Passage of a Particles through Hydrogen. 



829 



of 1*9 cm., so that the range in air o£ the a particle after passing 

 through the window was 6*9 — 1*9 = 5*0 cm. ; and according to 

 theory*, if a. particles and H particles of the same velocity are 



Foil. 



Atomic Weight 

 A. 



vr. 



Wt. per em 2 required 

 to reduce beam to 

 40 p. c. = W. 



W-f Va. 



Aluminium ... 



27-1 

 63-6 



119 



195-2 



5-2 



8-0 



10-9 



14-0 



•0108 

 •0132 

 •0180 

 •0266 



Mean 



2-1x10-3 

 1-65 „ 

 1-65 „ 

 1-9 „ 



l-8xl0- 3 



PP 



Tin 



Platinum 



absorbed similarly, the swiftest H particle produced as above 

 ought to have a range equivalent to about 4x5*0 = 20 cm. 

 of air. The hydrogen between the screen and source was 

 calculated from Taylor's f results to have an air equivalent of 

 70 cm., leaving 13 cm. an equivalent for the swiftest H 

 particle. Although the air equivalent of metal foils varies 

 somewhat with the velocity of a particles and therefore pre- 

 sumably for H particles, we may take for the sake of 

 comparison the weights per unit area per cm. air equivalent 

 given by Richardson % and the author for a particles 6 cm. 

 from the end of their range. Multiplying these numbers 

 by 13 we obtain 0*021 gm, Al, 0*029 gm. Cu, 0*041 gm. 

 Sn, 0*055 gm. Pt. As will be seen from fig. 3 these 

 numbers are not very different from the amounts of the 

 various substances required to completely absorb the H 

 particles, and the agreement verifies the fundamental 

 assumptions made. 



The foregoing method is somewhat unsuitable for a deter- 

 mination of the number of H particles produced by a given 

 number of a particles of known velocity, since a particles of 

 all velocities are present, and further, the minimum velocity 

 at which a H particle will produce a visible scintillation is a 

 matter of conjecture. However, the number actually ob- 

 served was about that to be expected from calculations on 

 Darwin's formula, making reasonable assumptions for the 

 difference between the theoretical and experimental 

 conditions. 



* Darwin, loc. cit 



t T. S. Taylor, Phil. Mag. xxvi. p. 402 (1913). 



X Marsden & Richardson, Phil. Mag\ xxv. p. 184 (1913). 



