SULPHURET OF POTASSIUM, — VOLTAIC CIRCLES. 
91 
exciting electrolytes, I will mention the varying and beautiful phenomena which occur 
when copper and silver, or two pieces of copper, or two pieces of silver, form a circle 
with the yellow solution. If the metals be copper and silver, the copper is at first 
positive and the silver remains untarnished ; in a short time this action ceases, and 
the silver becomes positive ; at the same instant it begins to combine with sulphur 
and becomes covered with sulphuret of silver ; in the course of a few moments the 
copper again becomes positive ; and thus the action will change from side to side 
several times, and the current with it, according as the circumstances become in turn 
more favourable at one side or the other. 
1912. But how can it be thought that the current first produced is due in any way 
to the contact of the sulphuret of copper formed, since its presence there becomes at 
last the reason why that first current diminishes, and enables the silver, which is ori- 
ginally the weaker in exciting force, and has no sulphuret as yet formed on it, to 
assume for a time the predominance, and produce a current which can overcome 
that excited at the copper (1911.) ? What can account for these changes, but che- 
mical action ? which, as it appears to me, accounts, as far as we have yet gone, with 
the utmost simplicity, for all the effects produced, however varied the mode of action 
and their circumstances may be. 
Royal Institution, 
December 12, 1839. 
