554 Sir E. Rutherford on Collision of 



arise from the absorbing material and source. These are 

 always present, and in number comparable with those pro- 

 duced by the admission of hydrogen. While the general 

 results show that, under the experimental conditions, the 

 number of H atoms becomes relatively small for ranges of 

 a particles between 2 and 3 cm., it is not possible to say 

 with certainty whether the number falls to zero for still 

 smaller velocities of the incident a. particles. We are unable 

 to continue observations with this experimental arrangement 

 for absorptions less than 7 cm. of air, so that no information is 

 available of the number of H atoms of range less than 7 cm. 

 In § 8, the absorption curves for H atoms produced by 

 a particles of different velocities have already been given. 



§ 10. Number of H atoms. 



We have already mentioned that the number of H atoms 

 is considerably greater than that to be expected on the 

 simple theory. It is important to determine the number as 

 accurately as possible, as it gives us important information 

 on the nature of the collision. The apparatus of fig. 1 was 

 employed. A thick copper plate with a hole 1*02 mm. 

 diameter was placed over the end silver plate of stopping 

 power about 6 cm., and the zinc sulphide screen placed 

 about 1 mm. away. Even allowing for possible scattering, 

 all the H atoms passing through the opening were counted 

 by the microscope, which had a field of view of diameter 

 2 mm. The source was part of a small hemisphere whose 

 outer surface was active, placed 2*85 cm. from the end of 

 the vessel. The space between was filled with hydrogen 

 at atmospheric pressure. The initial 7-ray activity of the 

 source was about 10 mg. Ua. 



The zinc sulphide screen was specially made for the 

 purpose and was estimated to have about 90 per cent, 

 efficiency in giving scintillations. As a result of three 

 separate concordant determinations, it was found that the 

 number of H atoms for hydrogen at N.T. P. falling on the 

 screen corresponded to 5*1 per minute per milligram of 

 activity, including an allowance of 10 per cent, for in- 

 efficiency of the screen. 



If 1 = length in cms. of path of a particles in hydrogen, 

 A = area of opening in sq. cms., 

 n — number of a particles emitted per second by one 



milligram of radium, 

 p = fraction of a particles which produce an H atom 

 per centimetre of path in hydrogen at N. T. P. 





