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



209 



column of water, the process will be greatly impeded ; 

 and if we interpose a non-conductor of electricity, such 

 as a rod of glass or of gutta-percha, or otherwise break 

 the continuity of the circuit, deposition will be com- 

 pletely arrested. 



38. Electric Conductivity. The following is the gen- 

 eral order of the conducting power of metals and alloys for 

 voltaic electricity at 60 Fah. , beginning with those which 

 conduct most freely viz. , silver, copper, gold, cadmium, 

 zinc, brass, tin, palladium, iron, steel, lead, platina, 

 German silver, antimony, mercury, bismuth, potassium. 

 The order is somewhat different at other temperatures. 



39. Direction of the Electric Currents. In every case 

 where a current of electricity is developed by the mutual 

 contact of liquids and metals, or where it merely passes 

 through them, as in a separate depositing vessel, the 

 current of positive electricity invariably passes from the 

 positive or dissolving metal, through the liquid, to the 

 negative or receiving one ; and the negative electricity 

 passes similarly in the opposite direction. When we 

 speak of "the current," without stating which is meant 

 the positive or negative electricity it is always in- 

 tended, for the sake of simplicity of expression, to indi- 

 cate the positive electricity : when we speak of the 

 positive metal or plate, the metal which is positive and 

 dissolves is meant ; but when the positive pole is men- 

 tioned, the metal from which the positive electricity 

 proceeds out of the arrangement or apparatus into the 



S and which is invariably the negative or receiving 

 metal, is intended: for instance, when a piece of zinc and a 

 piece, of silver or copper are immersed in dilute sulphuric 

 acid, the zinc is the negative pole and the positive plate, 

 whilst the copper or silver is the positive pole and the 

 negative plate. * 



40. In a separate depositing vessel it is the dissolving 

 metal which is called the positive plate, because it cor- 

 responds to the zinc or dissolving plate of the battery ; 

 while the receiving metal is called the negative plate : 

 the term pole is also sometimes applied to those plates, 

 but in an irregular manner. 



41. Electrical Decomposition of Liquids. In all cases 

 of electro-deposition, the elements of the liquids are 

 split asunder by electric action at the surfaces of the 

 metals; the electro-negative elements, such as metal- 

 loids and acids, either combine with, or are set free at 

 the surface of the dissolving or positive metal, and the 

 electro-positive elements, such as metals and alkalies, 

 either combine with, are set free, or deposited at the 

 surface of the receiving or negative metal : for instance, 

 if a piece of silver and a piece of copper are immersed 

 in a solution of sulphate of copper, and a piece of zinc 

 and a piece of platina are placed in dilute sulphuric 

 acid, the silver connected with the zinc by one wire, and 

 the copper with the platina by another wire, the negative 

 elements of the liquid namely, the sulphuric acid of 

 the sulphate of copper solution, will be split from its 

 associated copper, and will combine with the positive 

 metal, the copper, causing it to dissolve in the liquid ; 

 while the positive element of the liquid namely, the 



.<n of the salt, will be deposited at the surface of 

 the negative or receiving metal, the silver, but will not 

 combine with it ; but if we substitute a piece of platina 

 f'.r the piece of copper, and mercury for the silver 

 (Fig. 74), the effects will be reversed, the acid or nega- 

 tive element will collect around the positive platina, but 

 will not combine with it ; whilst the positive element of 

 liquid, the copper, will be deposited and combine 

 with the negative mercury. Fused salts yield the same 

 substances by electric decomposition as the same salts do 

 when dissolved in water. 



42. Electrical Terms in Deposition. For the more clear 

 remembrance of the different parts of the circuit and of 

 the direction of the electric forces, and for the better 

 understanding of the action of the currents, Faraday has 

 proposed the following terms, which have come into 



nj use: The liquid undergoing decomposition ho 

 terms an " electrolyte," from two Greek words, " electron," 

 meaning "electric," and "luo," to "set free;" the act 



Sec ante, p. 194. 

 VOL. 1. 



of electric decomposition ho terms "electrolysis;" the 

 metallic or other surfaces at which the electric forces 

 enter and leave a liquid, he terms " electrodes," from two 

 Greekwords, "electron "and "odos," meaning a "way;" 

 the positive electrode, or that point at which the positive 

 electricity enters a liquid, is termed "anode," from two 

 Greek words, " ana," meaning "upwards," and " odos," 

 a " way" the way in which the sxin rises ; and the 

 negative electrode, or that by which the positive electri- 

 city leaves a liquid, is termed "cathode," from two Greek 

 words, "cata," meaning "downwards," and "odos," a 

 "way" the way in which the sun sets ; the elements of 

 the liquid set free by electrolysis, he terms " ions," from 

 a Greek word meaning "going;" those which combine 

 with, or are set free at the anode, are termed "onions;" 

 and those which combine with, or are set free at the 

 cathode, are termed "cations."^ 



43. Anions and Cations. Under the head of Anions 

 may be classed oxygen, fluorine, chlorine, bromine, 

 iodine, and cyanogen; probably, also, sulpho-cyanogen, 

 and the various mineral acids. Cations include hy- 

 drogen (and ammonium), the alkaline metals, magnesium, 

 manganese, arsenic, antimony, bismuth, zinc, cadmium, 

 tin, lead, iron, cobalt, nickel, copper, mercury, silver, 

 gold, platina, palladium, and the salifiable bases. 



44. Electro-chemical Scale. The various elementary 

 substances have been arranged by Berzelius according to 

 their relative degrees of positive and negative electro- 

 chemical character, in a table or scale like the accom- 

 panying one, commencing with those substances possess- 

 ing the strongest electro-positive properties, and ending 

 with those of the strongest electro negative properties : 



POSITIVE END. 



K1XK. 



Potassium 

 Sodium ... 

 Lithium ., 

 Barium 

 Strontium 

 Calcium ... 



M u'li' i-L-nn 



Glue in urn 

 Yttrium .. 

 Aluminum 

 Zirconium 



Thorinum.. 

 Cadmium 



Zinc 

 Iron 

 Nickel 

 Cobalt 



f\ M::nT,. 



K. 



Na. 



L. 



Ba. 



Sr. 



Ca. 



::::: f 



" Al. 



7.r. 



Th. 



Mn. 

 Zn. 

 re. 

 Ni. 

 Co. 



It will be observed, that the division indicated in the 

 above table between gold and hydrogen, is, in a great 

 measure, an arbitrary one ; useful to assist the memory 

 in recollecting the general electro-chemical character of 

 the substances, but not really existing in nature : for 

 instance, sulphur and chlorine, two of the most negative 

 of substances, must be viewed as positive in relation to 

 oxygen. A still more negative substance, when combined 

 with that element, is hyposulphurous or hypochlorous 

 acid, which is negative in relation to arsenic, hydrogen, 

 zinc, potassium, when combined with those elements in 

 the various metallic sulphides and chlorides : in fact, 

 each substance throughout the scale may be viewed as 

 both positive and negative positive in relation to those 

 below it, and negative in relation to those above it ; those 

 of the upper end being strongly positive and feebly 

 negative, and those of the lower end strongly negative 

 and weakly positive. It has been objected that sulphur 

 and nitrogen occupy a position too near the negative end 

 of the sca'e, they being generally less negative than 

 chlorine and fluorine; also that hydrogen should bo 

 placed higher up in the positive division. 



45. A consideration of all the foregoing facts leads us 

 to conclude, that current electricity acts a most important 

 part in all the phenomena of electro-deposition. 



46. Thermal Conditions. Under this head compa- 

 ratively little can be said, because it is that portion of 

 the subject which has been the least investigated : but it 

 is highly probable that, as heat is generally evolved by 

 the chemical combination of metals with metalloids or 



t Sec ante, p. 194. 



