Chapter II: BLEACHING of =Linen=:--Linen may be bleached in a similar way to (31)

The relative decomposing effects produced by the same current in different electrolytes are exactly expressed by the chemical equivalents of the electrolytes. Thus, if a current be made to traverse acidulated water, iodide of potassium, and chloride of lead, these three electrolytes will suffer decomposition at the same time, but by no means to the same extent; for the current which decomposes but 9 parts of water will separate into their elements 166 parts of iodide of potassium and 139 parts of chloride of lead. The electrolysis of metallic salts is now carried out on a large scale in the beautiful arts, which we notice under the general head of ELECTROTYPE.

=ELECTROMOTIVE ESSENCE= (Romershausen). An embrocation for restoring the suspended functions of the skin by stimulating the flow of vital electricity and the functions of the nerves. A solution of oils of turpentine and rosemary in the ninth dilution of alcohol previously coloured red with some vegetable dye. (Reithner.)

=ELECTRO-PLA''TING and GILDING.= See ELECTROTYPE.

=ELECTROPH'ORUS.= A simple instrument for exciting electricity, generally used in the chemical laboratory for charging small Leyden jars when gases have to be exploded by the electric spark. To construct it, a plate of tinned iron is made into a circle of about 12 inches diameter; a raised border is then turned up for about half an inch, and the extreme edge is turned outwards over a wire to avoid a sharp border. A mixture of equal parts by weight of shell-lac, Venice turpentine, and resin, is made by gently heating them together with stirring until well fused and thoroughly incorporated. This composition is poured into the plate, to quite fill it, and kept melted until all bubbles have disappeared. Another portion of the instrument, serving the same purpose as the conductor of an electric machine, is a circle of wood, rather smaller than the resinous plate, rounded at the edge, and neatly covered with tin-foil. An insulating handle, formed of a piece of stout glass rod, is cemented into the centre of this wooden disc. Before using the instrument it must be carefully dried and slightly warmed. The resinous surface is excited by beating it obliquely with a folded piece of warm flannel. When this has been done for about a minute, the warm dry cover of the instrument is to be placed upon the resinous plate, and touched with the finger. If the cover is then raised a few inches, and the knuckle approached, a powerful spark of positive electricity will pass; and if the cover be again replaced, touched, and raised, a second spark will pass. This may be repeated many times without again exciting the resinous plate. By receiving the sparks with the knobs of a Leyden jar, a charge strong enough to give a powerful shock, or explode a gaseous mixture, may be rapidly obtained. Other forms have been given to the instrument, but the essential part of every one is a plate of some resinous substance.

=ELECTROTYPE.= _Syn._ ELECTRO-MET'ALLURGY, GALVAN'O-PLAS'TIC. The art of working in metals by the aid of electricity. Strictly speaking, the term electrotype is only applicable to one branch of 'electro-metallurgy'--that which relates to the production of copies of engraved plates, medals, coins, and other works--but it is now commonly employed in the sense indicated by our definition. According to this extended signification of the term, the art of electrotype includes ELECTRO-PLATING, and ELECTRO-GILDING.

_General Principles._--If a current from a voltaic battery be passed, by means of platinum electrodes, through water to which some sulphuric acid has been added, electrolysis, takes place, hydrogen appearing at the cathode, and oxygen at the anode. If into the acid liquid some crystals of sulphate of copper be now thrown, electrolysis will still go on, but only one of the elements of the water, namely oxygen, will be evolved; for the hydrogen, on being released, will take the place of the copper in the solution, and the copper thus liberated will be deposited on the platinum plate or wire which constitutes the negative electrode. This experiment may be continued until all the copper is extracted from the solution. Let this experiment be repeated with a copper plate for the positive electrode, and it will be found that neither of the gases will be evolved. The hydrogen, as before, will take the place of the copper in the solution; the oxygen, instead of escaping at the anode, will combine with the copper of the electrode and the sulphuric acid to form sulphate of copper. The chemical forces called into action by the current are so beautifully balanced, that in the last experiment the quantity of copper supplied by the positive electrode exactly equals the quantity withdrawn from the solution and deposited upon the negative electrode. The whole art of electrotype consists in applying the metals thus released from their solutions to artistic or useful purposes. To obtain compact and brilliant deposits, many precautions have to be observed. The solutions must be kept saturated, or nearly so; the mould to be copied, or object to be coated, must not be too small, or out of proportion to the size of the zinc plate of the battery; in fine, the power employed must be carefully regulated according to the work to be done. In all arrangements the moulds or objects which receive the deposits act as negative electrodes, and are consequently in connection with the zinc of the battery or generating cell.

_Electrotype Processes._ Although reguline deposits of many metals can be obtained through the agency of voltaic electricity, we shall only treat of those of copper, silver, gold, and platinum. When copper is deposited, the object is generally to produce a substantial copy of a medal, an engraved plate, or other work of art; but when solutions containing the precious metals are electrolysed, the deposits are nearly always used for covering the surface of inferior metals. We shall notice the operations connected with the deposition of copper, and those relating to electro-plating under separate heads.

1. DEPOSITION OF COPPER:

The moulds or models intended to receive the deposited metals may be formed of various materials. For medals and similar small works, moulds of fusible metal, white wax, stearine, stearic acid, and gutta percha, are commonly used. The first are formed by dropping or pressing the medals to be copied upon the melted metal, taking care that the former are quite cold, and that the surface of the metal is bright or free from oxide. To make a mould in gutta percha, the material must be softened in warm water, and then pressed upon the medal by means of a strong screw press. With the other materials the manipulation is very easy. A ribbon of cardboard or thick paper is placed round the medal, so as to form a rim; the material, which has been melted in an earthen vessel, is then poured on, and allowed to remain until quite cold and hard, when it is cautiously removed. For large works, moulds of plaster of Paris are usually employed; these require to be saturated with wax or tallow, by standing them in a shallow vessel containing these substances in a melted state. For copying seals and small coins, impressions in ordinary sealing-wax may be used as electrotype moulds. Non-metallic moulds must be coated with some substance which has the property of conducting electricity before they can be used as negative electrodes. The substance commonly employed is plumbago or black-lead. It must be in the condition of an impalpable powder. It is rubbed briskly over the surface of the mould (wax, stearine, plaster, &c.) by means of a strong fine camel-hair brush, till the whole presents the well-known black-lead polish. The adhesion of the plumbago may be often promoted by breathing slightly on the mould. To cause it to adhere to sealing wax impressions, the wax may be slightly moistened with spirits of wine, or exposed to the vapour of ether. Delicate moulds and objects, which cannot well be black-leaded, may be covered with a conducting film of silver, by first dipping them in bisulphuret of carbon holding about 1/20th part of phosphorus in solution, and then, after a few seconds, immersing them in a weak solution of nitrate of silver, and allowing them to dry in the light. Metallic moulds require no preparation.

The voltaic apparatus used may now be described. The single-cell arrangement, used for small works, is formed on the principle of Daniell's Constant Battery. It consists of a vessel of glass, earthenware, or wood, containing a smaller cell of thin biscuit ware, or other porous material; a rod or plate of amalgamated zinc, placed within the porous cell, and a wire connecting the zinc with the mould to be copied; the latter being placed in the outer vessel. The annexed figure represents a convenient form of the single-cell:--

The battery arrangement has many advantages over that described above, and should always be employed when large objects are to be electrotyped, or when a number of small moulds are to be operated upon. In this arrangement the copper solution is electrolysed in a separate vessel, termed the decomposition cell, and the current generated by one or more cells of a Daniell's or Smee's battery. This arrangement is shown in the following engraving:--

_a._ An oval vessel of salt-glazed earthenware or wood nearly
filled with a saturated solution of sulphate of copper.
_b._ A porous diaphragm, containing the cylinder or plate of zinc
(_c_), and filled with dilute sulphuric acid.
_d._ A small bar of brass or copper fastened to the vessel by the
binding screws (_e, e_), and supporting the plate of zinc
(_c_), by the hook of copper wire (_f_), and the mould (_g_),
by the hook (_h_).
_i._ A small shelf or partition to support crystals of sulphate of
copper, to keep up the strength of the solution.]

_a._ A constant battery cell.
_b._ Decomposition cell, a cubical vessel made of wood or
earthenware, and filled with a mixture of 1 part of dilute
sulphuric acid (1 acid + 9 water), and 2 parts of saturated
solution of sulphate of copper by measure.
_c, c, c._ Moulds suspended to the brass rod (_f_), and connected
with the zinc or positive element of the battery (_a_), by
means of the screw (_g_.)
_d, d._ Pieces of sheet copper suspended on the brass rod (_h_),
and connected with the zinc end of the battery, by means of
the screw (_i_), employed to keep up the strength of the
cupreous solution in the decomposition cell.]

To connect the moulds with the zinc or positive element, stout copper wires or strips of thin sheet copper are employed. In the case of a non-metallic mould, the wire must lead directly to the plumbagoed surface, or, what amounts to the same thing, the plumbago must be extended to the point of attachment. The connecting wires, and the backs and edges of metallic moulds, must be covered with sealing-wax varnish, or other non-conducting substance, to prevent them receiving the deposit. Before a mould is placed in the copper solution it is advisable that everything should be arranged, so that the immersion may occasion immediate voltaic action. If the connection between the zinc and the mould is not effected until after the immersion, the solution may act chemically on the surface of the mould, and cause the deposit to appear dark and dirty. When a mould has remained in the solution long enough to receive a complete coating of copper, it may be lifted out with impunity for examination. If everything is going on well, the deposited metal will present a brilliant, light, copper-coloured surface. When sufficiently thick, the deposit is removed with care, washed and placed to dry. Electrotype medals may be polished with wash-leather and the plate brush, or bronzed. Various natural objects such as insects, fruits, &c.; small works of art, such as busts and statuettes; chemical vessels, particularly glass flasks and retorts; and numerous classes of articles, may be rendered less fragile by coating them with copper by the electrotype process.