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



The aggregation of molecules, as described in the preceding t 

 sections, is one method by which the attractions between the j 

 oppositely electrified atoms in the molecule might be weakened 

 sufficiently to allow chemical action to proceed. Another 

 method by which the same end might be accomplished would 

 be for the molecules to be immersed in, or to be close to the 

 surface of, a conductor or substance of large specific inductive 

 capacity. The effect of this, as explained in the Philoso- 

 phical Magazine, vol. xxxvi. p. 320, would be to diminish the 

 attraction between the atoms and allow them to be easily 

 separated. The substance which acts as the bridge for the 

 interchange of the charges on the atoms might be the sub- 

 stance which when collected into lumps served by its electro- 

 static action to diminish the attraction between the atoms. 

 This method of producing chemical change closely resembles 

 the preceding — in each of them the presence of a third sub- 

 stance is necessary, and in each of them the formation of 

 molecular ao-o Te aates, but whereas on the first view the 

 aggregates are those of the substances which are combining 

 chemically, on the second they are formed by the third 

 substance. 



The hypotheses to which we are led by the preceding con- 

 siderations may be summed up as follows : — 



The forces between charged atoms placed very near together 

 are modified by the electrochemical properties of the atoms. 

 In consequence of this an electronegative atom if positively 

 charged cannot, in the absence of a strong external electric 

 field, interchange its charge with the negative charge on an 

 electropositive atom unless the two are connected by an 

 external conducting circuit. 



Chemical action does not (in general) take place between 

 a single pair of molecules alone in the field, but requires the 

 formation of aggregates either of the interacting molecules or 

 of some third substance which is either a conductor of elec- 

 tricity or has a large specific inductive capacity. 



In the preceding statement an element is called electro- 

 positive or electronegative according as its Volta coefficient is 

 negative or positive. 



Thus, on these hypotheses, chemical combination between 

 two elements involves in general two processes : — (1) a process 

 by which all the electropositive atoms, half of which were 

 originally positively and half negatively charged, all become 

 positively charged, while the electronegative atoms become all 

 negatively charged ; (2) the combination of these properly 

 charged atoms to form a molecule of the compound. 



Jn some cases the process of chemical combination seems 



