XX Proceedings. {^March yfli, igii, 



been experimentally tested by Geiger for a particles, and found 

 to hold within the limit of experimental error. 



From a consideration of general results on scattering by 

 'different materials, the central charge of the atom is found to be 

 very nearly proportional to its atomic weight. The exact value 

 of the central charge has not been determined, but for an atom 

 of gold it corresponds to about loo unit charges. From a 

 comparison of the theories of large and small scattering, it is 

 concluded that the effects are mainly controlled by the large 

 scattering, especially when the fraction of the number of 

 particles scattered through considerable angles is small. The 

 results obtained by Crovvther are for the most part explained 

 by this theory of large scattering, although no doubt they are 

 to a certain extent influenced by small scattering. It is con- 

 cluded that for different materials the fraction of particles 

 scattered through a large angle is proportional to NA" where N 

 is the number of atoms per unit volume, and A the atomic 

 weight of the material. 



The main results of large scattering are independent of 

 whether the central charge is positive or negative. It has not 

 yet been found possible to settle this question of sign with 

 certainty. 



This theory has been found useful in explaining a number 

 of results connected with the scattering and absorption of a and 

 /3 particles by matter. The main deductions from the theory are 

 at present under examination in the case of the a rays by 

 Dr. Geiger using the scintillation method. 



Dr. H. Geiger read a paper entitled " The Large 

 Scattering of the « Particles," of which the following is 

 an abstract. 



Geiger and Marsden have shown that a small fraction of 

 the o particles incident on a thin film of matter are so scattered 

 that they emerge again on the side of incidence. In the 

 present paper the fraction of the a particles scattered through 

 various large angles by a thin gold foil has been experimentally 



