"Study of the conduction of electricity in liquids became possible at the beginning of the nineteenth century, following the discovery of the electrolytic cell by Volta in 1800, which provided the first continuous source of electric current. It was soon discovered that the conduction of electricity by solutions is accompanied by chemical reactions at the electrodes which serve to conduct the current into and out of the solution. Nicholson and Carlisle demonstrated the decomposition of water into hydrogen and oxygen by a current in 1801. Davy's discovery of sodium and potassium metals by electrolysis of moist soda and [caustic] potash was a striking example of the novelty of electrochemical decomposition. Many of the phenomena of electrolysis were already known when Michael Faraday began his researches. It was the quantitative relationship between electrochemical change and current which interested Faraday and enabled him to correlate the mass of experimental data that had accumulated since 1800. Faraday's laws of electrolysis, which were published in 1833, state: (1) that the amount of chemical decomposition produced by an electric current (that is, the mass of substance deposited or dissolved at an electrode) is proportional to the quantity of electricity passed. (2) that the amounts of different substances released or dissolved at electrodes by the same quantity of electricity are proportional to their chemical equivalents. From the second law, it follows that the amount of electricity required to liberate or dissolve one equivalent weight of any substance by electrolysis is constant. It was not until after Faraday's death that the significance of his laws of electrolysis for atomic theory was realized. In 1881 von Helmholtz pointed out that if elementary substances are composed of atoms, it follows from Faraday's laws of electrolysis that electricity also is composed of elementary portions which behave like atoms of electricity. Investigations on the conduction of electricity by gases led to the identification of the electron as the fundamental unit of electricity at the end of the century. Faraday's positive and negative ions are therefore atoms (or groups of atoms or radicals)) with a deficiency or an excess of an integral number of electrons, where the integral number is the valency of the atom. The ions move in opposite directions through the solution to the electrodes where their charges are neutralised, causing them to be discharged to neutral atoms or radicals. These are the primary electrode reactions, of which the deposition of silver on a platinum cathode in the silver coulometer is a typical example."