METALLIC SURFACES IN AQUEOUS SOLUTIONS. 
131 
a current of considerable volume will flow from one to the other cell through the galva- 
nometer. If a grain of zinc be added to the mercury this effect will be very persistent, 
and is more powerful, the zinc maintaining the polarization. Only one platinum plate 
in each vessel is shown in fig. 3. 
When the funnel-shaped vessel C in fig. 1 was 5 inches in diameter, and the tube con- 
necting it with E the -g^- of an inch in diameter, and the galvanometer connexion between 
E and F was severed, it was found, after having polarized the mercury surface C by the 
power of rather less than one Daniell’s cell (a power too small to evolve hydrogen gas), 
that on tilting the vessel so as to let the mercury run from C to E, a few small bubbles 
of hydrogen gas were given off just as the last drop of mercury ran out of C: thus the 
contraction of the surface concentrated the polarization until it had power enough to 
evolve the hydrogen as gas. The other funnel might have been removed, the surface D 
having no influence on the result. 
This evolution of gas is better shown by floating a minute piece of fine platinum wire 
on the mercury, which gives off the gas as the surface of mercury becomes reduced. 
In this experiment the piece of platinum wire was about (b002 inch in diameter and 
0 - 5 in length. It was floated on the mercury by a small lump of shellac, thus: 
The exposed portion of the wire was as short as possible. The contracting polarized 
mercurial surface acted like zinc to the platinum, which evolved hydrogen as it would 
have done when in contact with a piece of zinc. 
The author had many times endeavoured to ascertain the electrostatic capacity of 
metallic surfaces exposed to an aqueous solution, but has only recently, and by the fol- 
lowing means, been able to get sufficiently reliable results. 
A reflecting galvanometer was constructed with copper wire, No. 18 gauge, and having 
20 Ohms resistance; this was again reduced by a shunt to 4 Ohms; the mirror and 
magnet are hung in water to destroy the oscillations rapidly. 
Two sets of platinum surfaces were used: — 1st. Two platinum spheres about f of an 
inch in diameter, and having each about 1'6 square inch of surface exposed to the acid 
and water, the two bulbs being placed in a glass of diluted sulphuric acid and water. 
2nd. To reduce the resistance as much as possible two platinum plates, each of 1 square 
inch surface, were coated on one side with bees’-wax and paraffin, so as to leave only the 
one surface exposed to the fluid ; these two plates.were made to face each other at a very 
T 2 
