146 SMELL, TASTE, ALLIED SENSES 



ferent metals are placed simultaneously on the tongue. 

 This observation was independently made by Volta in 

 1792 whO' believed these sensations to be produced by the 

 electrical stimulation of the organs of taste, for he ob- 

 tained the same effects by passing an electric current 

 through the tongue. Five years later, however, Humboldt 

 pointed out that the real stimulating agent in the 

 so-called electric taste might be the substances produced 

 by electrolysis at the region where the current passes 

 from the electrode into the tongue rather than the electric 

 current itself. Thus was established the two opposing 

 views concerning electrical taste. 



If an electric current is passed through the human 

 body in such a way that the anode is applied to the tongue 

 and the cathode to some other part, a sour taste develops 

 around the anode. If the electrodes are reversed in posi- 

 tion, an alkaline taste appears at the cathode. This con- 

 forms with what takes place when an electric current is 

 passed through an alkaline solution, such as the saliva; 

 hydrogen ions appear at the anode and hydroxyl ions 

 at the cathode. Why then are not these two substances, 

 the hydrogen and the hydroxyl, the stimuli for the char- 

 acteristic tastes ? 



But Rosenthal (1860) and, before him, Volta, found 

 that if the anode is a weak alkaline solution into which 

 the tip of the tongue is dipped, a sour taste nevertheless 

 arises, though the hydrogen ions under such a combination 

 might be expected to be neutralised immediately by the 

 hydroxyl present. Eosenthal also showed that if an elec- 

 tric current is passed through the bodies of two persons 

 and is completed by bringing the tip of the tongue of one 

 of these individuals into contact with that of the other, 



