5*>0 Sir E. Rutherford on Collision of 



as point charges, no advantage in explanation is gained by 

 supposing that the free charges carried by the nuclei are 

 greater than those usually supposed ; for while such an 

 assumption gives an increased number of H atoms of all 

 velocities, it fails to account for (1) above. 



If we suppose the central forces fall off more rapidly than 

 the inverse square law, the proportion of swift atoms in- 

 creases relatively. This can be deduced from consideration 

 of the calculations given by Darwin * for the case of the 

 inverse cube law, and it is not difficult to see that this 

 relative increase of high speed particles becomes more 

 marked the more rapid the law of variation of the central 

 fore. In all cases, however, the pencil of H atoms should 

 be widely heterogeneous for all velocities of the colliding 

 a particle. It thus seems clear that no theory of single 

 central forces can account for the experimental facts. 



This is not unexpected, for we have every reason to believe 

 that the a particle lias a complex structure consisting pro- 

 bably of four hydrogen nuclei and two negative electrons f. 

 If we assume, for simplicity, that the hydrogen nucleus 

 acts as a point charge for the distances under consideration, 

 we still have a complicated system of forces near the nucleus 

 of the a particle. 



Now we have seen that the anomalous effects in hydrogen 

 manifest themselves when the two nuclei approach within 

 about 3 x 10~ 13 cm. of each other, Geiger and Marsden 

 have shown that the scattering of a particles in passing 

 through atoms of a heavy element like gold, is consistent 

 within experimental error with an inverse square law of 

 repulsion, and in the case of a head-on collision, the closest 

 distance of approach is about 3xl0 -12 cm. or about 

 10 times the distance in the case of a close collision between 

 the a particle and the hydrogen atom. It appears significant 

 that, in the latter case, the closest distance of approach is 

 about the same as the accepted value of the diameter of the 

 negative electron, viz. 3 - 6 x 10~ 13 cm. The observed effects 

 are similar to those to be expected if the helium nucleus, for 

 example, consisted of a charged disk of radius about 

 3 x 10~ 13 cm. with its plane perpendicular to the direction 

 of motion, and it seems clear that the helium nucleus must 

 have dimensions of this order of magnitude. 



If the helium nucleus is composed of two electrons and 

 four hydrogen nuclei, we should expect a complicated field 



* Darwin (loc. cit.). 



t Eutherford, Phil. Mag-, xxvii. p. 488 (1914). 



