f 39 3 
that it had been obferved, that fometimes 
one metal was fufficient for the experi¬ 
ment. At prefent, I can give numerous 
proofs of this laft circumftance. The 
fails I am about to relate, are by no means 
uncommon; on the contrary, one may 
witnefs them in every frog, provided the 
experiment be made immediately after the 
animal is killed. The following is the 
method which I have obferved. 
Jdaving prepared one of thefe animals, 
I hold it fufpended by the foot with one 
hand, and with the other bring the fciflkrs 
gently to the fpine, or to the nerves. On 
this contail, the legs (hake and fly from 
the inftrument, and fometimes thefe 
movements are effeiled with conliderable 
force and vivacity. I find fmall frogs 
afford more movements than large ones, 
and particularly if the fpine be touched 
in (lead of the nerves; in fome of thefe 
the contail did not produce any effect. 
When I hold the frogs fufpended by a 
D 4 When 
