380 



Sir E. Rutherford and Mr. A. B. Wood on 



corresponding to 9 cm. of air. With the most intense source 

 available, about 20 scintillations per minute were counted on 

 the microscope. The number fell off rapidly with increase 

 of distance of the source, but an occasional a particle was 

 still observed at a distance of 2 cm. from the source. In 

 order to determine the variation of number of these particles 

 with distance of matter traversed, thin screens of aluminium, 

 each corresponding in thickness to 1*25 mm. of air, were 

 successively interposed between the source and screen. It 

 was found that the number of long-range particles remained 

 constant between S'6 and 9*3 cm. of air, but decreased in 

 number from 9*3 cm., vanishing at 11*3 cm. The grouped 

 average of all the observations in a large number of 

 experiments is shown in fig. 1. 



Fio-. 1. 



100 



80 



60 



40 



20 



<n _. Q 













so 



-?\ 











o 



v. 

 5 





/ 















\p 





























o\ 



8-6 3 1 9-6 10-1 



Range in cms. 



10-6 



n- l 



11-6 



It is seen that the curve shows evidence of two fairly 

 definite slopes AB, BC, as if there were two sets of alpha 

 particles present of different ranges. This important point 



