Alpha Particles from Light Atoms. 541 



1 mm. absorption in the box. Theory demands that the total 

 range of the particles be 3*35 cm., which is in agreement 

 with the above experiment, the range being less than 3*6 cm.. 

 Thus in this case, as Prof. Rutherford has shown for nitrogen 

 and hydrogen, the laws of momentum and energy seem to 

 hold for close encounters of the atoms. 



It is felt that if the number of particles deflected through 

 170° by aluminium be determined, fulfilling the conditions 

 specified above, the question at issue might be definitely 

 settled. If a gas-tight diaphragm of 2 mm. equivalent air 

 absorption can be obtained it is possible that success might 

 also be achieved with carbon at this angle. 



Summary. 



1. An attempt was made to verify the abnormal deflexion 

 of alpha particles, from light atoms for close encounters, 

 assumed by Prof. Rutherford from a study of the deflexions 

 of the atoms struck, by studying the deflexions of alpha 

 particles through large angles by light atoms. 



2. Lack of time prevented more than preliminary results 

 at an angle of 105°. The results as far as they go indicate 

 that collisions where the centres of the atoms of He and Al 

 approach within 7 x 10 -13 cm. of each other begin to show 

 the abnormalities found by Prof. Rutherford for nitrogen 

 and hydrogen. 



3. The range of the alpha particles deflected from Al at 

 105° lies below 3'6 cm., as would be expected if the laws of 

 momentum and energy hold. 



4. It is the writer's impression that in spite of the experi- 

 mental difficulties, viz. low counts, short range, gamma-ray 

 effects, contamination, and stray particles, a definite decision 

 as to the abnormality of aluminium ought to be possible at an 

 angle of 170°. 



In conclusion, the writer desires to express his sincere thanks 

 to Prof. Sir Ernest Rutherford, at whose suggestion the pro- 

 blem was undertaken, for his kind advice and the free use of 

 his radium in the course of these experiments. His thanks are 

 also due to the authorities of Manchester University for the 

 privilege of working in their laboratories. Finally, he desires 

 to express his thanks to Mr. W. Kay. whose able technique 

 in handling the source as well as whose confirmatory counts 

 taken from time to time were invaluable to the prosecution of 

 the work. 



Manchester University. 

 July 11, 1919. 



