' Supplement to “ Nature,” June 9, 1923 ix 

which will make the result symmetrical on either side 
of a plane. Neither do you find a crystal, built on 
such a basis, to have right and left symmetry. The 
crystal of tartaric acid, investigation of the properties 
of which established the fame of Pasteur, is an excellent 

Fic. 11.—Tartaric acid, a monoclinic 
sphenoidal crystal, having an axis of 
digonal symmetry, AB, and no other 
axis or plane of symmetry. 
example. Its peculiar form is shown in Fig. 11. A 
recent publication by Mr. Astbury gives the proof 
that there are two molecules in the unit cell. Their 
mode of arrangement in terms of “ shoes,’”’ which are 
of one kind only, is shown in Fig. 12. The arrange- 

Fic. 12.—Arrangement of “shoes” of one kind only representing the 
arrangement of molecules in tartaric acid. 
ment of the atoms in one molecule, represented in 
Fig. 12 by one shoe, is shown in Fig. 13. A model 
of the crystal is shown in Fig. 14; and diagrammatic 
representations of the atoms in two adjacent molecules 
are given in Tig. 15 (a) and (4): one of these figures is 
right-handed and the other left. The most striking 

Fic. 13.—Molecule of tartaric acid. 
physical property of the crystal is its power of rotating 
the plane of polarisation of light which traverses it. 
It has long been guessed that there must be some 
spiral arrangement in the structure: and this is 
beautifully confirmed in the model. There are, in 
fact, two spirals. ‘This is somewhat unexpected, but 
it explains in a delightful way a property which has 
been obscure. One of the spirals is in the interior 
of the molecule itself and is certainly permanent when 
the crystal is dissolved. That accounts for the fact 
that tartaric acid in solution is “ active,” that is to 
say, can exercise its rotatory power. But the second 
spiral is a twist brought in by the necessity of fitting 

Fic. 14.—Tartaric acid. The small balls represent hydrogen atoms, the 
larger black balls oxygen, and the largest two grey and two black balls 
carbon. Scale of the model is 10% to 1. 
the molecules in their places. It is a peculiarity of 
the crystal structure, not of the molecule : it is a right- 
handed screw if the first is a left-handed screw, and 
vice versa. Also it appears to be more powerful in 
its effect on the light ; so that when the tartaric acid 


c H Qa -¢ 
‘a @’ C Carbon 
oo <a’ Oo Ox gen 
0° H c H Hydrogen 
0 4 


Cc H 
, 
ep (b) S 
ion J 
H7 %o 
Fic. 15.—Tartaric acid; (a) and (b) represent two molecules end to end; 
(a’) and (b’) show corresponding cross-sections with the side attach- 
ment. The two sets of figures are right and left to each other. 
as a crystal rotates light in one sense, in solution it 
rotates light in the opposite sense. Here, again, 
the intricate effects of the whole crystal are directly 
referred to structural details. 
It is to be observed that in this case there could 
be no question of the existence in the crystal of two 
molecules related to one another as right to left. For 
