[ io ] 



II. On the Theory of the Decrease of Velocity of Moving 

 Electrified Particles on passing through Matter. By N. 

 Bohr, Dr. phil. Copenhagen *. 



WHEN cathode-rays or a- and /3-rays penetrate through 

 matter their velocity decreases. A theory o£ this 

 phenomena was first given by Sir J. J. Thomson f. In the 

 calculation of this author the cathode- and /3-rays are assumed 

 to lose their velocity by collisions with the electrons con- 

 tained in the atoms of the matter. The form of the law, 

 found by this calculation, connecting the velocity of the 

 particles and the thickness of matter traversed, has been 

 recently shown by Whiddington J to be in good agreement 

 with experiments. Somewhat different conceptions are used 

 in the calculation of Sir J. J. Thomson on the absorption of 

 a-rays, as the latter, on account of their supposed greater 

 dimensions, are assumed to lose their velocity by collisions, 

 not with the single electrons but with the atoms of the 

 matter considered as entities. 



According to the theory given by Professor Rutherford § 

 of the scattering of a-rays by matter, the atoms of the matter 

 are supposed to consist of a cluster of electrons kept together 

 by attractive forces from a nucleus. This nucleus, which 

 possesses a positive charge equal to the sum of the negative 

 charges on the electrons, is further supposed to be the seat 

 of the essential part of the mass of the atom, and to have 

 dimensions which are exceedingly small compared with the 

 dimensions of the atom. According to this theory an 

 a-particle consists simply of the nucleus of a helium atom. 

 We see that after such a conception there is no reason to 

 discriminate materially between the collisions of an atom 

 with an a- or /3-particle — apart of course from the differences 

 due to the difference in their charge and mass. 



An elaborate theory of the absorption and scattering of 

 a-rays, based on Professor Rutherford's conception of the 

 constitution of atoms, was recently published by C. G. 

 Darwin ||. In the theory of this author the a-particles 

 simply penetrate the atoms and act upon the single electrons 

 contained in them, by forces varying inversely as the square 



* Communicated by Prof. E. Eutherford, F.R.S. 

 + J. J. Thomson, ' Conduction of Electricity through Gases,' pp. 370- 

 382 



X K. Whiddinaton, Proc. Rov. Soc. A. lxxxvi. p. 360 (1912). 

 § E. Rutherford, Phil. Mag. xxi. p. 669 (1911). 

 j| C. G. Darwin, Phil. Mag. xxiii. p. 907 (1912). 



