1878.] 



Contact Theory of Voltaic Action. 



225 



this conductivity with the conductivity for that temperature which 

 we have otherwise determined, but we infer from the curve that the 

 above formula gives a result twice or three times as great as the true 

 conductivity: this, however, might have been expected from our 

 knowledge of the phenomena of absorption in dielectrics. 



Sir W. Thomson, in the " Proceedings of the Royal Society " for June 

 10th, 1875, describes an experiment in which plates of zinc and copper, 

 in contact with the two sides of a plate of glass, form a voltaic cell. 

 When the glass was sufficiently heated to become conducting there 

 was evident charging of the plates * In the glass cell the maximum 

 charge became less and less after every short-circuiting, whereas in the 

 paraffin cell the maximum charge is constant. In the one case all the 

 elements of the circuit were solid, the glass being very far indeed from 

 its liquefying point, so that the amount of possible chemical action at 

 the junctions was small, and after every successive short-circuiting, 

 there was a less and less supply until the arrangement was approach- 

 ing a state which had hitherto been supposed to be peculiar to a com- 

 pound metallic circuit ; whereas our paraffin possessed mobility, and 

 the amount of possible chemical action at the surfaces of contact was 

 practically infinite in comparison with the currents charging the 

 plates. 



It is stated above that when any two pure substances otherwise 

 insulated are brought into contact with one another, the amount of 

 chemical action must be practically nil, but, as perfectly pure sub- 

 stances do not exist, when two bodies are brought into contact small 

 complete circuits are formed by three or more different substances, and 

 currents are produced until the possible chemical actions at the surfaces 

 of contact are exhausted. Now if one or more of these substances 

 in such elementary circuits are non-conductors, this exhaustion can 

 only proceed very slowly, and if the substances at the surfaces of 

 contact have immobility, the total amount of possible chemical action 

 will be necessarily small. Hence we see that the amount of chemical 

 action between bodies depends upon their conductivity and mobility, 

 as well as on the electromotive forces of contact, and as the chemical 

 action equivalent to the energy of small charges of electricity is quite 

 incapable of being detected by the ordinary methods of chemical ana- 

 lysis, and as the charges are easily measurable with an electrometer, we 

 have, if our theory be correct, in electrical measurement a method of 

 chemical analysis which is infinitely more sensitive than any purely 

 chemical method. Thus it is evident that in our paraffin cell there is 

 chemical action, but our colleague, Dr. Divers, has found no traces of 

 zinc in English paraffin oil which came out to Japan in galvanized 

 iron cases, nor in paraffin oil heated to a temperature of 60° C, in a 



* Our experiments of the increase of conductivity of glass with temperature are 

 described in the " Proceedings of the Eoyal Society " for the same date. 



