which incites the Muscles of Animals to contract. 259 
muscular contractions by means of any disengaged electricity 
they contain. If my opinion were now asked, respecting the 
mode in which friction communicates such a power to them, 
I should say, that the part which has been rubbed is so far 
altered, in some condition or property, as to be affected dif- 
ferently, by the fluid exciters, from a part which has not been 
rubbed ; in short, that the rubbed part becomes, as it were, a 
different metal. There are two facts, beside those already 
mentioned, which support this conjecture. The first is, that 
when I have endeavoured to give an equal degree of friction 
to the two parts of the metal- whichT applied to the muscle 
and its nerve, little or no motion was excited by it ; so that it 
is reasonable to suppose, that, if precisely the same degree of 
friction were given to both the parts, no contractions would 
ever be produced by them, when used in this way. The se- 
cond 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 application of the rubbed metal between these simi- 
lar coatings will not be followed 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 re- 
specting the facts which have been mentioned, that very 
slight accidents may give the power of exciting contractions 
to a single metal, which had it not before ; and that we may 
hence easily account for the discordant testimonies of authors 
upon this point. 
Hitherto I have spoken only of the effects of friction upon 
metals. But to conclude this part of my subject, I must now 
