﻿by Moving Electrified Particles. 455 



the potential energy of the positively charged residue and a 

 corpuscle at an infinite distance from it. 1£ the work spent 

 on the corpuscle is greater than W, the corpuscle leaves the 

 atom with a finite amount of kinetic energy, but the state 

 of the positively charged ion and its potential energy with 

 respect to a distant corpuscle is not affected. Consider now 

 what happens when this positive ion and a corpuscle combine 

 and restore the atom or molecule to the state in which it 

 existed before ionization took place. Under their mutual 

 attraction the corpuscle and positively charged ion approach 

 each other and acquire kinetic energy. As the mass of the 

 corpuscle is much the smaller, the kinetic energy will be 

 almost entirely localized in the corpuscle. After the cor- 

 puscle has fallen back into the ion it will have an amount 

 of kinetic energy equal to W, if there has been no dissipation 

 of energy by radiation ; before the atom is restored to the 

 state it was in before ionization, this energy must be got 

 rid of. This dispersal of the energy is accomplished by its 

 radiation, which takes place as long as its motion is being- 

 accelerated. Thus from the time of commencement of the 

 recombination up to that when the atom has become normal, 

 a stream of radiation is emitted from the system, constituting a 

 pulse, which may be a linear one, of electromagnetic radiation, 

 whose duration and energy depend only on the properties of 

 the atom or molecule. Hence the character of the radiation 

 emitted during the recombination of the ions will be a series 

 of pulses, each pulse containing the same amount of energy, 

 and of such a character that if we were to analyse the electro- 

 magnetic disturbance by Fourier's theorem into a system of 

 harmonic vibrations, the distribution of energy among the 

 different periods would be the same for each pulse. In fact, 

 each of these pulses will form a unit or quantum, and the 

 total radiation will be built up of such units. 



Now, what will be the nature of this radiation ? If we 

 resolve it up into light-vibrations, where will the maximum 

 energy be? will it be far down in the infra-red or high up 

 in the ultra-violet ? This will depend on the energy required 

 to ionize a molecule. To fix our ideas let us suppose this is 

 represented by 10 volts, so that when the corpuscle regains 

 the atom or molecule it will be moving with the velocity 

 corresponding to the fall of the atomic charge through 

 10 volts. We may fairly compare the radiation it emits 

 with that emitted by a corpuscle moving with this velocity 

 when it strikes against a molecule. If we suppose that t ho 

 radiation from hot bodies is due to the electric waves gene- 

 rated when corpuscles in thermal equilibrium with the body 



