GALVANISM. 



wire, and, indeed, through the whole arrangement. 

 If we break the connecting wire, we may find, by 

 the gold-leaf electroscope, that the end of the wire 

 fixed to the copper is positively electrified, and the 

 other wire negatively, though each in an extremely 

 feeble degree. There the electricities remain shut 

 up, and the chemical action suspended, till we 

 bring the ends together. Then the opposite elec- 

 tricities combine and neutralise each other ; and 

 the whole, plates and wires, will be discharged or 

 reduced to electric rest. But only for the instant ; 

 the action of the acid water sends a fresh supply 

 of opposite electricities to each plate. So there 

 is an incessant production and combination of 

 the two electricities. 



This constant succession of electric discharges 

 is called a current, and it is said to flow in a 

 circuit through the liquid, plates, and wire. If 

 the circuit be anywhere broken, as by cutting the 

 wire or lifting one plate out of the liquid, \h&_ft 

 of electricity instantly ceases. But the ends of the 

 circuit, where the break is, become charged with 

 opposite electricities ; and the excitement is then 

 said to pass from the dynamic or motive state to 

 the static, or reposing state. 



Theory of Action of a Galvanic Couple. On 

 collecting some of the gas, which is seen to form 

 at the copper plate, we find it to, -be hydrogen. 

 Why it should appear there, and not at the zinc, 

 and why it should be formed at all, we shall now 

 explain. When the plates of a galvanic pair are 

 put in acidulated water, they are instantly thrown, 

 by the action of the acid on the zinc, into opposite 

 electrical states. The zinc becomes positively, and 

 the copper negatively, polarised on the face next 

 the liquid. But between the two plates, the liquid 

 is also thrown into a polarised state. Water, as 

 has long been known, is not a simple but a com- 

 pound body, formed by the chemical union of 

 oxygen and hydrogen. Each molecule of water 

 may be looked on as an atom of oxygen bound 

 to one of hydrogen, and these molecules are per- 

 fectly movable amongst each other, so that they 

 can take up any position. Zinc and copper have 

 each an affinity for oxygen, but zinc far more 

 than the copper. When a plate of each is put 

 into acid water, then we have this result along 

 each line of molecules. The zinc plate turns 

 the oxygen end of the first molecule to itself, 

 and weakens its attraction for its hydrogen fellow. 

 The latter turns its spare strength to attract 

 the oxygen of the next molecule, and so on, 

 till ultimately the positive hydrogen is presented 

 to the copper, for which it has no liking, and 

 induces on its face a negative polarity. In this 

 way, then, we see the action of the acid water 

 on the zinc, or the chemical affinity between the 

 two, is a force urging all the particles of the liquid 

 to set in a certain direction. In like manner, the 

 chemical affinity of the copper for the oxygen 

 tends to set the particles all in an opposite direc- 

 tion. The difference between these two forces 

 constitutes what is called the electro-motive force of 

 the couple. Had we taken a plate of silver with 

 the plate of zinc, the resulting electro-motive force 

 would have been still greater, because silver has 

 less affinity for oxygen than copper has. But had 

 we taken two plates of zinc, or two of copper, we 

 should have been setting two equal forces against 

 each other, and the electro-motive force would be 

 nil. As a general rule, the electro-motive force is 



greater the greater the difference of the metals in 

 their liability to be acted on by the liquid. 



The face of the zinc, then, has positive, and the 

 face of the copper negative, electricity ; and all 

 their molecules will be thrown into a polarised 

 state, in accordance with our previous notions of 

 electric action. By the doctrine of polarity, the 

 further ends of each wire attached to the plates 

 will have an opposite electricity to that on their 

 faces. That is to say, the end of the wire from 

 the copper will have positive, and the end of the 

 wire from the zinc negative electricity. But the 

 whole system, we are to remember, plates, wires, 

 and liquid, is really in a polarised state. 



When, now, we bring the ends of the two wires 

 together, the positive electricity in the end of the 

 copper wire tends to unite with the negative at the 

 end of the zinc wire. Yet so feeble is the degree 

 of polarity, or so weak is the tension of each elec- 

 tricity, that they are unable to unite unless there 

 be absolute contact When there is this, the two 

 combine, and an instantaneous electric discharge 

 is set up along the whole polarised course. The 

 zinc can now unite with the oxygen atom next it ; 

 the hydrogen atom thus set free unites with the 

 oxygen of the next molecule to re-form water ; 

 and so the hydrogen and oxygen atoms pair off, 

 till next the face of the copper is left an unmatched 

 atom of hydrogen. Having no affinity for copper, 

 the hydrogen escapes in its natural form of gas. 

 There is thus, with such a galvanic couple, an 

 incessant union of oxygen with the zinc plate to 

 form zinc oxide, and a setting free of hydrogen at 

 the copper. 



For the sake of simplicity, we have here sup- 

 posed the liquid to be pure water. But this would 

 only give a very feeble current, for the oxide of 

 zinc, being a bad conductor, would stick to the 

 surface of the plate and stop the action. A little 

 sulphuric acid added to the water serves to dis- 

 solve this zinc oxide, and keep the surface of the 

 plate clean and ready for action. Theoretically, 

 the action is the same as before. The liquid may 

 be regarded as formed of two atoms, one of hydro- 

 gen and one of sulphion, formed by the union of 

 the sulphuric acid with the oxygen of the water. 

 This sulphion atom behaves exactly as the oxygen 

 did : it unites with the zinc to form sulphate of 

 zinc, and hydrogen escapes at the copper plate. 



Electro-chemical Order of the Elements. We 

 have just seen that zinc takes positive, and copper 

 negative, electricity, when they form a galvanic 

 pair, simply because zinc is the more oxidisable of 

 the two. If we form a pair with copper and gold 

 plates, we find the copper plate is now the positive 

 one, and the gold the negative, because gold is 

 even less oxidisable than copper. The plate which 

 is more oxidisable, or more readily acted on by 

 the liquid, always becomes positive, and is there- 

 fore said to be electro-positive to any less oxid- 

 isable metal. Knowing the relative degree of 

 oxidability of any two metals, we can at once tell 

 how they would act as a galvanic pair. The 

 direction of the current is defined as being from 

 the positive plate to the negative within the liquid, 

 and from the negative to the positive without. 

 Therefore, we have a general rule that, if we form 

 a galvanic couple with any two metals and a 

 dilute acid, the current will flow outside the liquid, 

 or in the communicating wire, from the metal 

 which is least acted on by the acid to the other. 



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