ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 401 



The facts afforded by the X-ray observation of tartaric 

 acid are thus co-ordinated and linked up with the symmetry. 

 The exact position of the second molecule is then fixed very 

 nearly by measurement of the relative intensities of the various 

 orders of reflection from the faces OCDB, AGEF in Fig. 2. 

 It is found to lie just to the right of the centres of these faces. 

 There remains now the means by which the two interpene- 

 trating lattices are bound together. It would be impossible 

 to give here a detailed account of the investigation of this 

 part of the problem. The salient facts only may be enumerated . 

 The molecules, of which the chemical structural formula is 

 {a or a') 



lie lengthwise along the a axis, the length of the molecule 

 fitting almost exactly with the observed probable " diameters " 

 of the various atoms. The perfect cleavage (parallel to OCDB 

 in Fig. 2) thus passes directly across the junctions between the 

 hydrogens of the hydroxyl groups, a fact which at once recalls 

 the similar cleavage in those organic bodies investigated by 

 Bragg. The remaining junctions are effected between the 

 ( — H)'s and the ( || 0)'s, the ( — H) of each molecule of one 

 lattice linking up diagonally across the faces parallel to OCDB 

 (Fig. 2) with the ( || O) of each molecule of the other lattice 

 (see Fig. 5). 



But now let us consider the most important part of the 

 structure, the dispositions of the various atoms constituting 

 each molecule ; for in these must lie the explanation of those 

 characteristic properties of tartaric acid which have so long 

 excited scientific interest. As Pasteur pointed out over sixty 

 years ago, whatever the arrangement of these atoms, it must 

 be so ordered that an alternate arrangement is also possible, 

 such that the relation between the two forms is as the relation 

 of an object to its mirror-image. For tartaric acid, both in 

 solution and in crystals, shows two distinct forms, one of 

 which rotates the plane of polarisation of light to the right 

 and the other to the left. Corresponding to this purely optical 

 dimorphism is the crystallographic property of existing in 

 two enantiomorphous forms, that is, in two forms, either of 



