730 REPORT—1885. 
II. Tse Micration or Ions. 
Do the opposite ions in solutions travel at different rates ? And, in any case, 
at what rate do they travel ? 
Here, as is well known, we must draw a distinction between fused and 
dissolved compounds. Fused or homogeneous electrolytes, in which no 
solution occurs, must always remain of uniform composition; conse- 
quently, considering any bounded region, exactly equivalent quantities of 
either ion must pass out of it on the whole. It is not evident that the 
two constituents must pass out in opposite directions, but in so far as 
they pass out in the same direction there is no true electrolytic decom- 
position : there is only a kind of electric endosmose, affecting the level of 
the fluid as a whole. Given that the level remains uniform, and that 
ions are only liberated at electrodes, then it is plain that the opposite 
corresponding ions must pass through any imaginary plane in the fluid 
at the same rate in opposite directions; for, if not, the constitution of 
the fluid would not remain uniform. 
This argument fails to apply to dissolved salts, and it breaks down 
before long even with fused salts if they dissolve one of the liberated 
elements or any other body, because in a solution no necessity for uni- 
formity of composition exists—one portion may be concentrated, while 
another is quite weak; and itis well known that such weakening and 
concentrating actions do occur in electrolysed solutions: e.g., to specify a 
hackneyed but sufficiently instructive case, in the electrolysis of dilute 
sulphate of copper with copper electrodes the solution near cathode 
becomes weaker, while that near anode becomes stronger, than before. 
If the anode is arranged above the cathode visible blue streaks descend ; 
if the arrangement is inverted the top liquor gets almost clear. If 
platinum electrodes be used both sides get weaker, but the cathode side 
weakens two or three times as fast as the anode side. It is customary to 
explain these well-known and perfectly certain experimental facts by the 
obvious and plausible hypothesis that the two ions do not travel both at 
the same rate through the liquid, but that the SO, travels quicker than 
the Cu. Such an explanation accounts for the phenomenon, but it does 
not follow that it is certainly the true explanation ; and for the sake of 
examining it more particularly I may be allowed to suggest doubts con- 
cerning it. If these doubts have no sound physical basis their state- 
ment will only result in their removal, and can do no harm. 
First let us quickly see how the hypothesis of unequal velocity 
accounts for the unequal concentration of solution which is called migra- 
tion. Let SO, travel, say, three times as fast as Cu; and consider an 
imaginary partition about the middle of the cell. Three atoms of SO, 
travel unit distance through it towards the anode for every atom of Cu 
travelling unit distance in the other direction ; but the number of atoms. 
of Cu and of SO, liberated against the electrodes in the same time is. 
four of each: hence the anode region of the cell loses only one SO,, 
while the cathode region loses three Cu. 
The reasoning may be made to look more general. 
Consider a compound, AC, arranged in two vessels with a joining 
tube; let A be anion and C be cation, and let A travel » times as fast as 
C, so that through the tube we have n equivalents of A passing for every 
equivalent of C. 
