Stopping Poiuer and Atomic Number. 



479 



Bragg's rule. The limitations in one respect are the same 

 for both. They do not apply when large velocity variations 

 are permitted in the ol rays employed. Nevertheless Bragg's 

 rule is extremely useful over a considerable range of speeds, 

 and there is no reason to believe that the new rule should 

 be less applicable over the same range. The degree of 

 numerical agreement is shown by the figures in columns 

 V., VI., and VII. of the Table. The constancy of the 

 numbers in column VII. is the measure of the success 

 of the rule. The average deviation from the mean is 











I 



'ig- 



1. 



























' 



s® ? * 



3 

 2 

















/An 



















JJ Cu 















Air 





Al Jr 











S 





Li s 

























He 



















6 e io 



Atomic Number. 



5 per cent, for column VII. as against 8 3 per cent, for 

 column IV. The maximum deviation is 11 per cent, as 

 against 25 per cent, for the square root rule. The case 

 of the helium-hydrogen ratio is interesting. According to 

 the square root law, remembering that hydrogen is di- 

 atomic, the stopping powers of the two gases should be 

 equal. All the observations prior to 1914 had made the 

 ranges of a particles nearly the same for the two gases, 

 that in helium being slightly less than that in hydrogen. 

 In 1^14 Taylor found (Phil. Mag. vol. xxvi. p. 402, J 914) 

 that if special precautions were taken to purify the helium, 

 then- the range was actually 5 per cent, greater in helium 



