278 



and where it is a characteristic property of their structure. The 

 rotatory power of the last mentioned group may be quite different, 

 and in opposite directions in the crystals and in the liquid state, 

 as far as the rotation of the crystalline substance is an inherent 

 property, and not caused by pseudo-symmetrical intercalation. 



Thus, according to Wyrouboff 1 ), while for instance the 

 rubidium-salt of dextrogyratory tartaric acid deviates the plane of 

 vibration of the polarised light to the right, if dissolved in water, 

 the crystals of the substance are nevertheless laevogyratory. It is 

 difficult to give a rational explanation of such facts, as the 

 hypothesis of Liebisch 2 ), who attributes the phenomenon to 

 the circumstance that in aqueous solution the salt is evidently 

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

 be maintained, considering Rimbach's measurements 3 ) on the 

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

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

 totally dissociated, of <x, + 19,51, The crystals, however, are 

 strongly laevogyratory: # being: 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 4 ), in the same way 

 as rotatory power is exhibited by the wellknown mica-piles of 

 Von'Reusch. In the previous chapter (p. 186, etc.) we have dealt 

 witht hese facts more in detail. 



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

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



J ) G. Wyrouboff, Journ. de Phys., (-3), 3, 451, (1894). 



2 ) Th. Liebisch, Gvundzuge der physik. Krystallo graphic, (1896), p. 428. 



3 ) E. Rimbach, Zeits. f. phys. Chemie, 16, 671, (1895). 



4) G. Wyrouboff, Ann. de Chim. et Phys., (4), 8, 412, (1886). E. Mallard, 

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



