Electromotive Forces in the Voltaic Cell. 975 
The alternative calculated number sometimes given is 
merely to show the kind of variation probable in those cases 
from uncertainty of data. In each case of agreement the 
calculated number is a little higher than the observed, as was 
to be expected. No reason occurs to me for the breakdown 
and apparent interchange, in the case of lead and iron, but 
such vague guesses as may occur to every one. 
Measurements of the H.M.F. between clean metals plunged 
into distilled water or weak acid, have been made by Clifton 
and by Beetz*. I suppose one is justified in calling them:— 
Volta Effects in Water. 
Observed by 
. Calculated from 
Metal pair. GHD Or OM GanOs ee 
Beetz. Clifton. 
Zine :— 
WepPer) sic. .c..-s. 1-0 ‘98 82 to 92 
ReMRED Rs naan ccna irs 1-23 
Pinkinnm _....... 18 or less. 1°52 1-3 (Smee). 
Sodium amalgam :— 
Te eee 1-0 ‘78 
MRMRCE seccac.s, «. 2:0 1-79 
VET occa ta ce esds. 26 2°05 
Plating -.......- 2°8 2°31 
I do not wish to blink the fact that some of the numbers in 
the former of the above tables afford a rather poor support to 
my theory ; but it must be remembered, on the other hand, 
that they are not relative numbers only that we have calcu- 
lated, but absolute ; and the fact that the heats of combus- 
tion reduced to volts are numbers of the same order of 
magnitude as the Volta effects, is of itself a strong confirma- 
tion of the belief that chemical strain at the air-contacts is 
the real cause of the apparent contact-force at the junction 
of two metals. 
The agreement of the numbers, though not exact, seems to 
me too close to be the result of accident. One may, I think, 
claim that the hypothesis whence the calculated numbers are 
obtained is justified by the figures as far as they go. It is 
not put forward as a completed theory, but only asa first step 
to such a theory. I believe it to be a step in the direction of 
the truth, but it requires working out and elaborating by a 
scientific chemist. 
18. Not many measurements of metal pairs have been made, 
even in air; for mere permutations such as copper-tin, tin- 
silver, &c. follow at once from the numbers given above by 
Volta’s series-law ; but in gases other than air one has at 
* Ann. der Physik, x. p. 348 (1880). 
