( 363 ) 



=: \ 111 \, very broad and very liib-trous ; id = * I 10 { , hiiphlly 

 reflecting, but niut'li narrower than o ; co = \ W Ï \ very narrow and 

 mostly wanting. The crystals are thin or flattened upon JOOlj, and 

 sometimes elongated along a side c : o. 



Angular values : 



c:0=: (001) : (Dl) =i- 

 C:/y^=:(001):(110)=-- 

 0:0= (111) : (111) =- 

 O: ,'« = (111): (110) = 

 m: ??i = (lll):(lll) = 



An exceedingly perfect cleavage is present along {OOlj. On |0()JJ 

 diagonal extinction ; in convergent polarised light a beautiful biaxial 

 interference- image is observed. The pseudoletragonal character of 

 the compound is also shown in the fact, that the first bisectrix stands 

 nearly perpendicular on {OOlj, and that the axial angle is exceedingly 

 small. The dispersion is extraordinarily strong, has a weakly-inclined 

 character, while Q <C i' ; the axial plane is |010|. The double re- 

 fraction is negative. 



After some time some of the clear crystals become locally opatpie, 

 like porcelain. 



Although monoclinic, the whole character of the 6'.v-compound is 

 so different from that of the /?6-compound, that an isomorphism of 

 the two salts cannot be assumed. 



Topic parameters : / : ip : to r=i 6,21 70 : 5,9585 : 5,7835. 



From the last motherliquor of the caesium-salt, are sometimes 

 obtained large, rudimentary crystals^ which are flattened prisms and 

 generally exhibit no well developed termiiuil planes; on analysis 

 they appear to contain a trifle more than the quantity of caesium ') 

 required, but to show accurately 6V:6V=1:6. 



They are also monoclino-prismatic ; the parameters are : 



a:b:c = 0/J968 : 1 : 0,2538 



(^ = 80° 187,' 



The forms observed are: /«==:|210j, broad and lustrous; /9=:|110|, 

 narrower, but brigiitly reflecting; a = \iOO\ and /> = {010}, about 

 equally broadly de\eloped as /j ; c = |00J | small and very lustrous; 

 t>=:|211| very lustrous and sometimes fairly well developed. 



M Th? ciYslals contain Cs only; this was proved speclroscopically. 



25 

 Proceedings Royal Acad. Amsterdam. Vol. XIV. 



