698 Prof. E. Rutherford and Dr. H. Geiger on the 



the scintillations were as bright i£ not brighter than those 

 from a thin film of uranium. Boltwood has found that the 

 range of the a particle from ionium is 2 '8 cms., so that it 

 appeared probable that the range of the « particles from 

 uranium had been overestimated. This conclusion was 

 confirmed by finding that the a rays from a thin film of 

 uranium were more readily absorbed by aluminium than 

 those from ionium. By a special method, the range of the 

 a particle from uranium has been measured and found to be 

 about 2*7 cms., while the range of the a particle from ionium 

 is a millimetre or two longer. Further experiments are in 

 progress to determine the range of the a particle from 

 uranium accurately, and to examine carefully whether two 

 sets of a particles of different range can be detected. 

 University of Manchester, 

 July 1910. 



LXXVI. The Probability Variations in the Distribution of 

 a Particles. By Professor E. Rutherford, F.E.S., and 

 H. Geiger, Ph.D. With a Note by H. Bateman *. 



IN counting the a. particles emitted from radioactive 

 substances either by the scintillation or electric method, 

 it is observed that, while the average number of particles 

 from a steady source is nearly constant, when a large number 

 is counted, the number appearing in a given short interval 

 is subject to wide fluctuations. These variations are especially 

 noticeable when only a few scintillations appear per minute. 

 For example, during a considerable interval it may hnppen 

 that no a particle appears ; then follows a group of a par- 

 ticles in rapid succession ; then an occasional a particle, and 

 so on. It is of importance to settle whether these variations 

 in distribution are in agreement with the laws of probability, 

 i. e. whether the distribution of a particles on an average is 

 that to be anticipated if the a particles are expelled at random 

 both in regard to space and time. It might be conceived, 

 for example, that the emission of an a particle might pre- 

 cipitate the disintegration of neighbouring atoms, and so 

 lead to a distribution of a particles at variance with the 

 simple probability law. 



The magnitude of the probability variations in the number 

 of a particles was first drawn attention to by E. v. Schweidler f . 

 He showed that the average error from the mean number of 

 a particles was \/N • t> where N was the number of particles 

 emitted per second and t the interval under consideration. 

 This conclusion has been experimentally verified by several 



* Communicated by the Authors. 



t v. Schweidler, Oongres Internationale de Badiologie, Liege, 1905. 



