250 THE HISTORY OF SCIENCE 



takes place. After ten years of further investigation 

 Rutherford stated that this hypothesis affords a 

 satisfactory explanation of all radioactive phenom- 

 ena, and gives unity to what without it would seem 

 disconnected facts. Besides accounting for old ex- 

 perimental results it suggests new lines of work 

 and even enables one to predict the outcome of fur- 

 ther investigation. It does not really contradict, as 

 some thought might be the case, the principle of the 

 conservation of energy. The atom, to be sure, can 

 no longer be considered the smallest unit of matter, 

 as the mass of a /3 particle is approximately one 

 seven teen-hundredths that of an atom of hydrogen. 

 Still the new hypothesis is a modification and not a 

 contradiction of the atomic theory. 



The assumption that the series of radioactive sub- 

 stances is due, not to such molecular changes as chem- 

 istry had made familiar, but to a breakdown of the 

 atom seemed to Rutherford in 1913 at least justified 

 by the results of the investigators whose procedure 

 had been dictated by that hypothesis. He set forth 

 in tables these results (since somewhat modified), 

 indicating after the name of each radioactive sub- 

 stance the nature of the radiation through the emis- 

 sion of which the element is transformed into the 

 next-succeeding member of its series. 



List of Radioactive Substances 



URANIUM a particles 



Uranium X 

 Uranium Y 

 IONIUM 



