VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 655 



that this was not owing to any want of virtue in the metal, I kept the same part 

 of it still in contact with the nerve, while I applied another part to the muscles; 

 immediately on which contractions were excited. 



5. Admitting now the limb of an animal to be in such an experiment com- 

 pletely insulated, and that the metal actually becomes electrical from the friction 

 it undergoes, surely a very few applications can only be required to place them 

 both in the same state with respect to the electric fluid; and when this happens, 

 all motions depending on the transflux of that fluid must necessarily cease. I 

 have found however, that a piece of metal which has been rubbed will excite 

 contractions, after it has been many times applied to the limb. In one instance 

 vigorous contractions were occasioned by the 200th application; and if I had 

 chosen to push the experiment further, I might certainly have produced many 

 more. I may mention also, as connected with this fact, that I have frequently 

 observed a piece of metal to excite motions, an entire day after it had been 

 rubbed. 



What I have said will probably be thought more than sufficient to prove that 

 metals, after being rubbed, do not produce muscular contractions by means of 

 any disengaged electricity they contain. If my opinion were now asked, re- 

 specting 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 con- 

 dition or property, as to be affected differently, 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 2 facts, besides those already mentioned, 

 which support this conjecture. The first is, that when I have endeavoured to 

 give an equal degree of friction to the 1 parts of the metal which I applied to 

 the muscle and its nerve, little or no motion was excited by it; so that it is rea- 

 sonable 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 2d is, that though 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 similar 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 on this head be admitted as just or not, it must yet be granted, as I 

 think I cannot be mistaken respecting the facts which have been mentioned, 

 that very slight accidents may give the power of exciting contractions to a single 

 metal, which it had not before; and that we may hence easily account for the 

 discordant testimonies of authors on this point. 



Hitherto I have spoken only of the effects of friction on metals. But to con- 

 clude this part of my subject, I must now remark, that charcoal, though from 



4 B 2 



