General Radio-activity 581 



periods of time, and yet eventually spontaneously explode, as here an 

 atom and there an atom reached a condition of instability? 



The atomic theory of corpuscles or electrons fortunately was ready 

 to be applied to this new problem. Of the resulting speculations the 

 most detailed and suggestive is that of J. J. Thomson 1 . Thomson 

 regards the atom as composed of a number of mutually repelling 

 negative corpuscles or electrons held together by some central attrac- 

 tive force which he represents by supposing them immersed in a 

 uniform sphere of positive electricity. Under the action of the two 

 forces, the electrons space themselves in symmetrical patterns, which 

 depend on the number of electrons. Three place themselves at the 

 corner of an equilateral triangle, four at those of a square, and five 

 form a pentagon. With six, however, the single ring becomes un- 

 stable, one corpuscle moves to the middle and five lie round it. But 

 if we imagine the system rapidly to rotate, the centrifugal force 

 would enable the six corpuscles to remain in a single ring. Thus 

 internal kinetic energy would maintain a configuration which would 

 become unstable as the energy drained away. Now in a system of 

 electrons, electromagnetic radiation would result in a loss of energy, 

 and at one point of instability we might well have a sudden spon- 

 taneous redistribution of the constituents, taking place with an 

 explosive violence, and accompanied by the ejection of a corpuscle 

 as a /3-ray, or of a large fragment of the atom as an a-ray. 



The discovery of the new property of radio-activity in a small 

 number of chemical elements led physicists to ask whether the 

 property might not be found in other elements, though in a much less 

 striking form. Are ordinary materials slightly radio-active ? Does 

 the feeble electric conductivity always observed in the air contained 

 within the walls of an electroscope depend on ionizing radiations 

 from the material of the walls themselves? The question is very 

 difficult, owing to the wide distribution of slight traces of radium. 

 Contact with radium emanation results in a deposit of the fatal 

 radium-D, which in 40 years is but half removed. Is the " natural " 

 leak of a brass electroscope due to an intrinsic radio-activity of brass, 

 or to traces of a radio-active impurity on its surface ? Long and 

 laborious researches have succeeded in establishing the existence of 

 slight intrinsic radio-activity in a few metals such as potassium, and 

 have left the wider problem still unsolved. 



It should be noted, however, that, even if ordinary elements are 

 not radio-active, they may still be undergoing spontaneous disintegra- 

 tion. The detection of ray-less changes by Rutherford, when those 



1 Phil. Mag. March, 1904. 



