26 



SCIENCE PROGRESS 



proof that the a particles of one kind had all the same initial 

 speed : though it must be admitted that the first evidence was 

 somewhat over-valued and more satisfactory proof was not 

 found until later. This showed that the cataclysm which 

 resulted in the birth of the a particle was always the same 

 and that each radium atom in turn underwent exactly the same 

 experience, an idea which gave precision to conceptions 

 previously formed. 



There were certain new facts which appeared from the 

 observation of the behaviour of the a rays which wholly or in 



loft t &ot"«o rv. . 



Fig. 9. 



The three curves in the figure beginning with the top one were obtained with the radium film (1) 

 uncovered, (2) covered with a silver sheet weighing '0021 gr./cm. 2 , and (3) covered with a platinum sheet 

 weighing "0063 gr./cm. 2 . See Phil. Mag., Sept. 1905, p. 331, 



part had not been anticipated. Let us consider these for a 

 moment. 



It is natural to test radiations by considering their ability 

 to penetrate sheets of different materials : indeed it is really 

 the most interesting fact about all radiations. Let us place 

 therefore a thin sheet of some substance in the way of the 

 a rays of our experiment and examine the result. It is certainly 

 most surprising. It is found that every a particle loses a 



