Collisions of u Particles with Hydrogen Nuclei. 935 



minimum is reached at which the number is about that to be 

 expected on the inverse square law. As the velocity of the 

 a particle is further reduced, the number of H particles 

 increases in the way demanded by this law of force. 



ange of 

 particle. 



WV) 2 . 



No. of 



H particles 



per mgni. 



F(31°-3)-F(21°-4). 



Inverse 

 Square La^ 



66 



1-01 



5-5 



9-9X10- 6 



0-34x10" 



5-6 



116 



4-6 



8-4 



0-43 



4-6 



1-32 



3-7 



6-7 



0-55 



3-6 



1-55 



3-1 



5-6 



0'76 



3-3 



1-65 



2-8 



5-1 



0-86 



29 



1-80 



1-7 



3-0 



1-0 



2-0 



2-32 



1-1 



1-9 



18 



1-6 



2-70 



0-8 



1-8 



23 



1-0 



3-69 



14 



4-9 



4-3 



It appears from these results that the inverse square law 

 holds, at least approximately, for the collisions of & particles 

 of low velocity, that is, for large distances of collision. It 

 was clearly of great importance to make certain of this 

 point. The experiments with the low-range a-rays were 

 therefore repeated several times, all possible precautions 

 being taken. The observations were naturally difficult, 

 owing to the weakness of the scintillations produced by 

 the low-range H particles concerned, but we estimate 

 that the error is within 30 per cent. The fact that H par- 

 ticles due to a-rays of 1 cm. mean range could still be 

 counted consistently would indicate that the count with 

 a-rays of 2 cm. range must certainly be reliable ; and the 

 latter count agrees within the error of experiment with 

 the inverse square number. 



The variation of the number of H particles with the 

 velocity of the a-rays is shown very clearly in fig. 6, where 

 Darwin's p is plotted against (V /V) 2 - For a-rays of range 

 less than 2'9 cm. [(V /V) 2 >l-80], the numbers between 0° 

 and 21°*4 could not be determined directly, for the reason 

 stated above. To obtain the total numbers between 0° and 

 31°'3, it was assumed that within this range these numbers 

 follow the same law of variation with angle as that given 

 by the inverse square law, viz. raoc tan 2 6, and the results 



