EADIOACTIVITY AND ATOMIC THEORY ^ 



By LOED RUTHEKFORD, O. M., F. R. S. 



Nearly 40 years have passed since the spontaneous radioactivity of 

 uranium was shown by Becquerel in 1896. We know that the investi- 

 gations which led to this fundamental discovery were much influenced 

 by the discovery of X-rays by Roentgen in the preceding year. We 

 can now look back with some sense of perspective and recognize the 

 extraordinary importance of the discovery of radioactivity and the 

 profound influence on our knowledge of atoms and the relation of 

 the elements which has followed from a detailed study of the 

 radioactive bodies. 



In the course of this lecture, I have thought it of interest to give 

 a brief account of some of the earlier experiments in radioactivity 

 which pointed the way to the conclusion that the radioactive bodies 

 were undergoing spontaneous transformation. This will be followed 

 by a statement of the most significant of the discoveries that have 

 resulted from an examination of the chemical and radioactive prop- 

 erties of the radio-elements. But this in a sense is only the begin- 

 ning of the story. The use of swift a-particles to bombard matter 

 gave us the first proof that certain light elements could be trans- 

 formed by artificial methods. This has been followed in recent years 

 by experiments in which streams of other fast particles, like protons, 

 neutrons, and deuterons, have been artificially generated in order to 

 bombard matter. By these methods, we have been enabled to extend 

 widely our Imowledge of the modes of transformation of the ele- 

 ments. In some cases, the nuclei of the atoms can be caused to break 

 up with explosive violence, giving rise to new stable elements. In 

 other cases, new radioactive bodies are produced which correspond 

 to unstable isotopes of the elements. More than 50 of these arti- 

 ficially produced radioactive bodies are now known, and no doubt 

 many more will be found in the near future. 



The subject of radioactivity has indeed been born anew and has 

 entered again on a new and vigorous phase of life. It is of interest 



^ Sixteenth Faraday Lecture, delivered at the Royal Institution on Feb. 12, 1936. 

 Reorinted by pennission from the Journal of the Chemical Society, April 1936. 



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