94 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



Stances separated by the current are changed at the electrodes before their appearance, 

 then they give rise to secondary results, although in many cases the bodies evolved are 

 elementary. 



744. These secondary results occur in two ways, being sometimes due to the mutual 

 action of the evolving substance and the matter of the electrode, and sometimes to its 

 action upon the substances contained in the decomposing conductor itself. Thus, 

 when carbon is made the positive electrode in dilute sulphuric acid, carbonic oxide 

 and carbonic acid appear there instead of oxygen ; for the latter, acting upon the 

 matter of the electrode, produces these secondary results. Or if the positive elec- 

 trode, in a solution of nitrate or acetate of lead, be platina, then peroxide of lead ap- 

 pears there, equally a secondary result with the former, but now depending upon an 

 action of the oxygen on a substance in the solution. Again, when ammonia is decom- 

 posed by platina electrodes, nitrogen appears at the anode* ; but though an elementary 

 body, it is a secondary result in this case, being derived from the chemical action of 

 the oxygen electrically evolved there, upon the ammonia in the surrounding so- 

 lution (554.). In the same manner when aqueous solutions of metallic salts are de- 

 composed by the current, the metals evolved at the cathode, though elements, are 

 always secondary results, and not immediate consequences of the decomposing power 

 of the electric current. 



745. Many of these secondary results are extremely valuable ; for instance, all the 

 interesting compounds which M. Becquerel has obtained by feeble electric currents 

 are of this nature ; but they are essentially chemical, and must, in the theory of elec- 

 trolytic action, be carefully distinguished from those which are directly due to the 

 action of the electric current. 



746. The nature of the substances evolved will often lead to a correct judgement of 

 their primary or secondary character, but is not sufficient alone to establish that point. 

 Thus, nitrogen is said to be attracted sometimes by the positive and sometimes by the 

 negative electrode, according to the bodies with which it may be combined (554. 555.), 

 and it is on such occasions evidently viewed as a primary result -j~ ; but I think I shall 

 show, that, when it appears at the positive electrode, or rather at the anode, it is a 

 secondary result (748.). Thus, also, Sir Humphry Davy J, and with him the great 

 body of chemical philosophers, (including myself,) have given the appearance of cop- 

 per, lead, tin, silver, gold, &c., at the negative electrode, when their aqueous solutions 

 were acted upon by the voltaic current, as proofs that the metals, as a class, were 

 attracted to that surface ; thus assuming the metal in each case to be a primary 

 result. These however, I expect to prove, are all secondary results ; the mere conse- 

 quence of chemical action, and no proofs of the attraction or the law announced §. 



* Annales de Chimie, 1804, torn. li. p. 167. f Ibid. torn. li. p. 172. 



X Elements of Chemical Philosophy, pp. 144. 161. 



§ It is remarkable that up to 1804 it was the received opinion that the metals were reduced by the nascent 

 hydrogen. At that date the general opinion was reversed by Hisingee and Berzelius (Annales de Chimie, 



