m 



are mutually replaced and the former divided by 3, we get a 

 new axial ratio: 



a' :b' -.c' = 0-46813 : 1 : 0-71922. 



approaching very much to the axial ratio of the aeschynite. 

 According to Frankenheim M as great a correspondence may 

 be obtained by replacing the «-axis by the c-axis in the poly- 

 mignite, and this seems to be the more natural course, as 

 thereby an almost complete identity between the forms of the 

 two minerals may be obtained. If next the same transforma- 

 tion is made as in the aeschynite, we get for polymignite 



a:b :c = 0-38408 : 1 : 0-35602. 



Derbylite has the axial ratio 



a:b :c = 0-9661 : 1 : 0-5502 



by a very simple transformation we may get 



a' :b' :c' = ^c:b:\a = 0-3668 : 1 : 0-3220 



that is to say, an axial ratio approaching those of aeschynite 

 and polycrase very much. 



As shown above the erikite has the axial ratio 



a:b:c = 0-57552 : 1 : 0-75796 



as no single direction in the crystals is especially predominant 

 over the others, we may, by a replacing of the b- and c-axis 

 get a new axial ratio: 



a! -.b' -.c' = \a\c\\b = 0-3796 : t : 0-3298 



being not very far from the majority of the preceding minerals. 

 According to Winther^) the axial ratio of the britholite is 



a:b : с = 0-620 : 1 : 0-423 



1) Pogg. Ann. 91. 1855 p. 372. 

 ») Medd. om Grønl. XXIV, p. 192 



