58 Professor J, J. Thorrison [IMarcli 10, 



greater distances from the atom than D, the force is attractive and 

 remains attractive at all greater distances ; thus a positively charged 

 particle Avould be in equilibrium at D, and it is easy to see that the 

 equilibrium would be stable, for if the particle were made to approach 

 0, the repulsive force would drive it back to D, while, if the particle 

 were to recede from D, the attractive force would drag it back. If 

 we represent the relation between the radial force and the distance 

 by a graph, a point above the horizontal axis corresponding to repul- 

 sion, and one below it to attraction, we obtain a curve of the following 

 character. The curve crosses the axis at the point D, the place where 

 the force vanishes ; after passing D, the force which is now attractive 

 increases as the distance from the atom increases, until a point E is 

 reached when the force is a maximum ; beyond E the attraction 

 diminishes as the distance increases. Thus, since in the region D E, 

 the force is attractive and increases as the distance increases, a positive 

 particle, placed in this region, might be in stable equilibrium, while 

 outside this region the equilibrium would be unstable. 



There would, of course, by symmetry be similar regions on A^ 

 B^, the prolongations of A and B respectively. It will be seen 



Fig. 5. 



that the nature of the force lietwen the atom and the charged particle, 

 is of the type postulated by Boscovich, i.e. a repulsion at short distances 

 succeeded by an attraction at greater ones. With the very simple 

 type of atom we have been discussing, there is only one change from 

 repulsion to attraction ; with atoms containing more corpuscles, the 

 graph representing the relation between force and distance becomes 

 more complicated, and we may have several alternations between 

 repulsion and attraction instead of only one as in Fig. 5. 



However complicated the atom, a distribution of forces of this 

 kind will only occur in a limited number of directions, or rather only 

 along directions making small angles with a limited number of axes 

 drawn in definite directions. 



I have here an arrangement to show the change in direction of 

 the force due to an atom. The atom is supposed to be one with three 

 corpuscles ; these are represented l)y the negative ends of three electro- 

 magnets arranged radially on a board, the }K)sitive ends of the magnets 

 which represent the j)ositive electrification in the sphere being at the 

 centre. We see tliat along the lines A\ B\ C^ the magnetic 

 force on a positive pole changes from repulsion to attraction at a 



