GALVANISM. 



GALVANISM. 



action of Voluic and of magneto-electric currents. When the wire* 

 of the latter wore und for decompositions, but in the form of thin 

 lean* or lamina, then WM but little disengagement of gaa, and the 

 more the lamina wai plunged, the lees wu the gas evolved, which was 

 nut the caw in the oommon form of the wire : this doee not occur in 

 Voltaic electricity ; the ame experimentalist ha* Bought the quantity 

 of electricity Decenary to deoompoee a given quantity of water, and 

 hi* reeult i* that the product of the time multiplied by the intensity 

 of the current i* constant. 



Let u* now recapitulate the general phenomena of voltaic or electro- 

 chemical action. 



If two flat piece* or plates, one of zinc, the other of copper or 

 platinum, be immersed, without touching each other, in diluted iul- 

 phuric acid, chemical action, a* it is termed, will take place between 

 the tine and fluid ; the water of the latter undergoing decomposition, 

 it* oxygen unite* with the metal to form a protoxide, while the 

 equivalent of hydrogen ia set free, but adhere* to the surface of the 

 plate in small bubbles, which gradually coalescing into larger, detach 

 themselves and rise to the surface from their specific levity; the 

 protoxide combines with an equivalent of acid, forming a soluble salt, 

 which, being consequently removed, allows of renewed and continuous 

 action on the metal. In time however the fluid becomes saturated 

 with the sulphate of cine, which is then thrown down undissolved, and 

 i* also deposited on the surface of the plate, acting mechanically to 

 prevent that intimate contact between the pure metal and the water 

 which is essential to chemical action ; this obstruction is also in some 

 measure occasioned from the beginning by the adhesion of the bubbles 

 of hydrogen, and consequently that action is gradually retarded and 

 finally ceases. During this time the copper or platinum plate is per- 

 fectly inactive, neither of these metals being affected by sulphuric acid, 

 or more properly, owing to the affinity, as it may at present be still 

 called, between those metals and oxygen, being weaker than that which 

 exist* between the hydrogen and that element. 



Under these circumstances, if a perfect communication be made 

 between the two metals, by causing a wire soldered to the one, to touch 

 that connected with the other, a remarkable change takes place in the 

 phenomenon, the chemical action between the zinc and the fluid 

 become* more energetic, but the hydrogen, instead of being liberated 

 at the surface of that metal, appears solely at that of the other, although 

 not the slightest effect is produced on the copper or platinum itself ; 

 while the connecting wires will be found to exhibit, by their increased 

 temperature and their magnetic state, the usual indications of what is 

 termed an electrical current passing along them, and, as is well known, 

 an electric spark is visible at the instant of separating the wires, pro- 

 Tided the plates be sufficiently large. 



It is now generally admitted that all chemical is connected with 

 electrical action, and that they bear a direct relation to each other, 

 that is, a certain constant quantity of electricity is evolved by the 

 decomposition of each equivalent of any compound, though that 

 quantity varies for different bodies ; but it is not clearly known 

 whether the electricity is the cause or the effect of the chemical action, 

 they being perfectly contemporaneous and co-existent to all our means 

 of observation. It follows therefore that the quantity of electricity 

 varies with the extent of surfaces between which the action takes 

 place, as well as with the nature of those surfaces ; but with the same 

 two metals and fluid this quantity depends solely on the extent of 

 surface, that is, on the sixe of the zinc plate. 



It ha* been proved by Professor Daniell that under these circum- 

 stances our power of collecting or retaining the electricity evolved 

 from any given surface of metal, depends on the extent of that of the 

 other, or conducting metal, for in the arrangements under con- 

 sideration the copper or platinum plate simply acts in this capacity, 

 and that this second plate cannot be too Urge in proportion to the 

 former, to prevent the loss of any of the evolved electricity, which, 

 if it do not meet with a good conductor in its immediate proximity, 

 pu*e* off to some other.* The form of our arrangements puts a limit 

 to this inequality in the extent of surface* of the generating and 

 conducting metals, but a more important principle still further con- 

 tract* this limit, which must be briefly adverted to. 



All electrical action i* most easily and consistently explained as the 

 effect* of induction [ELECTBICITT, COMMON; POLARITY], or of an 

 action exerted by matter, itself in a polar state, producing polarity in 

 surrounding matter ; what is commonly called an electrical current is, 

 on this hypothesis, the momentoneous destruction and reproduction 

 of polar forces, acting along a chain of particles sufficiently approx- 

 imated to admit of the intensity of the forces in that line exceeding 

 that produced on contiguous particles in other and lateral directions. 

 In the voltaic circuit the particles of the fluid form a part nf the chain 

 through which the induction, originated by the cine on the fluid, is 

 propagated, the chemical decomposition i* connected with or occa- 

 sioned by this polarity, the hydrogen of one particle of water quitting 

 it* equivalent of oxygen to combine with that of the contiguous 

 particle, and so on throughout until the hydrogen of the particios in 

 contact with the conducting plate, having no oxygen wherewith to 



If a larg* *l*otrloal machine w*re provided with a null conductor, thl, 

 might become charged by half a torn of thl handle, and mart dlicharge lUtlf 

 laterally to the prarc.t conductor! before it oomld reoeiv. a freth charge. 



combine, is liberated at that surface in it* gaseous state ; the metallic 

 part of the circuit undergoes no chemical change except at the surface 

 in contact with the fluid, where the polar forces produce oxidation of 

 the metal, and consequent decomposition of that fluid. 



If this chain of polarised particles be broken, and the interposed 

 matter be a nonconductor, the current is at once arrested, and the 

 chemical action between the cine and the fluid nearly ceases; but 

 even a diminution in the conducting power of any part of the circuit 

 occasions a diminution or retardation of that action. The fluid of 

 the arrangement is an imperfect conductor, compared with the me- 

 tallic part, and if the distance between the metals be increased, the 

 increase of the intervening portion of imperfect conducting matter 

 may occasion a cessation of the action, and always diminishes it in an 

 inverse ratio to that distance. Hence the necessity in all forms of 

 rollaic balleriei for diminishing as much as possible the quantity of 

 fluid between the metallic elements : consequently the magnitude of 

 the negative or conducting plate cannot be indefinitely increased, if by 

 so doing it becomes necessary, owing to the peculiar form of the 

 arrangement, to increase the distance between the two plates. 



Although we are warranted in inferring by analogy that there may 

 be other chemical sources of electro-polar induction than oxidation of 

 a metal, yet at present we are not acquainted with any other that can 

 at all be compared with it in energy ; and, of all combination by which 

 this oxidation may be produced, the most efficient is that of a metal 

 and a solution of a metallic salt, the acid of which has a greater 

 affinity for the former metal than it has for that with which it is 

 combined. If therefore a solution of sulphate of copper be used, 

 instead of the simple diluted sulphuric acid of the arrangement, it 

 becomes far more powerful ; but this substitution necessitates a pre- 

 caution to prevent the deposition on the surface of the cine plate of 

 the metallic copper liberated by the decomposition, which deposition 

 would otherwise almost instantly take place, and thus, by causing 

 both surfaces to consist of the same metal, polar currents would be 

 produced in opposite directions, which would neutralise each other's 

 action. 



The precaution alluded to consists in interposing between the decom- 

 posing plate and the metallic solution a substance which, while it 

 admits of the passage of the current, and even that of the pure fluid 

 under the influence of that current [ENDOSMOSE], mechanically inter- 

 cepts the solid copper ; the arrangement of the " constant battery," as 

 it is technically termed, for which we are indebted to Professor 

 Daniell, consists of a small rod of amalgamated zinc, that is, of zinc 

 the surface of which ia coated over with mercury, placed in a mem- 

 branous bag, or in a porous earthenware cylinder, filled with dilute 

 sulphuric acid, this cylinder or bag being again placed in a copper one 

 filled with a saturated solution of sulphate of copper ; this external 

 copper vessel constitutes the conducting plate, and has the connect- 

 ing wire soldajxd to it, while the* other wire is attached in any con- 

 venient mode to the zinc rod. It must be observed that the membrane 

 or porous cylinder must be perfectly continuous : the least fissure 

 would admit of the passage of the copper to the nc, and destroy the 

 effect. 



As long as the two wires do not touch, this battery is nearly 

 quiescent, except the slight local action which takes place between t In- 

 line and its surrounding fluid ;* but when the circuit ia completed by 

 making the ends of the wires touch, the action becomes energetic, 

 the solution of the sulphate of copper is decomposed, the reduced 

 metal being deposited on the surface of the copper vessel ; it is con- 

 sequently necessary to maintain the supply by adding from time to 

 time solid sulphate to the solution, so as to keep it always saturated ; 

 for it must be distinctly understood as a fundamental principle, that, 

 without continuous chemical decomposition and recomposition, no 

 current or circle of electro-polar forces can be maintained. 



Let the ends of the two wires, not in contact, be plunged into a 

 liquid compound, such for example as sulphate of copper in solution, 

 which, being an imperfect conductor, is capable of decomposition ; 

 decomposition of it will accordingly take place, and in the same 

 direction as that in which it occurs in the fluid of the battery; that 

 is, the copper of the solution will be determined to or precipitated on 

 the -wire connected with the zinc plate, while the other wire will be 

 dissolved, uniting with the fne acid to produce a sulphate. It mny 

 be asked how it nappens that the copper wire is dissolved while the 

 plate of that metal in the battery is not acted on, and what becomes 

 of the hydrogen which was liberated in a free state, when diln- 

 alone was employed. This apparent contradiction is explained by on 

 attentive consideration of the constant direction of the current, and 

 the consequences of its passing through the two portions of fluid* in 

 opposite directions as regards them. At the zinc plate the action is 

 the some as before, but by virtue of the current-affinity the copper is 



The object of amalgamating the cine rod U to prevent thU local action, 

 which arlMn from the Inevitable wt of perfect homogeneity In the metal ; any 

 tkc alijhtcit difference In two portion* of which will catue them to act as 

 positive and negative element! of a mull circuit, and a great number of tlicae 

 caum that action on the zinc which takes place before the great or principal 

 circuit U completed ; the perfect conducting power of the mercury appears to 

 leitroy or neutralise these partial current*, M that of the battery Hwlf would 

 be dettroyed If the two plate* were connected within the fluid by a perfi > 

 duetor, uutesd of having an Imperfect one Interposed between them. 



