The current enters the end compartments by the carbon anodes and passes through the salt solution to the mercury layer which in these compartments are the cathodes. The current then passes through the mercury to the middle compartment, and then through the solution to the cathode, thence back to the dynamo. It is important to note that in the middle compartment the mercury becomes the anode. Chlorine is liberated at the carbon electrodes, and when no more can dissolve in the liquid it escapes and is conveyed away by the pipe P. Sodium atoms are formed at the surface of the mercury cathodes in the outside compartments and dissolve instantly in the mercury, forming sodium amalgam. While the current is passing, a slight rocking motion is given to the tank by the cam E. This is sufficient to cause the mercury containing the dissolved sodium to flow alternately into the middle compartment, and there the sodium amalgam comes into contact with water; the sodium is dissolved out of the mercury and caustic soda is formed. Water in a regulated stream is constantly admitted to the middle compartment, and a solution of caustic soda of about 20 per cent. strength overflows. The production of caustic soda by an electrical method still remains to be fully developed. A process which gives only a 20 per cent. solution cannot be looked upon as final. In the meantime, other methods have been tried, in some of which fused salt is used in place of brine in order to give caustic soda in a more concentrated form. For a description of these methods, the reader must consult some of the larger works mentioned in the preface. Here we can only say that very great difficulties have been encountered, particularly in the construction of a satisfactory porous diaphragm or, alternately, in devising methods in which this can be dispensed with. Another interesting application of electrolysis is furnished by the use of copper sulphate in industry. When this salt is dissolved in water, it breaks up into copper ions (positive) and an equal number of negative ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO″4). Under the influence of the current copper ions travel to the cathode, and there by the gain of two electrons become copper atoms. Now, since copper is not soluble in copper sulphate solution, and is not volatile except at very high temperatures, it is deposited on the cathode in a perfectly even and continuous film when the strength of the current is suitably adjusted. This film continues to grow in thickness as long as the conditions for its deposition are maintained. If the current employed is not suitable, the metallic film is not coherent, and the copper may appear as a red powder at the bottom of the cell. Any other metal or impurity which might be present in the unrefined copper falls to the bottom of the tank.