The negative ion consists of an electron with a cluster of molecules attached to and moving with it; the positive ion consists of a molecule from which an electron has been expelled, with a cluster of molecules attached. At low pressures under the action of an electric field the electron does not form a cluster. The positive ion is always atomic in size, even at low pressures of the gas. ”

“ If a thick layer of radio-active matter is employed, only the α particles from the surface have a range a. Those which reach the surface from a depth d have their range diminished by an amount ρd, where ρ is the density of the radio-active matter compared with air. This is merely an expression of the fact that the absorption of the α rays is proportional to the thickness and density of matter traversed. The rays from a thick layer of active matter will thus be complex, and will consist of particles of different velocity whose ranges have all values between 0 and a. ”

“ In comparing the uranium, thorium, and actinium series of transformations, one cannot fail to be struck by the many points of similarity in their modes of disintegration. Not only are the radiations similar in type and in energy, but, in all cases, the end product is believed to be an isotope of lead. This remarkable similarity in the modes of transformation is especially exemplified in the case of the "C" bodies, each of which is known to break up in at least two distinct ways, giving rise to branch products. For example, thorium C emits two types of α rays, 65 percent of range 8.6 cms. ”

“ It may be recalled that the constant of decay of the activity of a radio-active product has a definite fixed value under all conditions. It is independent of the concentration of the active matter, of the pressure, and of the nature of the gas in which the substance is placed, and it is not affected by wide ranges of temperature. The only observed exception is the product radium C. Its value of λ increases with temperature to some extent at about 1000° C., but at 1200° C. returns nearly to the normal value. ”

“ In the case of radium, for example, which has been most closely investigated on account of its great activity, radium itself, the emanation and radium A emit only α particles; radium B emits no rays at all; while radium C emits all three kinds of rays. It is difficult to settle with certainty whether the products thorium X and actinium X emit β particles or not, but the β and γ rays certainly appear in each case in the last rapid change in the active deposit, and, in this respect, behave in a similar manner to radium. ”

“ It may thus be possible that all matter is undergoing a slow process of transformation, which has so far only been detected in the radio-elements on account of the expulsion of charged particles with great velocity during the change. This process of degradation of matter continuing for ages must reduce the constituents of the earth to the simpler and more stable forms of matter.
The idea that helium is a transformation product of radium suggests the probability that helium is one of the more elementary substances of which the heavier atoms are composed. ”

“ Since electrons are continuously and spontaneously expelled from radium with enormous velocities, they must acquire their energy of motion from the matter itself. It is difficult to avoid the conclusion, that this velocity has not been suddenly impressed on the electron. Such a sudden gain of velocity would mean an immense and sudden concentration of energy on a small particle, and it is more probable that the electron before its expulsion has been in rapid orbital or oscillatory motion in the atom, and, by some means, suddenly escapes from its orbit. ”

“ Recent experiments (section 279) have shown that there is present at the surface of the earth a very penetrating type of rays, similar to the γ rays of radium. Even if it were supposed that the radio-elements possessed the power of absorbing this radiation, the energy of the rays is far too minute to account even for the energy radiated from an element of small activity like uranium. In addition, all the evidence so far obtained points to the conclusion that the radio-active bodies do not absorb the type of rays they emit to any greater extent than would be expected from their density. ”

“ A most important and striking property possessed by radium, thorium, and actinium, but not by uranium or polonium, is the power of continuously emitting into the surrounding space a material emanation, which has all the properties of a radio-active gas. ”

“ If a small vane, coated uniformly with radium on both sides, and mounted on an insulating support, be brought near a charged body kept at a constant potential, it acts like a water dropper and rapidly acquires very nearly the average potential which existed at that point before the vane was brought up. The mechanical force acting on the vane will, in consequence, be small. If, however, the vane is only coated with radium on the side near the charged body, the ionization and consequently the conductivity of the gas is much greater between the vane and the charged body than on the opposite side. ”

“ The nomenclature to be applied to the numerous radio-active products is a question of great importance and also one of considerable difficulty. Since there are at least seven distinct substances produced from radium, and probably five from thorium and actinium, it is neither advisable nor convenient to give each a special name such as is applied to the parent elements. At the same time, it is becoming more and more necessary that each product should be labelled in such a way as to indicate its place in the succession of changes. ”

“ TRANSFORMATION PRODUCTS OF RADIUM. 215. Radio-activity of radium. Notwithstanding the enormous difference in their relative activities, the radio-activity of radium presents many close analogies to that of thorium and actinium. Both substances give rise to emanations which in turn produce “excited activity” on bodies in their neighbourhood. Radium, however, does not give rise to any intermediate product between the element itself and the emanation it produces, or in other words there is no product in radium corresponding to Th X in thorium. ”