Collisions of a Particles with Hydrogen Nuclei. 925 



The maximum velocity of the H particles projected by 

 the « particles of RaC has been measured by Sir Ernest 

 Rutherford *, and found to be in accord with that relation. 

 He has also shown that the range of the H particle is pro- 

 portional to the cube "of its velocity. Therefore, if R is 

 the range given to an H particle by a direct impact, the 

 range R of a uarticle projected at an angle 6 is given by 

 R = R o cos 3 0. 



The validity of the assumption that the law of con- 

 servation of energy holds in these collisions can be tested 

 by observing simultaneously the range and direction of 

 the H particles. It will be shown later that the above 

 relation holds within the accuracy of the experiments, and 

 the assumption is therefore justified. 



The observations then consist in counting the number of 

 • H particles produced within a given angle by a known 

 pencil of a particles. This number is a direct measure 

 of the probability of a collision" which will project the 

 H particles within an angle #, and this probability will 

 depend on the structure of the a particle and of the 

 H particle. 



If both particles can be regarded as point charges, it can 

 be shown that the number of H particles projected within 

 an angle 6 by a single a particle in its passage through 

 1 cm. of hydrogen gas is 



n = 7rN^ 2 tan 2 0, 



where N is the number of H atoms per c.c, and 



As stated above, Sir Ernest Rutherford found that this 

 relation did not hold, and he attributed the discrepancy 

 between theory and experiment to the complex structure 

 of the u particle. On the nuclear theory, the c particle is 

 composed of four H nuclei and two electrons. For the 

 present, therefore, it is justifiable to regard the H particle 

 as a point charge and to ascribe the difference between 

 experiment and the simple theory to the complexity of the 

 a particle. 



The experiments give a relation between n and 6 for a 

 given velocity of the ol particle, or, if observations are made 

 with sets of a particles of different velocities, a relation 

 between the three quantities n, 0, and V. 



* Rutherford, loc, cit. 



