Experimental Philosophy. [Lecture 9- 



then interpose a piece of silver, as half-a-crown, 

 between the lower thigh and the nerves, so that 

 it may touch the former with one surface, and 

 the tin-foil which is wrapped round the spine 

 with the other, we shall find the lower leg con- 

 vulsively agitated, so as even sometimes to strike 

 against the hand which holds the other. 



Living animals, when thus placed between two 

 different metals which touch each other, will also 

 be convulsively agitated. Or you may make the 

 experiment upon yourselves in a very innocent 

 way, so that the taste and even the sight may be 

 affected by it. Take, for instance, a piece of 

 metal (zinc is the best), and lay it on^your 

 tongue, and another piece of metal, as a shilling 

 or half-crown under it, make the edges of the 

 two metals touch, and you will immediately ex- 

 perience a kind of irritation and a taste like cop- 

 per in your mouth. If, again, in a dark place 

 one of the metals is applied to your eye and the 

 other up your nostril or in your mouth, upon 

 bringing the metals in contact a faint flash of 

 white light will appear before your eyes. Nay 

 the same effect will be produced, and the light 

 will still appear, if one of the pieces of metal is 

 put up your nostril, and the other upon the 

 tongue ; or even if one is put between the upper 

 lip and the gums, and the other on the tongue ; 

 only remarking that the metals must be different 

 silver and zinc are the best for the purpose. 



These experiments have served to explain 



