a Particles icith L'ujlit Atoms. 577 



with an accurate determination. Since mica contains 

 oxygen as well as hydrogen we should obtain swift atoms, 

 and the number of scintillations observed was about that to 

 be expected from the amount of oxygen present, but was less 

 than the number observed in air. There appears to be no 

 doubt that the scintillations observed in air between the 

 ranges 7 and 9 cm. arise from collision of a particles with 

 N and atoms. The observation that the range of the swift 

 atoms, produced by a particles in their passage through 

 carbon dioxide, is equivalent to the range of atoms, indicates 

 that there are no carbon atoms carrying a single charge, for 

 in that case bright scintillations should have been observed 

 for ranges up to 12 cm. of air (see Table I.). 



It will be remembered that in the beautiful photographs 

 of Mr. C. T. R. Wilson * showdng the trails of a particles, an 

 example is given where the a particle in air shows a sudden 

 deflexion of 43°, and there is clear evidence of a well-marked 

 spur presumably showing the trail of the N or O recoil 

 atom. It is of interest to compare the length of this spur 

 with the range to be expected for a collision with an atom. 

 If (j> be angle of deflexion of the a particle and 6 the deflexion 

 of the atom, 



m sin 26 

 tan *=M-mcos20' 



where ??i=mass of atom and M — mass of a particle. 



Putting M = 4, m = 16, = 43°, then = 63°'55. 



If v = velocity of the a. particle before the collision, the 

 velocity of the atom 



= Ti . v cos 6 — "1/8 v, 

 M -f m 



while the velocity of the a particle after the collision 

 is *934 v. 



Range of recoil atom „ „ /-17 



.- 



Range of a particle after collision 



«G») =" 13 



This is based on the calculation that the maximum range 

 of atoms due to a particles from radium C is 7*8 cm., 

 while the observed range is 9*0 cm. Making this correction, 

 the value *13 becomes *15. 



It is possible to compare only roughly the ranges of the 

 a particle and recoil atom in the photograph, but the results 

 are in fair accord with the calculation. 



* C. T. R. Wilson, Proc. Roy. Soc. A. Ixxxvii. p. 277 (1912). 



