466 SCIENCE PROGRESS 



have different tastes, that of the ^-form being sweet, while the 

 /-isomer is insipid. Fischer and Hagenbach ^ state that laevo 

 leucine has a weakl}' bitter taste, while the dextro form is quite 

 sweet. A similar difference was noted by Menozzi and Appiani,^ 

 who found that rt'-glutaminic acid was sweet, while the laevo form 

 was tasteless. Glutaminic acid has the formula : 



NH,.CH.COOH 



CH, . CHj.COOH. 



In the case of mannose and glucose, the evidence is conflicting ; 

 for van Ekenstein ^ states that mannose is sweet, while glucose 

 is bitter; but this is contradicted by Neuberg and Mayer.^ 

 Sternberg ® considers that as a rule the difference between the 

 isomers is quantitative rather than qualitative. If this be 

 correct, it opens up a field for speculation. Marckwald and 

 McKenzie^ have shown that when an active acid is esterified 

 with a racemic alcohol, or vice versa, the reaction proceeds faster 

 with one component of the racemic body than with the other. 

 For instance, the}' found that laevo menthol combines more 

 rapidly with dextro-mandelic acid than with laevo-mandelic acid. 

 If we recall that Pasteur conceived nerves as asymmetric 

 organisms, the " quantitative difference " between the actions of 

 optical antipodes upon them might be explained by supposing 

 that the asymmetric nerve substance combines more readil}^ with 

 one antipode than with the other. This, however, is pure theory. 



The sense of sight is the last one with which we can deal. 

 Byk ^ has shown that a solution of potassium copper racemate 

 differs in tint from a solution of the potassium copper salt of 

 dextro tartaric acid. Thus, the only sense which is not yet 

 proved to be affected by optical isomerism is that of hearing. 

 Even this might conceivably be influenced by differences in 

 the rate of transmission of sound in active and racemic 

 substances ; but, so far, no experiments have been made in 

 this direction. 



We must now turn to another branch of the subject, and 



' Fischer and Hagenbach, Be?:, 34, 3764 (1901). 



- Menozzi and Appiani, Affi R. Accad. Li?icei [5], 2, II. 421 (1893). 



^ van Ekenstein, Rec. trav. chim., 15, 222 (1896). 



^ Neuberg and Mayer, Zeit. physiol. C/iem., 37, 545 (1903). 



* Sternberg, Archiv Anat. 11. Phys., 1898, 457. 



* Marckwald and McKenzie, Ber., 32, 2130 (1899). 

 '^ Byk, Zeit.physikal. Chetn., 59, 682 (1904). 



