534 Dr. L. B. Loeb on the Recoil of 



compare tins with the number to be expected from Darwin's 

 theory *. 



1. The number of alpha particles deflected through such 

 angles by light atoms is extremely small. Assuming the 

 most favourable conditions possible, the numbers of alpha 

 particles per minute deflected through 170° by An, Al,. 

 and C, arriving on the portion of the ZnS screen in the field 

 of the microscope, are on the Darwin theory, 



An = 115-5, 

 Al = 3-78, 

 C = 0-35. 



The conditions which were found to be the most favourable 

 in the course of the experiments and used in the above com- 

 putations are : — 



Source = 50 mg. RaC. 

 Area target = 3 cm. 2 

 Area zinc sulphide field = 3' 14 mm. 2 

 Distance source to target = 2*5 cm. 

 Distance target to screen = 4*3 cm. 

 Thickness of deflecting layer as determined from range 

 of recoiling alpha particle is approximately 



An = 3 cm. air equivalent. 



Al = 1*4 cm. „ 



= 0*5 cm. 



It is seen that, under the best conditions, carbon would o-ive 

 about 10 particles per minute and aluminium 90, assuming a 

 multiplying factor of 30 for particles thrown straight forward 

 in H 2 gas as found by Prof. Rutherford. This factor would 

 probably be distinctly reduced in the case of aluminium. 

 The nuclear charge on aluminium is so high that the closest 

 distance of approach for an alpha particle is already near 

 the outside limit of 7 x 10 -13 cm. The reduction of the 

 velocity of the alpha particles in the first cm. of the de- 

 flecting layer would be Sufficiently great so that the impacts in 

 the last 4 ram. of range w r ould not attain the critical distance 

 of approach. The deflexions from this layer would then be 

 in numerical accord with those demanded by the Darwin 

 theory, and the number deflected would be thus reduced. 



* The first observations of the recoil of « particles incident on matter 

 were made by Geiger and Marsden. (Pro'c. Roy. SSoc. A. lxxxii. p. 49o 



1909). 



