FERMENTATION OF STEREO-ISOMERS 141 



e. g. the /3-methylglucoside from cZ-glucose is easily con- 

 verted by emulsin into methyl alcohol and grape-sugar 

 whilst the 0-methyl -glucoside is unchanged. The com- 

 parison of a lock and key was used to illustrate this stereo- 

 chemical influence 1 . Quite recently a further step in this 

 direction was taken by Marckwald and McKenzie 2 , since 

 they found that cZ-mandelic acid is esterified more quickly 

 than /-mandelic acid by laevorotatory menthol, whose 

 constitution is known (whilst the constitution of the 

 enzymes is still quite in the dark). 



3. Certain Atomic Groupings give rise to entirely new 

 Chemical Properties. 



The relations between chemical properties and constitu- 

 tion may, in many cases, be referred satisfactorily, though 

 only in a qualitative manner, to the action of rules : that 

 elements bring into complex compounds those properties 

 which they show in their simple derivatives, but are 

 modified in their action by the other elements already 

 present in the molecule. That this amounts only to a pre- 

 liminary insight is shown by the failure of all attempts to 

 give a quantitative form to the rules in question, expressing 

 the facts numerically. Thus, e.g. the heat of formation 

 cannot be determined accurately by addition (p. 103), because 

 the intensity of combination of two atoms is influenced by 

 the others present. These influences again can be grouped 

 and arranged under definite points of view, it is true, but 

 again cannot be predicted quantitatively ; rather, on this 

 point only certain approximate rules can be brought for- 

 ward, which in many cases fail. For instance, the effect 

 of chlorine on the strength of the chlorinated acetic acid 

 suggests a quantitative investigation. The affinity of 

 chlorine for negative electricity is known (p. 90), and must 

 stand in a quantitative relation to the increase in the 



1 See, inter alia, Zeitschr. f. Physiol. Chem. 26. 60. 



2 Bert. Ber. 32. 2130 ; Fischer and Windans, 1. c. 33. 345. 



