404 SCIENCE PROGRESS 



on the one hand to the solid state on the other, the dextro- 

 rotatory power gradually gives place to the Isevo-rotatory 

 power. Dilution, rise of temperature, or " fixing " the hydroxy! 

 groups by the addition of boric acid to form boro-tartaric 

 acid, 



yO — CH — COOH 



HO B I 



^O CH — COOH 



reduces and finally destroys the Isevo-rotatory power. Cooling, 

 concentration, solidification, or crystallisation reduces and 

 finally destroys the dextro-rotatory power. 



The crystalline structure of tartaric acid offers a simple 

 but real physical explanation of all these facts. We have 

 already indicated the two opposing spiral formations which 

 are necessary and sufficient to account for the anomalous 

 rotatory dispersion. The perfect cleavage of the crystals 

 is a manifestation that the structure is ruptured across the 

 junctions of the hydrogen atoms of the hydroxyl groups. 

 In the complete structure these hydroxyl groups are linked 

 together in such a way as to form a twisted arrangement of 

 atoms. The act of solution or any departure from the crystal- 

 line form is sufficient to destroy the twisted arrangement. 

 But the fact of crystal growth itself points to an initial leaning 

 on the lart of the hydroxyl groups towards such an arrangement 

 as holds in the crystalline form, and it is fairly safe to assume 

 that the tendency of the molecules to build themselves up 

 into a lattice exists even in dilute solutions. There will 

 always be present strings of molecules, connected together 

 at the hydrogen junctions, which will show to a certain extent 

 those properties which are completely manifested only in the 

 crystalline state. On dilution, ionisation and hydration will 

 break up these strings more and more, and the dextro-rotatory 

 effect of the nucleus will increasingly predominate. 



In the paper mentioned above it is shown that it is impos- 

 sible by the diffraction of X-rays to detect directly the difference 

 between the dextro- and laevo-forms of the same chemical 

 compound. 



Some far-reaching theoretical considerations regarding the 

 true significance of crystal-structure have just been brought 

 forward by G. Shearer (Proc. Phys. Soc, now in press). They 

 mark the latest development and, in fact, a new era in the 

 analysis of crystalline form. The connection between the 

 symmetry of the molecule and of the lattice and the number of 

 molecules contained in each unit cell is stated clearly in a few 

 fundamental principles. This aspect of the subject has been 

 already hinted at in the discussion above of the crystalline 



