Collision of ol particles with Hydrogen Atoms. 307 



X 



Also, since — = const, for any gas, at the rotation stage 



we have, at this stage, 



XI 



In a previous paper I have shown that — =pco i where 



P 

 o) = the angular rotation of electric discharge under electric 



intensity X. We have accordingly pco x -, a result which 

 has been experimentally verified. P 



Moreover, experiment has shown that (at the rotation 



X 



stao-e) —varies as the effective voltage of the induction-coil 



° J p fe 



producing the discharge. This voltage, therefore, must vary 

 inversely as the density of the gas used. It stands to reason, 

 therefore, that in the case of hydrogen, the effective voltage 

 required will be much higher than in air, and accordingly, 

 the effect will be much more difficult to observe. 



XXVIII. On the Collision of oc particles with 



Hydrogen Atoms. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN two papers in your Journal (June 1919) I gave an 

 account of some experiments on the collision of a. 

 particles with hydrogen atoms, using the scintillation method 

 for determining the number of H atoms set in swift motion. 

 With the microscope used at that time the counting of 

 scintillations was a difficult and trying experience, but 

 during the last year by the use of holoscopic lenses of large 

 aperture and suitable eyepieces, recently constructed in this 

 country, it has been found possible to improve greatly the 

 ease and certainty of counting such weak scintillations. 

 With the old arrangement, using a complex beam of H 

 atoms of different velocities, the observer was very liable to 

 miss the weaker scintillations due to lower velocity H atoms 

 in the presence of the brighter scintillations due to swifter 

 ones. 



The accurate determination of the relation between the 

 number and angle of ejection of the H atoms for different 

 speeds of the a, particles is of great importance in order to 

 throw light on the nature and distribution of the forces in 



