CH. xvi.] U COHERER 157 



electrons on each other across molecular distances. 

 It may be said to be a result of "residual affinity."* 



Ions cannot thus combine ; because if they were 

 oppositely charged their combination would be 

 chemical, and if they were similarly charged they 

 would strongly repel each other. But if ions arrive 

 at a metallic electrode, or are provided with other 

 means of passing on their free charges, they cease 

 to be ions ; and then the diselectrified atoms can and 

 do combine molecularly with each other. 



It is of course possible for an ion to have more 

 than one free electron, forming a dyad or a triad 

 radical ; and the way in which a neutral group can 

 receive, and by rapid re-adjustment pass on, an^ extra 

 foreign electron, reminding one of the re-adjustment 

 of the films in a lather when one compartment bursts, 

 is doubtless instructive. 



The effect of electric polarisation on such a 

 neutral group of electrons is noteworthy. The effect 

 of a charged body in the neighbourhood is at once to 

 disturb the equilibrium, and to perturb the grouping 

 throughout the atom, more or less : it will cause the 

 negative electrons to protrude slightly on one side 

 and the positive on the other (see fig. 21 where two 

 different but very complete kinds of polarisation are 

 shown). 



If two molecules were beyond each other's molecular 

 range, and if the neighbouring surfaces could by any 

 means as by the supply of electricity from without 

 be oppositely electrified, the forces of cohesion 

 would be intensified momentarily, by something akin 

 to chemical affinity, and cohesion would set in over 

 ultra-molecular distances. This appears to be what 

 goes on in a " coherer." The opposite charges 



* See Lodge in Nature, 1904, vol. 70, p. 176. 



