344 BOLTW'OOD— RADIOACTIVITY. [April 22, 



The point can be definitely settled, however, by a comparison of 

 Rutherford's standard with a standard of indisputable purity. Such 

 a standard is in prospect in the not distant future and its prepara- 

 tion has been undertaken by ]Mme. Curie on behalf of the Interna- 

 tional Radium Standards Committee appointed at the recent Radio- 

 logical Congress in Brussels. 



A very interesting action which has been observed to accompany 

 radioactive transformations is known as the recoil plicnouicnon. 

 When a plate bearing a thin layer of very active material is placed 

 in close proximity of another plate which is inactive, a portion of 

 the active matter becomes detached from the film and is deposited on 

 the surface of the second plate. 



The efifect is increased considerably if the receiving plate is 

 charged negatively with respect to the plate bearing the active coat- 

 ing. This action is apparently due to the fact that, when the alpha 

 or beta rays are expelled at a high velocity from a radio-atom under- 

 going transformation, the reaction on the residual atom causes this 

 to move in the opposite direction with sufficient force to detach it 

 from the plate. The action is analogous to the recoil of a rifle 

 attending the expulsion of a high velocity bullet. When, for exam- 

 ple, the active coating on the first plate consists of radium A then 

 the active matter received on the second plate is composed almost 

 exclusively of radium B ; and when the film consists of radium B the 

 material thrown ofif is for the most part radium C. This and other 

 similar effects which have been noted are all of such a nature as to 

 suggest that the explanation proposed for this interesting phenom- 

 enon is the correct one. The effect of the electric field indicates that 

 in some way these " rest atoms " acquire positive electric charges. 



From the standpoint of the disintegration theory, it is evident, 

 when we consider the three principal grouj^s of radioactive sub- 

 stances, the uranium-radium group, the actinium group, and the 

 thorium group, that the radioactive phenomena exhibited by the 

 atoms abruptly disappear after they have passed through a certain 

 series of transformations, which terminates with radium F in one 

 instance, with actinium C in another and with thorium D in the 

 third. The apparent explanation of this circumstance would seem 

 to be, that, following the last active change, the residual atomic 



