HISTORY OF GALVANISM. 17 



cessary that a body from one of these classes be 

 placed between two bodies from the other class ; by 

 this means the equilibrium is destroyed, and it is 

 again restored when the two are united by a con- 

 ductor. The same effects proceed from this ar- 

 rangement as when the equilibrium is destroyed 

 by the contact of the two metals ; but two con- 

 ductors of the same class, whatever be their dif- 

 ference, are not sufficient to form a circuit with- 

 out the intervention of one of the other class. 

 For the purpose of illustrating his theory, he 

 brings forward what he calls an experimen- 

 tum crucis ; he applies the ends of two metal- 

 lic rods, one formed of silver and the other of 

 tin, to the two disks of Nicholson's doubler, and 

 unites their other ends by a moist conductor ; after 

 they have remained for some time in this situation, 

 he finds that the disk in contact with the silver 

 rod indicates the positive electricity, and that in 

 contact with the tin the negative. As was the 

 case in his former paper, he does not attempt to 

 explain the cause of this change in the electric 

 state of the substances, but is satisfied with 

 pointing it out as a matter of fact which he had 

 discovered, and as constituting a well-established, 

 although empirical law of electricity.* 



A very ample and elaborate memoir on the Committee 

 subject of animal electricity was drawn up by a French in- 

 cbmmittee of the French Institute, which, besides JViS te> 



* Ann. de Chim. xxiii. 276. 

 c 



