The vessel containing the liquid is known as an _electrolytic cell._ In fig. 89, A is the cell, which may be of glass or of any other suitable material, and B is the liquid which is to be electrolyzed. Current enters by the _positive electrode_ C, also known as the _anode_, traverses the liquid, and leaves by the _negative electrode_, or _cathode_, D. [Illustration: FIG. 90.--Modern apparatus for decomposing water by electrolysis. Platinum electrodes P and P′ are placed at the bottom of two upright tubes O and H, and are connected to the terminals T and T′ by platinum wires, which are fused through the glass of the tubes. These tubes have glass stop cocks S and S′ at their upper ends, and at their lower ends are connected by a short glass tube, from the center of which rises the large central tube which expands with a bulb at its upper end, which is open at the top. The three tubes can be filled with acidulated water from the central tube, the previously contained air being allowed to escape through the stop cocks, which are afterwards closed. If it be so filled, and the terminal T be attached to the positive and T′ to the negative pole of a suitable battery, bubbles of gas will be observed to rise from the plates P and P′, and finding their way to the top of the respective tubes, will displace the liquid, which will be driven into the open central tube. The gas rising from the anode P is oxygen (O), and that rising from the cathode P′ is hydrogen (H). If the tubes are graduated, the latter will be found to occupy about twice the volume of the former. The proportion is theoretically 2 to 1; however, on account of the different solubilities of the two gases in water, oxygen being the more soluble of the two, is deficient in quantity.] The passage of current through the water splits up its molecules into their constituent atoms of oxygen and hydrogen, the former being given off in bubbles at the anode, and the latter at the cathode. When current is passed through a solution of copper sulphate between platinum electrodes, the liquid is decomposed, atoms of copper being deposited at the cathode, bubbles of oxygen being given off at the anode, and sulphuric acid being formed in the liquid, which latter becomes more and more acid as the copper is withdrawn.