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XCI. Ilie Passage of a Particles through Hydrogen. By 

 E. Marsden, Lecturer in Physics, University of Manchester*. 



ON the nucleus theory of an atom f , Sir Ernest Rutherford 

 and C. Gr. Darwin have recently discussed the relative 

 motions of an a particle and the nucleus of an atom in an 

 intimate collision. In the case of the encounter of an 

 a. particle of velocity Y with the nucleus of a hydrogen atom, 

 if the latter is projected in a direction making an angle 9 

 with the original direction of the a particle, then the velocity 

 of the "H" particle is given by w=l'6 Y cos 6. From 

 Bohr's 1 formula for the "velocity curve" of a charged 

 particle, it can be deduced that in an end-on collision, i. e. # = 0, 

 the " H " particle will have about four times the range of the 

 a particle producing it. Consequently, in the passage of a 

 particles through hydrogen the " H" particles maybe looked 

 for well beyond the range of the ordinary a particles, and in 

 the experiments to be described evidence of their existence 

 has been found. More detailed experiments are in progress 

 to investigate whether the distribution of their velocities 

 and relative motions with regard to the « particles producing 

 them is in agreement with the calculations from the simple 

 assumptions ; but, owing to the probable lengthy series of 

 observations necessary, it appears advisable to publish a 

 preliminary account of the experiments so far made. 



Fis-. l. 



b 



The apparatus used in the first experiments is shown in fig. 1, 

 and consists of a wide tube about one metre in length and 

 9 cm. in diameter, closed at one end by a zinc-sulphide screen 

 Z and at the other by a glass plate P on a ground flange. A 



* Communicated hv Sir E. Rutherford, F.R.S. 



t E. Rutherford, Phil. Mag. xxi. p. 699(1911); Phil. Mag. xxvii. 

 p. 488 (19U). C. G. Darwin, "Phil. Mag. xxvii. p. 499 (1914). 

 I N. Bohr, Phil. Mag. xxv. p. 10 (1913). 



