80 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



If the upper and lower halves of each molecule be con- 

 sidered, it can be seen that in formulae (1) and (2) the upper 

 and the lower halves are not mirror images of each other ; both 

 upper and lower halves therefore represent the same optical 

 isomer. Which is actually the formula for dextro or for Ia3vo 

 tartaric acid is a matter of indifference, but both will be 

 optically active and their mixture will form racemic acid. 

 On the other hand, in formula (3) the upper and lower halves 

 of the molecule are related as object and mirror image and 

 represent therefore optically opposite groups. We have here 

 intra-molecular compensation and such an acid is optically 

 inactive ; it is known as meso-tartaric acid. These relations 

 of the four acids can be summarised as follows, d and I 

 being the opposite optical activities of the two portions of the 

 molecule : 



d I did 



II III 



d I d I I 



Dextro-rotatory Lsevo-rotatory Eacemio Meso-tartario 



The cases of the lactic and tartaric acids will serve to 

 illustrate the character of the isomerism which is to be found 

 in more complex substances and especially among the sugars, 

 a field of organic chemistry which has been worked out in 

 great detail, mainly by Emil Fischer. 



Before passing on to a brief sketch of the chemistry of the 

 sugars it is important that the reader should understand that 

 although the conception of space-isomerism owes its origin 

 to observations connected with the optical activity of sub- 

 stances, yet once the spatial arrangement of the atoms is 

 conceded, and the carbon atom considered always as being 

 the centre of a tetrahedral space, a number of conclusions 

 follow, quite unconnected with the subject of optical activity. 

 Remarkable relations have been found to obtain between the 

 structure of compounds and their stability, which become 



