LAWS OP DEPOSITION.] UNDULATORY FORCES. ELECTRO-METALLURGY. 



205 



deposit of copper is produced ; whilst in the second in- 

 stance, the platina or gold having a weaker affinity for the 

 acid of one liquid than copper for the acid of the other 

 liquid, cannot separate the copper, or cause deposition. 



4th. With Two Metals and Two Liquids. If we im- 

 merse a piece of silver in a strong solution of cyanide of 

 potassium, and a piece of copper in a solution of the 

 double cyanide of copper and potassium, the liquids 

 being separated by a porous partition, and the metals 

 connected by a wire, the silver will dissolve, and the 

 copper receive a metallic deposit ; but if a piece of iron 

 be substituted for the silver, no deposit will occur. In 

 the first instance, the one solution has a stronger affinity 

 for the silver than the other has for the copper con- 

 sequently the former is dissolved, a current or depositing 

 force is generated, and copper deposited; but in the 

 second case, the one liquid has a weaker affinity for iron 

 than the other has for copper, and therefore no iron is 

 dissolved, no depositing force generated, and no copper 

 deposited. 



5th. With a Separate Depositing Liquid connected with 

 any Source of Depositing Power. If we connect two 

 pieces of silver with any of those sources of power, and 

 immerse them in a solution of the double cyanide of 

 silver and potassium, one piece will quickly dissolve, and 

 the other receive a deposit of silver ; but if pieces of iron 

 are substituted for those of silver, neither will dissolve 

 or receive a metallic deposit. 



In every case where a separate depositing liquid is 

 used, the two pieces of metal immersed in it have a 

 mference of chemical affinity imparted to them by virtue 

 of their connection with some arrangement which 

 develops depositing force; and this difference of affinity 

 is manifested most when the liquid has a strong affinity 

 for the imnv:rsod metal, and least when it has a weak 

 affinity for that metal : so in the first of the two imme- 

 diately preceding instances, the liquid having a strong 

 affinity for silver, allows this difference of affinity to be 

 freely exercised at the immersed surfaces of the two 

 pieces of metal, and consequently one dissolves, and the 

 other receives a deposit ; but in the second of these 

 instances, the liquid having a very weak affinity for iron, 

 does not admit of the exercise of this difference of affinity, 

 and hence neither piece dissolves or receives a deposit. 

 From these instances it is manifest, that whenever de- 

 position occurs, there is a difference of chemical affinity be- 

 tween the dissolving and receiving surfaces forthe different 

 elements of the liquid ; that the dissolving metal has a 

 stronger affinity for the acid elements than the receiving 

 one ; and without this condition no deposition will occur. 

 24. It must be mentioned, also, that the metals which 

 have the greatest difference in their degrees of affinity 

 for acids, are those which evolve the greatest strength of 

 depositing power. For instance 



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

 a piece of silver in a solution of terchloride of gold, it 

 slowly becomes gilded ; but if we immerse a piece of zinc 

 in it, gold is almost instantaneously deposited ; because, 

 in the former case, the difference of affinity between gold 

 and the immersed metal for the acid of the liquid is very 

 much smaller than in the latter case. 



2nd. With Two MetaU and One Liquid. If we im- 

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

 contact, in a solution of nitrate of silver, the platina will 

 become silvered, but much more slowly than if iron or 

 zinc were used in place of the copper, because there is a 

 greater difference of affinity between platina and iron or 

 zinc for the acid of the liquid, than between platina and 

 copper ; or, if we immerse a piece of zinc and a piece of 

 platina or platinised silver in dilute sulphuric acid, and 

 connect them separately with two pieces of copper im- 

 mersed in a separate solution of sulphate of copper, 

 copper will be dissolved and deposited in the separate 

 liquid more rapidly than if we used zinc and copper in 

 place of zinc and pliitina, and much more rapidly than if 

 we used iron and copper, because the difference of 

 affinity between zinc, platina, or platinised silver for 

 dilute sulphuric acid, is more than zinc and copper for 

 that liquid, and much more than between iron and copper. 



3rd. With Two Metals and Two Liquids. If we 

 immerse a piece of zinc in dilute sulphuric acid, and a 

 piece of copper in a solution of sulphate of copper, the 

 two liquids touching each other, and the two metals 

 connected with two pieces of copper in a separate solution 

 of sulphate of copper, the amount of metal dissolved and 

 deposited in a given time in the separate liquid, will be 

 much smaller than if we used zinc in dilute sulphuric 

 acid, and platina in strong nitric acid ; because the dif- 

 ference of affinity between the two metals in the two 

 liquids, in the first instance, is less than in the second 

 instance. 



25. Acid and Basic Affinities necessary. Chemical 

 affinity differs not only in degree; but also in kind ; 

 basic substances, such as metals, alkalies, alkaloids, and 

 most metallic oxides, have a great tendency to combine 

 with acids ; whilst oxygen, sulphur, phosphorus, chlorine, 

 iodine, bromine, fluorine, mineral and vegetable acids, 

 and some metallic oxides, tend to combine with bases. 

 In a similar manner, the difference of affinity between 

 the dissolving and receiving surfaces in electro-depo- 

 sition is not wholly one of degree, but is also one of 

 kind: the dissolving metal, in a separate depositing 

 liquid (13), acquires, by virtue of its connection with 

 some source of depositing power, an affinity for the acid 

 elements of the liquid; whilst the receiving metal ac- 

 quires, by the same means, an affinity of an opposite 

 kind : for instance (Fig. Fig. 74. 



74), if a piece of platina 

 A, and a piece of zinc B, 

 are immersed in dilute sul- 

 phuric acid C, some mer- 

 cury D placed at the bottom 

 of a separate solution of 

 protosulphate of iron E, a 

 piece of iron F immersed 

 in this liquid and connected 

 by a wire with the platina, 

 and the mercury connected by an iron or platiua wire G, 

 which is prevented from touching the liquid by a tube 

 of glass or gutta-percha, the immersed piece of iron will 

 exercise one kind of affinity, combine witli the acid of 

 the dissolved salt, and form a definite chemical com- 

 pound (protosulphate of iron), containing one equivalent 

 of iron and one equivalent of sulphuric acid ; while the 

 mercury will exercise an opposite kind of affinity, ami 

 combine with the metal or base of the salt, forming 

 likewise a definite chemical compound (Fo Hg), con- 

 taining one equivalent of iron and one of mercury. If 

 a solution of sulphate of copper be substituted for that 

 of sulphate of iron, and a piece of copper for the pieco 

 of iron, a similar definite compound of copper and 

 sulphuric acid will be formed at the dissolving plate, and 

 of copper and mercury (Cu Hg) at the receiving metal. 



These experiments prove, in a most satisfactory manner, 

 that, in the act of deposition, the surface of the dis- 

 solving metal possesses one kind of affinity by virtue of 

 which it tends to attract acid substances, and combine 

 with them in definite proportions ; and the surface of the 

 receiving metal possesses an opposite kind of affinity, by 

 virtue of which it tends to attract and combine with 

 icwic substances, also in definite proportions. Mercury 

 is the only metal which has been observed to manifest 

 this definite affinity at the receiving surface, probably 

 because it is the only metal fluid at ordinary tempera- 

 tures, fluidity being an essential condition of such 

 affinity ; but it is likely that other metals would also 

 manifest this tendency, if kept in a melted state in 

 contact with suitable fused salts, and properly acted upon 

 depositing force. 



26. Fluidity essential to Electro- Deposition. The affi- 

 nities of electro-deposition, like those of ordinary che- 

 mic:il action, require, generally speaking, one at least of 

 the combining bodies to be in the liquid state ; and they 

 act, like them, wholly at insensible distances, being con- 

 ined in their exercise to the immediate surfaces of 

 mutual contact in the opposed substances. The coin- 

 Dound formed at those surfaces becomes diffused through 

 ihe fluid masses by capillary attraction and mechanical 



