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



Suppose that the combination begins, ZnO being formed. 



+ — 

 Since a positive charge is withdrawn from the zinc and a 

 negative one from the oxygen, the zinc and oxygen, which 

 were originally electrically neutral, will now be charged nega- 

 tively and positively, so that at the surface of contact of the 

 zinc and the oxygen a charged condenser will be formed with 

 its negative charge carried by the zinc atoms, its positive 

 charge by the oxygen atoms. According to our hypothesis, 

 zinc and oxygen atoms in these electric states do not combine 

 unless connected by conducting circuits, so that, unless such 

 circuits are present, for each molecule of ZnO formed there 

 will be an increase in the charge on the condenser, and con- 

 sequently an increase in its potential energy. The charge on 

 the condenser is proportional to the number of molecules of 

 ZnO formed, so that the electrostatic potential energy will be 

 proportional to the square of the number of molecules ; the 

 diminution in the chemical potential energy due to the 

 formation of ZnO is only proportional to the number of 

 molecules of this substance. Thus the increase of the potential 

 energy increases faster with the number of molecules of ZnO 

 formed than the diminution in the energy; so that after a 

 certain amount of chemical combination has occurred, any 

 further combination will be attended by an increase in the 

 total potential energy. When this point has been reached 

 chemical combination will stop ; so that under these circum- 

 stances only a limited amount of chemical combination will 

 occur. Suppose now, however, that in addition to the zinc 

 and oxygen other substances are present which can form con- 

 ducting circuits between the negative zinc and the positive 

 oxygen atoms, and thus enable these atoms to interchange 

 their charges, so that the zinc becomes positively, the oxygen 

 negatively electrified. In this state they can combine so that 

 the condenser at the surface of the zinc will be discharged. 

 As this condenser is constantly being discharged, there will be 

 no increase in the potential energy due to it, and thus the 

 progress of the chemical action will be accompanied by a 

 diminution in the potential energy, and will therefore proceed 

 until one or other of the substances is exhausted. 



Similar considerations will show why but little chemical 

 action goes on if zinc is immersed in acid, while the action 

 proceeds freely when the zinc is connected to the acid by an 

 external conducting circuit. 



It is easy to rind an expression for the difference of 

 potential reached between the plates of the condenser formed 

 at the surface of the zinc when there is no external con- 



