1905.] on the Structure of the Atom. 57 



forces give stability for displacements along the lines of magnetic 

 force. For example, if at any point near an atom the magnetic force 

 were radial, then a second charged atom at this point wonld be in 

 stal)le e({nilil>rium, provided the radial attraction lietwecn the atoms 

 at that point increased as the distance between the atoms increased. 



Let us now consider the forces produced by an atom of the kind 

 we have described. Take the case of an uncharged atom, i.e. one where 

 the sum of the charges on the negatively electrified corpuscles is just 

 equal to the positive charge in the sphere in which the corpuscles are 

 supposed to be placed. Let us consider the radial force to the 

 centre due to such an atom. Since there is as much positive as neg- 

 ative electricity in the atom, the average radial force taken over the 

 surface of a sphere with its centre at the atom is zero ; this does not 

 mean that the radial force is everywhere zero, but that at some places 

 it is directed towards the centre, and at others away from it. There 



B! 



Fig. 4. 



may be, as we shall see, certain directions in which the force changes 

 from attraction to repulsion, or vice-versa, as we travel outwards from 

 the sphere. 



Thus take the case of three corpuscles placed in a sphere. The 

 corpuscles, when in equilibrium, are at the corners of an equilateral 

 triangle iV B C ; let be the centre of the atoms of which these cor- 

 puscles form a part. Consider the force on a positively charged 

 particle. As we travel from A radially outwards, we find that the 

 force is always towards 0, and gets smaller and smaller as we get 

 further and further away. As the attraction diminishes as the distance 

 increases, there is no place at which the particle would be in equilib- 

 rium, stal)le or unstable. Suppose, however, we travel outwards along 

 C\ the prolongation of C 0, then when the particle is just outside A B, 

 the force on the particle is repulsive. This repulsive force diminishes 

 as we recede from the atom and vanishes at a certain distance D ; at 



