_=372. Composition of Water.=_ The two gases liberated in Exp. 147 were hydrogen (H) and oxygen (O). The chemical formula for water is H_{2}O, which means that it is composed of two parts, by volume, of H and one part of O. With proper apparatus these gases may be collected, tested, and the amounts measured. _=373. Electromotive Force of Polarization.=_ We know that H and O have a strong chemical attraction, or affinity, for each other. In order, then, for the current to decompose water, this attraction between the gases must be overcome; and as soon as the current ceases, these gases try to rush together again to form water. This sets up an electromotive force of almost 1.5 volts; in fact, a current is produced if the H and O be allowed to form water again (See Storage Cells). To decompose water the current must have an E. M. F. of over 1.5 volts to overcome this E. M. F. of polarization. It was seen in the study of simple cells that the current became rapidly weaker as hydrogen was deposited upon the copper plate, on account of this opposing electromotive force. In decomposing other compounds, the anode is made of the metal which is to be deposited at the cathode. If copper is to be deposited from a solution of copper sulphate the anode should be a copper plate; this keeps the solution at same strength, and avoids the opposing E. M. F. of polarization; that is, a very weak current will do the work (See Exp. 149), because the electrodes are of the same metal. =EXPERIMENT 148. To coat iron with copper.= _Apparatus._ Iron nail, solution of copper sulphate (§ 283). =374. Directions.= (A) Clean the nail with sandpaper, then hold it in the copper solution for a few seconds. Machinists often cover iron or steel tools with a thin coating of copper in this way. [Illustration: Fig. 114.] =EXPERIMENT 149. To study the electrolysis of a solution of copper sulphate.= _Apparatus._ Galvanoscope, G V; two-fluid cell, 2-F C; a tumbler, T, containing about an inch of copper sulphate solution (§ 283); a wooden cross-piece to which is fastened a copper strip; carbon rod, C; wire 2 is held to C by a rubber band. _Arrange_ as in Fig. 114, so that Cu will be the _anode_ (§ 370), the current passing as shown by arrow. A dry cell may be used for short experiments instead of the 2-F C. =375. Directions.= (A) The carbon being clean, allow the current to pass, C and Cu being kept about 1/2 in. apart. Watch the surface of C, and note the beautiful color of the deposited copper. Save the coated rod for the next experiment. Has the Cu plate been acted upon?