210 PROFESSOR DANIELL ON THE ELECTROLYSIS OF SECONDARY COMPOUNDS. 
Now it seemed to me that this combination of the oxygen and metal was a secondary 
result, due to the local affinity of the two elements thus brought into juxtaposition; 
and was in no way connected with the primary phenomena of the current, which would 
have completed its course whether this combination had taken place or not, i. e. 
whether magnesium and oxygen had been separately evolved, or whether magnesia 
had been formed by the combination of the two. It also seemed probable that, al- 
though in the very slow action of this experiment this combination invariably took 
place, by varying the experiment so as to evolve metals possessing different degrees 
of affinity for oxygen, and particularly by shortening the time in which the evolution 
might take place, instances might be found of some portion of the metal escaping 
this combination, which would thus afford the most incontrovertible proof of the 
point to be established. 
I was happy enough to succeed perfectly in my first experiment: nothing could 
be more simple than the arrangement. 
Experiment 18 . — A small glass bell, with an aperture at the top, had its mouth 
closed by tying a piece of thin membrane over it. It was half-filled with a dilute so- 
lution of caustic potassa, and suspended in a glass vessel containing a strong neutral 
solution of sulphate of copper, below the surface of which it just dipped. A platinum 
electrode, connected with the last zinc rod of the large constant battery of twenty 
cells, was placed in the solution of potassa; and another, connected with the copper 
of the first cell, was placed in the sulphate of copper immediately under the diaphragm 
which separated the two solutions. The circuit conducted very readily, and the ac- 
tion was very energetic. Hydrogen was given off at the platinode, or the electrode 
which dipped into the potassa, and oxygen at the zincode in the sulphate of copper. 
A small quantity of gas was also seen to rise from the surface of the diaphragm. In 
about ten minutes the lower surface of the membrane was found beautifully coated 
with metallic copper, interspersed with oxide of copper of a black colour, and hydrated 
oxide of copper of a light blue. 
The explanation of these phenomena is obvious. In the experimental cell we have 
two electrolytes separated by a membrane, through both of which the current must 
pass to complete its circuit. The sulphate of copper is resolved into its compound 
anion, sulphuric acid -j- oxygen, and its simple cathion copper: the oxygen of the 
former escapes at the zincode, but the copper on its passage to the platinode is 
stopped at the surface of the second electrolyte, which for the present we may regard 
as water improved in its conducting power by potassa. The metal here finds nothing 
by combining with which it can complete its course, but, being forced to stop, yields 
up its charge to the hydrogen of the second electrolyte, which passes on to the plati- 
node, and is evolved. The corresponding oxygen stops also at the diaphragm, giving 
up its charge to the anion of the sulphate of copper. The copper and the oxygen, 
thus meeting at the intermediate point, partly enter into combination, and form the 
black oxide ; but from the rapidity of the action, there is not time for the whole to 
