•622 Dr. H. Geiger and Mr. E. Marsden on the Laws of 



fraction of the original beam that different methods of 

 measurement have to be employed in the two cases. The 

 number of scattered a particles was determined from the 

 number of scintillations observed on the zinc-sulphide screen, 

 a correction being necessary owing to the fact that with the 

 particular screens used only about *S5 per cent, of the incident 

 a. particles produce scintillations. The number of a particles 

 in the main beam was in one case in which an emanation 

 tube was used (as shown in fig. 1, p. 607) determined directly 

 by the scintillation method, several weeks being allowed to 

 elapse, so that the emanation had decayed to a small value. 

 In other experiments Ra C deposited on the inside of a conical 

 glass tube (as in fig. 2, p. 611) was used, and the number of 

 a particles was calculated from its 7-ray activity and the 

 distance and area of the diaphragm determining the beam. 



The results showed that, using a gold foil of air equivalent 

 1 mm. (actual thickness 2*1 x 10~ 5 cm.), the fraction of 

 incident Ra G a particles (y = 2 # 06xl0 9 cm./sec.) scattered 

 through an angle of 45° and observed on an area of 1 sq. mm. 

 placed normally at a distance of 1 cm. from the point of 

 incidence of the beam, was 3' 7 x 10 ~ 7 . Substituting this 

 value in the equation given at the commencement of this 

 paper, it can be calculated that the value of the number of 

 elementary electric charges composing the central charge of 

 the gold atom is about half the atomic weight. This result 

 is probably correct to 20 per cent., and agrees with the 

 deduction of Prof. Rutherford from the less definite data 

 given in our previous paper. 



From the results of this and the previous sections it is 

 possible to calculate the probability of an a particle being- 

 scattered through any angle under any specified conditions. 

 For materials of atomic weight greater than that of 

 aluminium, it is sufficiently accurate to put N equal to half 

 the atomic weight in the equation given at the commence- 

 ment of the paper. 



It will be seen that the laws of " single scattering " found 

 in this paper are quite distinct from the laws of " compound 

 scattering " previously deduced by Geiger. It must be 

 remembered, however, that the experiments are not directly 

 comparable. In the present paper we are dealing with very 

 thin sheets of matter, and are measuring the very small 

 fraction of a particles which are deflected by single collisions 

 through relatively large angles. The experiments of Geiger, 

 however, deal with larger thicknesses of scattering foils and 

 angles of deflexion of a few degrees only. Under these 

 conditions the scattering is due to the combination of a large 

 number of deflexions not only by the central charges of the 



