180 SUMMARY OF ELECTRON THEORY [CH. xvm. 



characteristic or specific quality in the radio-activity 

 of the different substances : a result tending to nega- 

 tive the idea that it is due to some residual effect 

 of a common impurity, such as an excessively 

 minute trace of radium common to all metals ; 

 the indication is rather in favour of a specific 

 radio-activity belonging to each metal, not due to 

 any impurity. 



It is not yet absolutely proved that this is identical 

 with orthodox radio-activity, of the kind which is 

 accompanied by atomic change or transformation of 

 substance ; but, inasmuch as the rays appear to consist 

 to a great extent of alpha-rays, there is not much 

 doubt but that the complete identity of the process 

 will be established before long. 



Population Analogy. 



Since radio-activity is a sign of, and is accompanied 

 by, disintegration and loss of material, it is manifest 

 that substances of exceedingly high radio-activity 

 must be comparatively scarce. Ordinary permanent 

 materials cannot be violently radio-active, though 

 each gramme of them might lose a few thousands of 

 atoms per second without any probability of our 

 being able to detect the loss by weighing not even 

 by weighings continued through a century. The 

 plentifulness of a substance must depend on its rate 

 of production, its life time, and its rate of decay; just 

 as the population of a circumscribed area is deter- 

 mined by the birth rate, death rate, and average age. 



