SULPHURET OF POTASSIUM, — VOLTAIC CIRCLES. 
85 
produces a current ; but when the sulphuret which is formed, by investing the metal, 
shuts off the fluid and prevents further chemical action, then the current ceases also. 
Nor is it necessary that it should be a non-conductor for this purpose, for conducting 
sulphurets will perform the same office (1885. 1894.), and bring about the same re- 
sult. What, then, can be more clear, than that whilst the sulphuret is being formed 
a current is produced, but that when formed its mere contact can do nothing towards 
such an effect ? 
1885. Lead. — This metal presents a fine result in the solution of sulphuret of po- 
tassium. Lead and platinum being the metals used, the lead was at first highly posi- 
tive, but in a few seconds the current fell, and in two minutes the galvanometer- 
needle was at 0°. Still the arrangement conducted a feeble thermo current extremely 
well, the conducting power not having disappeared, as in the case of tin ; for the in- 
vesting sulphuret of lead is a conductor (1820.). Nevertheless, though a conductor, 
it could stop the further chemical action; and that ceasing, the current ceased also. 
1886. Lead and gold produced the same effect. Lead and palladium the same. 
Lead and iron the same, except that the circumstances respecting the tendency of 
the latter metal under common circumstances to produce a current from the elec- 
trolyte to itself, have to be considered and guarded against (1826. 2049.). Lead 
and nickel also the same. In all these cases, when the lead was taken out and 
washed, it was found beautifully invested with a thin polished pellicle of sulphuret 
of lead. 
1887- With lead, then, we have a conducting sulphuret formed, but still there is no 
sign that its contact can produce a current, any more than in the case of the non- 
conducting sulphuret of tin (1882.). There is no new or additional action pro- 
duced by this conducting body ; there was no deficiency of action with the former 
non-conducting product ; both arealike in their results, being, in fact, essentially alike 
in their relation to that on which the current really depends, namely, an active che- 
mical force. A piece of lead put alone into the solution of sulphuret of potassium, 
has its surface converted into sulphuret of lead, the proof thus being obtained, even 
when the current cannot be formed, that there is a force (chemical) present and active 
under such circumstances ; and such force can produce a current of chemical force 
when the circuit form is given to the arrangement. The force at the place of excite- 
ment shows itself, both by the formation of sulphuret of lead and the production of 
a current. In proportion as the formation of the one decreases the production of the 
other diminishes, though all the bodies produced are conductors, and contact still 
remains to perform any work or cause any effect to which it is competent. 
1888. It may perhaps be said that the current is due to the contact between the 
solution of sulphuret and the lead, (or tin, as the case may be,) which occurs at the 
beginning of the experiment; and that when the action ceases, it is because a new 
body, the sulphuret of lead, is introduced into the circuit, the various contacts being 
then balanced in their force. This would be to fall back upon the assumption before 
