Circles with Sulphuret of Potassium  267
other metal, a result quite in opposition to the idea, that the
mere presence of the sulphuret on it could have caused the
former powerful current and positive state of the copper (885,
866). A further proof that it is not the mere presence, but
the formation, of the sulphuret which causes the current, is,
that if the plate be left long enough for the solution to pene-
trate the investing crust of sulphuret of copper and corne into
activity on the metal beneath, then the plate becomes active,
and a current is produced.
 887.	I made some sulphuret of copper, by igniting thick
copper wire in a Florence flask or crucible in abundance of
vapour of sulphur.   The body produced is in an excellent
form for these experiments, and a good conductor;   but it is
not without action on the sulphuretted solution, from which
it can take more sulphur, and the consequence is, that it is
positive to platinum or iron in such a solution.    If such sul-
phuret of copper be left long in the solution and then be washed
and dried, it will generally acquire the final state of sulphura-
tion, either in parts or altogether, and also be inactive, as
the sulphuret formed on the copper was before (886);   i.e.
when its chemical action is exhausted, it ceases to produce a
current.
 888.	Native grey sulphuret of copper has the same relation
to the electrolyte:   it takes sulphur from it and is raised to a
higher state of combination;   and, as it is also a conductor
(808), it produces a current, being itself positive so long as
the action continues.
 889.	But when the  copper  is fully sulphuretted,  then  all
these actions cease;  though the sulphuret be a conductor, the
contacts still remain, and the circle can carry with facility a
feeble thermo current.   This is not only shown by the quiescent
cases just mentioned (886), but also by the utter inactivity of
platinum and compact yellow copper pyrites, when conjoined
by this electrolyte, as shown in a former part of this paper (828).
• 890. Antimony.—This metal, being put alone into a solution
of sulphuret of potassium, is acted on, and a sulphuret of
antimony formed which does not adhere strongly to the metal,
but wipes off. Accordingly, if a circle be formed of antimony,
platinum, and the solution, the antimony is positive in the
electrolyte, and a powerful current is formed, which continues.
Here then is another beautiful variation of the conditions under
which the chemical theory can so easily account for the effects,
whilst the theory of contacts cannot. The sulphuret produced