Page:Radio-activity.djvu/177

 The kinetic energy of each projected particle is enormous, compared with its mass. The kinetic energy of each [Greek: alpha] particle is

(1/2)mV^2 = (1/2)(m/e)V^2e = 5·9 × 10^{-6} ergs.

Taking the velocity of a rifle bullet as 10^5 cms. per second, it is seen that, mass for mass, the energy of motion of the [Greek: alpha] rays is 6 × 10^8 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 ).

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 [Greek: alpha] 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 [Greek: alpha] 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 [Greek: alpha] particle is atomic in size, it is