356 Prof. Oliver Lodge on the Seat of the 
which we find about 320 microvolts ; and for Cu/air, which 
again we don’t find. Add them all up with their proper signs, 
and we have the total E.M.I’. of the circuit. | 
Again, consider the case of a Daniell cell at a given tem- 
perature producing a current; we shall have to look in each 
series for Zn/ZnSQ,, for ZnSO,/CuSO,, for CuSO,/Cu, and 
for Cu/Zn, and add them all up. It is true that these tables 
of numbers have practically yet to be made, for at present 
they include so few substances ; but that does not affect the 
question of the existence and independence of these two kinds 
of series. 
It is, of course, a question how far all E.M.F. of contact 
may be found to depend on chemical tendency. For instance, 
when bismuth and antimony are put into contact, does the 
H.M.F. developed measure the alloying affinity of these two 
metals? When sodium is dropped into mercury, does the 
heat produced represent the thermoelectric power of a sodium- 
mercury junction? When metal touches glass, does the 
tremendous H.M.I’. developed represent a tendency of the 
metal to combine with the glass? These are questions for 
experiment to decide; but to me it does not seem probable 
that it will reply in the affirmative. 
We know that Sir W. Thomson, and Davy before him, con- 
sidered the apparent contact-force at the junction of zine and 
copper to be due to the chemical affinity between these two 
metals, and to be measured by the heat of formation of brass; 
but this we have seen strong reasons for disbelieving. It 
sounds more probable that the real contact-force at a junction 
of bismuth and antimony should be due to the chemical 
affinity between these metals; but perhaps it is no more true. 
The greater part of a contact-force of this kind is probably 
due to a physical difference between the metals, such as 
difference in atomic velocity, and has no close relation to their 
chemical affinities for each other. It is, however, just possible 
that part of a metallic junction-force is due to chemical ten- 
dency between the two metals in contact. Tor instance, take 
the case of zinc and copper. ‘There is, I suppose, an undoubted 
affinity between them, as shown by the formation of brass 
under proper conditions. [If chemists assume the right to 
demur to this, on the ground that the two metals mix equally 
well in any proportions, one can choose any other pair of 
metals—say, perhaps, copper and tin—for which the statement 
does not hold.| Now does this affinity result in any H.M.F. 
between them on making contact? This question, I appre- 
hend, is to be answered by passing a current for a long time 
across a copper-zine junction and seeing if any brass does, 
