Passage of a Particles through Hydrogen. 241 



and e the charge of the hydrogen nucleus ; V is the velocity 

 of the a particles, and 6 is the angle between the direction 

 of their initial motion and the direction of projection of the 

 H particles considered ; Q is the number of particles in 

 the main beam, N the number of hydrogen atoms per unit 

 volume and t the thickness of the absorbing layer, and co the 

 solid angle over which the H particles are observed. 



It will be seen that the number of H particles projected 

 in a direction making an angle 6 with the direction of the 

 a particles should vary as see ;} #, a very different law from that 

 for the scattering of a particles in single collisions with heavy 

 atoms, where the number is found to vary as cosec 4 (/>/2. 



To make measurements, the same apparatus was used as 

 in the experiments of Geiger and Marsden * on the scattering 

 of a particles, minor alterations being introduced owing to 

 the smallness of the effect to be expected. A pencil of u 

 particles from an a-ray tube containing radium emanation 

 fell on a thin sheet (10//.) of wax, and the H particles 

 projected from the hydrogen in the wax were counted in 

 different directions by means of a zinc-sulphide screen. The 

 a particles were eliminated by placing sheets of aluminium, 

 sufficient to absorb them, between the wax and the zinc 

 sulphide. 



The effects observed were several times greater than 

 anticipated by formula. It was found, however, that the 

 source itself was emitting long-range particles capable of 

 scintillating, and that these were scattered in the wax, thus 

 causing the disturbance. Consequently an investigation of 

 these long-range particles had to be undertaken. 



Long- Range Particles emitted from the source of 

 Radium Emanation. 



The a-ray tube was placed in air at a distance of 8 cm. 

 from a zinc-sulphide screen, the path between the two being 

 in a transverse magnetic field to get rid of disturbing- 

 luminosity due to /3 particles. Although the a particles did 

 not penetrate more than 5'8 cm. from the a-ray tube, yet 

 scintillations were observed on the screen. These scintilla- 

 tions were similar in appearance to those produced by H 

 particles. On moving the zinc-sulphide screen further from 

 the source, the numbers fell off in the same way as 



* Phil. Mag. xxv. p. 604 (1913). 

 Phil Mag. S. 6. Vol. 30. No. 176. Aug, 1915. R 



