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LXX. On the Ionization Tracks of Alpha Rays in Hydrogen. 

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

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



[Plate XIII.] 



I. Introduction* 



IN a paper by Marsden in the Philosophical Magazine of 

 May 1914, experiments are described in which it was 

 found that when alpha particles from radium are projected 

 into hydrogen, velocities are given to particles 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 scintilla- 

 tions 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 produce 

 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 pro- 

 ducing scintillations in a zinc-sulphide screen, and. since 

 scintillations are now considered to be the results of ioniza- 

 tion, it would appear that these " H" particles are capable 

 of ionizing the atoms in the zinc sulphide. The question, 

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

 capable of ionizing 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 ex- 

 perimental study of the ionization 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. K. Wilson |, and used by him in obtaining 



* Communicated by the Authors. Read "before the Eoyal Society of 

 Canada, May 26th, 1915. 

 fC.T.E. Wilson, Proc. Roy. Soc. A. vol. lxxxvii. pp. 277-292 (1912). 



