oxygen, chlorine, iodine, sulphur, and the other so-called electro-negative substances--and liberates hydrogen, metals, and other so-called electro positive substances,-positive electricity goes directly from the passie to the active metal, and-if the current be supposed to continue through the liquid-from the acting metal through the liquid to the passive metal,

[a. The so-called electro-negative bodies, viz. oxygen, chlorine, bromine, sulphur, &c. contain positive electricity as well as heat in the combined state, and the so-called electro-positive bodies, as hydrogen and metals, contain negative electricity similarly combined. In the combination of an electro-positive with an electro-negative body, the opposite electricities neutralize one another more or less completely and produce fire; in water, e. g. the oxygen has lost its positive, and the hydrogen its negative electricity (p. 157, 158).

b. When a metal, such as zinc, comes in contact with water, its great affinity for oxygen causes the oxygen-atoms of the contiguous atoms of water to turn towards the zinc, and the hydrogen-atoms the contrary way. This arrangement is propagated throughout the whole mass of water, so that the hydrogen-atom of each atom of water is turned towards the oxygen-atom of the next. (App. 12.)

c. The oxygen-atoms lying nearest to the zinc unite with it and form oxide of zinc. This sets free the negative electricity of the zine. At the same time, the nascent hydrogen has to recover the negative electricity which it had lost by combination with oxygen in the form of water: it therefore takes this negative electricity from the zinc. But the oxygenatom situated between the zinc and the hydrogen-atom interferes with this transference of electricity: consequently, an extremely small quantity of negative electricity becomes accumulated in the zine, and a corresponding quantity of positive electricity in the water; because that portion of the nascent hydrogen which does not receive negative electricity from the zinc takes it from the caloric of the water, and therefore sets free some of the positive electricity of that liquid.

d. When another metal, copper for example, is placed in the water near the zinc, but without touching it, this metal likewise attracts the oxygen-atoms of the water,-and we may suppose that, since the atoms of water turn their oxygen-atoms towards both metals, a limit x will be found between the two metals, but nearer to the copper than to the zinc, at which every two hydrogen-atoms come in contact with each other.

e. If, on the other hand, zinc and copper-or another metal whose affinity for oxygen is less than that of zinc-are placed in metallic contact, either directly or by means of a wire (e. g. that of the galvanometer), the following change takes place. The negative electricity set free in the zinc passes over, in greater or less quantity, directly to the copper; thence it attracts the hydrogen-atom of the adjacent atom of water in every series of atoms of water situated between the zinc and copper,-passes over to it, and causes it to escape in the form of hydrogen gas. In this case, all the oxygen-atoms are turned towards the zinc, and all the hydrogen-atoms towards the copper, in consequence of the attraction exerted upon them by the negative electricity issuing from that metal. It is immaterial whether we say that negative electricity goes through the metallic are from the zinc to the copper, or positive electricity from the copper to the zinc.

f. In each series of atoms of water between the zinc and copper, a mutual displacement of the oxygen-atoms from the copper towards the zinc, and of the hydrogen-atoms from the zinc towards the copper, must be

supposed to take place, the water remaining unaltered in composition throughout its whole mass; while the fixation of oxygen takes place only at the surface of the zinc, and the liberation of hydrogen at the surface of the copper. Suppose, for the sake of illustration, that only four atoms of water are situated in a row between the zinc and copper (the actual number, even with a small interval between the metals, would probably be several millions):—then, the zinc takes up the oxygen-atom 1, and the hydrogen-atom 4 escapes at the surface of the copper. Next, the hydrogen-atom 1 unites with the oxygen-atom 2; and similarly, H2 with 03, and H 3 with O 4,-and, notwithstanding the separation of the two elements of the water in different places, every thing in the middle remains apparently tranquil and unaltered, and no visible current is formed in the water. (4pp. 15, a.) Since now one atom of water has disappeared from the series, another atom, 5, is transferred to the series from the surrounding water. (App. 15, b.) In this series, all the hydrogenatoms are turned towards the zinc; but the affinity of that metal for oxygen immediately causes the atoms of water to turn half round, so that all the oxygen-atoms may be directed towards the zinc. (App. 15, c.) The zinc then combines with O 2, and H 5 is evolved at the surface of the copper; and thus the process is repeated, as long as the oxidation of the zinc goes on. The hydrogen-atoms move in semi-circles towards the right, first upwards and then downwards,-the oxygen-atoms towards the left alternately upwards and downwards, till the former reach the surface of the copper and there escape, the latter, the surface of the zinc and combine with that metal. This simultaneous transposition of the two kinds of atoms may be represented by the lines in App. 16.

g. It appears then that there are two kinds of chemical action to be distinguished: the purely-chemical and the electro-chemical. The former takes place when zinc is placed alone in contact with water (b, c,): it does not give rise to an electric current, because the negative electricity passes directly from the zinc to the hydrogen. But when the zinc is connected with another perfect conductor-which in this case plays only the passive part of transferring the negative electricity to the hydrogen (e, f)-electro-chemical action is produced, the electricity passing along from the zinc to the copper-even when the connecting wire is of great length and giving rise to a mutual transposition of the atoms.

h. The cause of both pure chemical and electro-chemical action is to be found in the affinity of zinc for oxygen; that of the latter, more especially, in the obstacle which the oxygen-atom situated between the zinc and the hydrogen-atom opposes to the transference of negative electricity from the zinc to the hydrogen. For, the electricity being able to traverse metals with the greatest facility and rapidity, prefers the circuitous road from the zinc to the copper, and transposes the atoms of the water, which -since it possesses the peculiar mobility of a liquid, and since the new compounds formed are equal in number to the atoms decomposed-appears to offer no great opposition to the change.

i. This last-mentioned opposition is, however, in most cases sufficiently great to cause a part of the purely chemical action to go on simultaneously with the electro-chemical. The negative electricity avails itself of both paths to pass from the zinc to the hydrogen-the direct route and that through the copper. The stronger the purely chemical action, the weaker is the electric current. The electro-chemical action, and therefore the electric current, is strengthened, and the purely chemical action proportionally weakened: 1. By diminishing the distance between

the copper and zinc within the liquid,-because the difficulty caused by the transposition of the atoms is thereby somewhat lessened.-2. By increasing the surface of copper in contact with the liquid,-because the transference of the negative electricity from the copper to the hydrogen of the water is thereby facilitated.-3. By facilitating the mechanical removal of the bubbles of hydrogen which accumulate on the copper and diminish its acting surface.-4. By purifying the zinc, and consequently equalizing the power with which the different points of its surface attract the oxygen of the water. For even if the zinc contains but a small quantity of other metals (except mercury), these impurities, in combination with a small portion of the zinc, form alloys, which, being mechanically diffused through the rest of the metal, and having less affinity for oxygen than pure zinc, produce an action similar to that of copper-that is to say, while the oxygen goes over to the pure zinc, these alloys transfer the liberated negative electricity to the hydrogen, thereby producing a local electrochemical action, by which the current from the zinc to the copper is weakened.-5. An opposite effect is produced by covering the surface of the zinc with mercury. Amalgamated zinc exhibits with water and dilute acids, only electro-chemical, not purely chemical action. What it is that here prevents the direct passage of the negative electricity from the zinc to the hydrogen remains yet to be discovered.-6. The nature of the liquid has likewise some influence. Thus, water is more inclined to electro-chemical, nitric acid to purely chemical action-possibly, because the oxygen-atom offers a more effectual obstacle to the transference of electricity from the zinc to the minute atoms of hydrogen, than to the much larger atoms of nitric oxide evolved from the nitric acid,-which indeed, from their greater size, must project beyond the atoms of oxygen.

k. Zinc connected with copper takes up the oxygen of a watery liquid more easily and quickly than zinc alone, and consequently liberates a greater quantity of hydrogen gas in a given time: for the points of transference of negative electricity to hydrogen are multiplied, and the transference thereby facilitated.

7. Since, in the action of water upon zinc, the oxide, as it forms, is deposited on the surface of the metal, and thereby offers a continually increasing obstacle to its contact with the water, the oxidation proceeds very slowly, both when the zinc is alone, and when it is connected with copper: hence, even in the latter case, the quantity of negative electricity which passes over in a given time is but small. The presence of an acid and certain other substances accelerates the oxidation of the zinc1. By the predisposing affinity of the acid for the oxide of zinc produced, and 2. By dissolving the oxide and keeping the surface of the zinc clean. By this addition, the quantity of negative electricity which passes over in a given time from the zinc to the copper is much increased -in other words, a current of much greater Quantity is produced; but the Intensity of the current,-that is to say, its velocity and power of overcoming the obstacles which interfere with its motion-is not on that account necessarily increased: for the intensity depends on the quantity of negative electricity which can accumulate in zinc unconnected with copper, before it acquires a sufficient force to overcome the obstacle presented by the oxygen-atom between the zinc and hydrogen-atom, and pass directly to the hydrogen. It may likewise be supposed that the atom of zinc-salt formed when an acid is present, moves onward, together with the adjacent hydrogen-atom, from the zinc towards the copper in the manner described in f-whilst the free acid, in connection with the

oxygen-atoms of the water, is transferred from the copper towards the zine side.

m. Similar actions take place when zinc and copper are immersed in other liquids. From nitric acid zinc takes oxygen, while negative electricity passes over to the liberated nitric oxide gas. From sulphuret of potassium, containing several atoms of sulphur, zinc takes sulphur,-and negative electricity passes over to the sulphuret of potassium, which has thus been deprived of part of its sulphur: and so on.

n. The rest of the metals and other perfect conductors exhibit relations similar to those of zinc and copper, at least when one of the two conducting bodies immersed in a liquid is capable of decomposing it. The direction of the current is always determined by the difference of affinity of the two conductors for the electro-negative element of the liquid. Negative electricity always proceeds from the metal, which by its greater affinity appropriates the electro-negative element, through the connecting wire to the other metal,-or, what comes to the same thing— positive electricity goes from the latter metal to that which is in the act of combination, and possibly, from this through the liquid to the inactive. metal. The quantity of the electric current is directly proportional to the strength of the electro-chemical action. The greater the quantity of the electro-negative element taken up by the metal, the greater also will be the quantity of electricity which passes over,-provided always that no purely chemical action take place at the same time,-for the effect of this would be to weaken or arrest the electric current. The following circumstances accelerate the combination of the metal with the electronegative element of the liquid, and may therefore increase the electric current: Increase of affinity and diminution of cohesion in the metal,— rise of temperature,-increased facility of conveying the negative electricity from the electro-negative metal to the liquid, depending upon cleanliness and extent of surface.-The tension or intensity of the electric current is perhaps greater in proportion to the difference between the affinities of the two metals for the electro-negative element; the influence of the circumstances noticed in i, 1, must however be taken into account.

o. The metal endowed with the greater affinity for the electro-negative element of the liquid may, by its action, become covered with a product which may interrupt wholly or partially the contact between the liquid and the metal; in such a case, the affinity of the other metal may come into play, and give rise to a reversal of the current.

p. Two perfect conductors may produce a current, even without abstracting any element of the liquid, provided one of them give up one of its own elements to the liquid or the metal. Thus, from peroxide of manganese, lead, or silver, immersed in different liquids, positive electricity passes over through the galvanometer to platinum or copper. For, when a body of this nature gives up oxygen to the liquid, or transmits it by displacement of particles to the metal, the oxide, being more or less. reduced, recovers its lost negative electricity by appropriating that which flows to it through the connecting wire from the platinum or the copper,the metal either receiving this negative electricity from the liquid which gives it up on combining with the oxygen, or evolving it directly, when it takes up the oxygen itself.]

Experiments with Water. On placing one above the other, brass, zinc, dry wood, damp wood, zinc, and brass, the last-mentioned brass imparted negative electricity to the condenser of the electrometer. (De la Rive.)

The same result was obtained with copper, zinc, four sheets of dry paper, one sheet of damp paper, zinc, copper;-the two copper-plates may also be omitted; the first zinc gave positive, the second negative electricity to the electrometer. Again, when a bar of zinc is wrapped round with paper and one-half of the paper wetted, the wet end imparts positive electricity to the electrometer; the dry end-if a damp sheet of paper be laid on the condenser to facilitate the conduction-negative electricity. (Fechner.) [Negative electricity accumulates in the last zinc (p. 342, c), positive electricity in the damp wood or paper; and these are communicated to the electrometer, when it is brought in contact with the zinc or the moisture.]

A solution of 1 part of potassium in 100 parts of mercury is, when placed in water, strongly positive towards platinum; polished lead is also positive towards platinum. Potassium being soluble in water, and oxide of lead likewise soluble though in a slighter degree, these substances can produce a strong current without the intervention of an acid. (Faraday*.)

If a strip of zinc and a strip of platinum in water deprived of air give at the commencement of the action a deflection of 15°, this deflection will be reduced to 1°, when the circuit has been closed for some time; but will again rise to 15°, after the circuit has been left open for 24 hours. Zine which has become inactive by immersion, together with platinum, in water free from air, is negative towards zinc freshly immersed. In water containing air, on the contrary, a pair of zinc and copper gives a constant deflection of 15°. (Buff.)

Iron, which rusts when placed by itself under water, remains bright when in contact with zinc, the latter alone becoming oxidated. (Wetzlar.) -The so-called Galvanized Iron is iron protected by a coating of pulverized zinc.

Polished lead is positive towards iron in pure water, tarnished lead negative. (Yorke.)-In pure water, lead is constantly, but very feebly, positive towards iron,-the iron likewise oxidating, though in a less degree. In spring-water, the lead is positive at first; but the iron soon becomes positive, remains so, and oxidates. (Wetzlar.) [Perhaps a deposit forms on the surface of the lead (p. 345, 0).]

Gold and platinum together produce no current in pure water (Walcker, Becquerel); neither does gold or platinum produce a current with magnetic iron ore, specular iron, red oxide of copper, or iron pyrites (Becquerel.)

If the wet and dry ends of a piece of peroxide of manganese be connected with the galvanometer, positive electricity passes from the dry end through the galvanometer to the wet end. A crystal of peroxide of manganese is feebly positive towards platinum in water, as shown by the galvanometer; the current soon ceases, but reappears if the circuit be left open for not less than five minutes. Platinum exhibits similar relations with graphite or anthracite, excepting that the latter becomes positive.

*These and most of the following experiments were so arranged that the metals in contact with the liquid were connected with the ends of the galvanometer. When it is said, in accordance with the received phraseology, that one metal is positive with regard to another, and the latter negative with regard to the former, it is to be understood that positive electricity goes from the negative metal through the galvanometer to the positive metal (and, as is sometimes assumed, from the positive metal through the liquid to the negative metal). In short, the positive metal is that which is chemically active-and, by taking up the electro-negative element of the liquid, developes negative electricity within itself, and sends it through the galvanometer to the inactive, negative metal,—or receives positive electricity from the latter (p. 342, c).