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LII. Collision of a Particles with Light Atoms. II. Velocity 

 of the Hydrogen Atom. By Professor Sir E. Rutherford, 

 F.R.S* 



IN the first paper giving an account of the number of 

 H atoms produced by a. particles and their absorption 

 by matter, it has been implicitly assumed that the long-range 

 scintillations observed in hydrogen are due to swift hydrogen 

 atoms set in motion by close collisions with a particles. This 

 is supported by the observations that the range of the atoms 

 is in good accord with the value calculated by Darwin from 

 Bohr's theory of absorption of charged particles. 



Taking into account, however, the intense forces developed 

 in such collisions and the possibility of the disruption of the 

 structure of the nuclei involved in the collisions, it was 

 thought desirable to determine experimentally the mass and 

 velocity of these riving atoms, and to compare the values 

 with those deduced from the collision theory. Such a deter- 

 mination was rendered the more necessary by certain apparent 

 anomalies observed in connexion with the brightness and 

 distribution of H atoms, an account of which will be given 

 later in this paper. 



To determine the mass and velocity of the H atom, it was 

 necessary to measure the deflexions of a stream of H atoms 

 both in a magnetic and in an electric field. The experiments 

 were somewhat tedious and difficult on account of the small 

 number of H scintillations present under the experimental 

 restrictions. 



^Magnetic deflexion of H atoms. 



In these experiments it was necessary to produce the 

 H atoms at a definite point, and for this purpose a film of 

 paraffin wax of convenient thickness, exposed to an intense 

 beam of a rays, was used. The method finally adopted was 

 to compare directly the deflexion of a pencil of H atoms 

 produced from the film of paraffin wax, with the deflexion of 

 a pencil of a. rays using the same source of a rays in both 

 cases. 



The experimental arrangement is shown in fig. 1. 



The horizontal slits A and B, about 1 cm. broad and 1 mm. 

 wide, were mounted on a rectangular brass bar C. The 

 source R, consisting of a circular brass disk coated on one 

 side with radium C, was mounted on a vertical block D, close 

 to the slit A and making a small angle with the horizontal. 



* Communicated by the Author. 



