189 



Fig. 145. 



Potassium- ferrocyanide. 

 Plate parallel to 



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 may be demonstrated 

 in the following by some detailed examples. 



One of the instances of this kind most studied l ) is potassium- 

 ferrocyanide : K 4 {Fe(CN) Q } , 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 = 



>3947 ' * ' >3983> w i tn |3 = 90 1'; from 

 these numbers the approximate tetragonal 

 character of the space-lattice is imme- 

 diately clear. 



The optical properties are those of a 

 biaxial crystal; the rather large angle of 

 the optical axes is about 1 20 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. 145), 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 

 axes are perpendicular to each other; all four sectors become simul- 

 taneously dark between crossed nicols, if the sides of the quadratic 

 plate include an angle of respectively about 34 or 56 with the 

 planes of vibration of polarizer and analyser. 



The whole behaviour of these crystals is in full agreement with 

 the supposition that they are composed by monoclinic lamellae, 

 crossing at 90, and intercalated in such a way that an apparently 

 tetragonal crystal is produced. There cannot be the least doubt 

 as to the correctness of Mallard's view in this case. 



The same is valid for the case of autunite 2 ): Cfl(70 2 ) 2 (PO 4 ) 2 + 



J ) G. Wyrouboff, Ann.de Chim.et Phys., (4), 16, 293, (1896) ; 29,335, (1906) ;R. 

 Brauns, DieOptischen 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. W; Ref.: Zeits. f. Kryst., 24, 512, (1895). 



2) A. Brezina, Zeits. f. Kryst., 3,273, (1897); A. Madelung, ibid., 8, 75, (1884); 

 R. Brauns, loco cit., p. 63. 



