March yf/i, iqit.] Proceedings. xix 



84, p. 226, 1 910). On this theory, the atom is supposed to 

 consist of a positive sphere of electrification containing an equal 

 quantity of negative electricity in the form of corpuscles. By 

 comparison of theory with experiment, Crowther concluded- 

 that the number of corpuscles in an atom is equal to about three 

 times its atomic weight in terms of hydrpgen. There are, how- 

 ever, a number of experiments on scattering, which indicate 

 that an a or /? particle occasionally suffers a deflexion of more 

 than 90" in a single encounter. For example, Geiger and 

 Marsden {Proc. Roy. Soc, 82, p. 495, 1909) found that a small 

 fraction of the a particles incident on a thin foil of gold suffers 

 a deflexion of more than a right angle. Such large deflexions 

 cannot be explained on the theory of probability, taking into 

 account the magnitude of the small scattering experimentally 

 observed. It seems certain that these large deviations of the n 

 particle are produced by a single atomic encounter. 



In order to explain these and other results, it is necessary 

 to assume that the electrified particle passes through an intense 

 electric field within the atom. The scattering of the electrified 

 particles is considered for a type of atom which consists of a 

 central electric charge concentrated at the point and surrounded 

 by a uniform spherical distribution of opposite electricity equal 

 in amount. With this atomic arrangement, an « or /3 particle, 

 when it passes close to the centre of the atom, suffers a large 

 deflexion, although the probability of such large deflexions is 

 small. On this theory, the fraction of the number of electrified 

 particles which are deflected between an angle and + d(p is 



given by -jifi)"' cot 0/2 cosec" (/>/2^0 where /i is the number of atoms 



4 

 per unit volume of the scattering material, t the thickness of 



material supposed small, and 6= , where iVe" is the charge at 



mir ° 



the centre of the atom, E the charge on the electrified particle, 

 m its mass, and // its velocity. 



It follows that the number of scattered particles per unit 

 area for a constant distance from the point of incidence of the 

 pencil of rays varies as cosec''(/)/2. This law of distribution has 



