=283. Electrolysis of Water.=--Two glass tubes (Fig. 263), _H_ and _O_, are attached at the bottom to a horizontal glass tube. To the latter is also connected an upright tube _T_. At the lower ends of _H_ and _O_ are inserted, fused in the glass, platinum wires, _A_ and _C_. The tubes are filled with a weak solution of sulphuric acid. The tops of _H_ and _O_ are closed with stopcocks, _T_ being open; a current of electricity is sent in at _A_ and out at _C_. A movement of the ions at once begins, the positive hydrogen ions appearing at _C_. These accumulate as bubbles of hydrogen which rise to the top of _H_ and displace the liquid. At the same time bubbles of oxygen appear at _A_. These rise in _O_ and also displace the liquid which rises in _T_. After the action has continued some time it may be noticed that the volume of hydrogen is just twice that of the oxygen. This was to have been expected since the formula for water is H_{2}O. The nature of the gas in _H_ or _O_ may be tested by opening the stopcock and allowing the gas to escape slowly. The hydrogen gas can be lighted by a flame while the oxygen gas will cause a spark upon a piece of wood to glow brightly, but does not burn itself. [Illustration: FIG. 263.-Electrolysis of water; oxygen collects in _O_, hydrogen in _H_.] =284. Evidence that ions are necessary to conduct a current in a liquid= is furnished by the following experiment. A quart jar is carefully cleaned, and half filled with distilled water. Two pieces of zinc 5 cm. square are soldered to pieces of rubber-insulated No. 14 copper wire. The zincs are placed in the distilled water (Fig. 264) and the wires are connected to a 110 volt circuit with a 16 candle-power incandescent lamp in _series_ with the cell, as in the figure. If the zincs have been carefully cleansed and the water is pure, no current flows as is shown by the lamp remaining dark. If a minute quantity of sulphuric acid or of common salt is placed in the water the lamp at once begins to glow. Ions are now present in the liquid and conduct the current. That some substances in solution do not form ions may be shown by adding to another jar of pure water some glycerine and some cane sugar, substances resembling the acid and salt in external appearance but which do not _ionize_ when dissolved as is shown by the lamp remaining dark after adding the glycerine and sugar. The acid and salt are of _mineral_ origin while the glycerine and sugar are _vegetable_ products. This experiment illustrates the principle that the water will conduct only when it contains ions. [Illustration: FIG. 264.--The current passes only when ions are present in the liquid.] =285. Laws of Electrolysis.=--These were discovered by Faraday in 1833, and may be stated as follows: _I. The mass of a substance deposited by an electric current from an electrolyte is proportional to the intensity of current which passes through it._