which incites the Muscles of Animals to contract. 255 
tions ; in confirmation of which, many persons have asserted, 
that they have never observed muscles to move from the em- 
ployment of two such pieces of metal, or of one piece of metal 
having the same fineness through its whole extent. Others, 
however, upon the authority of their observations, have main- 
tained the contrary; and to the testimony of these I must add 
my own, as I have frequently seen muscular motions produced 
not only by a single metal, but likewise by charcoal alone. 
Nor will credit be denied me on this head, after I have pointed 
out certain practices, by which any one of those substances 
may at pleasure be made to produce contractions. The most 
proper way of mentioning these practices will, perhaps, be to 
relate in what manner they came to my knowledge. 
I one day placed a piece of silver, and another of tin-foil, at 
a small distance from each other upon the crural nerve of a frog, 
and then applied a bent silver probe between them; with the 
view of ascertaining, whether contractions would arise, agree- 
ably to Mr. Volta's declaration, from the influence passing 
through a portion of the nerve without entering the muscles. 
Having finished this experiment, I immediately after applied 
the same probe between the silver coating of the nerve and 
the naked rSbu^les, and was surprised to see these contract. A 
second and third application were followed by the same effects, 
but further applications were of no avail. It then occurred 
to me that motions might re-appear, if I again touched the 
two coatings with the probe, and the event proved the conjec- 
ture to have been fortunate ; for after every application of the 
probe to the two coatings, contractions were several times ex- 
cited by it. The fact being thus established, that under certain 
circumstances contractions could be produced by silver alone, 
LI 2 
