220 INDUCTIVE AFFINITY. 



It is likewise necessary that the fluid DC be of easy decompo- 

 sition, so as to yield to the polarizing power of the single circle. 

 In this arrangement, however, it is obvious that the zinc itself 

 forms a complete polar element, of which A is the zincous, and 

 D the chlorous pole ; and the copper also an entire polar ele- 

 ment of which B is the chlorous, and C the zincous pole. 



The preceding table exhibits the relation which the metals 

 enumerated assume to each other, in the acid and saline solu- 

 tions usually employed as exciting fluids. But the relation of 

 any one metal to another is not the same in all exciting fluids. 

 Thus when tin and copper are placed in acid solutions, the 

 former is most rapidly corroded and becomes the positive metal, 

 according to its position in the series, but if they are put into a 

 solution of ammonia which acts most upon the copper, then the 

 latter becomes the positive metal. Copper is positive to lead in 

 strong nitric acid, which oxidizes the former most freely, where- 

 as in dilute nitric acid, by which the lead is most rapidly dis- 

 solved, the lead is positive. 



LIQUID ELEMENTS OF THE VOLTAIC CIRCLE. 



With the view of simplifying the statement of the chemical 

 changes which occur in the voltaic circle, the exciting fluid has 

 hitherto always been supposed to be hydrochloric acid (chloride 

 of hydrogen), and this compound is a fair type of the class of 

 bodies which are available for the purpose of bringing these 

 changes into play. The exciting fluid is always a saline body in 

 the general sense, that is, a compound of a salt-radical, such as 

 chlorine, with a basyle, such as hydrogen or a metal. The 

 chloride of copper, chloride of sodium, chloride of ammonium, 

 or the chloride of any other basyle may be substituted for hy- 

 drochloric acid, although not all with the same advantage ; and 

 the chlorides of basyles may be replaced by their iodides, sul- 

 phatoxides (sulphates), nitratoxides (nitrates) and salts of other 

 acids, as exciting fluids, provided they have the condition of 

 liquidity, which gives mobility to their particles and permits 

 that disposition of them which is assumed in a polar chain. 

 The liquids, which yield in the cell of decomposition, are of the 

 same nature, although the liquid which forms the best exciting 

 fluid is not always the most easily decomposed in the decom- 

 posing cell. 



