570 Professor Sir Ernest Rutherford [June 6, 



In the exiDeriments of Geiger and Marsden, the nnml)er of 

 <x-particles scattered through 5' was observed to be about 200,000 

 times greater than the number through 150'. The variation with 

 angle was in close accord with the theory, showing that the law of 

 inverse squares holds for distances between oMi x 10~^^ cm. and 

 4- 8 X 10"^^ cm. in the case of the gold atom. The experiments of 

 Crowther in 1910 on the variation of scattering of /?-rays with 

 velocity indicate that a similar law holds also in that case, and for 

 even greater distances from the nucleus. 



We have seen that Marsden was able by the scintillation method 

 to detect hydrogen atoms set in swift motion by a-particles up to 

 distances about four times the range of the incident a-particle. In 

 Marsden's experiments a thin-walled glass tube filled with radium 

 emanation served as an intense source of rays. Since the lack of 

 homogeneity of the a-radiation and the absorption in the glass are 

 great drawbacks in making an accurate study of the laws controlling 

 the production of swift atoms by impact, I have found it best to use 

 for the purpose a homogeneous source of radium C by exposing a 

 disc in a strong source of emanation. Fifteen minutes after removal 

 from the emanation the a-rays from the disc are practically homo- 

 geneous, with a range in air of 7 cm. By special arrangements very 

 intense sources of a-radiation can be produced in this way, and in 

 the various experiments discs have been used the y-ray activity of 

 which has varied between 5 to 80 milligrams of radium. Allowance 

 can easily be made for the decay of the radiation with time. 



In the experiments with hydrogen the source was placed in a 

 metal box about 3 cm. away from an opening in the end covered by 

 a thin sheet of metal of sufficient thickness to absorb the a-rays 

 completely. A zinc sulphide screen was mounted outside about 

 1 mm. away from the opening, so as to allow for the insertion of 

 absorbing screens of aluminium or mica. The apparatus was filled 

 with dry hydrogen at atmospheric pressure. The H atoms striking 

 the zinc sulphide screen were counted by means of a microscope in 

 the usual way. The strong luminosity due to the y8-rays from 

 radium C was largely reduced by placing the apparatus in a powerful 

 magnetic field which bent them away from the screen. 



If w^e suppose, for the distances involved in a collision, that the 

 a-particle and hydrogen nucleus may be regarded as point chai-ges, 

 it is easy to see that oblique impacts should occur much oftener than 

 head-on collisions, and consequently that the stream of H atoms set 

 in motion by collisions should contain atoms the velocities of which 

 vary from zero to the maximum produced in a direct collision. The 

 slow-velocity atoms should greatly preponderate, and the number of 

 scintillations observed should fall off rapidly when absorbing screens 

 are placed in the path of the rays close to the zinc sulphide screen. 



A surprising effect was, however, observed. Using a-rays of 

 range 7 cm., the number of H atoms remained unchanged when the 



