110 Prof. E. Rutherford on the Velocity and 



tension, and enable the thinned film to form in stable equi- 

 librium with the thicker parts. If this be the case, the practical 

 process of thinning will take place by the sudden removal 

 of a finite thickness from a limited area, forming a spot with 

 a sharply defined edge. From the point of view of this 

 explanation there is no necessity to limit to a single case the 

 number of films, having thicknesses finitely different from each 

 other, which might be capable of being formed in this way. 

 It is well known indeed that black films occur in two different 

 thicknesses ; and Stansfield. * and Johonnottf have recently 

 drawn attention fo the existence of a series of at least three 

 grey films of thicknesses intermediate between those of th<* 

 white and the black. 



The University, Sheffield. 

 August 1906, 



V. The Velocity and Energy of the a particles from Radio- 

 active Substances. By E. Rutherford, F.R.S., Macdonald 

 Professor of Physics, McGilt University, Montreal %. 



IN two previous papers §, an account has been given of 

 measurements of the velocity and mass of the a particle 

 expelled from selected products of radium, actinium, and 

 thorium. It has been shown that, within the limit of experi- 

 mental error, the a particles expelled from each of the 

 substances examined have the same mass but differ in the 

 initial velocity of their projection. 



Under the normal conditions of experiment, the a particle 

 carries a positive charge and behaves as if it were a charged 

 atom of matter projected from the radioactive matter at very 

 great velocity. The value of ejm — the ratio of the charge of 

 the « particle to its mass — was found to be 507 X 10 s . The 

 probability that the a particle is an atom of helium is discussed 

 in detail in one of the papers already mentioned. 



The range of ionization in air of the a particles from a 

 thin layer of active matter is a definite and easily measured 

 quantity, which is a characteristic of each individual product. 

 The ranges in air of the radium products have been accurately 



* Proc. Royal Soc. March 1906, vol. lxxvii. p. 314. 

 t Phil. Mag. June 1906, p. 746. 

 % Communicated by the Author. 



§ Rutherford, Phil Mag. Oct. 1906. Rutherford and Hahn, Phil. 

 Mag. Oct. 1906. 



