18 SCIENCE AND LIFE 



With the discovery of radioactivity the Rubicon 

 was crossed, and physical science found itself in a 

 new world, in the presence of giant-like primary 

 manifestations of energy which proceed in absolute 

 indifference to and completely unaffected by any of 

 the pygmy second-order influences of the world 

 external to themselves, the old world of chemistry 

 and physics. 



These radioactive disintegrations of the atom 

 proceed in a long sequence of successive changes at 

 characteristic rates. The primary parent-elements, 

 uranium and thorium, each stand, as it were, at the 

 head of a long genealogical table, comprising some 

 fourteen members in the first, and twelve members 

 in the second case, before the processes come to 

 an end and the outflow of energy accompanying 

 them ceases. 



Each of the changes proceeds at definite rates 

 which, so far as has been ascertained, are absolutely 

 independent of every known consideration, and so 

 it comes about that each of these successive products 

 has a characteristic average period of life. Its atom 

 remains in existence for a period of time which is, 

 on the average, definite, and which varies among 

 the various successive members between the ex- 

 tremes, estimated indirectly in a variety of ways, 

 of a hundred-thousand-millionth of a second on the 

 one hand and twenty thousand million years on the 

 other. The two parent-elements are the longest 

 lived, and preserve the strain of their less enduring 

 children throughout the ages, over periods which 

 exceed those covered even by the utmost estimates 

 of the duration of geological time. 



Radium is but one of the products of the uranium 

 series, and its special interest is chiefly to be ascribed 

 to the fact that the rate at which it changes, esti- 

 mated as one-two thousand five hundredth part per 



