RADIO-ACTIVITY 183 



they then emit seems to be an inseparable property 

 of the elements themselves, and is of the a type only. 

 To the separated products the names of uranium- A' 

 and thorium-^ have been given. They may be 

 analogous to emanations as far as the series of 

 radio-active changes is concerned, being, however, 

 solid instead of gaseous at ordinary temperatures. 



The important point is this : if these X pro- 

 ducts be kept for some weeks or months, they will 

 be found to have lost their radio-active properties, 

 while the original samples of uranium or thorium 

 will have become as active as they were before 

 the separation, and will again emit all three types 

 of radiation. The rates at which the processes of 

 loss and gain of activity occur have been studied 

 carefully by Rutherford and Soddy, and shown 

 to correspond accurately with each other. This 

 correspondence is clearly shown by the curves in 

 Fi^- 33^ which give the decay of activity of the 

 separated uranium-X, and the recovery of the 

 residual uranium. Again we see that the total 

 amount of activity remains constant, and is not 

 affected by the processes of chemical action. 



These experiments lead to a definite view as 

 to the source of the radiations. Whenever radio- 

 activity exists, the active material is always slowly 

 changing into some other substance, which has 

 distinct chemical properties, and can be separated 

 by chemical means from the original material. 

 Thus, in the case of thorium compounds, the radio- 

 active body producing most of the effects usually 

 observed is not really thorium, but a definite 

 substance we may call thorium-X, which is being 

 formed at a constant rate from the bulk of the 

 thorium, and, after its formation, gradually loses 



