Page:Elementary Text-book of Physics (Anthony, 1897).djvu/338

324 called ions. One of them appears at the anode and is called the anion, the other at the cathode and is called the cation.

For the sake of clearness we will describe some typical cases of electrolysis. The original observation of the evolution of gas when the current was passed through a drop of water, made by Nicholson and Carlisle, was soon modified by Carlisle in a way which is still generally in use. Two platinum electrodes are immersed in water slightly acidulated with sulphuric acid, and tubes are arranged above them so that the gases evolved can be collected separately. When the current is passing, bubbles of gas appear on the electrodes. When they are collected and examined, the gas which appears at the anode is found to be oxygen, and that which appears at the cathode to be hydrogen. The quantities evolved are in the proportion to form water. This appears to be a simple decomposition of water into its constituents, but it is probable that the acid in the water is first decomposed, and that the constituents of water are evolved by a secondary chemical reaction.

An experiment performed by Davy, by which he discovered the elements potassium and sodium, is a good example of simple electrolysis. He fused caustic potash in a platinum dish, which was made the anode, and immersed in the fused mass a platinum wire as cathode. Oxygen was then evolved at the anode, and the metal potassium was deposited on the cathode. This is the type of a large number of decompositions.

If, in a solution of zinc sulphate, a plate of copper be made the anode and a plate of zinc the cathode, there will be zinc deposited on the cathode and copper taken from the anode, so that, after the process has continued for a time, the solution will contain a quantity of cupric sulphate. This is a case similar to the electrolysis of acidulated water, in which the simple decomposition of the electrolyte is modified by secondary chemical reactions.

If two copper electrodes be immersed in a solution of cupric sulphate, copper will be removed from the anode and deposited on the cathode, without any important change occuring in the character or concentration of the electrolyte. This is an example of the