Put a teaspoonful of sulphate of soda into a cup, and dissolve it in
hot water; pour a little cabbage blue into the solution, and put a
portion into two glasses, connecting them by a piece of linen or cotton
cloth previously moistened in the same solution. On putting one of the
wires of the galvanic pole into each glass, the acid accumulates in the
one, turning the blue to a red, and the alkali in the other, rendering
it green. If the wires be now reversed, the acid accumulates eventually
in the glass where the alkali appeared, while the alkali passes to the
glass where the acid was.