substance was one containing such elements and in such proportions as made it amenable to the power of the electric current.  This effect I have already given reasons for supposing may be due to the presence of a true protoxide, consisting of single proportionals (696. 693.).  The action soon diminished, and finally ceased, because of the formation of a higher oxide of the metal at the positive electrode.  This compound, which was probably the peroxide, being infusible and insoluble in the protoxide, formed a crystalline crust around the positive electrode; and thus insulating it, prevented the transmission of the electricity.  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 opening the tube it was found that a little antimony had been separated at the negative electrode; but the quantity was too small to allow of any quantitative result being obtained[A].

  [A] This paragraph is subject to the corrective note now appended to
  paragraph 696.—­Dec. 1838.

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 percompound; nor does crystallization from its hot aqueous solution 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 decomposed 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 carried by the rapid currents in the liquid into contact with the cathode.

803.  This view of the result 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 immediately 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 admitted as included in it.