NATURAL HISTORY. 
wasiexcited by it; so that it is rea- 
sonable to suppose, that, if precisely 
the Same degree of friction wer 
given to both the parts, no contrac- 
tions would ever be produced by 
them, when used in this way. The 
second is, that, although only one 
part of the metal be rubbed, still, if 
both the muscle and nerve be coated 
with’some other metal, the applica- 
tion of the rubbed metal between 
these similar coatings will not be fol- 
lowed by motions ; which, however, 
will immediately be produced, by 
touching the’ naked muscle and nerve 
with the same piece of metal. But, 
whether any part of my reasoning 
upon this head be admitted as just 
or not, it must yet be granted, as I 
think I cannot be mistaken respect- 
ing the facts which have been men- 
tioned, that very slight accidents 
may give the power of exciting con- 
tractions to a single metal,which had 
it not before; and that we may hence 
easily account for the discordant tes- 
timonies of authors upon this point. 
Hitherto I have spoken only of 
the effects of friction upon metals. 
But to conclude this part of my sub- 
ject, I must now remark, that char- 
coal, though from its friability not 
very fit for the experiment, may yet 
be rendered capable by the same 
means of producing contractions, 
without the assistance of any of the 
metals. 
My next and last objcet is to en- 
quire, whether the influence, which 
in all these experiments immediate- 
ly excites the muscles to act, be 
electrical or not. 
The points of difference between 
any two species of natural bodies, 
even those which, from the similarity 
of some of their most obvious quali- 
ties have once been thought the 
same, are found, upon accurate ex- 
[*69 
amination, greatly to exceed in 
number those of their agreement. 
When, therefore, two. substances 
are known to have many properties 
in common, while their differences 
are few, and noneof these absolutely 
contradict such a conclusion, we 
infer with ‘considerable confidence, 
that they are the same, though we 
may not be immediately able-to 
explain why their resemblance is 
not complete. After Mr. Walsh, for 
instance, had discovered, that. the 
influence of the torpedo was trans- 
mitted by all the various bodies 
which are good conductors of the 
electric fluid, philosophers made 
little hesitation in admitting them to 
be one and the same. substance, 
though some of their apparent diffe~ 
rences could not then be accounted 
for. In like manner, the inquirers 
into the nature of the influence, the 
effects of which are so evident in 
Mr. Galvani’s experiments, have 
very generally, and in my opinion 
justly, allowed it to be electrical, 
on the ground that its conductors 
and those of electricity are altogee 
ther the same. To this, however, 
an objection has been made by Dr. 
Fowler, which, if well founded, 
would certainly prove them to be 
different substances ; -for he has as- 
serted that charcoal, which is so 
good a conductor of electricity, Te= 
fuses to transmit the influence, upon 
which the motions in Mr. Galvani’s 
experiments depend. In. reply, I 
shall only say, that Dr. Fowler.must 
have been unfortunate with respect 
to the charcoal he employed ; since 
all the pieces I ever tried, and I 
have tried many,’ were found to 
conduct this influence. 
Other arguments have likewise 
been urged against the identity of 
the two influences; all of which, 
[* E 3] how- 
