STEREO-CHEMISTRY AND PHYSIOLOGY 4 53 



These represent the right-handed and left-handed varieties of 

 the grouping XCH(OH)COOH. But now we have still to 

 consider the grouping of the atoms in X, the second radicle 

 — CH(OH)COOH. They also may be arranged in dextro or 

 laevo form. Now, if we call the rotatory power of the one half 

 of the molecule A, we shall find that first we have 



+ A 



- A 



and to each of these we can add a dextro or a laevo form of the 

 second half molecule, which has also a rotatory power, A. This 

 gives us : 



Form (i) and form (4) represent dextro- and laevo-rotatory 

 bodies having a rotatory power of twice A. But when we 

 examine forms (2) and (3) we find that they have no rotatory 

 power at all, one half of the molecule counteracting the other 

 half, just as one molecule of dextro camphor counteracts a 

 molecule of laevo camphor and produces inactive camphor. The 

 inactivity of the tartaric acid, however, depends upon com- 

 pensation within the molecule itself, and the case is said to 

 be one of internal compensation. Internally compensated tartaric 

 acid is usually termed meso-tartaric for the sake of convenience. 

 The tetrahedron symbol does not lend itself to easy repro- 

 duction in printing, so other means were devised to represent 

 the configuration or space arrangement of atoms in a compound. 

 The method suggested by Fischer is the most generally employed. 

 To use his formulae, the tetrahedron model is first supposed to 

 be built up in the usual way ; and then, after laying it on a sheet 

 of paper, the positions of the groups of atoms are projected upon 

 the paper. As an example, we may draw the formulae of the 

 tartaric acids, giving a sketch of the tetrahedra in each case : 



