of the plate, even to the tiny scratches on its surface. Three years later, Jacobi, in St. Petersburg, and Jordan, in London, starting from this fact, devised a means of obtaining impressions, of medals, woodcuts, stereotypes, and ornaments by the electro-deposition of copper. Even non-metallic bodies could be reproduced in copper by employing moulds of wax lined with a film of plumbago, in order to get a conducting surface to serve as a cathode, and a ground for the deposit.

This process, originally termed "galvano-plasty" by Jacobi, is now known as electrotyping. Electrotypes of objects are formed by hanging a mould of it, lined with plumbago, in a bath of saturated solution of sulphate of copper, and making the lining of this mould the cathode of the current, while the anode is a plate of copper, which will be decomposed at the same rate as the copper is deposited on the mould, and thus keep up the strength of the solution. The process is largely used in multiplying copies of woodcuts and stereotyping pages of printed matter; and the copies wear better than the original wooden blocks or leaden types.

Electro-plating, which is the art of covering one metal with another, was begun by Brugnatelli in 1805, when he took a silver medal and coated it with gold by making it the cathode in a solution of gold.

In 1840 Messrs. Elkington, of London, introduced German-silver articles electro-plated with gold and silver. The electro-plating bath is shown in Fig. 81, and consists of the vat A, containing a solution в of the double cyanide of gold and potassium when gold is to be deposited, and the double cyanide of silver and potassium when silver is to be overlaid. The electrodes

CD in this case are rods of metal, laid across the mouth of the vat, and the articles to be coated are suspended from the cathode into the solution в, like the spoons in the figure. A plate E of the kind of

metal to be deposited is also hung from the anode D, so as to keep up the strength of the solution. In silver plating this metal is, of course, silver, and a small quantity of bisulphide of carbon is added to the bath to give the deposited metal a bright surface. A frosted, or crystalline, deposit is produced by a rapid deposition. The gilding of cups inside is effected by filling them with the solution, and suspending in it an anode of gold, the vessel itself being the cathode.

Iron or pewter can be gilded in the same way, but in order that the deposit may adhere the better, they are first coated with copper. Even non-metallic objects can be electro-plated by first covering them with a conducting film. The best film is made of fine grains of silver deposited on the surface of the object

by plunging it into a solution of nitrate of silver and then into one of phosphorus. The recent Paris Electrical Exhibition contained many curious and beautiful specimens of this kind encased in gold, silver, and copper, for example, ferns and foliage of all sorts, flowers, sabots, bonnets, dragon-flies, lizards, beetles, and even human hands and brains, which showed every wrinkle and convolution. The use of the latter objects was of course for anatomical purposes; but it also suggests a new mode of mummifying unknown to the Egyptians.

Nickel-plating is a comparatively new branch of the industry we are considering. Iron or brass articles may be coated directly with nickel and preserved from rusting on the surface. The Americans nickel-plate most of their small iron tools, and also many of the exposed parts of larger engines. Nickel is deposited from the double sulphates or chlorides of nickel and ammonium; and the powerful currents derived from the dynamo-electric machine are generally used in the process, as also in the coating of carbon rods with copper for electric lamps. The thermo-electric pile is also employed as a generator of the current and substitute for the battery in electro-plating.

Electrolysis, as the process of decomposing a solution by the electric current is called, has given rise to many other practices. It is the principle of the accumulator in which electricity is "stored up," as we have before described; and inferior alcohols can be rectified and made purer by the electricity breaking up the adulterating oils in them. Wine, too, is improved in quality by the same action. Moreover, at the Paris Exhibition a great variety of brilliant aniline

dyes which had been deposited by electrolysis from allied solutions were displayed.

Metallurgists are also turning their attention to the reduction of gold, silver, and other precious metals from their ores by electrolysis, and promising results have been obtained by MM. Blas and Miest. While upon the subject of purification by means of electricity, we may mention the magnetic ore-separator of Edison, by which iron is separated from the black sands of Rhode Island, U.S., by means of a powerful electromagnet. Porcelain clays are also purified of ferruginous particles by the same means, a wash of the clay being passed between the poles of a magnet so as to enable the particles to adhere to the poles, from which they are afterwards scraped. A similar sifting has recently been applied to the separation of bran from flour in America, but in this case the light flakes of bran are attracted by ebonite rollers which have been rubbed with wool, just as chaff or shreds of cotton are attracted by a rod of sealing-wax that has been rubbed upon the sleeve.

ELECTRIC ALARMS.

Turning now to the minor uses of electricity, and first to those in the household, we come to the electricbell, which is superseding the old pull-bell by reason of its convenience. The power for ringing the bell is supplied by a small battery of two or three Leclanché elements (L, Fig. 82), and all the ringer has to do is simply to complete the electric circuit by pressing two metal contacts together by a press-button p let into the wall. The bell itself consists of the sounder в and its clapper

B

c. The clapper projects from a soft iron armature a, which is pressed by a spring nearly into contact with the poles of an electro-magnet E. When the current by completion of the circuit flows through the electromagnet E, the armature is attracted, and the clapper

strikes the bell. There are bells of a single stroke, but that illustrated is what is called a "trembler," and continues to ring as long as the current flows-that is to say, as long as the person makes contact by pressing the button. This is effected by making the clapper break the circuit every time it strikes the bell. By causing the forward movement of the clapper to draw the spring T away from a contact-point p in the circuit, this is easily done. The stoppage of the current of course leaves the clapper free to spring back and re-establish the circuit, so that the current flows afresh, and the electro-magnet again pulls it forward.