Velocity of the a -particle upon the Stopping Power. 511 



But in fig. 1 the two curves are almost identical ; in fig. 2 

 they are *23 cm. apart. 



In another experiment 28 thicknesses of gold-leaf were 

 placed over the radium, and the whole ionization curve was 

 determined. It was found that the drop for RaO was 

 1*13 cm. for EaA 1*01 cm., for the emanation *98 cm., and 

 for Ra "93 cm. 



In yet another experiment the velocity of the a particles 

 of RaO was cut down by putting an Al sheet over the radium, 

 so that the average range became 3*63 ; the same gold-foils 

 then cut it down to 2*70, the drop being- therefore *93 cm. 

 In this case, therefore, the two methods agreed, and there 

 w r as not, as usual, a greater effect obtained when using the 

 particles of Ra than when using those of RaC whose velocity 

 had been reduced to that of the particles from Ra. 



Thus it is clear that if the effect under discussion is small 

 in the case of A\, it is larger in the case of Au. The stopping- 

 power of Au increases with the speed of the a particle. 



It is now of some interest to consider certain results 

 obtained by Kleeman and myself two years ago ; they relate 

 to the drop in various parts of the full radium ionization 

 curve, and are set out in the following table : — 



Table. 

 Showing drop in various parts of the ionization curve when 



different metal sheets were placed over the radium. 





EaC. 



Ea A. 



Emanation. 



Ea. 



1. Al 



2. Sn 



231 



192 



1-91 



•86 



163 



2-25 

 1-77 

 1-81 

 •81 

 1-46 



1-67 

 •80 -73 



3. Sn 



4. Aff 



5. Au 



1-36 











From these results it appears to be not improbable that the 

 effect under consideration increases with atomic weight. 



If the stopping-powers of metals are affected in different 

 degrees by variations in the speed of the particle, then the 

 stopping-pow-er of a pair of metal sheets, say Al and Au, 

 must depend on the order in wmich the a particle passes 

 through them. This deduction furnishes a good test of the 

 accuracy of the preceding experiments and arguments. I 

 have therefore tried to examine the matter carefully ; the 

 more so, as Kucera and Masek did not find the effect. 



