SECTION XXIX.

GALVANISM is a peculiar kind of electricity, elicited by the force of chemical action, instead of friction. It is connected with one of the most brilliant periods of British science, from the splendid discoveries to which it led Sir Humphry Davy; but it has acquired additional interest since it has been proved, by the reciprocal action of galvanic and magnetic currents, that magnetism has no existence as a distinct or separate principle, but is only an effect of electricity: therefore, galvanism, as immediately connected with the theory of the earth and planets, forms a part of the physical account of their nature.

The disturbance of electric equilibrium, and a development of electricity, invariably accompanies the chemical action of a fluid on metallic substances, and is most plentiful when that action occasions oxidation. Metals vary in the quantity of electricity afforded by their combination with oxygen; but the greatest abundance is developed by the oxidation of zinc by weak sulphuric acid; and in conformity with the law, that one kind of electricity cannot be evolved without an equal quantity of the other being brought into activity, it is found that the acid is positively, and the zinc negatively electric. It has not yet been ascertained why

equilibrium is not restored by the contact of these two substances, which are both conductors, and in opposite electrical states; however, the electrical and chemical changes are so connected, that unless the equilibrium be restored, the action of the acid will go on languidly, or stop as soon as a certain quantity of electricity is accumulated in the acid. The equilibrium, however, will be restored, and the action of the acid will be continuous, if a plate of copper be placed in contact with the zinc, both being partly immersed in the fluid; for the copper, not being acted upon by the acid, will serve as a conductor to convey the positive electricity from the acid to the zinc, and will at every instant restore the equilibrium, and then the oxidation of the zinc will go on rapidly. Thus three substances are concerned in forming a galvanic circuit, but it is indispensable that one of them be a fluid. The electricity so obtained will be very feeble, but it may be augmented by increasing the number of plates. In the common galvanic battery, the electricity which the fluid has acquired from the first plate of zinc exposed to its action, is taken up by the copper plate belonging to the second pair, and transferred to the second zinc plate with which it is connected. This second plate of zinc having thus acquired a larger portion of electricity than its natural share, communicates a larger quantity

of electricity to the fluid in the second cell. This increased quantity is again transferred to the next pair of plates; and thus every succeeding alternation is productive of a further increase in the quantity of the electricity developed. This action, however, would stop unless a vent were given to the accumulated electricity, by establishing a communication between the positive and negative poles of the battery, by means of wires attached to the extreme plate at 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