354 Dijcwery of Gaham. [Book IX. 



nutely into the caufe of thefe contractions, he found 

 that he could produce them at pleafure, by touching, 

 the animals with two different metals at the fame time 

 in contact with each other. 



From later obfervations it feems to be fufficiently 

 afcertained that thefe involuntary contractions cannot 

 be excited by any fubftances whatever, whether folid 

 or fluid, except the metals, and that the mutual con- 

 tact of two metals with each other is, in every cafe, 

 necefTary to the effect. Zinc has been found by far 

 the mod efficacious, efpecially when in contact with 

 gold, filver, molybdena, fteel or copper, although 

 thefe latter excite feeble contractions when in contact 

 ,. only with each other. Next to zinc, tin and lead feem 

 to be the befl exciters. When the pieces of metals 

 employed, and the furface of the animal fibre with 

 which they are in contact, are large, the contractions 

 excited are in general more confiderable, but by no 

 means in proportion to thefe circumftances. 



In order to obferve the phenomena in queftion, cut 

 off the head of a frog. When it has ceafed to flruggle, 

 apply a plate of zinc under its body, and a plate of 

 gold to the fuperior furface. Then Hide the gold plate 

 ' till it comes in contact with the zinc, when the mufcles 

 which are further from the brain and fpinal marrow 

 than the metals, will be vifibly agitated. 



This effect will take place, although the frog, with 

 the metals are placed on an inverted glafs jar, and a 

 Hick of fealing-wax is fnterpofed between -the hand 

 of the operator and the metals, that is, although the 

 animal as well as the metals is infulated. I men- 

 tioned gold as being the moft' powerful of the metals, 

 but a plate of filver, a crown-piece for inftance, will 

 anfwer the purpofe. 



Cut off the thigh of a frog, juft killed, clofe to the 

 i body, 



