410 Sir Humphry Davy on the relations of 
tinum. That the effect does not depend upon any circum- 
stance connected with conducting power is evident; for 
charcoal, which is a very imperfect conductor, acts like an 
oxidable metal ; and a very fine wire of platinum, terminated 
by a small piece of oxidable metal, acts more efficiently 
when the oxidable metal is opposite the negative pole, than 
if the whole chain had been composed of oxidable metal ; 
but entirely destroys the effect when the oxidable metal is 
opposite the positive pole. 
If the contact of the metals only was necessary for conti- 
nued electro-motion, these results, in which a simple homo- 
geneous chain is interposed between the fluids, v/ould be 
impossible ; but they are a necessary consequence of the 
electro-chemical theory, in which the destruction of the posi- 
tive surface by the chemical negative agent is regarded as a 
necessary condition ; and platinum and tellurium acted like 
zinc, when their surfaces opposite to the platinum were 
plunged into diluted nitro-muriatic acid. 
If two, three, or four glasses are used, and two, three, or 
four arcs of platinum and zinc, the extreme metals of which 
are connected through the multiplier, a piece of platinum 
used instead of one of the arcs will not now entirely destroy 
the electro-motive effect : it will be diminished as if one arc 
had been removed. The two will act as a single combina- 
tion ; the three as two arcs, and the four as three ; and of 
course in a Voltaic combination of 100 arcs, a single piece 
of platinum substituted for any one of the arcs, will diminish 
the power of the apparatus only part. 
In attempting to protect copper by zinc, in a separate 
