KM 



UNDULATORY FORCES. ELECTRO-METALLURGY. [LAWS OF DKPOBITIOH. 



metal or hue alone was present, and no deposition 

 occurred. 



With Tun Jfetali atui One Liquid. If rino and 

 platina are immoraed in mutual contact in a solution of 

 nitrate of mercury, the platina will receive a metallic 

 deposit ; bnt if they are immersed in pure dry mercury, 

 no depotrit will occur. 



3rd. With One Metal and Two Liquids. If we im- 

 merse one end of a platina wire in a strong solution of 

 cyanide of potassium, and its other end in a solution of 

 nitrate of mercury, the two liquids being in mutual 

 contact by means of a porous partition, the end in the 

 metallic solution will soon receive a deposit of mercury; 

 but if perfectly dry mercury be substituted for the 

 nitrate solution, no such deposit will occur. 



4th. With Two Metals and Two Liquid*. If we im- 

 merse zinc in dilute sulphuric acid, and platina in a 

 solution of nitrate of mercury, the two liquids being 

 separated by a porous partition, and the metals con- 

 nected together by a wire, the platina will quickly re- 

 ceive a deposit of mercury ; but with dry mercury 

 instead of the metallic solution, the platina will not 

 receive a deposit. 



6th. With a Separate Depositing Liquid. If we take 

 two pieces of platina wire, connect them, as already 

 described (13), with either of the foregoing Arrange- 

 ments, or with a magnet and coil (15), and immerse their 

 free ends in a solution of nitrate of mercury, one of the 

 wires will receive a deposit of mercury ; but if the sepa- 

 rate liquid consist only of dry mercury, no deposit will 

 be obtained. 



It is evident from these facts, that in every case 

 where deposition occurs, the depositing liquid contains 

 both acid and basic substances, and that without the 

 presence of both no deposition will take place. 



21. Degrees of Chemical Affinity of Metals and Liquids. 

 At the present point it is necessary to mention a few 

 instances of the different degrees of chemical affinity 

 manifested by different metals and liquids, that the 

 reader may be able to understand their general chemical 

 relations in electro-deposition more clearly. 



1st. If we immerse a piece of potassium in almost any 

 liquid, very violent chemical action takes place, wlii.-h 

 is stronger in mineral and vegetable acids than in water 

 or organic liquids ; if we place a small piece of it upon 

 water, violent chemical action occurs, the water is de- 

 composed, heat is produced, gas is evolved which takes 

 fire, the metal melts and rolls about on the surface of 

 the water, oxidates and dissolves. 



Jii'l. If we immerse a piece of zinc in any of the 

 strong mineral acids (sulphuric, hydrocliloric, hydro- 

 fluoric, or nitric acids), strong chemical action takes 

 place, gas is freely evolved, and the metal oxidates and 

 dissolves; with solutions of the ordinary vegetable 

 acids, i.e., oxalic, tartaric, citric, formic, and acetic 

 acids, the same effects occur in a much weaker degree ; 

 bnt with pure water there is no visible decomposition, 

 no gas evolved, nor any perceptible chemical action. 

 These instances show that potassium has a much stronger 

 affinity for water or its oxygen than zinc, and that both 

 potassium and zinc have a stronger affinity for acids, 

 especially mineral ones, than for water. 



3rd. If we immerse a piece of zinc successively in each 

 of the ordinary mineral and vegetable acids diluted with 

 water, it will be quickly dissolved, with evolution of 

 hydrogen gas in nearly all of them : but if we immerse a 

 piece of copper in those liquids, it will be quickly dis- 

 solved in only one of them viz., nitric acid ; and from 

 this we conclude that copper has generally a much 

 weaker affinity for acids than zinc. 



4th. If we immerse either gold or platina in any of 

 the strong mineral or vegetable acids, or even in cold 

 vjua regia, it will be quite unaffected in all of them ; 

 whilst copper would be rapidly acted upon by nitric acid 

 or by aqua regia, and slowly by several of the others, 

 thereby showing that the affinity of gold or platina for 

 acids, is generally much weaker than that of copper. 



From the foregoing, and many other instances that 

 might bo mentioned, wo conclude that the general order 



of affinity of the metals for acids occurs in the following 

 order namely, potassium, zinc, copper, gold, and 

 platina. Hydrocyanic an. I and cyanogen appear to 

 differ in one respect, in then chemical relations towards 

 ordinary metals, from oxygon, chlorine, ami tho ordinary 

 mineral and vegetable acids, in having a much stronger 

 affinity for noble metals, and a weaker for bane metals : 

 this is probably one of the chief reasons for the extn 

 adoption of cyanogen com]* unds in electro-deposition. 

 Those compounds are highly suitable for the deposition 

 of noble metals, because of the great affinity of cyanogen 

 for them ; but not, as some persons assert, for the de- 

 position of many of the base metals, on account of a 

 comparatively weak affinity for them. 



22. Potassium is an alkali metal; einc, cadmium, tin, 

 lead, iron, cobalt, nickel, and copper, are base metals ; 

 and mercury, silver, gold, platina, palladium, .t-c. , are 

 noble metals; and of these three classes, the alkaline 

 metals possess the strongest affinity for acids, base metals 

 intermediate, and uoble metals the least. The same 

 order prevails in their degrees of depositing po 

 potassium and the alkaline metals generally deposit 

 nearly all others from their solutions ; zinc, and the base 

 metals generally, deposit a smaller number; and gold, 

 platina, and the other noble metals deposit a very few 

 from their solutions. Thus we perceive that those metals 

 which have the strongest chemical affinity for acids, 

 possess the greatest depositing power; and those whieh 

 have the least affinity for acids, have also the least de- 

 positing power. 



23. Difference of Chemical Affinity necessary to De- 

 position. The second chemical condition which wo have 

 to observe upon is, that, in every case of dt position, there 

 is a difference of chemical affinity at the dissolving ami 

 receiving surfaces for the different elements of the liquid, 

 and that the dissolving metal has a stronger affinity for 

 the acid elements of the liquid than either the metal in 

 solution or tho receiving; metal. For instance 



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

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

 deposit takes place upon it; but if we immerse a piece of 

 platina in the liquid, it receives no deposit : in the first 

 case, the iron having a stronger affinity for the sulphuric 

 acid of the salt than copper, combines with it and 

 dissolves, and the copper thereby set free from the acid 

 is deposited upon the iron ; whilst in tho second case, 

 platina having a much weaker affinity for the acid than 

 the copper, cannot separate the acid and copper, and 

 therefore cannot cause deposition. 



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

 merse copper and iron in mutual contact, in a solution 

 of sulphate of copper, tho iron dissolves, and deposition 

 of copper takes place upon both metals; but if we 

 immerse copper and platina in mutual contact in this solu- 

 tion, no deposition will occur. In the first instance, the 

 iron possessing a stronger chemical affinity for sulphuric 

 acid than the copper, combines with it, and sets tho 

 copper free ; by this action, a current of depositing force 

 is generated, which circulates through the iron, liquid, 

 and copper, at their points of contact, and causes tho 

 metal of tho liquid to bo deposited upon the piece of 

 copper ; but in the second case, neither the copper nor 

 platina possessing a stronger affinity for the acid of the 

 salt than its associated metal, there is no copper set free, 

 no current of depositing force generated, and con- 

 sequently no deposition takes place. 



3rd. li'i7/i One Mi'iiil and Two Liquids. If wo im- 

 merse one end of a piece of copper in dilute sulphuric 

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

 tho two liquids touching each other, copper will bo 

 deposited upon the end immersed in the metallic solution, 

 whilst the other end will combine with tho acid and 

 dissolve; but if a piece of platina or gold be substituted 

 for tho copier, neither of its ends will dissolve or re. 

 a metallic deposit: in the first instance, the dilute sul- 

 phuric acid, having a stronger affinity for copper than a 

 solution of sulphate of copper, combines with it, causes 

 it to dissolve, develops a current of depositing f 

 which circulates through tho metals and liquids, and a 



