V. BAKBBIAM LKOTUME. The Succession of Changes in Radioavtire. Undies. 



lit/ Professor E. RUTHERFORD, F.R.S., Macdoixdd Professor of Physics, 



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Lecture delivered May 19, MS. received August 20, 1904. 



1. IN previous papers by RUTHERFORD and SODDY* it has been shown that the 

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

 -lies of new substances with some distinctive physical and chemical properties. For 

 example, thorium produces from itself an intensely radioactive substance, ThX, 

 which can be separated from the thorium in consequence of its solubility in ammonia. 

 In addition, thorium givi-s rise to a gaseous product, the thorium emanation, and also 

 to another substance which is deposited on the surface of bodies in the neighbourhood 

 of the thorium, where it gives rise to the phenomenon known as ' excited activity.' 



A close examination of the origin of these products shows that they are not 

 produced simultaneously, but arise in consequence of a succession of changes 

 originating in the radio-element. Thorium first of all gives rise to the product ThX. 

 The ThX produces from itself the thorium emanation, and this in turn is transformed 

 into a non-volatile substance. A similar series of changes is observed in radium, with 

 the exception that there is no product in radium corresponding to the ThX in the 

 case of thorium. Radium first of all produces an emanation, which, like thorium, is 

 transformed into a non-volatile substance. In uranium only one product, UrX, has 

 been olraerved, for uranium does not give off an emanation and does not in consequence 

 produce excited activity on bodies. 



2. As a typical example of the evidence, from which it is deduced that one 

 sulwtance is the parent of another, we will consider the connection of the two 

 products ThX and the thorium emanation. After the separation of ThX from a 

 thorium solution, by precipitation with ammonia, the precipitated thorium hydroxide 

 has lost t<> a large extent its power of emanating. This cannot be ascribed to a 

 prevention of escape of the emanation produced in it, for very little emanation is 

 oKsrrvcd when ,i current f air is drawn through the hydroxide in a state of solution, 



* ' I'hil. Mag.,' Sept., Nov., 1902 ; April and May, 1903. 

 CCIV. A 376. / 29.11.04 



