282 



large and lustrous; to = |lll|, small, but giving good reflexes; 

 =1 {lllj, larger than oj, but duller; ?«=: jllO}, verj small, yielding 

 sharp images. The habit of the crystals is elongated parallel to the 

 a-axis. 



Perfectly cleavable parallel to |()11|. 



The plane of the optical axes is !100|; probably the c-axis is first 

 bissectrix. 



§ 4. From these measurements it is obvious that both isomerides 

 possess the same symmetry, and, — within the limits of experimental 

 errors, — tlie same parameter a : b (= 0,4944). Such relations are 

 met with often in the case of polymorphic modifications of the same 

 substance ^). The identity of the cyclic structure in both molecules 

 compels us to believe that it is this cyclic of atoms common to both, 

 which determines tlie value of a:b. In no cases of polymorphism 

 where up till now such analogy in the values of one of the para- 

 meter-ratios was observed, there could be indicated the existence of 

 a reversible transition between the two modifications. In the case of 

 «- and i^-d-Fruciose-penta-acetates too, as Hudson and Braln demon- 

 strate, no such reversibility seems to exist either {inonotropy). It 

 may seem probable that in cases like this, no ordinary polymorphism 

 is present, but that in last instance the differences observed between 

 such modifications are always caused by true chemical isomerism, 

 in which a great part of the molecule is common to the two modifications. 



Laboratory for Inorganic and Physical Chemistry . 

 of the University. Groningen, Holland. 



1) F. M. Jaeger, Zeits. f. Kryst. u. Miner. 40. 131 . (19051 : thus in the case of « and ^- 

 1-3-4 Dinitro dieihylaniline ; of «- and ^^-Benzyl-phtalimide (ibid. 40. 371.(1905); 

 of «- and (i-Mannite, (Ghoth's Chem. Kryst. III. 431. (1910); etc. 



