552 Sir E. Rutherford on Collision of 



of H atoms due to a particles of range &'2 cm. lias a maximum 

 range 24'8cm. The corresponding absorption curve is given 

 in curve B. The intermediate curve shows the distri- 

 bution to be expected for the hydrogen column, supposing the 

 H atoms are all projected in the direction of the a particles 

 with a velocity proportional to the velocity of the a particle 

 at each point of the hydrogen column. 



This theoretical curve C is very similar in all respects to 

 the experimental curve (fig. 4, curve A), showing that the 

 H atoms produced in a thin film of hydrogen are nearly 

 homogeneous in velocity and are thrown forward in the 

 direction of the colliding a. particles. 



It d( es not follow that the direction of the H atom coincides 

 with the direction of the a. particles, but the average deflexion 

 cannot be much more than 10° or 15°. For an angle of 

 deflexion of 0, the range of the H atom in the direction 

 of the a particles is l» max x co* 4 #. The value of cos 4 # is 

 0-94 lor 10°, 0-87 for 15°, and 078 for 20°. An average 

 value of 6 of 20° would make the decrease in the number 

 begin about 13 cm. instead of 19 cm. 



It is to be anticipated that the average angle of deflexion 

 should increase rapidly with decrease of the velocity of the 

 a particle. The rapid changes in shape of absorption curve 

 with change of velocity of u particle are at any rate partly 

 due to this cause. 



It is difficult to determine directly the actual average angle 

 of deflexion of II atoms, since the H aton s are scattered con- 

 siderably in passing through the minimum 7 cm. of air or 

 other absorbing material required to stop the a particles. 



There seems to be little doubt that if a film of hydrogen 

 were exposed to a particles of greater initial velocity than 

 those from radium C, a nearly homogeneous beam of H rays 

 would be obtained, all of which w ould travel nearly in the 

 direction of the a particles. 



§ 9. Variation of number of H particles with velocity of 

 a particles. 



In order to reduce the velocity of the a. particles, the 

 vertical source in the apparatus shown in fig. 1 was com- 

 pletely covered with different thicknesses of gold toil whose 

 stopping power in tfrms of air Mas accurately determined. 

 The distance between the source and screen was 3*3 cm., and 

 the apparatus filled with hydrogen at atmospheric pressure. 

 The gold loils were pressed lightly against the source to 

 prevent production of H atoms between the source and 

 foils. The number of H particles was determined after 



