METALLIC SUEFACES IN AQUEOUS SOLUTIONS. 
133 
accompanied by decomposition of the water. If there be such a film between the water 
and platinum, the following considerations will enable us to get an idea of its depth. 
The electrostatic capacity of a layer of pure gas is sensibly the same as that of a vacuum. 
Gutta percha has a capacity of between three and four times this amount. 
If we assume that the film between the platinum and the water is a pure gas, the 
comparison of the capacity of the 1 inch of platinum in water with the surface of un- 
necessary to produce at a distance of 1 inch a similar capacity will give an idea of the 
thickness, or rather the extreme thinness, of this hypothetical film separating the water 
from the platinum. 
The capacity of the French Atlantic Telegraph-Cable, Brest to St. Pierre, is 0-4 mi- 
crofarads per nautical mile ; each nautical mile has 400 lbs. of gutta percha, specific 
gravity 0-98, and 400 lbs. of seven-strand conductor filled with gutta-percha compound, 
giving it a specific gravity of about 8- 8. 
Calculating from this how large a sheet of gutta percha, 1 inch in thickness, and 
coated on each side with metal, is necessary to give an inductive capacity of one micro- 
farad, we shall find that a sheet built up out of 1,040,000 cubic inches of gutta percha 
will have the capacity sought. 
The condensers used for comparison with the two sheets of platinum, offering each 
1 square inch of surface, had a capacity of 311 microfarads — that is to say, they were 
equal to a sheet of gutta percha 1 inch in thickness, and having a surface of 323 mil- 
lions of square inches ; and as gutta percha has about 3y times the capacity of pure gas, 
1131 million square inches of metal separated from another such surface by 1 inch of 
pure gas would about give the capacity sought, viz. 311 microfarads. 
At the commencement of the experiment with the platinum plates in fluid, 
when the potential was very small, the capacity was about 175 microfarads per square 
inch of (double) platinum surface ; and as a sheet of air one inch thick and having a 
surface of 1,040,000 X 3-^=3,640,000 square inches has a capacity of 1 microfarad, 
3,640,000x175 = 637,000,000 square inches will have the same capacity as the two 
platinum plates 1 inch square. 
If the two hypothetical films have the same thickness on each platinum plate, the film 
on each will, if a pure gas, be i a 74,000,000 °f an i nc h, with small potentials, and de- 
crease to y of this amount with a potential of 1-6 volt. 
A useful inference can be drawn from these experiments by the telegraph engineer. 
It has been repeatedly proposed to telegraph by means of a naked wire laid in the 
Ocean. 
The speed of a long telegraph-cable varies inversely as the square of its length for 
similar sectional dimensions. 
If L be its length, I its inductive capacity, B its resistance, and A a constant, its 
A 1 
