Section III., 1915 [2li] Trans. R.S.C. 



On the Ionisation Tracks of Alpha Rays in Hydrogen. 



By Professor J. C. McLennan, F.R.S. and Mr. H. N. Mercer, 

 M.A., B.Sc, University of Toronto. 



(Read May Meeting, 1915). 



I. Introduction. 



In a paper by Marsden in the Phil. Mag. of May, 1914, experi- 

 ments are described in which it was found that when alpha particles 

 from radium are projected into hydrogen velocities are given to par- 

 ticles of the gas which enable them to pass through a thickness of 

 aluminium foil capable of stopping the fastest alpha particles. These 

 "H" particles, as Marsden designated them, have been shown by him 

 to be capable of producing visible scintillations on a zinc sulphide 

 screen when this is placed at a distance away from their source more than 

 three times the distance at which the alpha particles themselves pro- 

 duce scintillations. 



On account of the high velocities possessed by these "H" particles 

 and for various other reasons it is believed that they are the positively 

 charged nuclei of hydrogen atoms just as alpha particles are the charged 

 nuclei of helium atoms and that they are expelled from the atoms of 

 hydrogen when these are subjected to a direct impact by the alpha 

 rays. 



Moreover since these "H" particles are capable of producing 

 scintillations in a zinc sulphide screen and since scintillations are now 

 considered to be the results of ionisation it would appear that these 

 "H" particles are capable of ionising the atoms in the zinc sulphide. 

 The question, then, which naturally arises is: Are these "H" particles 

 capable of ionising the atoms of a gas through which they may be 

 projected ? In order to throw some light on this matter it was thought 

 well by the writers to make an experimental study of the ionisation 

 tracks of alpha rays in hydrogen and the following paper contains an 

 account of this investigation. 



II. Apparatus. 



The method followed was precisely the same as that devised by 

 C.T.R. Wilson^ and used by him in obtaining his exceedingly beautiful 



1 C. T. R. Wilson, Proc. Roy. Soc. A Vol. 87, pp. 277-292, 1912. 



Sec. HI. 1916—16 



