236* Dr. It. D. Kleeman on 



e 2 a 3 



comparison with a the expression becomes —^- . Now a 3 is 



proportional to the volume of the atom, and this, we have 

 seen, is proportional to ??i 1/2 , and the attraction is thus pro- 

 portional to the square root of the atomic weight of the 

 atom and inversely proportional to the fifth power of the . 

 distance of separation of the electron from the centre of the 

 atom. It is of interest that the "chemical" attraction 

 between two atoms follows a similar law. Thus the writer 

 has shown in the paper mentioned above that the "chemical" 

 attraction between two atoms is proportional to Ihe product 

 of the square roots of their atomic weights, or, if one atom 

 is always the same, proportional to the square root of the 

 other atom, and inversely proportional to the fifth power of 

 their distance of separation. The above result is of interest 

 and importance in connexion with the passage of a or j3 

 particles through matter. 



It is also interesting to note that the forces are of the same 

 order of magnitude. Thus if we substitute 3*4 X 10" 10 for e 

 and 10~ 8 for a, the expression for the electric attraction 



becomes ^ . The constant K relating to the 



chemical attraction between two atoms — say of lead, corre- 

 sponding to the above constant (tf 2 a 3 ), was calculated to be 

 equal to 4*14 x 10 -44 , which is of the same order of magnitude 

 as the above value. 



When a (3 particle in passing through matter encounters 

 an atom it gets deflected from its course and also produces 

 secondary (3 rays from the atom. The amount of secondary 

 radiation, and its direction of propagation and that of the 

 primary fi ray after an encounter, will depend on the nature 

 of the encounter. Let us suppose the secondary radiations 

 from a large number of atoms taken at random are made 

 from the same atom with the paths of the primary rays 

 parallel to one another, the deflected primary rays being 

 included in the secondary radiation. The relative distribution 

 of the secondary radiation round the atom in direction of 

 motion wnth respect to the direction of motion of the primary 

 electrons, and the distribution of velocity among the electrons 

 moving in any given direction, will be independent of the 

 number of atoms considered if a sufficiently large number is 

 taken. The various angles made by the secondary rays with 

 the direction of propagation of the primary (3 rays will be 

 grouped about a mean somewhat like the molecular velocities 

 according to Maxwell's law. The value of this mean angle, 



