each end. When the wires are brought into contact, the galvanic circuit is completed, the electricities meet and neutralize each other, producing the shock and other electrical phenomena, and then the electric current continues to flow uninterruptedly in the circuit, as long as the chemical action lasts. The stream of positive electricity flows from the zinc to the copper, but as the battery ends in a zinc plate which communicates with the wire, the zinc end becomes the positive, and the copper the negative poles of a compound battery, which is exactly the reverse of what obtains in a single circuit.

Galvanic or voltaic, like common electricity, may either be considered to consist of two fluids passing in opposite directions through the circuit, the positive stream coming from the zinc, and the negative from the copper end of the battery; or, if the hypothesis of one fluid be adopted, the zinc end of the battery may be supposed to have an excess of electricity, and the copper end a deficiency.

Voltaic electricity is distinguished by two marked characters. Its intensity increases with the number of plates— its quantity with the extent of their surfaces. The most intense concentration of force is displayed by a numerous series of large plates, light and heat are copiously evolved, and chemical decomposition is accomplished with extraordinary energy; whereas, the electricity from one pair of plates is so feeble, whatever their size may be, that it gives no sign either of attraction or repulsion; and, even with a battery consisting of a very great number of plates, it is difficult to render the mutual attraction of its two wires sensible, though of opposite electricities.

The action of voltaic electricity differs materially from that of the ordinary kind. When a quantity of common

electricity is accumulated, the restoration of equilibrium is attended by an instantaneous violent explosion, accompanied by the development of light, heat, and sound. The concentrated power of the fluid forces its way through every obstacle, disrupting and destroying the cohesion of the particles of the bodies through which it passes, and occasionally increasing its destructive effects by the conversion of fluids into steam from the intensity of the momentary heat, as when trees are torn to pieces by a stroke of lightning even the vivid light which marks the path of the electric fluid is probably owing to the sudden compression of the air and other particles of matter during the rapidity of its passage; but the instant equilibrium is restored by this energetic action, the whole is at an end. On the contrary, when an accumulation takes place in a voltaic battery, equilibrium is restored the moment the circuit is completed; but so far is the electric stream from being exhausted, that it continues to flow silently and invisibly in an uninterrupted current supplied by a perpetual reproduction; and although its action on bodies is neither so sudden nor so intense as that of common electricity, yet it acquires such power from constant accumulation and continued action, that it ultimately surpasses the energy of the other. The two kinds of electricity differ in no circumstance more than in the development of heat. Instead of a momentary evolution, which seems to arise from a forcible compression of the particles of matter during the passage of the common electric fluid, the circulation of the voltaic electricity is accompanied by a continued development of heat, lasting as long as the circuit is complete, without producing either light or sound; and this appears to be its immediate direct effect, inde

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pendent of mechanical action. Its intensity is greater than that of any heat that can be obtained by artificial means, so that it fuses substances which resist the action of the most powerful furnaces. The temperature of every part of a galvanic battery itself is raised during its activity.

When the battery is powerful, the luminous effects of galvanism are very brilliant; but considerable intensity is requisite to enable the electricity to force its way through the air on bringing the wires together from the opposite poles. Its transit is accompanied by light, and in consequence of the continuous supply of the fluid, sparks occur every time the contact of the wires is either broken or renewed. The most splendid artificial light known is produced by fixing pencils of charcoal at the extremities of the wires, and bringing them into contact. This light is the more remarkable as it appears to be independent of combustion, since the charcoal suffers no change, and likewise because it is equally vivid in such gases as do not contain oxygen. Though nearly as bright as solar light, it differs from it in possessing some of those rays of which the sunbeams are deficient, according to the. experiments of M. Fraunhofer. Voltaic electricity is a powerful agent in chemical analysis; numerous instances might be given, but the decomposition of water is perhaps the most simple and elegant. Suppose a glass tube filled with very pure water, and corked at both ends: if one of the wires of an active galvanic battery be made to pass through one cork, and the other through the other cork, into the water, so that the extremities of the two wires shall be opposite and about a quarter of an inch asunder, chemical action will immediately take place, and gas will continue to rise from the extremities of both wires till the

water has vanished. through the tube, the water will reappear. By arranging the experiments so as to have the gas given out by each wire separately, it is found that water consists of two parts of hydrogen and one of oxygen. The positive wire of the battery has a stronger affinity for oxygen than oxygen has for hydrogen; it consequently combines with the oxygen of the water, and sets the hydrogen free; but as the negative wire has a stronger affinity for hydrogen than hydrogen has for oxygen, it combines with the hydrogen of the water, and sets the oxygen free. If, therefore, an electric spark be sent through a mixture consisting of two parts of hydrogen and one of oxygen, the gases will combine and form water. The decomposition of the alkalies and earths by Sir Humphry Davy, and all chemical changes produced by the electric fluid, are accomplished on the same principle, and it appears that, in general, combustible substances go to the negative wire, while oxygen is evolved at the positive. The powerful efficacy of voltaic electricity in chemical decomposition arises from the continuance of its action, and its agency appears to be most exerted on fluids and substances which, by conveying the electricity partially and imperfectly, impede its progress. But it is now proved to be as efficacious in the composition as in the decomposition or analysis of bodies.

If an electric spark be then sent

It had been observed that, when metallic solutions are subjected to galvanic action, a deposition of metal, generally in the form of minute crystals, takes place on the negative wire by extending this principle, and employing a very feeble voltaic action, M. Becquerel has succeeded in forming crystals of a great proportion of the

nature.

mineral substances precisely similar to those produced by The electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles, when in solution, within the narrow sphere of molecular attraction already mentioned as the great agent in the formation of solids. Both light and motion favor crystallization. Crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light; and it is a well known fact that still water, cooled below 32°, starts into crystals of ice the instant it is agitated. Light and motion are intimately connected. with electricity, which may therefore have some influence on the laws of aggregation; this is the more likely, as a feeble action is alone necessary, provided it be continued for a sufficient time. Crystals formed rapidly are generally imperfect and soft, and M. Becquerel found that even years of constant voltaic action were necessary for the crystallization of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond!

Several fish possess the faculty of producing electrical effects. The most remarkable are the gymnotus electricus, found in South America, and the torpedo, a species of ray, frequent in the Mediterranean. The absolute quantity of electricity brought into circulation by the torpedo is so great that it effects the decomposition of water, has power sufficient to make magnets, and gives very severe shocks; it is identical in kind with that of the galvanic battery, the electricity of the under surface of the fish being the same with the negative pole, and that in the upper surface the same with the positive pole: its manner of action is, however, somewhat different, for, although the evolution of