UNDULATORY FORCES. ELECTRO-METALLU ROY. [LAWS or DMOSITH.N. 



mixture. Thia afford* a reaaonatfle explanation why 

 fluidity of the receiving metal iaeaaential to the formation 

 of definite compounds at its surface, at well aa why flu- 

 idity of the metallic salt is essential to ita decomposition, 

 ami for the formation of definite compounds at the dis- 

 solving surface. If chemical action took place at sensible 

 and considerable distances, t. e. , throughout the whole mass 

 of the opposed fluid bodies, combination would probably be 

 in all cases violent and instantaneous ; and if fluidity were 

 not essential tocombination, the substances deposited upon 

 the receiving surface would probably, in most cases, enter 

 into the mass of the receiving metal, and combine with it 



27. Alternation and Circulation of Chemical Affinities 

 necettary. The next chemical condition to be mentioned 

 is, that in every case of deposition, the surfaces at which 

 the acid and basic affinities are manifested, alternate 

 with each other in the circuit, and that the acid affinity 

 circulate* one way, while the basic affinity circulate* in 

 the opposite direction. For instance 



1st. With, One Metal and One Liquid. If we immerse 

 a piece of copper in a solution of double cyanide of silver 

 and potassium, it becomes silvered ; but if we immerse 

 a piece of iron therein, it receives no deposit. In the 

 first of these instances, it is considered that, immediately 

 upon the immersion of the metal, the superior affinity of 

 copper for cyanogen over that of silver, causes it to 

 combine with that compound, and sets the silver free ; 

 at the same time, an immense number of minute currents 

 of depositing force or chemical affinity are developed all 

 over the immersed surface of the piece of copper, leaving 

 it at innumerable minute points, passing a very small 

 distance into the liquid, and re-entering the copper at 

 numberless other points; and thus the affinities cir- 

 culate, the copper dissolves, and receives a deposit 

 simultaneously. In each of these atomic circuits, as they 

 are termed, acid affinity is exercised where the depositing 

 force leaves the metal, and basic affinity where it re- 

 enters it ; but in the second instance, where iron is used, 

 there is no circulation of those affinities, no dissolving 

 of metal at one point, or deposition of it at another. 



2nd. With Two Metal* and Oiie Liquid. If we im- 

 merse a piece of iron and a piece of copper, in mutual 

 contact, in a solution of sulphate of copper, a deposit of 

 copper will take place upon the iron and upon the copper ; 

 but if we immerse a piece of gold in place of the iron, no 

 deposit will occur upon either. In the first of these in- 

 stances, in addition to the circulation of atomic currents 

 of affinity all over the immersed surface of the iron, as 

 already explained, and which causes it to dissolve and 

 receive a deposit, there are separate and distinct currents 

 of the same force circulating through the liquid ; and the 

 two metals, by their points of mutual contact, cause the 

 surface of the iron to combine with the acid, and that of 

 the copper to receive a metallic deposit; but in the 

 second case there is no circulation of affinities, and no 

 solution of the gold, or deposition of the copper. 



3rd. With One Metal and Two Liquids. If we immerse 

 two pieces of silver, one in a solution of cyanide of 

 potassium, and the other in a solution of double cyanide 

 of silver and potassium, the two pieces being connected 

 together by a wire, and the liquids being divided only 

 by a porous partition, chemical affinities will circulate 

 by the metals, wire, and liquids, and silver will be dis- 

 solved and deposited ; but if we substitute pieces of iron 

 for the pieces of silver, there will be no circulation of 

 affinities and no deposition. 



Wf. 75. 



siating of solutions of sulphate of copper, with separate 

 pairs of copper wires C 1>, K F, (J II, immersed in thrm, 

 and with end pieces of copper B and I. B being connected 

 with a piece of zinc J immersed in dilute sulphuric and, 

 and I connected with a piece of copper A immersed in a 

 solution of sulphate of copper, the two liquids being 

 separated by a porous diaphragm K, chemical ailii. 

 will circulate in opposite directions through the whole of 

 the circuit, every alternate piece of immersed metal, J, 

 I, G, E, and C, will exercise one kind of affinity and 

 dissolve, and every other alternate piece, A, B, D, F, 

 and II, will exercise an opposite kind of affinity, and 

 receive a deposit of copper. If in either of the fore- 

 going instances, where deposition occurs, we break the 

 continuity of the circuit, either by separating the metals 

 from each other at their points of contact, lifting them 

 out of the liquid, or by cutting through the roiino 

 wires, no deposition will occur ; but if we immerse the 

 free ends of the divided wire in a suitable liquid, such 

 as a solution of sulphate of copper, if the wires are of 

 copper, deposition will immediately recommence through- 

 out the circuit. 



28. A consideration of such facts as these, leads us 

 to conclude that the process which causes every alternate 

 metal in a series to combine with acids and dissolve, 

 and every other alternate metal in the circuit either to 

 combine with bases, or receive a deposit, is of a che- 

 mical character, and occurs in opposite directions through 

 the circuit, which has led to the application of the term 

 "current affinity," to designate the depositing force, 

 when viewed only in a chemical aspect. 



29. Two or any other even number of vessels containing 

 combinations of liquids and metals, each similar, and 

 capable of generating current affinity, may be so arranged 



Fig. 76. 



in a complete conducting circuit, that no current affinity 

 will circulate or deposition occur : for instance, if we 

 take two separate vessels A and B (Fig. 76), each con- 

 taining dilute sulphuric acid, and a separate piece of 



Vig. 77. 



4th. With Separate Depotiting Liquids 



zinc Z, and silver (S), and connect them together thus, 

 zinc-silver silver-zinc, no depositing power will be mani- 

 fested if we immerse the free ends of their copper 

 wires C C in a separate solution of sul- 

 phate of copper D ; but if they are con- 

 nected thus, zinc-silver zinc-silver (Fig. 

 77), and the copper wires immersed 

 us In foil', current affinity will circulate, 

 and deposition will proceed in the sepa- 

 miil. In the first of these oases 

 the affinities set in motion by the metals 

 ami liquid in the vessel A are opposite 

 in direction to those generated in vessel 

 B, and the two arrangements, being 

 in power, exactly neutralise each other, preventing 



