PHYSIOLOGY OF GUSTATION 143 



What occasions the sweet taste of lead acetate seems 

 never to have been ascertained. On the other hand glu- 

 cinum chloride and glucinum sulphate, both of which 

 break into ions in water, have been shown by Hober and 

 Kiesow (1898) to owe their sweet taste to their common 

 constituent, the glucinum ion. Thus ions are one means 

 of exciting this taste. 



But the sweet taste, like the bitter one, is primarily 

 associated with organic compounds. It centers about the 

 alcohols and especially the sugars in much the same way 

 that the bitter taste does about the alkaloids. Although 

 the halogenated hydrocarbon chloroform and the aromatic 

 compound saccharine are both sweet, the latter about 500 

 times as much so as cane sugar, the great majority of sweet 

 substances are aliphatic alcohols and their derivatives. 

 Ethyl alcohol is sweetish in taste as well as the trihydric 

 alcohol glycerol, but the type of sweet substances is cane 

 sugar or sucrose. This can be tasted in aqueous solution 

 to about 0.02 molar; in weaker concentrations it is dim- 

 cult to distinguish it from pure water. Ethyl alcohol 

 cannot be tasted in solutions much weaker than 3 molar, a 

 relatively high concentration (Parker and Stabler, 1913). 



What determines the sweet taste in carbohydrates is 

 by no means settled. It apparently turns upon very 

 slight differences. These are sometimes sterioisomeric 

 in character. Thus, as already stated, dextro-asparagine 

 is sweet and laevo-asparagine is tasteless. Dextro-man- 

 nose is sweet and its stereoisomer dextro-glucose is bitter. 

 Other such examples are known. In some instances 

 slight changes in composition are accompanied by con- 

 siderable changes in taste. Thus, according to Thorns 

 and Nettesheim (1920), dulcin loses its sweetening power 



