More than a century later Galvani’s attention was drawn to the subject by his observation of spasmodic contractions in the legs of freshly killed frogs under the influence of the “return shock” experienced every time a neighboring electric machine was discharged. The limbs of the frog, prepared as directed by Galvani, are shown in fig. 92. After the animal has been killed the hind limbs are detached and skinned; the crural nerves and their attachments to the lumbar vertebrae remaining. For some hours after death the limbs retain their contractile power. The frog’s limbs thus prepared form an excessively delicate galvanoscope. =Electroplating.=--This is the process of depositing a layer or coating of a rarer metal upon the surface of a baser, or of a metal upon any conducting surface, by electrolysis. The electric current used may be obtained from a battery or other source. The battery has its positive plate connected to a rod extending across a trough or tank containing the plating bath. Suspended from the rod are anodes of gold, silver, or copper or whatever metal from which a deposit is desired. The other plates of the battery or the negative elements, are connected with another rod across the trough, to which are suspended the articles to be plated. =Electrotyping.=--This is the process by which, type, wood cuts, etc., are reproduced in copper by the process of electroplating. A mould is first made of the set type in wax; this mould is next coated with black lead to give it a metallic surface, as the wax is an insulator; the mould is then subjected to the process of electro deposition, resulting in the formation of a film of copper on the prepared surface. The copper shell is removed from the mould by applying hot water; the shell is then backed up with electrotype metal to render it strong enough for use. Almost all the illustrations in this book, for example, are printed from electrotype copies, and not from the original wood blocks, which would not wear so well. CHAPTER IX MAGNETISM =Magnetism.=--The ancients applied the word “magnet,” _magnes lapes_, to certain hard black stones which possess the property of attracting small pieces of iron, and as discovered later, to have the still more remarkable property of pointing north and south when hung up by a string. At this time the magnet received the name of _lodestone_ or “leading stone.” It is commonly, though incorrectly, spelled loadstone. [Illustration: FIG. 93.--Simple compass. It consists of a magnetic needle resting on a steel pivot, protected by a brass case covered with glass, and a graduated circle marked with the letters N, E, S, W, to indicate the cardinal points; _a b_ is a lever which arrests the needle by pushing it against the glass when the button _d_ is pressed.] =Ques. Describe two kinds of magnetism.=