264 



dissociated, while in the crystalline state it is not, can hardly 

 be maintained considering Rimbach's measurements 1 ) on the 

 subject. This author found that rubidium-d-tartrate, if not dissociated, 

 shows in solution a rotatory power of: # = + 25,63; and if 

 totally dissociated, of at = -f 19,51, The crystals, however, 

 are strongly laevo-gyi&tory : a = - - 10,7, so that the influence 

 of the spiral molecular arrangement in the crystals far outweighs 

 that of the molecules themselves. 



With respect to group a, the absence of rotatory power in the 

 crystals of these substances may be considered to be a quite 

 accidental case, and of only secondary importance. As a rule we 

 may suppose that substances of -this kind will ordinarily behave 

 like those of group c. 



We have now therefore only to draw attention to the compounds 

 of group b. Their number is not very great, and of many of 

 them it could be shown to be at least probable, that the rotatory 

 power of their crystals is in numerous cases really caused by 

 pseudo-symmetrical intergrowth of lamellae 2 ), in the same way 

 as rotatory power is exhibited by the well-known mica-piles of 

 Reuse h. In the previous chapter (p. 181, etc.) we have dealt 

 with these facts more in detail. 



In every case, - - as soon as crystals of sodium-chlorate, sodium- 

 bromate, sodium-dihydrophosphatc , Schlippe's salt, sodium-uranyl- 

 acetate 3 ), etc. are dissolved in water, the optical activity entirely 

 disappears. From this the conclusion, has often been drawn that 

 the optical activity is merely due to the spiral arrangement of the 

 crystal-molecules, this involving a laevo-, respectively a dextro- 

 gyratory helicoidal assemblage which breaks down as soon as the 

 crystals are dissolved 4 ) . 



1) E. Rim bach, Zeits. f. phys. Chemie 16. 671. (1895). 



2) G. Wyrouboff, Ann. de Chim. et Phys. (4) 8. 412. (J886). E. Mallard, 

 Bull. Soc. Miner. 7. (1884); 8. (1885); cf. also Chapter VII. 



3) For sodium-w any I- acetate, however, the pseudo-symmetrical character seems 

 also to be proved beyond any doubt. 



i) It was discussed, however, by the author (Proceed. Kon. Acad. Amsterdam. 

 Vol. 19. (1917); Chem. Weekblad 14. 723. (1917), that in systems of true anti- 

 podes sometimes phenomena may be observed which are completely analogous to 

 those found in the case of sodium-chlorate; etc. The optically active forms of 

 potassium-rhodium-tri-oxalate show abnormal rotation-dispersion, their specific 

 rotation passing through zero for a wave-length of 5970 A. U. For light of this 

 special wave-length the ternary system of solvent and both antipodes behaves in 



