12 



M. Levin — 



Absorpt 



ion of the a- 



-Bays from Polonium 







Table II. 



Distance in air 



Number of 





Distance in 



corresponding to 



aluminium foils. 



centimeters. 



one aluminium foil. 









3-87 





1 





3'37 



•50 



2 





2-84 



•51 



3 





2-33 



•51 



4 





J -83 



•51 

 Mean -51 







Table III. 



Distance in air 



Number of 





Distance in 



corresponding to 



aluminium foils. 



centimeters. 



one aluminium foil, 









3-85 





1 





3-36 



•49 



2 





2-86 



•48 



3 





2-35 



•50 



4 





1-82 



•50 

 Mean -49 



The break in each of the curves is very sharply marked, so 

 that the point where the ionization begins could be obtained 

 from the curves with an error of about •2 mm . There is, however, 

 a little uncertainty about the actual values, because the gauze 

 could not be obtained quite plane. The mean of these experi- 

 ments gives the range of the a-particles of polonium to be 

 3'86 cm . The average, from the scintillation measurements, is 

 3*77, and from the extrapolated values 3*81. 



Wigger* found that the ionization due to the a-rays of 

 polonium extended in air to a distance of about 4 cm . He used, 

 however, a large plane surface of polonium as a source of rays 

 and did not obtain the ionization-distance curve, for which, as 

 we have seen, a definite narrow cone of rays must be employed. 



Using the a-rays of radium as a source, Bragg has shown, 

 that the stopping power of the a-particle, passing through an 

 atom, is proportional to the square root of the atomic weight. 

 Adams (loc. cit.) states that he has verified the law with the 

 a-rays of polonium, using the scintillation method. As the 

 scintillation and electrical method give probably identical 

 results, there was no object to be gained in determining by the 

 electrical method the range of the a-rays of polonium in other 

 gases besides air. 



The results of this investigation have shown, that polonium 

 is a homogeneous source of a-rays, and that the a-particles are 

 initially projected with the same velocity. The range in air 

 of the a-particles is 3'86 cm , which is slightly greater than the 

 range of the a-particles (3-50 cm ) of radium itself, but much less 

 than that for the a-particles from radium C (range 7'06 (:m ). 



I wish to express my best thanks to Professor Rutherford 

 for the kind interest he took in this work, and for the advice 

 received from him. 



McDonald Physics Building, McGill University, 

 Montreal, 20 May, 1906. 



* Jahrbuch der Radio-aktivitat, ii, 391, 1906. 



