Definite Chemical Action of Electricity, 339 



and thus insulating it, prevented the transmission of the elec- 

 tricity. Whether if it had been fusible and still immiscible it 

 would have decomposed, is doubtful, because of its departure 

 from the required composition (697.)* It was a very natural 

 secondary product at the positive electrode (779.). On open- 

 ing the tube it was found that a little antimony had been se- 

 parated at the negative electrode; but the quantity was too 

 small to allow of any quantitative result being obtained. 



802. Iodide of Lead. — This substance can be experimented 

 with in tubes heated by a spirit-lamp (789.) ; but I obtained 

 no good results from it, whether I used positive electrodes of 

 platina or plumbago. In two experiments the numbers for 

 the lead came out only 75*46 and 73*45, instead of 103*5. 

 This I attribute to the formation of a periodide at the positive 

 electrode, which dissolving in the mass of liquid iodide, came 

 in contact with the lead evolved at the negative electrode, and 

 dissolved part of it, becoming itself again protiodide. Such 

 a periodide does exist; and it is very rarely that the iodide of 

 lead formed by precipitation, and well washed, can be fused 

 without evolving much iodine, from the presence of this per- 

 compound ; nor does crystallization from its hot aqueous so- 

 lution free it from this substance. Even when a little of the 

 protiodide and iodine are merely rubbed together in a mortar, 

 a portion of the periodide is formed. And though it is de- 

 composed by being fused and heated 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 car- 

 ried 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 in- 

 creased 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 immediately described (818.). 



The experiments on iodide of lead, therefore, offer no ex- 

 ception to the general law under consideration, but, on the 

 contrary, may, from general considerations, be admitted as in- 

 cluded in it. 



804. Protiodide of Tin. — This substance, when fused (402.), 

 conducts and is decomposed by the electric current, tin is 

 evolved at the anode, and periodide of tin as a secondary re- 

 sult (779. 790.) at the cathode. The temperature required 

 for its fusion is too high to allow of the production of any re- 

 sults fit for weighing. 



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