Imagini ale paginilor
PDF
ePub

from the bottom of the cell or vessel. These plates are connected by wires to the poles of the voltaic battery c, as shown, and therefore they act as electrodes and pass the current from the battery through the water. That in connection with the positive pole + of the battery is termed the “anode," and that in connection with the negative – pole is the “cathode.” Now, as the water is decomposed the hydrogen gas is found to collect on the cathode, by which the current is supposed to leave the water, while the oxygen collects on the anode, by which the current is believed to enter the water; and being a gas, and lighter than the water, it rises into the upper ends of the tubes. The volume of hydrogen at the cathode is always twice the volume of oxygen at the anode, and this agrees with the known constitution of water. Further, the quantity of water decomposed in a given time is proportional to the strength of the electric current, and hence if the tubes are graduated to show the volume of gases collected in them, the instrument becomes a voltameter, or current-measurer. This important discovery of electro-chemical decomposition was made in 1800 by Carlisle and Nicholson, and from it a great number of industries take their source.

We have, for example, the Bain electro-chemical telegraph, in which the current is caused to decompose a solution of iodide of potassium in starch and water. Paper is moistened with the solution, and the current is passed through it. The result is that the iodine is separated from the potassium and leaves a blue stain on the paper to mark the passage of the current, and be interpreted by the clerk as a signal of the message.

[ocr errors]

ELECTRO-PLATING.

The principal outcome of the discovery is, however, the art of electro-plating the baser metals with gold, silver, nickel, and other metals of a nobler kind, so as to improve the appearance of articles made from the baser metals, and at the same time prevent their surfaces from oxidising. When a metallic solution -say, a solution of sulphate of copper-is substituted for the water in the voltameter experiment which we have described, the sulphate of copper is decomposed, and metallic copper is deposited on the cathode, while an oxide of sulphur, known as “sulphion,” is deposited on the anode. The sulphur, however, soon combines with water to form sulphuric acid and free oxygen. If the electrodes are of copper instead of platinum, one copper plate becomes heavier in this process by the deposition of fresh copper on it, and the other becomes lighter because the newly-formed sulphuric acid eats it away. The rate at which this process of building and wasting goes on is a measure of the strength of the current, and Mr. Edison has constructed on this plan an ingenious current-meter for measuring the electricity consumed in electric lamps, just as gas is measured

now.

The important fact, however, is that metals can be deposited in this way from solutions, and hence we pass at once to the apparatus employed in electro deposition. It was in 1836 that Mr. De la Rue observed that in a Daniell's cell the copper deposited on the copper plate took on its under-side the impress

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 B 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 C D 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 B, like the spoons in the figure. A plate E of the kind of

[graphic][subsumed][subsumed][merged small]

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

« ÎnapoiContinuați »