Rules respecting the use of Batteries. 725 the common commercial article containing several impurities, such as arsenic, iron, and carbon; but it has been found, though the reason does not seem very obvious, that rubbing the zinc plates over with some mercury makes them as good for the purpose as if the zinc were chemically pure. In all cases, the zinc plates should be well rubbed in this way or amalgamated as it is technically termed.” (ii.) The wires used for conducting the current should be of pure metal. Copper is commonly used, and it was discovered by Matthiesen that the impurities, usually present in this metal, seriously affected its conductivity. Thus the presence of a small quantity of arsenic in copper, greatly destroys its conducting power. (iii.) All connections between pairs, should be as perfect as possible; and should be looked to occasionally, as the corrosion of these would very strongly impede the current. (iv.) The proper strength of acids must be maintained, and, instead of dilute sulphuric acid, a solution of sulphate of zinc may be employed with advantage in the zinc cell of any of the batteries we have described. The deposition of sulphate of zinc crystals on the porous cells should be prevented; and porous cells after having been in use for some time should not be emptied and set aside, but should be kept full of clean water. The conducting power of the different metals for the electric current nearly corresponds to the order observed for conducting heat (Art. 591, p. 404). Among non-metals it is remarkable that charcoal, which does not conduct heat, is a very good conductor of electricity. Among metals, silver, copper, and gold are the best conductors, and lead, platinum, and mercury, among the worst. A current which will pass through a silver wire without producing any apparent change, will heat a platinum wire of the same diameter red-hot. In current electricity, the force passes through the entire thickness of the conducting metal, and not, as in static electricity, by the surface only. Small wires of platinum are thus easily made red-hot by the current, and are used for exploding gunpowder in military mining and submarine operations. In reducing the thick * The amalgamation of the plates renders the battery extensively applicable to many purposes in the arts. It was first suggested by Mr. Kemp. According to Mr. Smee, the mercury used in amalgamation envelopes the carbon and foreign metals, and therefore the first gas evolved adheres so firmly to these that every foreign point of metal becomes coated in such a manner as to prevent further action. Of all metals known, there is none to which the hydrogen adheres so firmly as to mercury. 726 Galvanic deposition of Metals. ness of the conducting wire we thus intensify the heat, by making a larger quantity of electricity traverse it in the same time. Heated wires do not conduct so well as those at a low temperature, so that the heat acquired by a platinum wire tends to retard or even to destroy its conductivity.* For general purposes, where constancy of current more than power, or electro-motive force, is required, as, for instance, for telegraphic purposes, there seems to be no better form of battery than that devised by Daniell; which when properly attended to, will keep for months in action. In the cellars of the Central Office, in Telegraph Street, there are some thousands of cells, and the form of battery adopted there, is a modification of the Daniell; the fumes arising from the Grove and Bunsen batteries render the employment of these in such circumstances impossible. Galvanic deposition. Electro-plating. 964. The deposition of metallic copper from the cupric sulphate solution, which, as we have explained, takes place in the Daniell cell, has been turned to practical use in the deposition of other metals as well as copper, such as gold and silver, from solutions of their salts, and the extensive modern art of electro-plating has grown up from the application of this simple fact. Electro-plating is the deposition of a thin layer of one metal on the surface of another, either, as in electro-silvering, or electro-gilding, to give an inferior metal all the appearance and lustre of the more valuable ones, or for protective purposes, as when nickel is deposited on iron or steel to prevent their oxidation. In any case, the process employed is very much the same. Thus, in electroplating a copper spoon with silver, the copper spoon is immersed in a solution consisting of I part cyanide of silver, 10 parts cyanide of potassium, 100 parts distilled water; alongside of it is immersed a plate of silver; the copper is then connected with the zinc pole of a Daniell's battery, or cell, while the silver plate is Intensely heated metals are absolute non-conductors of electricity. Mr. Sandy states that at a great fire in Tooley Street, some years since, when he was chief of the Telegraph Department at Brighton, the flames reached the wires and made them red-hot. While they were in that condition the communication with London Bridge Station was as completely cut off as if the wires had been actually severed. Electro-plating and Electro-typing. 727 joined to the copper pole of the Daniell, and by an action precisely akin to that of the simple cell itself, the spoon is gradually covered with a strongly-adhering layer of silver. The process will Occupy a day or two, according to the strength of the exciting cell or battery; but the slower the process, the more cohesive will the deposited coating become. Electro-typing, electro-casting, or electro-moulding, is the depositing of a thick layer of a metal, such as copper, from a solution of its salt, the object being a substantial copy or re-production of a coin, engraving, or other object The general process differs from the former only in minor details, and we need but remark that where the original cannot be employed as the actual mould for the electro deposit, a cast of it may be taken in plaster-of-Paris, or in gutta-percha, wax, or paraffin, the surface of the mould being then carefully brushed over with finely-powdered plumbago or blacklead, to give to it conducting power. In this way faithful copies of antique coins or seals are easily produced, and copper-plate engravings may be multiplied to almost any extent, without losing much of the delicacy of the original. Even wood engravings can thus be turned into copper-plates; and, more wonderful still, the microscopic definitions and elevations of the silverized surface of a daguerreotype plate can be reproduced in relief by this same process. The applications of this process of depositing a thin metallic layer on a body prepared for its reception, have developed into an extensive art, the art of electro-metallurgy, relating to which hundreds of patents have been taken out in this country: patents for coating steel pens and pen-holders, for coating the soles of boots and shoes, for coating chairs, bedsteads, and other household articles, for gilding thread and wire-gauze, for making copper tubes and vessels, for coating the hulks of ships and for making coffins, for printing and engraving, for protecting telegraph-wires and cables, for ornamenting sepulchral monuments, for metallizing fibrous materials, leaves, or fruits, and even for metallizing a human corpse.* Electrolysis or Electro-analysis. 965. The study of the actions going on at the two poies of a galvanic cell or battery has also led to applications of as great im * See a volume printed by order of the Commissioners of Patents, entitled 'Abridgments of Specifications relating to Electricity and Magnetism. 728 Decomposition of Water. portance to the chemist, as those of metallurgy to the world at large; it has, in fact, created the distinct department of chemical science known as Electro-chemistry. When an electric current passes through a practically perfect conducting liquid, such as a molten metal or mercury, there is no alteration in the molecular structure of the liquid; but when the liquid is of a compound nature, and offers more or less resistance to the passage of the current, then decomposition of the liquid invariably occurs, one of the elements appearing at the positive pole, and the other at the negative. In accordance with the analogy of electric attraction generally, the element which is drawn to the positive pole is called the electro-negative one, and the element drawn to the negative pole, the electro-positive one. 966. Decomposition of Water.-Of course the two poles, that is, the extremities of the two wires from the end plates of the battery, must be of a kind not to be chemically acted on by either of the liberated elements. They are generally of platinum, and the decomposing apparatus may be arranged as in the figure 270. Here we have the wires from the zinc and copper poles of a plates be immersed in pure water, no decomposition takes place at all, the reason being that the water has not a sufficient conducting power. On the addition, however, of a small quantity of vitriol or sulphuric acid, the necessary conducting power is acquired, and hydrogen and oxygen bubble up at the two poles, the hydrogen at the zinc pole, or platinum connected with the zinc end of the battery, and the oxygen at the positive or copper pole. We may suspend two glass tubes over the two plates, provided, as in the figure, with india-rubber tubes and clamps at the upper ends, by which means we can easily fill at pleasure the glass tubes with the liquid under examination. With acidulated water we shall find that pure hydrogen rises up into the one tube, and pure oxygen into the other, the hydrogen forming twice as fast as the oxygen. If both Laws of Electrolysis. 729 gases be collected in one receiver, and combined again by explosion with a light or otherwise, they disappear entirely, and pure water is formed, showing that the sole effect of the sulphuric acid in the liquid is to conduct the current, and that the gaseous products are the decomposed elements of the water. If two copper plates, or the ends of two copper wires, were simply immersed in the cell of acid, the nascent or new-born oxygen would attack the positive pole, and combine with the metal as oxide (or sulphate) of copper, while the hydrogen alone would appear at the other. Suppose, again, that we substitute hydro-chloric acid for the water in this apparatus, it is found that hydrogen appears, as before. at the negative pole, and chlorine gas at the positive pole. If solutions of metallic chlorides be substituted for the hydrochloric acid, as, for instance, chloride of copper, or of gold, the metal appears as a precipitate at the negative pole where the hydrogen appeared, and the chlorine at the positive pole. Thus, hydrogen, copper, and gold are all classed as electro-positive elements, while oxygen and chlorine are electro-negative elements. The decomposition of other salts as well as of alkaline solutions, may be effected in the same way. It was by this means that Sir Humphry Davy, after effecting the decomposition of water with a battery of 250 cells, was led to the discovery of the metals potassium and sodium, as the bases of the alkalies potash and soda, formerly supposed to be elements. It is to the illustrious Faraday, however, that we owe the full development of the laws of electrolysis. He showed that if the same electric current traverse a series of different chemical compounds, the quantities of the different elements decomposed in its passage, are exactly proportional to the chemical or atomic equivalents of the elements. Thus, for example, if an apparatus such as that represented in fig. 270, and several U tubes containing, say, chloride of silver, chloride of tin, and oxide of lead, be all inclosed in one circuit, through which a strong current is passed, the electronegative elements will be simultaneously separated at the positive poles, and the electro-positive elements at the negative poles, while the quantities decomposed, will be in the proportion of 1 hydrogen to 108 silver, 581 tin, 1031 lead, 551⁄2 chlorine, and 8 oxygen, that is, in the proportion of the atomic weights of the elements. This theory of electrolysis, and the atomic theory of chemical action, thus mutually strengthen each other. Faraday also established the law that the quantity of any compound decomposed in a |