60 Professor J. J. Thomson [March 10, 



be possible ; if, however, D came inside the atom, the axes A\ B\ 

 C\ would cease to be axes of possible equilibrium. 



In some cases, the communication of a charge to the atom might, 

 in addition to affecting the position of equilibrium along the axes for 

 the uncharged atom, introduce axes of stahility which did not exist 

 when the atom was uncharged ; thus, in the case of a double pyramid 

 Fig. 7, if we gave a positive charge to the atom, the axes E, D, 

 which were not axes of equilibrium for the uncharged atom, would 

 become so for the charged one ; for if the atom had a positive charge, 

 the force on the negatively electrified particle would at a point a 

 great distance from the centre along E be an attraction, while close 

 to E it would be a repulsion ; there must be some point then when 

 the force changes from repulsion to attraction, so that this axis will 

 be one of equilibrium. 



In the case of a more complicated atom giving a distribution of 

 force changing from repulsion to attraction more than once, as in 

 the case represented in Fig. 5, there would be places along this axis 

 w^here a negatively electrified particle would be in stable equilibrium 

 and other places where a positively electrified particle would be in 

 stable equilibrium. The effect of giving a positive charge to this alone 

 would be to make the positions of equilibrium for the negative 

 particles approach the atom, those for positive particles recede from 

 it ; the effect of a negative charge would displace those positions in 

 the opposite directions. 



The forces we have been considering are those exerted by an atom 

 on a charged particle ; they would be a part (and in many cases, I 

 think, the most important part) of the forces acting on a second 

 atom, if that atom had an excess of one kind of electricity over the 

 other. Eemembering, however, that there is an electric field round 

 an atom, even when it is uncharged, and that an uncharged ^tom is 

 not an atom in which there is no electricity, but one where the negative 

 charge is equal to the positive, we easily see that two uncharged atoms 

 may exert forces on each other ; the calculation of these forces is, on 

 account of the complex nature of the atom, very intricate, and I shall 

 not go into it this evening. I shall treat the subject from the experi- 

 mental side. I have here two systems, each built up of magnets, each 

 containing as many positive as negative poles, and thus analogous to 

 an uncharged atom ; one of them is suspended from the arm of a 

 balance. Fig. 8. You see that I can place these systems so that they 

 repel each other when close together and attract each other when further 

 apart, so that these atoms would be in stable equilibrium under each 

 other's influence when separated by the distance at which repulsion 

 changes to attraction. 



The force which an atom A exerts on another atom B may be 

 conveniently divided into two parts : the first part, which we shall 

 call the force of the E type, depends upon the charge on B ; it is pro- 

 portional to tliis charge and independent of the structure of B, and 

 we might, without altering this force, replace B by any atom we pleased, 



