118 CONCEPTION OF THE CHEMICAL ELEMENT 



of the scintillations separately visible is due to the 

 impact of a single a-particle on the zinc sulphide 

 screen. On the same principle, methods have been 

 developed and are in regular use for counting- the 

 number of atoms disintegrating per minute, whereas 

 to the spectroscope at least 3- io 13 atoms as a minimum 

 must be present, 25,000 times as many atoms as 

 there are human beings alive in the world, before any 

 element can be so detected. By the most curious 

 compensation, almost of the nature of a providential 

 dispensation which some may have found difficult 

 to believe, the quantity of matter of itself is not 

 of importance in investigating radioactive change. 

 The methods depend on the rate of emission of 

 energy, and this is proportional to the quantity of 

 the changing element multiplied by its rate of change. 

 In the disintegration series, the various members 

 accumulate in quantities inversely proportional to the 

 rates of change, and so it comes about that all 

 changes within the series are equally within the scope 

 of the method whether, as in the case of the parent 

 elements, they involve periods surpassing the most 

 liberal estimates of the duration of geological time or, 

 as in the case of the C' members, are estimated to 

 run their course in a time so short that light itself 

 can travel but a very few millimetres, before the next 

 change overtakes the changing atom. 



The condition of radioactive equilibrium in which 

 the quantities of the successive products assume the 

 above stationary ratio is of course entirely different 

 from chemical equilibrium, and is the condition in 

 which for each member of the series except the first 

 as much is produced as changes further in the unit 

 of time. 



The foregoing applies so long as the changes 

 continue. When they are finished and it is a 

 question of ascertaining the ultimate products, the 



