Page:Text-book of Electrochemistry.djvu/128

 of the machine. After some time the paper became blue immediately below Ay proving that alkali had been formed.

A similar experiment described by Gubkin {1) is carried out as follows: — A solution of copper sulphate is placed imder Ay and a wire from B passes into the solution. When the negative electricity has passed across for some time, a thin film of copper is formed on the surface of the solution just below A,

Faraday proved, too, that chemical decomposition by an electric current may take place at a considerable distance from the poles. A layer of pure water was placed over a solution of magnesium sulphate, and an electric current was passed from a positive pole at the bottom of the solution to a negative pole near the surface of the water ; it was found that a precipitation of magnesium hydroxide took place at the surface of separation of the solution and water. (This experiment was later studied more completely, and explained, by F. Kohlrausch.)

The Ions. — Faraday assumed that the ions were held together in the molecule by chemical forces, and that, further, the positive ion of one molecule w«is attracted by the negative ion of another molecule. This latter attraction, acting in conjunction with the electric force, was sufficient to overcome the attraction within the molecule.

Faraday, however, was astonished to find that those substances, like potassium or sodium chloride, or salts in general, which are the best conductors, are those in which the ions were supposed to be most firmly bound in the molecule. If aU the molecules were really held together in the form of a Grotthuss chain so that a certain force would be necessary to break it, then a certain electromotive force would be required before electrolysis could take place. Apparently this is really the case, because polarisation often occurs at the electrodes.

If, however, the electrodes consist of unpolarisable metals, i.e. of the same metal as the positive ion of the salt, during the passage of the electric current the electrodes are

I

�� �