241 
of Edinburgh, Session 1863 - 64 . 
And by the formulge (2), Art 5. 
a b' c 
^3 1 tan a ’ 
tan = ^3 , 
1 
tan a 
^ tan a’ 
1 
cot a. 
tan a, = — tan a . 
■ V3 
(«,) 
a^=60°; c6^ = |-o5. 
The other angular element a, is given by equation (a.) 
Hence one of the angular elements is always equal to 60°. 
15. The results of this investigation may therefore be summed 
up by the following statement : — All crystals included in the rhom- 
bohedral system are merely the particular forms which crystals of 
the prismatic system assume when one angular element is equal 
to 60°. 
16. Tables are given in the paper containing the symbols of the 
forms of the prismatic system corresponding to the forms observed 
in the various minerals belonging to the rhombohedral system. 
The following is an example selected from the tables 
Apatite. 
Angular elements — 
100, 110-60°; 010, 011-34° 20'; 001, 101-40° 13'. 
Simple forms — 
100, 010, 001, oil, 101, 110, 111, 012, 021, 102, 201, 310, 112, 
114, 311, 312, 314. 
The following forms are alwaj^s hemihedral with parallel faces — 
210, 130, 320, 350, 510, 910, 121, 122, 532, 534, 712, 714, 615, 
3 7 10, 9 5 10. 
Cleavage — 
100 , 001 , 110 . 
001 not so easily obtained as 100, 110. The angles between the 
faces are of course the same whatever be the symbols used to re- 
present them. For Apatite they are, therefore, those given on p. 
485 of Miller’s ‘‘ Mineralogy.” 
