=83. Electrotyping.= It was observed by De La Rue in 1836 that in the Daniell cell an even coating of copper was deposited upon the copper plate. From this was developed the process of electrotyping, which consists in making a copy in metal of a wood-cut, page of type, etc. A mould or impression of the type or coin is first made in wax, or other suitable material. These moulds are, of course, the reverse of the original, and as they do not conduct electricity, have to be coated with graphite. This thin coating lines the mould with conducting material so that the current can get to every part of the mould. These are then hung upon the cathode in a bath of copper sulphate as described in § 82. The electric current which passes through the vat deposits a thin layer of metallic copper next to the graphite. When this copper gets thick enough, the wax is melted away from it, leaving a thin shell of copper, the side next to the graphite being exactly alike in shape to the type, but made of copper. These thin copper sheets are too thin to stand the pressure necessary on printing presses, so they are strengthened by backing them with soft metal which fills every crevice, making solid plates about ¼ in. thick. These plates or _electrotypes_ are used to print from, the original type being used to set up another page. CHAPTER IX. THE STORAGE BATTERY, AND HOW IT WORKS. =84. Polarization.= It has been stated that a simple cell polarizes rapidly on account of hydrogen bubbles that form upon the copper plate. They tend to send a current in the opposite direction to that of the main current, which is thereby weakened. [Illustration: Fig. 81.] =85. Electromotive Force of Polarization.= It has been shown, Fig. 71, that water can be decomposed by the electric current. Hydrogen and oxygen have a strong attraction or chemical affinity for each other, or they would not unite to form water. This attraction has to be overcome before the water can be decomposed. As soon as the decomposing current ceases to flow, the gases formed try to rush together again; in fact, if the water voltameter be disconnected from the cells and connected with a galvanoscope, the presence of a current will be shown. This voltameter will give a current with an E. M. F. of nearly 1.5 volts; so it is evident that we must have a current with a higher voltage than this to decompose water. This E. M. F., due to polarization, is called the E. M. F. of polarization. =86. Secondary or Storage Batteries=, also called _accumulators_, do not really store electricity. They must be charged by a current before they can give out any electricity. Chemical changes are produced in the storage cells by the charging current just as they are in voltameters, electroplating solutions, etc.; so it is potential chemical energy that is really stored. When the new products are allowed to go back to their original state, by joining the electrodes of the charged cell, a current is produced.