148 PHYSIOLOGY CHAP. 



out that this relation, as expressed by Hay craft, must have been 

 different if he had considered the taste of the salt as a whole, 

 and not that of the kations. Heiiitzka employed Earnsay's 

 scheme to bring out the relation between the taste of the kations 

 and their position in the periodic system. From his researches 

 as a whole he was led to formulate the hypothesis that the 

 excitation of the peripheral taste organs by means of the salts 

 is produced (like the excitation of other elements of the body) 

 by a change in the state of the colloids, with alteration of their 

 electrical potential. 



According to Herlitzka the metallic sensation (metallic taste, 

 more correctly metallic smell) is characteristic only of a small 

 number of salts, all belonging to the heavy metals ; and that 

 only when the metal is present in the form of elementary ions, 

 never in the form of complex ions. With some metals, moreover, 

 the metallic smell is perceived when they are present in the form 

 of a certain ion, but is absent with forms of ions of a different 

 valency. According to this author the threshold of stimulation 

 for the metallic sensation is extremely low, and varies for the 



different salts between QQQQ and ^ * ; their solutions not only 



have no taste, but many of them do not give the characteristic 

 reaction of the respective ions. In these the molecule is com- 

 pletely dissociated. For the salts of the weak acids in which the 

 process of hydrolysis is more active, the threshold is slightly 

 higher. 



The metallic smell consequently appears to be a property of 

 elementary, dissociated ions. 



According to von Frey the alkaline taste (smell) depends on 

 the liberation by the alkalis of the volatile bases (methylamine) 

 that result from the products of the decomposition of the epi- 

 thelium from their salts. 



VI. Special attention should be paid to the fact that the 

 end-organs of taste differ specifically from the other sense organs 

 possessed by the tongue in common with the skin, in being 

 incapable (so far as is known) of excitation by mechanical and 

 thermal stimuli. Among inadequate stimuli of the taste sense 

 we need therefore only consider the action of the electrical current. 



Sulzer (1752) first noted that on applying two different metals 

 to the tongue a special gustatory sensation resulted which he 

 compared to the taste of rust. Volta (1792) repeated Sulzer's 

 experiment without knowing of it, and found that the special 

 taste was due to the passage of an electrical current ; in fact 

 he obtained the same effect on stimulating the tongue with 

 the current from his pile. Volta inclined to the opinion that 

 the electricity acted directly upon the taste organ. A few years 

 later Humboldt (1797) suggested that the electrical taste was 



