tina crucible, but found great
difficulty, from the high temperature required for perfect fusion, and the
powerful fluxing qualities of the substance. Green-glass tubes repeatedly
failed. I at last fused the oxide in a small porcelain crucible, heated
fully in a charcoal fire; and, as it is was essential that the evolution of
the lead at the _cathode_ should take place beneath the surface, the
negative electrode was guarded by a green-glass tube, fused around it in
such a _manner as to expose only the knob of platina_ at the lower end
(fig. 70.), so that it could be plunged beneath the surface, and thus
exclude contact of air or oxygen with the lead reduced there. A platina
wire was employed for the positive electrode, that metal not being subject
to any action from the oxygen evolved against it. The arrangement is given
in fig. 71.

798. In an experiment of this kind the equivalent for the lead came out
93.17, which is very much too small. This, I believe, was because of the
small interval between the positive and negative electrodes in the oxide of
lead; so that it was not unlikely that some of the froth and bubbles formed
by the oxygen at the _anode_ should occasionally even touch the lead
reduced at the _cathode_, and re-oxidize it. When I endeavoured to correct
this by having more litharge, the greater heat required to keep it all
fluid caused a quicker action on the crucible, which was soon eaten
through, and the experiment stopped.

799. In one experiment of this kind I used borate of lead (408. 673.). It
evolves lead, under the influence of the electric current, at the _anode_,
and oxygen at the _cathode_; and as the boracic acid is not either directly
(408.) or incidentally decomposed during the operation, I expected a result
dependent on the oxide of lead. The borate is not so violent a flux as the
oxide, but it requires a higher temperature to make it quite liquid; and if
not very hot, the bubbles of oxygen cling to the positive electrode, and
retard the transfer of electricity. The number for lead came out 101.29,
which is so near to 103.5 as to show that the action of the current had
been definite.

800. _Oxide of bismuth._--I found this substance required too high a
temperature, and acted too powerfully as a flux, to allow of any experiment
being made on it, without the application of more time and care than I
could give at present.

801. The ordinary _protoxide of antimony_, which consists of one
prop