case? for nothing happens here that does not happen in any other instance of a current produced by the same exciting electrolyte.
872. On the other hand, how beautiful a proof the result gives in confirmation of the chemical theory! Tin can take sulphur from the electrolyte to form a sulphuret; and whilst it is doing so, and in proportion to the degree in which it is doing so, it 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 (873, 882), and bring about the same result. 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?
873. Lead.—This metal presents a fine result in the solution of sulphuret of potassium. Lead and platinum being the metals used, the lead was at first highly positive, 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 investing sulphuret of lead is a conductor (808). Nevertheless, though a conductor, it could stop the further chemical action; and that ceasing, the current ceased also.
874. 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 electrolyte to itself, have to be considered and guarded against (814, 1037). 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.
875. 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 (870). There is no new or additional action produced by this conducting body; there was no deficiency of action with the former non-conducting product; both are alike in their results, being, in fact, essentially alike in their relation to that on which the current really depends, namely, an active chemical
