504 eepokt— 1884. 



8 in the ordinary way, and say that it is in the present case charged with 

 the E.M.F. of a volt. 



The quantity of electricity on either plate of such a condenser is 

 5(1-8 --8) or 6*; hence 



s(l-8-> = s' 0--S) = 8 (1-8--8). 



And the general relation between the three 5 's is like that of two 

 Leyden jars in cascade, viz. 



s + s 

 If the two pieces of metal are circular discs each of radius r, and at a 



r 2 

 distance z apart, the value of S is of course — . 



In Volta's form of the experiment, two plates are put near each other, 

 connected for an instant, then removed from each other, and the poten- 



tial of one of them observed. This measures — i — -, where % is the 



s+x 



capacity of the electrometer used. 



In Pellat's form the charges acquired by contact are neutralised, and 

 the oxygen atoms driven back to their normal distance, by an extraneous 

 E.M.F. carefully adjusted until the plates, after separation, exhibit no 

 charge. This measures 1*8 — '8 directly. 



16. So far I have spoken as if I were sure that (granting the hypothesis) 

 the potential of clean zinc is 1*8 volts below the air ; but I am not really 

 sure that this is anything better than an approximation. The fact that 

 no actual combination occurs makes the matter perhaps a little indefinite. 

 If an oxygen atom unites with a zinc atom, one has a right to say dis- 

 tinctly that i] has stepped down v ; but suppose they are only facing one 

 another, and wishing to combine, are we justified then in asserting that 

 the step v is ready for q to go down, and that it is the same v as before ? 

 It almost seems to depend on whether chemical attraction becomes greater 

 as two atoms approach one another, or becomes loss. 



Suppose, first, it becomes greater, which is the natural hypothesis, 

 then the v calculated from data obtained by permitting the com- 

 bination to occur will be too large for the step of potential caused by 

 the attraction of metal for ox3 r gen over a standard distance. On the 

 other hand, the differential force urging electricity across a junction 

 of two metals, which is observed in the Volta effect, may be somewhat 

 greater than simply the differences in their pull reckoned at standard 

 distances, because the approach of atoms to the zinc will increase 

 it on this side, and the recession of atoms from the copper will decrease 

 it on that. Hence the Volta effect may perhaps be expected to agree 

 better with calculation than the air/metal potential-difference does, if this 

 latter could be experimentally observed, which it never yet has been. 



Next, suppose that chemical attraction becomes less as atoms approach; 

 the step of potential between a metal and air will now be greater than 

 that calculated from chemical data ; nevertheless the Yolta effect will be 

 somewhat less than that due to the differences of such steps for two metals, 

 and may thus possibly agree pretty well with calculation. 



The agreement or non-agreement of Volta effects with calculation 

 doe* not therefore quite establish the accuracy of our calculated metal- 



