Deflexion of a Particles through Large Angles. 621 



the scattering was found to vary at a rate more nearly 

 proportional to the inverse fourth power of the velocity than 

 to any other integral power. Owing to the comparative 

 uncertainty of the values of the velocity for small ranges, 

 however, the error of experiment may be somewhat greater 

 than appears from column V. of the table. 



In these experiments it proved essential to use a source 

 possessing a high degree of homogeneity. In earlier experi- 

 ments, where we were not able to fulfil this condition, the 

 scattering apparently increased much more rapidly than the 

 inverse fourth power of the velocity of the Ra C a particles. 

 Even with a source of Ra with which only a small quantity 

 of emanation was associated, the amount of scattering first 

 rapidly increased on interposing the sheeis of mien, then 

 showed a slight decrease, and finally increased again. This 

 irregularity was due to the a particles of the emanation and 

 RaA, which are of shorter range than those of Ra C, and 

 therefore more easily scattered. 



The measurements could not easily be extended to a. 

 particles of lower velocity than corresponds to a range of 

 about 1 centimetre, owing to the difficulty of observing the 

 faint scintillations at lower ranges. However, in one 

 particular experiment, by adding sheets of mica to cut down 

 the velocity the number of scattered « particles appearino- 

 on the screen was increased 25 times, showing how easily 

 the u particles of low velocity are scattered. 



The results of the examination of the homogeneity of the 

 scattered a particles showed that at least in the case of gold 

 they remained practically homogeneous after the scattering. 

 Experiments of this nature in the case of scattering foils of 

 low atomic weight would be very interesting, but are some- 

 what difficult. 



Determination of Absolute Number of Scattered 

 a Particles. 



In the previous sections we have completely verified the 

 theory given by Prof. Rutherford. Since, according to this 

 theory, the large deflexion of an a particle is the result of a 

 close encounter with a single atom of matter, it is possible to 

 calculate the magnitude of the central charge of the atom 

 when the fraction of a particles scattered under definite con- 

 ditions is determined. We have made several attempts under 

 different conditions to obtain a quantitative estimate of the 

 scattered particles, but the results so far have only given us 

 an approximate value. The main difficulty arises from the 

 fact that the scattered particles consist of such a small. 



