The kinetic energy of each projected particle is enormous, compared with its mass. The kinetic energy of each α particle is
1 1 m
--- mV2 = --- --- V2e = 5·9 × 10-6 ergs.
2 2 e
Taking the velocity of a rifle bullet as 105 cms. per second, it is seen that, mass for mass, the energy of motion of the α rays is 6 × 108 times as great as that of the rifle bullet. In this projection of bodies atomic in size with great velocity probably lies the principal cause of the heating effects produced by radium ([chapter XII]).
95. Atomic disintegration. The radio-activity of the radio-elements is an atomic and not a molecular property. The rate of emission of the radiations depends only on the amount of the element present and is independent of its combination with inactive substances. In addition, it will be shown later that the rate of emission is not affected by wide variations of temperature, or by the application of any known chemical or physical forces. Since the power of radiating is a property of the radio-atoms, and the radiations consist for the most part of positively and negatively charged masses projected with great velocity, it is necessary to suppose that the atoms of the radio-elements are undergoing disintegration, in the course of which parts of the atom escape from the atomic system. It seems very improbable that the α and β particles can suddenly acquire their enormous velocity of projection by the action of forces existing inside or outside the atom. For example, the α particle would have to travel from rest between two points differing in potential by 5·2 million volts in order to acquire the kinetic energy with which it escapes. Thus it seems probable that these particles are not set suddenly in motion, but that they escape from an atomic system in which they were already in rapid oscillatory or orbital motion. On this view, the energy is not communicated to the projected particles, but exists beforehand in the atoms from which they escape. The idea that the atom is a complicated structure consisting of charged parts in rapid oscillatory or orbital motion has been developed by J. J. Thomson, Larmor and Lorentz. Since the α particle is atomic in size, it is natural to suppose that the atoms of the radio-active elements consist not only of the electrons in motion, but also of positively charged particles whose mass is about the same as that of the hydrogen or helium atom.
It will be shown later that only a minute fraction of the atoms of the radio-element need break up per second in order to account for the radiations even of an enormously active element like radium. The question of the possible causes which lead to this atomic disintegration and the consequences which follow from it will be discussed later in [chapter XIII].
96. Experiments with a zinc sulphide screen. A screen of Sidot’s hexagonal blend (phosphorescent crystalline zinc sulphide) lights up brightly under the action of the α rays of radium and polonium. If the surface of the screen is examined with a magnifying glass, the light from the screen is found not to be uniformly distributed but to consist of a number of scintillating points of light. No two flashes succeed one another at the same point, but they are scattered over the surface, coming and going rapidly without any movement of translation. This remarkable action of the radium and polonium rays on a zinc sulphide screen was discovered by Sir William Crookes[[152]], and independently by Elster and Geitel[[153]], who observed it with the rays given out from a wire which has been charged negatively either in the open air or in a vessel containing the emanation of thorium.
In order to show the scintillations of radium on the screen, Sir William Crookes has devised a simple apparatus which he has called the “Spinthariscope.” A small piece of metal, which has been dipped in a radium solution, is placed several millimetres away from a small zinc sulphide screen. This screen is fixed at one end of a short brass tube and is looked at through a lens fixed at the other end of the tube. Viewed in this way, the surface of the screen is seen as a dark background, dotted with brilliant points of light which come and go with great rapidity. The number of points of light per unit area to be seen at one time falls off rapidly as the distance from the radium increases, and, at several centimetres distance, only an occasional one is seen. The experiment is extremely beautiful, and brings vividly before the observer the idea that the radium is shooting out a stream of projectiles, the impact of each of which on the screen is marked by a flash of light.
The scintillating points of light on the screen are the result of the impact of the α particles on its surface. If the radium is covered with a layer of foil of sufficient thickness to absorb all the α rays the scintillations cease. There is still a phosphorescence to be observed on the screen due to the β and γ rays, but this luminosity is not marked by scintillations to any appreciable extent. Sir William Crookes showed that the number of scintillations was about the same in vacuo as in air at atmospheric pressure. If the screen was kept at a constant temperature, but the radium cooled down to the temperature of liquid air, no appreciable difference in the number of scintillations was observed. If, however, the screen was gradually cooled to the temperature of liquid air, the scintillations diminished in number and finally ceased altogether. This is due to the fact that the screen loses to a large extent its power of phosphorescence at such a low temperature.