212 



1 NDULATORY FORCES. ELECTRO-M KT A 1.1. 1 KG V. IOKKERAL PRINCIPLES. 



every 49 parti, or one equivalent, of hydrated sulphuric 

 acid let free at the cathode by the deposition of tho 

 t Hi i \.ilent of copper, one equivalent of acid will com- 

 bine with a like amount of copper at the anode. 



4th. With Tiro Metal* ami Tiro Liquids. If we im- 

 merse a piece of sine in dilute sulphuric acid, and a 

 piece of silver in a solution of double cyanide of itilvi r 

 and potassium, the two metals being connected by a 

 wire, and the liquids separated by a porous partition, 

 one equivalent, or 32*0 parts, of zinc will combine with 

 one equivalent of the acid, and one equivalent, or 108 '1 

 parta, of silver will bo deposited, setting one equivalent, 

 < . parts, of cyanogen free. 



6th. H'l'/A a Separate Depositing Liquid. If the plates 

 of a battery are connected by wires with two piece* of 

 gold in a hot solution of double cyanide of gold and 

 potassium, for every atomic equivalent, or 197 parts of 

 gold deposited, and one equivalent of cyanogen set free, 

 one equivalent of gold will combine with one equivalent 

 of cyanogen, and will be dissolved : and not only this ; 

 but for each of these actions, one equivalent of zinc will 

 combine with one equivalent of acid, and one equivalent 

 of hydrogen will be evolved in each of the battery cells 

 supplying the current of electricity. 



tith. It'ith n ir/wle Seriei of Depositing Liquids. Such, 

 for instance, as solutions of sulphate of copper with 

 electrodes of copper arranged as in Figs. 73 and 75 

 connected with a piece of zinc immersed in dilute sul- 

 phuric acid, and a piece of copper immersed in a solution 

 of sulphate of copper, the last two liquids separated by 

 a porous diaphragm, the whole of tho combinations, de- 

 compositions, and depositions throughout the scries, will 

 take place in the proportions of their atomic weights or 

 chemical equivalents. These, and many other instances 

 which might bo adduced, prove that all tho electro- 

 chemical actions taking place in any given circuit, occur 

 in certain definite proportions, and that this definite 

 electro-chemical action is one very important condition 

 of electro-deposition. 



68. Jiinary Theory of Electrolysis. The law of definite 

 rochcmical action was first established by Faraday ; 

 iind, in addition, he has advanced what is termed the 

 biliary theory of electrolysis that "only those com- 

 pounds of the first order are directly decomposible by the 

 electric current, which contain one atom of one of their 

 elements for each atom of the other : for instance, com- 

 pounds containing one atom of hydrogen or metal witli 

 one atom of oxygen, iodine, bromine, chlorine, fluorine, 

 or cyanogen;" whilst "boracic acid (B O s ), sulphurous 

 acid (S O f ), sulphuric acid (S O 1 ), iodide of sulphur, 

 chloride of phosphorus (P Cl') and (P Cl 1 ), chloride of 

 sulphur (S' Cl), chloride of carbon (C 1 Cl 6 ), bichloride of 

 tin (Sn Cl'), terchloride of arsenic (As Cl'), quintochloride 

 of antimony (Sb Cl 1 )," are non-conductors of electricity, 

 and incapable of electrolysis. Some substances, which 

 are not of the simple binary character mentioned, are 

 decomposed by current electricity, and yield their posi- 

 tive and negative elements in equivalent proportions at 

 the respective electrodes ; but, according to this theory, 

 they are indirectly decomposed, i.e., they are decomposed 

 by tho chemical action of some of the elements set free 

 by the direct action of the current upon other substances 

 present For instance, "fused borax (biborato of soda 

 N.;i >. L'Ki >"'; \ ;.-!>U ( \y . r. .n at tin- anode, ami I.IUMII at 

 the cathode : now, since fused borax is not decomposiblo 

 by the electric current, tho separation of the boron must 

 be attributed to indirect action ; tho current resolves the 

 soda (NaO) into oxygen and sodium, and tho latter sepa- 

 rates boron from the boracic acid."* Again, an aqueous 

 solution of ammonia (HO, N IP) yields, by electrolysis, ni- 

 trogen gas at the anode, and hydrogen at the cathode. In 

 this case, according to the theory, it may be supposed that 

 only tho water (BIO) is directly decomposed, and that its 

 oxygen, set free at the anode, combines chemically witli 

 some of tho hydrogen of the ammonia, again forming 

 water, and thus indirectly the nitrogen is set free. 



1 Mathematical Jiiea (if Electro-depositing Farce. A 

 consideration of tho law of definite electro-chemical action, 

 Faradijr. 



and the binary theory of electrolysis, leads us to \ 

 tho electric current or depositing force in a mathematical 

 aspect, as "an axis of forces equal in power, but oppo- 

 site in direction ;" because, for every atom of an electro- 

 positive substance attracted or transferred in one direc- 

 :m atom of an electro-negative character is attracted 

 in the opposite direction. It also suggests to us the 

 idea that an intimate connection exists between those 

 equivalents or mathematical relations of matter and tho 

 development of current electricity by chemical a> 

 and its transference through liquids by electric 

 because these phenomena only occur when tho mathe- 

 matical conditions are present 



CO. Sizes of Electrodes, Liquids, and Wires. The 

 rapidity of deposition is influenced by the area i 

 electrodes, the length and area of tho intervening liquid, 

 and of the connecting wire ; the larger the immersed 

 surfaces of the metals, tho shorter the len-th and the 

 greater the transverse area of the liquid between them ; 

 and the shorter and thicker their connecting wires, the 

 more rapid is the process of deposition. If the anodo 

 bo very large, and immersed in the lower part <>! 

 liquid, and the cathode very small and suspended near 

 the surface, much more metal will be dissolved than U 

 deposited, gas being generally evolved at the cathode in 

 place of some of the metal being deposited. 



01. liapidity of Deposition. The character of tho 

 deposited metal is very much influenced )>y tho rapidity 

 of deposition : if it be deposited very rapidly, it will bo 

 in the state of a perfectly black, soft, non-coherent 

 powder; if deposited more slowly, it will possess the 

 ordinary characters of tho particular metal ; and if de- 

 posited very slowly, it will be crystalline, because the 

 atoms are then allowed sufficient time to arrange them- 

 selves in the crystalline form. 



62. Logical L'undttions. The logical conditions arc 

 1st, that there is a difference at the dissolving and re- 

 ceiving surfaces, either in the "inutvrial substratum" of 

 metal, of liquid, in the forces involved, or in all com- 

 bined, both of chemical affinity and electricity probably, 

 also, of heat and of motion ; and, 2nd, 'that in all casts of 

 deposition there are certain chemical, electrical, thermal, 

 dynamical, and mathematical conditions invariably pro- 

 Bent, and certain other conditions of each of those kinds 

 invariably absent, both of which classes of condition. 

 necessary to the production of the phenomena; tho 

 whole of tho necessarily present circumstances consti- 

 tuting its causes, and all the necessarily absent ones con- 

 stituting its preventives. Thus, if all tho causes of de- 

 position are present, and all its preventives absent, 

 deposition will invariably occur ; but if only one of its 

 causes is absent, or one of its preventives present, depo- 

 sition cannot take place. 



03. Ontoloyii-nl Condition. The last, the most neces- 

 sary, most evident, and most simple condition of all 

 deposition is that metals, liquids, aud forces are re- 

 quired in order to produce it 



THE PRACTICE OF ELECTRO-DEPOSITION. 



64. Objects of Practical Deposition. In the theoretical 

 division we have brought forward a largo number of 

 instances, both of deposition and non-deposition; and 

 from a consideration of them, we liave drawn conclusions 

 both as to what substances, and what o and 



arrangements of those substances, really exist in all cases 

 of deposition, and in similar cases of non-deposition; 

 and we have pointed out in what respect the conditions 

 varied. Our object in pursuing this course 

 impress the reader with an extended knowledge of the 

 theoretical principles on which deposition proceeds, that 

 ho may fuel himself perfectly able to apply his know I 



move tho difficulties certain to arise in his 

 Now, however, our object is different ; it is to inst 

 tho reader how ho is to apply those principles in daily 

 working; to give him praetical rules, recipes, and direc- 

 tions for carrying out the various minute points in 

 workshop manipulation; and to enable him to obtain 

 the greatest degree of practical success. 



