Structure of the Atom, 



493 



Mr. Marsden has kindly made experiments for me to test 

 whether the presence of such hydrogen atoms can be detected. 

 A detailed account of his experiments will appear later, but 

 it suffices to mention here that undoubted evidence has been 

 obtained by him that some of the hydrogen atoms are set in 

 such swift motion that they are able to produce a visible 

 scintillation on a zinc sulphide screen and are able to travel 

 through hydrogen a distance three or four times greater than 

 the colliding a particle. The general method employed was 

 to place a thin a-ray tube containing about 100 millicuries of 

 purified emanation in a tube filled with hydrogen. The 

 scintillations due to the a. particle from the tube disappeared 

 in air after traversing a distance of about 5 cm. When the 

 air was displaced by hydrogen, the great majority of the scin- 

 tillations disappeared at about 20 cm. from the source, which 

 corresponds to the range of the a particle in hydrogen. A 

 small number of scintillations, however, persisted in hydrogen 

 up to a distance of about 90 cm. The scintillations were of 

 less intensity than those due to the ordinary a particle. The 

 number of scintillations observed is of the order of magnitude 

 to be anticipated on the theory of single scattering, supposing 

 that the nucleus in hydrogen and helium has such small 

 dimensions^ and that they behave like point charges for 

 distances up to 10 ~ 13 cm. 



There appears to be no doubt that the scintillations observed 

 beyond 20 cm. are due to charged hydrogen atoms which 

 are set in swift motion by a close encounter with an a par- 

 ticle. Experiments are at present in progress by Mr. Marsden 

 to determine the number of hydrogen atoms set in motion, 

 and the variation of the number with the scattering angle. 



It does not appear possible to explain the appearance of 

 such swift hydrogen atoms unless it be supposed that the 

 forces of repulsion between the a. particle and the hydrogen 

 atom are exceedingly intense. Such intense forces can only 

 arise if the positive nuclei have exceedingly small dimensions, 

 so that a close approach between them is possible. 



Dimensions and Constitution of the JS T ucleus. 



In my previous paper I showed that the nucleus must 

 have exceedingly small dimensions, and calculated that in 

 the case of gold its radius was not greater than 3 x 10" 12 cm. 

 In order to account for the velocity given to hydrogen atoms 

 by the collision with a. particles, it can be simply calculated 

 (see Darwin) that the centres of nuclei of helium and hydrogen 

 must approach within a distance of 1'7 x 10" 13 cm. of each 

 other. Supposing for simplicity the nuclei to have dimensions 



Phil. Mag. S. 6. Vol. 27. No. 159. March 1914. 2 L 



