184 



phism. They should all be explained by the repeated twinning of 

 pseudo-symmetrical space-lattices and their combination into aggregates 

 of apparently higher symmetry x ). 



10. It may be asked if any indication of such lamellar structure 

 in crystals showing optical anomalies, is really found? Experience 



has indeed plainly established its existence, 

 as by some more detailed examples may 

 be demonstrated in the following. 



One of the instances of this kind 

 most studied 2 ) is potassium-ferrocyanide : 

 KI {Fe(CN) 6 } , the optical anomalies of 

 which were discovered by Brewster. The 

 crystals are monoclinic, but they are so 

 nearly tetragonal, that for a long time they 

 were considered really to belong to the 

 last mentioned system. 



Indeed the axial ratio is: a : b : c = 

 0,3947 J ' >39 S 3> with /3 = 90!'; from 



these numbers the approximate tetragonal character of the space- 

 lattice is immediately clear. 



The optical properties are those of a biaxial crystal; the rather 

 large angle of the optical axes is about 120 for sodium-light, and 

 the character of birefringence is positive. In compound crystals 

 a plate parallel to the planes of {010} appears between crossed nicols 

 to be divided into four sectors (fig. 143), two of them diametrically 

 opposed of negative, the other two of positive character. The 

 boundary-lines of the fields are parallel to the edges of the qua- 

 dratic plates. In every two adjacent sectors the planes of the optical 





Fig. 143- 



Potassium-ferrocyanide. 

 Plate parallel to (010). 



1) That this explanation of "polymorphism" cannot always hold, is evident, 

 as has already been recognised by Mallard himself. The mere difference in specific 

 weight between the two modifications, as it is observed frequently, cannot be plau- 

 sibly explained by his theory. However there is a certain number of substances 

 which behave just as Mallard's view demands. At present these substances are 

 classified as those, which are pseudo-symmetrical, to distinguish them from those 

 in which true dimorphism occurs. Cf. P. Groth, Einleitung in die Chemische 

 Krystallographie, (1904), p. 4 7; G. Wyrouboff, Bull, de la Soc. Miner. 29. 

 335. (1906); F. Wallerant, ibid. 24. 159. (1901); 27. 184. (1904). 



2) G. Wyrouboff, Ann. de Chim. et Phys. (4). 16. 293. (1869); 29. 335. (1906); 

 R. Brauns, Die optischen Anomalien, (1891) ; p. 58; G. W. Wulff, Verh. der Kais. 

 russ. Miner. Ges. Petersburg, (2). 29. 65. (1892); A. Karnojitzki, Zeits. f. Kryst. 

 19. 571. (1891) ; Russ. Berg-Journ. (1892) No. 10; Ref. Zeits. f. Kryst. 24. 512. (1895). 



