670 Prof. E. Rutherford on the 



explain the scattering of electrified particles in passing through 

 small thicknesses of matter. The atom is supposed to consist 

 of a number N of negatively charged corpuscles, accompanied 

 by an equal quantity of positive electricity uniformly dis- 

 tributed throughout a sphere. The deflexion of a negatively 

 electrified particle in passing through the atom is ascribed to 

 two c;iuses — (1) the repulsion of the corpuscles distributed 

 through the atom, and (2) the attraction of the positive 

 electricity in the atom. The deflexion of the particle in 

 passing through the atom is supposed to be small, while 



the average deflexion after a large number m of encounters 

 © © 



was taken as \ ; m . 0, where is the average deflexion due 

 to a single atom. It was shown that the number N of the 

 electrons within the atom could be deduced from observations 

 of the scattering of electrified particles. The accuracy of this 

 theory of compound scattering was examined experimentally 

 by Crowther * in a later paper. His results apparently 

 confirmed the main conclusions of the theory, and he deduced, 

 on the assumption that the positive electricity was continuous, 

 that the number of electrons in an atom was about three 

 times its atomic weight. 



The theory of Sir J. J. Thomson is based on the assumption 

 that the scattering due to a single atomic encounter is small, 

 and the particular structure assumed for the atom does not 

 admit of a very large deflexion of an a particle in traversing 

 a single atom, unless it be supposed that the diameter of the 

 sphere of positive electricity is minute compared with the 

 diameter of the sphere of influence of the atom. 



Since the a and yS particles traverse the atom, it should be 

 possible from a close study of the nature of the deflexion to 

 form some idea of the constitution of the atom to produce 

 the effects observed. In fact, the scattering of high-speed 

 charged particles by the atoms of matter is one of the most 

 promising methods of attack of this problem. The develop- 

 ment of the scintillation method of counting single a particles 

 affords unusual advantages of investigation, and the researches 

 of H. Geiger by this method have already added much to 

 our knowledge of the scattering of a rays by matter. 



§ 2. We shall first examine theoretically the single en- 

 counters f with an atom of simple structure, which is able to 



* Crowther, Proc. Roy. Soc. lxxxiv. p. 226 (1910). 



t The deviation of a particle throughout a considerable angle from 

 an encounter with a single atom will in this paper be called " single*' 

 scattering. The deviation of a particle resulting from a multitude of 

 small deviations will be termed '•compound " scattering. 



