Long-Range A Iplia Particles from Thorn 



381 



was very carefully examined, and the results o£ a special 

 series of observations are shown in fig. 2. It will be seen 

 that there appears to be a fairly definite change in the 

 slope when the number of scintillations is reduced to about 

 two-thirds of the total. 



Fiff. 2. 



It will be seen from the curves that the alpha particles 

 start decreasing in number from about 2 cm. of the maximum 

 range. The variation of number with distance is much 

 slower than that to be expected for a single group of alpha 

 particles of corresponding range. This is brought out in 

 fig. 3, which shows the results obtained when the scintillation- 

 distance curve in air was obtained for the two well-known 

 groups of alpha particles emitted from thorium C of ranges 

 5'0 and 8*6 cm. respectively. In these cases, the scintillations 

 fall off rapidly, beginning at about 1 cm. from the end of the 

 corresponding range. The difference between the slopes of 

 the scintillation- curve for the long-range alpha particles and 

 those from thorium cannot be explained by the oblique 



