DEFINITE CHEMICAL ACTION OF ELECTRICITY. 107 



to dull redness for a few minutes, and the whole reduced to protiodide, yet that is not 

 at all opposed to the possibility, that a little of that which is formed in great excess of 

 iodine at the anode, should be carried by the rapid currents in the liquid into contact 

 with the cathode, 



803. This view of the results was strengthened by a third experiment^ where the 

 space between the electrodes was increased to one third of an inch ; for now the 

 interfering effects were much diminished, and the number of the lead came out 89*04 ; 

 and it was fully confirmed by the results obtained in the cases of transfer to be im- 

 mediately described (818.). 



The experiments on iodide of lead, therefore, offer no exception to the general law 

 under consideration, but, on the contrary, may, from general considerations, be ad- 

 mitted as included in it. 



804. Protiodide of tin. — This substance, when fused (402.), conducts and is decom- 

 posed by the electric current, tin is evolved at the anode, and periodide of tin as a 

 secondary result {77^- 790.) at the cathode. The temperature required for its fusion 

 is too high to allow of the production of any results fit for weighing. 



805. Iodide of potassium was subjected to electrolytic action in a tube, fig. 13. (789.). 

 The negative electrode was a globule of lead, and I hoped in this way to retain the 

 potassium, and obtain results that could be weighed and compared with the volta- 

 electrometer indication ; but the difficulties dependent upon the high temperature 

 required, the action upon the glass, the fusibility of the platina induced by the pre- 

 sence of the lead, and other circumstances, prevented me from obtaining such results. 

 The iodide was decomposed with the evolution of iodine at the anode, and of potas- 

 sium at the cathode, as in former cases. 



806. In some of these experiments several substances were placed in succession, 

 and decomposed simultaneously by the same electric current: thus, protochloride 

 of tin, chloride of lead, and water, were thus acted on at once. It is needless to 

 say that the results were comparable, the tin, lead, chlorine, oxygen, and hydrogen 

 evolved being definite in quantity and electro-chemical equivalents to each other. 



807. Let us turn to another kind of proof of the definite chemical action of electricity. 

 If any circumstances could be supposed to exert an influence over the quantity of the 

 matters evolved during electrolytic action, one would expect them to be present when 

 electrodes of different substances, and possessing very different chemical affinities for 

 the evolving bodies, were used. Platina has no power in dilute sulphuric acid of 

 combining with the oxygen at the anode, though the latter be evolved in the nascent 

 state against it. Copper, on the other hand, immediately unites to the oxygen, as the 

 electric current sets it free from the hydrogen ; and zinc is not only able to combine 

 with it, but can, without any help from the electricity, abstract it directly from the 

 water, at the same time setting torrents of hydrogen free. Yet in cases where these 

 three substances were used as the positive electrodes in three similar portions of the 

 same dilute sulphuric acid, specific gravity 1-336, precisely the same quantity of water 



p2 



