1904] The Succession of Changes in Radio-active Bodies. 493 



Bakerian Lecture. — " The Succession of Changes in Radio-active 

 Bodies." By E. Rutherford, E.R.S., Macdonald Professor of 

 Physics, McGill University, Montreal. Lecture delivered 

 May 19, 1904. 



(Abstract, Received May 27, 1904.) 



It has been shown by Rutherford and Soddy that the radio-activity 

 of the radio-elements is always accompanied by the production of a 

 series of new substances, possessing some distinctive physical and 

 chemical properties. These new substances are not produced simul- 

 taneously, but arise in consequence of a succession of changes originating 

 in the radio-elements. The radio-activity of these products is not 

 permanent, but diminishes, in most cases according to an exponential 

 law, with the time. Each product has a distinctive rate of decay of 

 activity, which has not, so far, been altered by any physical or 

 chemical agency. The law of decay has been explained on the suppo- 

 sition that the product undergoes change according to the same law as 

 a mono-molecular change in chemistry. The change occurs in conse- 

 quence of the expulsion of an a or p particle, or both, and the 

 activity of a product is thus a measure of its rate of change. While the 

 products like the emanations, and UrX, lose their activity according to 

 an exponential law, the matter emanation X, which gives rise to the 

 phenomena of excited activity, doe^ not lose its activity according to a 

 simple law. The experiments of Miss Brooks and the author, and of 

 Curie and Danne, have shown that the decay of the excited activity of 

 radium is very complicated, and depends upon the time of exposure to 

 the exciting cause, viz., the emanation. The author has shown that the 

 excited activity produced in a body by a short exposure in the presence 

 of the thorium emanation increases at first for a few hours, passes 

 through a maximum value, and then decays with the time according to 

 an exponential law. 



In the paper the curves of decay of excited activity of radium and 

 thorium are given for both short and long exposures to the emanations, 

 and it is shown that the law of change of activity with time can be 

 completely explained on the theory that emanation X of thorium and 

 radium is complex and undergoes a series of successive changes. 



The mathematical theory of successive changes is given in detail, 

 and a comparison is made of the theoretical and experimental curves 

 obtained for the variation with time of the excited activity. In the 

 case of thorium, two changes are found to occur in emanation X. The 

 first change is a "rayless" one, i.e., the transformation is not accom- 

 panied by the appearance of a, or y rays. The second change gives 

 rise to all three kinds of rays. 



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