x] RADIO-ACTIVE PROCESSES 341 
the simple case of a negatively charged electron revolving round 
an atom of mass relatively large but having an equal positive 
charge and held in equilibrium by electrical forces. This system 
will radiate energy, and since the radiation of energy is equivalent 
to motion in a resisting medium, the particle tends to move 
towards the centre, and its speed consequently increases. The 
rate of radiation of energy will increase rapidly with the speed 
of the electron. When the speed of the electron becomes very 
nearly equal to the velocity of light, according to Lodge, another 
effect supervenes. It has been shown (section 76) that the 
apparent mass of an electron increases very rapidly as the speed 
of light is approached, and is theoretically infinite at the speed 
of light. There will be at this stage a sudden increase of the 
mass of the revolving atom and, on the supposition that this stage 
can be reached, a consequent disturbance of the balance of forces 
holding the system together. Lodge considers it probable that, 
under these conditions, the parts of the system will break asunder 
and escape from the sphere of one another’s influence. 
It seems probable that the primary cause of the disintegration 
of the atom must be looked for in the loss of energy of the atomic 
system due to electro-magnetic radiation. Larmor! has shown 
that the condition to be fulfilled in order that a system of rapidly 
moving electrons may persist without loss of energy is that the 
vector sum of the accelerations towards the centre should be 
permanently null. While a single electron moving in a circular 
orbit is a powerful radiator of energy, it is remarkable how rapidly 
the radiation of energy diminishes if several electrons are revolv- 
ing in a ring. This has recently been shown by J. J. Thomson?, 
who examined mathematically the case of a system of negatively 
electrified corpuscles, situated at equal intervals round the circum- 
ference of a circle, and rotating in one plane with uniform velocity 
round its centre. For example, he found that the radiation from 
a group of six particles moving with a velocity of #1, of the velocity 
of light is less than one-millionth part of the radiation from a 
single particle describing the same orbit with the same velocity. 
When the velocity is ;45 of that of light the amount of radiation 
1 Larmor, Aether and Matter, p. 233. 
2 J. J. Thomson, Phil. Mag. p. 681, Dec. 1903. 
