580 Collision of a Particles with Light Atoms. 



It is clear from these results that the nuclei under con- 

 sideration can no longer be regarded as point charges for 

 distances of approach of the order of the diameter of the 

 electron. As far as experiment has so far gone, it is difficult 

 to fix with certainty the distance at which the forces between 

 the nuclei become abnormal, but a rough estimate can be 

 made. Regarding the nuclei as point charges, the closest, 

 distance of approach in a collision is 1*9 x 10 ~ u cm. in the 

 case of a hydrogen atom and 3*8 xlO -13 cm. in the case of 

 the oxygen atom. Taking into account the close similarity 

 of the effects produced by a particles in hydrogen and 

 oxygen and the greater repulsive ibices between the nuclei 

 in the latter case, it seems probable that the abnormal forces 

 in the case of oxygen manifest themselves at about twice the 

 distance observed in the case of hydrogen. This would mean 

 that the rapid variation in the magnitude and direction of 

 the forces between the nuclei which lead to the recoil 

 of >\\it't atoms mainly in the direction of the a particle 

 should begin at a distance about 7xl0~' 3 cm. Such a 

 result is to be anticipated on general grounds, for pre- 

 sumably the oxygen nucleus is more complex and has larger 

 dimensions than that of helium. 



In a paper published three years ago Mr. A. B. Wood and 

 the writer * described experiments which showed that the 

 active deposit of thorium gave rise to a few a particles of 

 range 113 cm. in addition to the main group of ranges 

 5"0 and 8"(> cm. In these experiments, the a rays of range 

 S'6 cm. were absorbed in mica. In the light of the present 

 experiments, the oxy r gen present in the mica should give 

 rise to scintillations like a. particles of range 



, 9*0 ,. , 

 <S*6 X— =li'l cm, 

 • *0 



This range is nearly the same as that observed in the 

 thorium experiment, and raises the question whether these 

 long range a particles are not in reality due to collision of 

 a particles with the oxygen atoms in the mica. A fraction 

 of the scintillations must undoubtedly have been due to this 

 cause, but on the other hand the number of scintillations 

 observed, about 1/10000 of the number of a particles, is 

 considerably greater than is to be expected from the experi- 

 ments with radium C. Further experiments to clear up this 

 important point have been undertaken by Professor Marsden 

 in New Zealand. 



* Rutherford and Wood, Phil. Mag. xxxi. p. 379 (1916). 



