RADIOACTIVITY 17 



continued decay of the internal energy of the atom, arising from 

 the radiation spreading away through the surrounding ether. 

 The radiation was ascribed to the action of the spinning 

 electrons in the atomic system, for any motion involving acceler- 

 ation can be shown to be the origin of radiation and therefore 

 a cause of the emission of energy. But an atom which has 

 come into existence as the result of a radioactive disintegration 

 has no process to work through before its turn comes to pass 

 away. The cataclysm may come at any time and needs no 

 preparation to be made for it ; and is all the more mysterious 

 on this account. This is a convenient time to state also that 

 the transformation is not to be precipitated or delayed by any 

 of the agencies which physical and chemical science consider. 

 Neither cold nor heat, nor even the most searching chemical 

 action can affect its chance of occurrence. This alone places 

 the new science by itself: it is more fundamental than any 

 followed hitherto. 



Now let us try to illustrate and verify some of these points 

 by considering the properties of the a rays. We must, of course, 

 suppose some way of detecting their presence and measuring 

 their effect ; and there are three such ways. One depends on 

 the action of the rays on a photographic plate but we shall not 

 use this. Another depends on the power of the a rays to excite 

 phosphorescence in a screen of proper material ; and we may 

 observe by the way that in examining this effect carefully we 

 shall find evidence of the material nature of the rays. For 

 when the screen is examined under the microscope while the 

 rays are acting on it, the illumination is seen to be due to 

 many minute sparkles which appear on the screen, distributed 

 irregularly in time and space and reminding us of the result 

 of throwing a shower of pebbles into a phosphorescent sea. 

 The third way depends on the production of ionisation by the 

 rays as they traverse the air. There is no need whatever to 

 examine or know anything about the exact nature of this 

 process for the present. It is sufficient that the air, when 

 traversed by the rays, is thrown into such a state that the 

 application of an electromotive force across it causes a current 

 to flow through it, which current does not depend, within wide 

 limits, upon the magnitude of the electromotive force but does 

 depend on the strength of the action exercised by the a rays. Thus 

 the usual apparatus of battery, ionisation chamber and electro- 



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