5. The four ends of two metallic arcs dip into two separate vessels filled with liquid.

In these cases, one of the liquids is decomposed by the metal, the other by the electric current thus produced.

A. The same liquid in both vessels.

a. Two Metals.-The ends of both metals dip into one vessel, the ends of one metal only into the other vessel.

In the following experiments, Faraday made use of App. 19. The platinum wire o which dips into the glass vessel a, is continued by the galvanometer, and connected by the iron wire y z with the platinum wire p, which dips into the vessel b. The iron, nickel, or silver wire dipping into a is connected at x with the platinum wire i which dips into b.-The current excited by the iron, nickel, or silver, in connection with the platinum in the liquid of the vessel a, has to make its way through the liquid in the vessel b, which weakens and, in some cases, completely stops it.

If a, b, contain nitric acid freed by boiling from nitrous acid, the iron appears slightly positive (i. e., positive electricity goes from o through y, z, p, i, to g); but the current is so feeble, that it is far exceeded by the opposite current excited by the introduction of a piece of paper moistened with hydrochloric acid, between the iron and platinum at x. A similar result is obtained when the end of the iron wire which dips into a has been made passive by ignition in the air. If the acid is diluted with four measures of water, the current is still weaker; and in both cases it is surpassed by a thermo-electric current produced by a single pair of bismuth and antimony.

When a, b, contain fuming nitric acid-which conducts better than the ordinary acid-iron produces a stronger current, which is only reduced one half by the opposite thermo-electric current of a single pair of bismuth and antimony.

If both vessels contain a mixture of equal parts of hyponitric acid and water, both ignited and unignited iron are at first slightly positive; but the current soon ceases. Unignited iron in a becomes passive; but the circuit still conducts a thermo-electric current well (though not nearly so well as sulphuret of potassium)—and moistened paper between the platinum and iron at x excites an opposite current. When the end of the iron dipping into a has been ignited, the circuit, after it has become inactive. conducts the thermo-electric current with great facility. [Iron rendered passive by hyponitric acid is covered with peroxide of iron, which is not quite so good a conductor as the black oxide (Fe3 O1), with which iron rendered passive by ignition is coated.]

In concentrated aqueous solution of potash, silver gives scarcely any current; iron or nickel, a feeble current which soon diminishes. When the solution of potash is diluted with six measures of water, silver gives no current, nickel or iron a feeble one, which, however, continues for a longer time. In these cases, paper wetted with nitric acid, introduced at x, excites an opposite current of greater strength.

If the cups a, b, contain yellow solution of sulphuret of potassium, the iron appears negative towards the platinum for ten minutes, on account

of the oxide which adheres to it. After this, the current ceases altogether. Nevertheless, the circuit conducts admirably-and produces a thermo-elec tric current on the application of heat at x, y, or z (where the iron and platinum touch one another)—and a galvanic current of still greater power on the introduction of paper moistened with dilute acid between the two metals. It appears then that no current is excited by the contact of two heterogeneous metals at x (the effects of contact at y and z would neutralize each other); but when the metals are separated at x by a moist conductor, a current is produced. Just as platinum behaves with iron in solution of sulphuret of potassium, so also does gold behave with iron or nickel, and palladium with iron or nickel: the slight deflection of the needle which always takes place at first, soon ceases. Only in the case of platinum with palladium, is the very feeble current of longer duration— the palladium after some time becoming covered with sulphuret of palladium. (Faraday.)

If the cups a, b (App. 2) contain solution of nitrate of silver-the arc gi consisting of silver, the arc op of zinc at o and of silver at pa silver tree is formed at p and g, and likewise [by purely chemical action] on the zinc, whilst the silver end i dissolves. (H. Rose.)

The cups a, b (App. 2), containing solution of nitre-o being zinc, p platinum, and gi zinc-the current is stronger than when a and b are connected by a bundle of threads saturated with solution of nitre, instead of by the arc of zinc. (H. Davy.) In Faraday's experiments, three ends of the less oxidable metal are used; in those of Davy, three ends of the more oxidable metal. (Vid. Decompositions by the electric current, Diaphragms.)

b. Three Metals and one Liquid. One metallic arc consists of two different metals, the other of the third metal.

The cups a, b (App. 2), containing a solution of copper, and o consisting of zinc, p of silver,-copper is precipitated on the silver, provided the second metallic arc gi is formed of one of the more oxidable metals, not of gold or platinum. (Singer.)

The vessels a and b (App. 2), containing solution of nitrate of silver, and o consisting of zinc, p of platinum, gi of silver,-silver is reduced at o, p, and g, while the silver end i dissolves. [The zinc as it oxidates sends negative electricity to the platinum-which negative electricity combines with the silver there reduced; the silver reduced by zinc in a is deposited, partly on the zinc by purely chemical action, partly on the silver end g by electro-chemical action. The negative electricity which has to combine with the silver reduced at the end g receives it from the silver end i, from which negative electricity is set free by the combination of the silver with oxygen.]-When the silver arc gi is replaced by an arc of platinum, the action is much slower [because the platinum end i takes up no oxygen, and therefore gives rise to no transposition of atoms in b].-If the vessels a, b, contain solution of sulphate of copper, and g i consists of silver, copper is deposited on the zinc [by purely chemical action], and likewise, after twenty-four hours, on the platinum and on the silver end g [silver probably also dissolving at i-When the arc gi consists of platinum, the platinum b does not become covered with copper till after the lapse of two days; but the action is much more energetic when the arc gi is formed of zinc, tin, lead, or iron. (Pfaff.)

If the vessels a, b (App. 2) contain a mixture of 1 part of oil of vitriol and 100 parts of water, op being an iron wire coated at p with

peroxide of lead, and gi a platinum wire,-hydrogen gas is evolved at the platinum end g, and oxygen at the platinum end i. (Schönbein.)

The metallic arc op (App. 2), consisting of platinum dipping into a by a point, but into b by a large surface, and the cups a, b containing water, or a mixture of 1 part oil of vitriol and 10 parts water, or 1 part oil of vitriol, 5 parts common salt, and 100 parts water, or a mixture of sea-water and nitric acid,-a larger deflection is produced when the arc gi consists of zinc at i and copper at g, than when it is arranged the contrary way. But with a mixture containing from 1 to 18 parts oil of vitriol with 1000 parts water, or 1 part hydrochloric acid and 80 parts water, the strongest deflection is produced with zinc in a and copper in b. With I part of common salt in 10 of water, the deflection is equally strong both ways. (Marianini.)

Various combinations with three and four metals.

The vessels a, b (App. 2), containing either pure or acidulated water, the current produces the same deflection of the needle in each of the following three combinations: 1. o zinc, g platinum, p and i copper;-2. o zinc, p platinum, g and i copper;-3. o zinc, p copper, g platinum, i iron. (Poggendorff.)

If both vessels contain water acidulated with sulphuric acid, o being cadmium, g iron, p platinum, and i copper, the current excited by copper with platinum has the advantage over that produced by cadmium with iron; whereas a simple circuit of cadmium and iron produces an electric current 400 times as strong as that excited by a copper and platinum circuit.

B. Two Liquids and two Metals.

a. Each metallic arc consists of one metal.

This arrangement may be regarded as a combination of two circuits of two metals and one liquid—or, according to Pohl, as a combination of two circuits of one metal and two liquids.

Experiments of this kind may be made with App. 2;-App. 20. (The upper end of one of the metals is bent round, and either beaten out flat or hollowed into a cup; and in or upon this is placed a liquid or a piece of paper saturated with a liquid, with which the bent wire i of the other metallic plate g is connected);-App. 21. (The two bent tubes are filled with the two liquids, in which are immersed the four ends of the two metallic arcs.

Hence the

If the cup a (App. 2), contains oil of vitriol, and b nitric acid-the arc op consisting of platinum, gi of zinc, tin, lead, iron, copper, brass, or silver-positive electricity goes from the oil of vitriol, through the galva nometer interposed in the platinum arc, to the nitric acid. galvanic action of the oil of vitriol on the more oxidable metal overcomes that of the nitric acid. The contrary effect is generally produced, if the cup a contains solution of chloride of calcium instead of oil of vitriol; but the current is feeble. (De la Rive.)

The cup a (App. 20) containing water mixed with sulphuric acid and a little nitric, the arc op being formed of zinc and g i of platinum, and a piece of paper saturated with iodide of potassium being placed between p and i-iodine is separated upon the platinum; and when turmeric paper is used, it is turned brown from the liberation of alkali on the zinc p.

The same decompositions of the second liquid are obtained with zinc, platinum, and aqueous solution of potash or common salt; and likewise (as Andrews obtained them with App. 19), with amalgamatedz inc, platinum, and water merely acidulated with sulphuric acid. In these cases, the action of the liquid containing acid or common salt upon the zine overcomes that of the solution of iodide of potassium on the same metal. In this or a similar manner, zinc with platinum in dilute sulphuric acid decomposes aqueous solution of iodide of potassium, fused protochloride of tin, and fused chloride of silver-but not water acidulated with sulphuric acid, hydrochloric acid, solution of Glauber's salt, fused chloride of lead, or fused iodide of lead. In those cases in which no decomposition takes place, there is likewise no current produced.-Zinc and platinum in aqueous solution of potash, or a mixture of ammonia and sulphate of ammonia, decomposes aqueous solution of iodide of potassium, sulphate of soda, nitrate of silver, and hydrochloric acid.-Zinc and platinum in water containing both sulphuric and nitric acid, decomposes not only the above named compounds, but also fused nitre, fused chloride of lead, and fused iodide of lead, but not solution of acetate of lead. It is not the quantity but the intensity of the current that determines the decomposition of these compounds: large plates of zinc and platinum in water containing sulphuric acid do not decompose the last mentioned substances; but the decomposition takes place readily, even with wires of these metals, if the water contains nitric in addition to the sulphuric acid. At all events it follows from these experiments, that contact of metals is not essential to the production of an electric current.-Polished lead with platinum in water likewise produces a current capable of decomposing iodide of potassium. (Faraday.)

Water containing sulphuric acid being placed in the vessel a (App. 2), aqueous solution of potash in b, and the ends of the zinc arc op and of the copper or platinum arc g i dipping into both liquids,- -a current is at the first instant produced in favour of the potash (from o to p)—but it ceases immediately. (Faraday.)

The arc op (App. 21) consisting of platinum, the arc gi of some other metal, and the tubes a, b, containing different liquids an electric current, generally a strong one, is produced in the direction ao pb, when the liquids and metal are disposed in the following order: gi iron, a dilute sulphuric acid, b fuming nitric acid;-gi iron, a hydrochloric acid, b hyponitric acid diluted with water sufficient to make it green;—g i zinc, cadmium, lead, or copper, a hydrochloric acid, 6 iodide of potassium;gi iron, a dilute hydrochloric acid, b fuming nitric acid or yellow solution of sulphuret of potassium;-gi iron, a dilute nitric acid, b concentrated colourless nitric acid, fuming nitric acid, hydrated green hyponitric acid, or yellow solution of sulphuret of potassium;-gi iron, zinc, cadmium, lead, or copper, a dilute nitric acid, b iodide of potassium;-g i iron, a water or solution of common salt, b hydrated green hyponitric acid.-If the arc op consists of iron instead of platinum, the current takes the same direction with the following arrangements: gi zinc or lead, a oil of vitriol, b dilute sulphuric acid;-gi copper, a yellow sulphuret of potas sium or strong nitric acid, b dilute nitric acid or iodide of potassium. The galvanic action of the liquid contained in a always predominates. (Faraday.)

Poggendorff, by the use of App. 2, obtained currents, feeble for the most part, and varying, not only according to the liquids used, but likewise Recording to the nature of the metals. The acids used were very dilute;

the

+ denotes that the galvanic action is strongest in the vessel a, contrary; denotes that the current takes the former direction at first and afterwards the latter; the contrary.

In Vessel.

Fe. +1

Zu. +

Sn. I

Cu

Ag

Pt. +1

Sn.

Cu

Pt.
Ag.

+

| | | + | | | 10 + 110111

+11+
1. | + + | | | + | + + | | | |│

+ + + |+|| |

The direction of the current varies with the degree of concentration. With zinc and platinum, hydrochloric acid produces a stronger current than iodide of potassium; but with dilute acid the contrary effect is observed. With zinc and copper, dilute hydrochloric acid also has the preponderance, [because the copper takes up iodine]. Sulphuric acid with platinum and zinc is overcome by iodide of potassium [Faraday asserts the contrary]; and with a mixture of one measure of oil of vitriol and two measures of water, no current is produced. (Poggendorff.)

The arc op consisting of platinum, the arc gi of zinc, and the two vessels a, b, containing water-then, if from any inequality in the zinc plates, a feeble current is produced proceeding from a to b, this current is actually strengthened by the introduction of common salt into b-although this substance, by increasing the chemical action in b, might be expected Nitric acid introduced into b likewise to produce an opposite current. strengthens the current in many cases, but sometimes changes its direction. (Poggendorff.)

When the vessels a, b (App. 2), contain water, and op consists of platinum, gi of zinc, no current is apparent, even on heating one of the vessels to the boiling point, although the chemical action must be thereby increased: but the addition of a small quantity of acid or salt to the water in one of the glasses immediately excites a current. (Poggendorff.)

If the cup a (App. 2) be divided into two parts by a membranous diaphragm the left hand division in which the platinum arc op is immersed containing nitric acid, while the right hand division into which the arc gi is made to dip, and likewise the cup b, are filled with a mixture of one measure of oil of vitriol and three measures of water-a feeble current is excited in favour of the cup a; but the zinc in b is most quickly dissolved. (Poggendorff.)