2 56 Dr. Wells’s Observations on an Influence 
it next 'became a subject of inquiry, whether this was owing to 
any disposition of the muscles and nerve, which had been in- 
duced upon them by Mr. Volta’s experiment, or whether, 
the condition of the muscles and nerve being unaltered by 
that experiment, the silver had gained some new property by 
coming into contact with the tin-foil. The point in doubt was 
soon determined, by applying the probe to a piece of tin-foil, 
which had no connection with any part of the animal ; for, 
when this was done, it was again enabled to produce con- 
tractions. As these experiments, however, frequently did not 
succeed when made upon other frogs, I afterwards varied the 
metals, and found in consequence, that zinc, particularly if 
moistened, communicated an exciting power pretty constantly 
to silver, gold, and iron. If any of these metals were slightly 
rubbed on the zinc, they almost always acquired such a. 
power. 
It will, perhaps, be thought from the last-mentioned cir- 
cumstance, that, in every instance of motion being in this 
way produced, it was in truth owing to some part of one of 
the metals having been abraded by the other ; so that, under 
the appearance of one metal, two were in reality applied. But 
it can scarcely be supposed, that, from touchingujhe polished 
surface of tin-foil in the gentlest manner witlrhhe smooth 
round end of a silver probe, any part of the former metal was 
carried away by the latter ; and even when friction was used, 
as the zinc was much harder than the gold and silver, it is not 
probable that it was in the least abraded by them. Besides, 
moisture, as I have already said, increases this effect of fric- 
tion, though it lessens friction itself. 
The most powerful argument, however, in favour of my 
