ortional of metal and one and a half of oxygen, was subjected to the
action of the electric current in a green-glass tube (789.), surrounded by
a jacket of platina foil, and heated in a charcoal fire. The decomposition
began and proceeded very well at first, apparently indicating, according to
the general law (679. 697.), that this 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 deco