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



203 



separate depositing liquids, may be connected together 

 in a series of any number, and may include each of the 

 arrangements in the same series, or include any number 

 of depositing liquids, and deposition may be obtained 

 either in the whole or in any portion of them at the 

 same time : for instance, the vessel A (Fig. 73) contains 

 a piece of zinc B, and copper C immersed in dilute 



Fig. 73. 





sulphuric acid ; vessel D contains zinc E, in dilute sul- 

 phuric acid, and copper F, in a solution of sulphate of 

 copper ; vessel G contains a series of separate depositing 

 liquids, consisting of solutions of sulphate of copper, 

 connected together by bent pieces of copper; the ex- 

 treme pieces being attached to zinc B and copper F ; 

 here deposition takes place upon every alternate piece 

 of copper throughout the whole series, except that in 

 vessel A. 



17. General Observations. We may make the following 

 :iieral observations upon the whole of the foregoing 



Facts : 1st, that negative as well as positive instances 

 occur in all classes of facts of electro-deposition ; 2nd, 

 that almost any of the ordinary metals, both noble and 

 base, may be deposited by each of the methods or ar- 

 rangements described ; 3rd, that the particular result of 

 deposition or nou-dej>osition occurring, ap]>ears to depend 

 chiefly upon the particular combination of liquids aud 

 metals, their arrangement and connections ; 4th, that 

 the size or shape of the containing vessels, the bulk or 

 depths of the liquids, the size, form, or position of the 

 metals, appear to exercise little or no influence upon 

 the result ; 5th, that in all cases of deposition there is a 

 difference in kind, either of the metal, of the liquid, or 

 of both ; 6th, that in all such cases a metal dissolves in 

 a liquid ; for instance, in Arrangement No. 1, with a 

 piece, of iron immersed in a solution of sulphate of 

 copper, a portion of the iron is dissolved as the copper 

 is deposited ; in Arrangement No. 2, with iron and 

 copper together in a solution of sulphate of copper, the 

 iron dissolves and the copper receives a deposit ; in Ar- 

 rangement No. 3, with copper in dilute sulphuric acid, 

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

 the acid dissolves, whilst that in the metallic solution 

 receives a deposit ; in Arrangement No. 4, with zinc in 

 dilute sulphuric acid, and copper in sulphate of copper, 

 the former dissolves, whilst the latter receives a deposit ; 

 in Arrangements Nos. 5 and (>, one piece of metal in 

 the separate depositing liquid dissolves, whilst the other 

 receives a metallic deposit, and the same with the com- 

 pound Arrangement No. 7 ; 7th, that in Arrangements 

 Nos. 1 and 2, the same piece of metal which dissolves 

 also receives a metallic deposit ; and in Arrangements 

 3, 4, 5, 6, and 7, the pieces of metal which receive a 

 deposit do not dissolve ; 8th, that, as a general rule, in 

 all methods, and in all solutions, except alkaline 

 metallic cyanides, zinc, among common metals, has the 

 greatest, and platina the least power of producing de- 

 position ; 9th, that zinc generally deposits metals most 

 rapidly from their solutions, and most frequently in the 

 state of a dark-coloured or black powder ; and 10th, 

 that among solutions of the salts of ordinary metals, 

 those of the salts of noble metals yield their metal 

 most easily, and those of the base metals, zinc espe- 

 cially, with the greatest difficulty. 



18. In looking over the foregoing instances, we also 

 observe : 1st, that when several metals are used, they 

 must either touch, each other or be connected together 

 by wires or other pieces of metal; 2nd, that when 

 several liquids are used, they also must touch each other, 



either by means of a porous diaphragm, or otherwise ; 

 3rd, that when a series of metals and liquids are used, 

 they must together form a complete circuit, and all thtir 

 points of contact be perfectly clean ; 4th, that a separate 

 depositing liquid possesses no power of deposition by 

 itself, but derives its power by means of the wires from 

 the other arrangements with which it is connected ; and 

 5th, that the length of the connecting wire, if not great, 

 has no influence on the result. These ob- 

 servations have led us to conclude that de- 

 position is caused by some force which is 

 generated in some part of the apparatus, 

 and circulates through the liquids, metals, 

 and wires, which compose the circuit. 



19. Principles. Conditions of Electro-De- 

 position. From the simple facts of electro- 

 deposition, and the general observations 

 made upon them, we proceed to consider 

 the causes of deposition, and the conditions 

 or circumstances under which deposition occurs, in the 

 following order : 



1st. The Chemical Conditions. If we immerse a clean 

 iron wire in a solution of nitrate of mercury, it receives 

 a deposit of that metal ; but if we immerse it in per- 

 fectly dry metallic mercury, it receives no deposit, be- 

 cause in the former case the necessary chemical con- 

 ditions of deposition are present, whilst in the latter 

 case they are absent. 



2nd. The Electrical Conditions. If we connect together 

 a piece of iron and a piece of copper, by means of a 

 metal wire, and immerse them in a solution of sulphate 

 of copper, the copper will receive a metallic deposit ; 

 but if we connect them together by a cord of gutta- 

 percha, or rod of glass, no deposit will take place, 

 because, in the former case, all the electrical conditions 

 are present, whilst in the latter case, one of them 

 viz. , a complete conducting circuit, is absent. 



3rd. The Thermal Conditions remain almost unknown. 

 4th. The Mechanical Conditions. If a piece of iron 

 be immersed in a solution of sulphate of copper, it 

 receives a copper deposit ; but if a piece of plutina be 

 so immersed it receives no such deposit, because, for one 

 reason, in the former instance the mechanical conditions 

 of attraction and repulsion at the dissolving and deposit- 

 ing surfaces are present, but in the latter they are 

 absent. 



5th. The Mathematical Conditions. If we immerse 

 two pieces of carbon in fused j>ro<o-chloride of tin, and 

 connect them with a voltaic battery, tin will be de- 

 posited : but if we immerse them in fused 6-chloride of 

 tin, no deposition will occur, because, in the first in- 

 stance, all the mathematical conditions are present ; the 

 fluid salt contains one atom of chlorine for each atom of 

 tin ; whilst in the latter case one of them is absent, 

 the salt contains two atoms of chlorine to one atom of 

 tin; and, according to Faraday's law, "only those sub- 

 stances of the first order are directly decomposable which 

 contajn one atom of one of their elements for each atom 

 of the other." 



These several heads are capable of including all the 

 known circumstances or conditions under which deposi- 

 tion occurs ; and under each of them will be given a 

 few instances, both of deposition and non-deposition, to 

 illustrate the principle. It would be advisable for the 

 reader to try for himself most of the experiments given, 

 in order to fix the principles firmly in his memory. 



20. Chemical Conditions of Deposition. The first che- 

 mical condition to be observed is, that in every case of 

 deposition, the depositing liquid contains acid and basic 

 elements namely, a salt, the acid of which is to dissolve 

 or combine with one metal, and its metal or ba k> be 

 deposited upon the other. 



1st. Deposition by One Metal and One Liquid. With 

 the first class of facts, if we immerse a piece of clean 

 iron in a solution of nitrate of mercury, it will receive a 

 deposit of that metal ; but if we immerse it in mercury 

 alone, it will receive no deposit : in the first instance an 

 acid as well as a basic substance was present, and de- 

 position took place ; but in the second instance the 



