ON ELECTROLYSIS. 747 
IV. Discusson OF THE LAW (c), AND QUESTIONS CONCERNING 
POLARISATION. 
The chemical changes which go on in a circuit wholly electrolytic, or 
in any homogeneous portion of a circuit, are decomposition and identical 
recomposition, and consume no energy ; accordingly no fresh H.M.F. is 
needed to send a current through a circuit wholly electrolytic or through 
a homogeneous electrolyte, when the force is really applied to it (i.e., not 
merely applied to electrodes), and Ohm’s law is possibly obeyed by elec- 
trolytes exactly as by metals. 
But at junctions of metals with electrolytes, or of electrolytes with 
one another,! permanent chemical changes may occur, and at such places 
a finite E.M.F. must be situated: and the resultant of these may be 
negative, when it is called polarisation, or positive, when the whole 
arrangement is called a battery. 
Total polarisation may be regarded as the sum of two kinds: 
(a) Temporary polarisation, existing during continuance of current. 
(b) Residual polarisation, existing afterwards. 
Residual polarisation is caused by a more or less permanent alteration 
of the surface of the electrodes by clinging or combined ions. Under 
this head comes the whole subject of secondary batteries. 
Concerning temporary polarisation less is known.? On Helmholtz’s 
theory it is caused by the deposited but unliberated and still charged 
ions being unwilling to part with their charges. Whenever the ions are 
able to combine with the electrode or with the liquid, and thus retain 
their charges, temporary polarisation is very small. It seems un- 
doubtedly due to the same circumstances as cause the extra energy of 
the nascent condition of a substance; and it varies very much with the 
particular state of combination, or uncombination, into which the ions 
enter when set free from their former union.* 
xe” Dm 
46,000" ©” 23,0000" 
be used to calculate the theoretical E.M.F. of a battery cell formed of 
specified constituents in which known reactions go on. 
Now is the law really true? Is H. really the E.M.F. needed to de- 
compose a given substance? Or is it only a partial statement of a truth ? 
It may at once be admitted that whatever direct thermal effects the 
The above expression, in the form = (Je@) or 
1 Du Bois Raymond proved polarisation to be possible at liquid-liquid junctions. 
He also asserts the existence of ‘innere Polarisation’ in a uniform liquid, which 
would be contradictory of what is said above; but what he observed is plainly con- 
ae with capillary E.M.F. and endosmose, and in the light of these is natural 
enough. 
2 The experiments of Bernstein, as well as some by Beetz and Edlund, have 
shown how quickly it can rise and decay with make and break of current; so it is 
scarcely a valid proof of its complete non-existence when a galvanometer switched 
into a circuit is not deflected. At the same time no sharp distinction can be drawn 
between temporary and permanent polarisation, any more than between temporary 
and permanent magnetism. 
8 Mr. J. Larmor gave a short communication to the meeting, ‘On the molecular 
theory of galvanic polarisation,’ which he has expanded to a most interesting paper, 
and published in the Phil. Mag. for November 1885. He shows that polarisation, 
whether natural or artificial, must necessarily diminish surface tension, without any 
question of exact reversibility ; and he calculates the distance apart of the consti- 
tuent molecules in the electrical layers of a voltameter-condenser, whose capacity is 
becoming incipiently non-constant. 
