Electromotive Forces in the Voltaic Cell. 267 
14. The oxidation energy of zinc per gramme-equivalent 
(2. e. 65 grammes of zine or 16 of oxygen) is, according to 
the determinations of Julius Thomsen, Andrews, and Favre 
and Silbermann, 85,430, 84,825, and 83,915 respectively.. 
The amount of electricity needed to deposit a gramme- 
equivalent of zinc, or of any dyad- element, is, according to 
the modern determination of Lord Rayleigh *, 19,320 units. 
Hence the value of " which is a ratio evidently independent 
of the number of atoms dealt with, lies between $2498 and 
$4000 probably. Let us say it is $399° or 4:4. 
Now J in absolute measure is 42 x 10°; so the value of »v, 
which is (—) according to the above hypothesis, comes out 
1°85 x 108, that is 1°85 volt. 
This then, I say, is the step of potential between zinc and 
air. (To avoid circumlocution I will henceforward speak as 
if the above hypothesis were admittedly true, and all I now 
say stands or falls with it.) 
All clean bright zinc is thus about 1°8 volt below the 
potential of the air near it; tarnished or oxidized zine will 
exhibit less difference; and it is perhaps possible that perfectly 
oxidized zinc need show no difference of potential at all be- 
tween itself and the air. The step of potential by no means 
therefore depends upon the occurrence of oxidation ; it is the 
oxidation tendency which causes it, but so far as oxidation 
actually takes place the step diminishes. 
Proceed to consider a piece of copper similarly. Oxygen 
molecules are straining at it also, but with less force. The 
combustion energy of copper per gramme-equivalent is given 
by the three authorities already quoted as 37,160, 38,290, and 
43,770 respectively. These do not agree well, and it is difficult 
to know which to take ; but Thomsen’s results are, I believe, 
generally relied on; so, assuming his, the step of potential 
"42 x 37200 
19320 
8 yolt. This then is the amount by which clean bright copper 
differs from the air. Oxidized copper will differ less. Com- 
paring this value for copper with that just obtained for zinc, 
we perceive that a piece of zinc and a piece of copper are, 
when separate, not at the same potential ; they differ by about 
a volt from each other. If the zinc is oxidized, they differ 
less : if the copper is oxidized, they differ more. 
between copper and air will be volts, that is about 
* “4-025 ovammes of silver are deposited by an Ampére current in an 
hour” (Montreal Address). This gives the electrochemical equivalent of 
silver ‘01118, and of hydrogen ‘00010352. 
