400 SCIENCE PROGRESS 



The present author has recently commenced an investigation of 

 several members of the other great division of organic chemistry, 

 the fatty series. 



In view of the interest which attaches to it in chemistry, 

 physics, and crystallography, and its association with the 

 great names of Biot and Pasteur, the problem of tartaric acid 

 was attacked first {Proc. Roy. Soc, now in press). The crystals 

 of this compound belong to the monoclinic sphenoidal class, 

 that is, they possess a dyad axis of symmetry only. Conse- 

 quently the unit cell is shaped like that of naphthalene in 

 Fig. 2, but the plane of symmetry parallel to OCGA is absent. 

 The crystallographic data are : 



a : b : c = 1-2747 : i : 1-0266, /3 = 100° 17', sp. grav. = 1-759. 



X-ray analysis shows at once that there are two molecules 

 to the unit cell, one at each of the corners and one lying some- 

 where along the line passing through the centres of the faces 

 OCDB, AGEF. Because of the position of this second mole- 

 cule, the observed lengths of the b and c axes are half the 

 calculated lengths. The accurate lengths of the three axes 

 are 



rt = 7-693 A.U., ^ = 6-037 A.U., c = 6-195 A.U. 



All the other observed spacings agree with the theoretical 

 values, which fact means that either (i) the second molecule 

 does not lie exactly half-way between the faces OCDB, AGEF ; 

 or (2) if it does, its orientation is quite different from that of 

 the molecules at the corners of the cell ; or (3) both (i) and (2) 

 hold. As a matter of fact, consideration makes it clear that 

 if the tartaric acid molecule itself is completely devoid of 

 symmetry, a single unit cell, with molecules at each of its 

 corners, cannot possibly possess a dyad axis of symmetry. 

 For the completed structure to show this element of symmetry, 

 it requires the presence of two interpenetrating monoclinic 

 lattices, either of which may be obtained from the other by 

 a certain displacement and a rotation through 180°. Either 

 of these two lattices alone is asymmetrical, but once they inter- 

 penetrate and extend through space, the appearance of the 

 final arrangement is clearly unaltered by a rotation through 

 1 80° about the b axis. And that is what we mean when we say 

 that tartaric acid shows a dyad axis. The second molecule 

 must be the same as those at the corners of the cell, but 

 rotated through 180° about the dyad (b) axis, because it is 

 part of the second simple monoclinic lattice which inter- 

 penetrates the first and may be obtained from it by a certain 

 displacement and a rotation through 180° about the dyad 

 axis. 



