concerning Volta s Contact Force, 369 



accident. On the other hand, it is undeniable that (either 

 accidentally or otherwise) the order of magnitude of the Volta 

 force can be calculated from the differential heats of com- 

 bination of the metals for oxygen ; which may be written 

 (Zn, 0) — (Cu, 0) . Hence, if it turns out that it can likewise 

 be calculated from the heat of formation of brass (Zn, Cu), it 

 will be either an odd coincidence or it will have an important 

 chemical meaning with regard to heats of combination or 

 chemical affinities in general. 



One way of {jutting its meaning may be that it tends to 

 strengthen what I may call von Helmholtz's hypothesis'*, 

 which can be stated roughly thus : — That the metals do not 

 really attract oxygen, nor do they really attract each other, 

 but that what they attract is electricity; that all chemical 

 attraction, apparently of matter for matter, is the consequence 

 of this real attraction of matter for electricity, the atoms being- 

 electrical ly charged. Further, that the Volta effect is caused, 

 simply enough, by the differential attraction of zinc and copper 

 for electricity. The orthodox way of putting it is as 

 follows : — The zinc attracts positive electricity most ; hence, 

 when they are put into contact, electricity flows across the 

 junction and the zinc becomes positive and the copper nega- 

 tive. Another way of putting it is to say that the zinc free 

 surface attracts negative electricity most, and accordingly 

 pulls it in from the space around as soon as the uniformity 

 of the force is anywhere interfered with by an inert metallic 

 contact. In either case its ready oxidisability by negative 

 ions is thus explained, and has not to be taken as an inde- 

 pendent fact or basis. Whereas, then, 1 had proposed to 

 calculate the Volta effect as (Zn, 0) — (Cu, 0), on this view 

 it would have to be reckoned as (Zn,E) — (Cu,E) ; though 

 what is to be considered precisely as meant by E may be 

 matter for further consideration. I am not going to say 

 anything against this far-reaching hypothesis in a hurry. It 

 demands separate treatment in the light of ' electron ' theory. 



This is what I intended when I admitted it possible that, as 

 Lord Kelvin said, I had too great a veneration for the need 

 for oxygen in this connexion. The sole reason why 1 think 

 that it is probably the oxygen film and not the tether dis- 

 continuity which causes the force, is that its magnitude cor- 

 responds so closely, not only in order of magnitude but in 

 relative details, with the appropriate energy of oxidation. At 

 the same time, if the electrical attraction is the cause of 

 oxidation tendency, this quantitative relation is not likely to 

 afford any means of discrimination. 



* Summarised in Phil. Mag. October 1885, p. 378. 



