CH. xix.] INSTABILITY OF ATOM 185 



occurrence is accompanied, and indeed demonstrated, 

 by radio-activity. 



A list of the series of changes actually observed in 

 radio-active substances will be found above in Chap. 

 XVIII., p. 170. 



Cosmic Analogy. 



An analogy may be drawn between the occasional 

 disruption of one of a large group of atoms, and the 

 phenomenon observed from time to time in the sky 

 called a new or temporary star. Both are outbursts 

 of a kind of radio-activity, though they may be 

 excited by different causes ; both are comparatively 

 rare occurrences, when the whole available number 

 of bodies capable of such outburst or collision is 

 contemplated. 



Assuming that in a gramme of average terrestrial 

 material there are a thousand such eruptions every 

 second, that would correspond to about one new 

 star per century among a cosmic assemblage of ten 

 thousand million bodies. 



Another Account of Atomic Instability. 



A different idea of the nature of Instability was 

 suggested by myself. An electron which is rotating 

 outside an atom, being attracted thereto by a force 

 varying as the inverse square of the distance, will,, 

 as it tends to lose energy by radiation, get drawn 

 nearer and nearer to the atom ; and will increase in 

 speed inversely with the square root of the distance. 

 As the speed thus increases, the effective inertia of the 

 particle will increase, and accordingly it will be more 

 and more difficult to retain it in an orbit by the centri- 

 petal force ; since this force, being merely an electrical 



