188 REPORT—1890. 
last two cases give rise to ‘secondary’ chemical products. These second- 
ary actions are quite independent of the direct effect of electrolysis. 
If we consider this chemical action more in detail, we may regard the 
electrolytic liquid as composed of a number of molecules, and the action 
will then be the separation of a number of these molecules each into two 
constituent parts or ions: these ions are deposited at the electrodes only. 
Considering a single molecule, the one part is deposited at the cathode, and 
is called the cation ; the other part (or the corresponding part of a similar 
molecule) at the anode, and is called the anion. The terminology of the 
subject was introduced by Faraday (‘ Exp. Res.’ Ser. VII. 1834). What 
is precisely to be understood by the ‘ molecule’ which is decomposed is 
not yet clear. ven if we suppose the electrolyte a solution of a salt so 
pure that the decomposition of impurity could not in any case be 
detected, we cannot now say that all the molecules decomposed are 
similar. To take a definite instance, in a solution of sodium chloride the 
molecule decomposed may be the simple chemical molecule NaCl, or it 
may be a molecular aggregate of sodium chloride [n(Na(l) ], or an aggre- 
gate of salt and water [n(NaCl), m(H,O)], or some molecules of one 
kind and some of another may be decomposed. The primary results of 
the separation of the molecules, each into two parts, are the true ions, 
and are deposited at the electrodes. But, however complicated may be 
the molecules of the electrolyte which are regarded as individually 
decomposed, in cases in which there is visible deposit on the electrodes, 
or direct combination with the electrodes, the deposit or combination 
could have been produced by the decomposition of the simple molecules 
[NaCl] of salt in the solution. 
(b.) The volume of the liquid in the cathode vessel increases ; that in 
anode vessel diminishes. This phenomenon, which is known as electric 
endosmose, is attributed to the action of the porous diaphragms, and is 
regarded as independent of the more strictly electrolytic phenomena. 
(c.) The percentage composition of the solution in the anode and cathode 
vessels is altered, generally unequally in the two, while that in the inter- 
mediate vessel remains unaltered. This phenomenon is usually attributed 
to the migration of the ions with unequal velocities through the solution, 
and is equivalent, if the ions be the result of decomposition of simple 
molecules, to a transfer of those molecules, which in the end are left in 
combination, through the body of the solution, in one direction or the 
other. 
(d.) There is a rise in temperature of the liquid owing to the develop- 
ment of heat by the current, just as there would be in the case of a 
metallic conductor. 
(e.) The deposit of ions upon the electrodes causes an electromotive 
force opposed in direction to the decomposing electromotive force applied. 
This E.M.F. of polarisation is in some cases sufficiently great to balance 
the latter and prevent the further flow of current. 
The current may also be considerably reduced by the resistance of 
a layer of non-conducting material produced by the action of the ions 
on the electrodes. 
(f.) Thermo-electric effects are produced at the junctions of the 
different substances in the circuit, including the junctions of metal and 
liquid. 
