60 Professors Ayrton and Perry on the Seat 



one of which was warmed up to 26° C, and which were 

 joined together by a syphon-tube containing mercury, ought 

 to have an E.M.F. of half a volt. But, as was to be anti- 

 cipated, we found experimentally that no such E.M.F. could 

 be obtained with such a thermopile, unless there were an 

 oxide present at the hot junction separating two of the metals, 

 and thereby introducing a high resistance. 



Contrasting these results with those given in our papers 

 read before the Royal Society (and which prove conclusively 

 that when a cell is formed of a number of substances, solid or 

 liquid, at the same temperature, its E.M.F. is equal to the 

 algebraical sum of the various contact-potential differences, 

 measured inductively, in air, of each of the pairs of substances 

 in contact in the cell), we are justified in concluding that 

 when layers of oxides or their salts are formed on the surfaces 

 of the substances employed, as in Mr. Brown's experiments, 

 the measured potential-differences do not represent the true 

 contact-potential differences of the substances employed. 



We are not disposed to think that Dr. Lodge has discovered 

 any striking coincidence between measured chemical heats of 

 combination and those heats as calculated on his theory. 

 Such coincidence as he has discovered is not more wonderful 

 than that the electro-positivity of the metals should be in the 

 order of their tendencies to combine with oxygen ; there can 

 be no doubt that an exceedingly important advance in the 

 sciences of Chemistry and Electricity will be made by the 

 philosopher who explains this and the twenty other coin- 

 cidences of a like nature which have been discovered. We 

 are quite sure, too, he would regard the seeming coincidences 

 which he has discovered as trivial, had he not at the beginning 

 made up his mind that the existence of contact-difference of 

 potential was absurd, and that what we and many others have 

 measured are due to air-effects. 



We are in the habit of using the expression " difference of 

 potentials between two points " as meaning, in Electro- 

 kinetics, the Electrostatic difference of potentials between two 

 copper wires, coming from the points. This practice is very 

 convenient; and there can be no objection to it, so long as 

 we do not imply that it is always the same as the Electrostatic 

 difference of potentials between the points. Now it seems to 

 us that Dr. Lodge, in following this convenient custom, has 

 fallen into the error of imagining that what he has been in 

 the habit of calling " difference of potential " is really the 

 same as " Electrostatic difference of potential." Indeed, we 

 believe that the root of the whole difficulty which prevented 

 Clerk Maxwell, and which now prevents Dr. Lodge, from 



