524 Prof. J. J. Thomson on the Relation between the 



conducting circuits, able to connect atom with atom, were 

 present, the circumstances would be much more favourable to 

 the formation of hydrochloric acid. For if the conducting 

 circuit stretched from a positive hydrogen atom to a negative 

 chlorine one, these atoms would retain their charges; whereas 

 when the circuits stretched from a negatively electrified 

 hydrogen atom to a positively electrified chlorine one, the 

 atoms would interchange their charges. Thus the effect of 

 these conducting circuits would be to cause all the hydrogen 

 atoms to be positively electrified, and all the chlorine ones 

 negatively ; this would of course increase the tendency for 

 the hydrogen and chlorine to combine. 



In the preceding case wo have supposed the molecules to 

 be already split up into atoms ; when, however, we consider 

 the case of a mixture of molecules not already decomposed, 

 we see that something more than the stretching of a conductor 

 from one atom to another is required to effect the interchange 

 of their charges. For suppose H, H, CI, CI (fig. 4) represent 



Fig. 4. 

 H H 



CI CI 



+ 



respectively a hydrogen and a chlorine molecule, and suppose 

 that the negative hydrogen atom is connected with the positive 

 chlorine one by a conducting circuit. Then, if the negative 

 charge of the hydrogen and the positive one of the chlorine 

 were interchanged, the diminution in the potential energy due 

 to the Volta effect would be 2(cr cl — cr H ), where <r cl , o- H are the 

 Volta coefficients of chlorine and hydrogen respectively, <r K 

 being negative. To set off against this diminution in the 

 potential energy due to the Volta effect, we have the increase 

 in the energy produced by tearing the — charge on the H 

 atom from its proximity to the + charge, and forcing it 

 close to the — charge on the CI atom. The increase in the 

 potential energy due to this cause will be of the order 

 2(Vi + V 3 ), where V x and V 2 are the potential differences 

 between the atoms in the hydrogen and chlorine molecules 

 respectively. Thus the diminution in the potential energy 

 when the charges are interchanged is 



8(*a-«H)-*lVi-yf), 



and the interchange will not 'go on unless this quantity is 

 positive. Now the potential difference due to the contact of 

 two substances is equal to the difference of their Volta 



