LAWS OP DEPOSITION 1 JNDULATORY FORCES.^-ELECTRO-METALLURGY. 



207 



the currents of affinity and their effects ; but, in the 

 second case, the direction of the affinities evolved by the 

 vessel A coincide with that of those from vessel B, the 

 currents circulate, and deposition is effected. In the 

 case of an unequal portion of the metals of a series 

 being connected in, the opposite way, those which are 

 wrongly connected will neutralise and be neutralised by 

 an equal number of the remainder, provided that all the 

 pairs of metals and the liquids are similar : for instance, 

 if three out of twelve are wrongly connected, they will 

 neutralise the power of three more, and only the re- 

 maining six will act in the desired direction. 



30. During the act of deposition, a salt is generally 

 formed at the dissolving surface by the union of the 

 metal with the acid elements of the liquid, which is dis- 

 solved in the liquid ; at the same time the acid, which 

 combines with the dissolving surface, is generally set 

 free at the receiving surface by the deposition of the 

 metaL For instance 



1st. With One Metal and One Liquid. When iron 

 coats itself with mercury by simple immersion in a 

 solution of nitrate of mercury, nitrate of iron is formed 

 by the union of the iron with the nitric acid of the 

 nitrate of mercury, and nitric acid is at the same time 

 set free by the deposition of the mercury. 



2nd. With Two Metal* and One Liquid. When cop- 

 per receives a deposit of copper by immersion, in contact 

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

 sulphate of iron is formed at the immersed surface of 

 *he iron, by the union of the sulphuric acid of the 

 sulphate copper salt, with the iron ; at the same time 

 sulphuric acid is set free at the copper surface by the 

 deposition of the copper. 



3rd. With One Metal and Two Liquids. When copper 

 receives a deposit of copper in a solution of sulphate of 

 copper, by connection with another piece of copper im- 

 mersed in dilute sulphuric acid, the two liquids being 

 separated by means of a porous partition or otherwise, 

 sulphate of copper is formed at the surface of the metal 

 in the dilute acid, by the union of the copper with the 

 acid, and, at the same time, sulphuric acid is set free at 

 the surface of the other piece by the deposition of the 

 copper. 



4th. Silver. When silver or any other metal receives 

 a coating of silver in a solution of double cyanide of 

 liver and potassium, by connection with a piece of zinc 

 in dilute sulphuric acid, the two liquids being separated 

 by a porous diaphragm, sulphate of zinc is formed at the 

 surface of the zinc, by the union of the acid and zinc, 

 and cyanogen (a substance of an acid character) is at the 

 same time set free at the receiving surface, by the de- 

 position of the silver. 



6th. With a Separate Depositing Liquid, connected 

 with any source of current affinity : for instance, when 

 two silver plates, immersed in a solution of the double 

 cyanide of silver and potassium, are connected by wires 

 with a piece of zinc, and a piece of copper immersed 

 in dilute sulphuric acid in a separate vessel, cyanide of 

 silver is formed at the surface of one of the pieces of 

 silver by the union of that metal with the cyanogen of 

 the liquid ; and, at the same time, either cyanogen or 

 hydro-cyanic acid is set free at the surface of the other 

 piece by the deposition of the silver. 



31. Proper Proportion of Free Acid in the Depositing 

 Liquid. If a solution contain a large excess of uncom- 

 bined acid, metallic deposition will not always occur : 

 for instance, if two pieces of zinc are immersed in a 

 neutral solution of sulphate of zinc, and connected by 

 wires with another piece of zinc, and a piece of silver 

 immersed in dilute sulphuric acid, in a separate vessel, 

 one piece of zinc will dissolve, and the other receive a 

 deposit of metal ; but if a rather large quantity of sul- 

 phuric acid be added to the depositing liquid, no deposit 

 of zinc will occur. In the same manner, if we connect 

 two pieces of silver, immersed in a strong solution of 

 cyanide of potassium, with the zinc and silver in dilute 

 sulphuric acid, as just described, one of the pieces of 

 silver will combine with the cyanogen of the cyanide of 

 potassium, and form cyanide of silver, which will com- 



bine with a portion of the remaining cyanide and then 

 dissolve ; but the other piece of silver will not receive a 

 deposit of silver, until the remaining uncombined cyanide 

 of potassium has decreased to a certain proportion bj 

 the working of the process. If, on the other hand, a 

 depositing solution contain no free combining substance, 

 deposition will either proceed very slowly, or be entirely 

 stopped, in consequence of an insoluble salt being formed 

 upon the surface of the dissolving metal, which will 

 impede the action : for instance, when two silver plates, 

 immersed in a solution of double cyanide of silver and 

 potassium, are connected with some source of depositing 

 power, one of the plates will receive a deposit of silver, 

 whilst the other will gradually become covered with a 

 white layer of insoluble cyanide of silver, which eventu- 

 ally stops deposition. 



32. Necessity of a Proper Proportion of Water. If a 

 depositing solution be diluted with water to a very large 

 extent, deposition will progress very slowly ; but if, on 

 the other hand, it contain insufficient water, crystals of 

 metallic salts will collect upon the dissolving metal, and 

 gradually stop the action : for instance, if two pieces of 

 copper, immersed in a saturated solution of sulphate of 

 copper containing free acid are connected with a piece of 

 zinc, and a piece of silver immersed in dilute sulphuric 

 acid in a separate vessel, one piece will receive a deposit 

 of copper, whilst the other will slowly dissolve and gra- 

 dually become covered with crystals of sulphate of 

 copper, first at its lower part and then at the edges, 

 which will gradually stop the action. 



33. All the foregoing facts prove that chemical affinity 

 plays a very important part in the phenomena of electro- 

 deposition. 



34. Electrical Conditions of Deposition Positive and 

 Negative Substances necessary. The first electrical con- 

 dition to be observed is, that in every case of deposition, 

 the liquid contains both substances of an electro-positive 

 and of an electro-negative character ; metals and alka- 

 lies are electro-positive, and metalloids (oxygen, sulphur, 

 chlorine, iodine, bromine, <bc.) and acids are electro- 

 negative ; cyanogen is also electro-negative.* 



1st. With One Metal and One Liquid. In the first 

 instance, if we immerse a piece of copper in a solution 

 of nitrate of mercury, deposition occurs, mercury being 

 thrown down ; but if we immerse it in dry mercury, 

 there is no deposition : in the former case the liquid 

 contains both electro-positive mercury and negative 

 nitric acid, but in the latter case it only contains the 

 positive mercury. 



2nd. With Two Metals and One Liquid. Immerse 

 zinc and platina in mutual contact in a solution of 

 nitrate of mercury, the platina will receive a deposit ; 

 but if we immerse them in dry mercury, it will receive 

 no deposit. 



3rd. With One Metal and Two Liquids. If one end <ji 

 a platina wire be immersed in a solution of cyanide of 

 potassium, and the other end in a solution of nitrate of 

 mercury, the two liquids being separated by a porous 

 partition, the end in the mercurial solution will receive 

 a metallic deposit ; but if dry mercury be substituted 

 for the nitrate solution, no deposition will occur. 



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

 merse zinc in dilute sulphuric acid, and platina in a 

 solution of nitrate of mercury, the metals touching each 

 other, and the liquids separated by a porous partition, 

 the platina will receive a mercurial deposit ; but when 

 dry mercury is substituted for the nitrate solution, it 

 will receive no deposit. 



6th. With a Separate Depositing Liquid. If we im- 

 merse two platina wires in a solution of nitrate of mer- 

 cury, and connect them with any source of depositing 

 power, one of the wires will receive a metallic deposit ; 

 but if we use dry mercury instead of the nitrate solu- 

 tion, there will be no deposit. 



These and many other instances prove, that unless the 

 depositing liquid contain both electro-positive and elec- 

 tro-negative substances, no deposition will occur. 



35. Electric Polarity of the Dissolving and Receiving 



See ante, p. 194. 



