u Particles with Light Atoms. 571 



The calculated numbers are the sum of each observation 

 worked out separately. 



It will be seen that while there is a very satisfactory 

 agreement between theory and experiment for the a rays 

 from polonium, the agreement is not so good for the H atoms. 

 In the case of the a rays, the number of doubles shows that 

 the eye cannot distinguish an interval less than 1/10 second ; 

 while in the case of H atoms the number of doubles is nearly 

 twice the theoretical number calculated on this power of 

 distinction. Whether this difference is apparent or real is 

 difficult to decide, for it must be remembered that counting 

 such weak scintillations and at the same time distinguishing 

 time intervals make a difficult task. 



It is clear that under the experimental conditions, only a 

 small fraction of the number of scintillations can be regarded 

 as possible instantaneous doubles, and the effect is too small 

 and uncertain to draw any very definite conclusions. It may 

 be urged that a question of this kind could be settled more 

 definitely by arranging that a small number of scintillations 

 fell on the screen per minute when the probability of short 

 intervals becomes very small. On the other hand, it takes a 

 long time to count a sufficient number to compare theory 

 with experiment^ and it is very fatiguing to the eye and 

 unreliable to count for long under such conditions. 



I am much indebted to Professor Marsden for his valuable 

 help in obtaining and analysing data for me on this 

 important point. 



LIII. Collision of a. Particles with Light Atoms. III. Nitrogen 

 and Oxygen Atoms. By Professor Sir E. Putherford, 

 F.R.S* 



BOHP f has worked out a general theory of the absorp- 

 tion of electrified atoms in passing through matter, 

 and has verified his conclusions by consideration of the 

 absorption of a particles. On this theory, Darwin J has 

 shown that the range of a swift hydrogen atom in hydrogen 

 can be calculated, and the value so found is in good accord 

 with experiment. It is not difficult to deduce by the same 

 method that the range x in hydrogen of an electrified atom 

 of charge e and mass m moving with a speed equal to an 



* Communicated by the Author. 



t Bohr, Phil. Mao-. xxv . p. 10 (1913). 



J Darwin, Phil. Mag. xxvii. p. 499 (1914). 



2R2 



