582  PKOFESSOE  PLOCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CETSTALS. 
3.  a « X diamagnetic,  Aragonite. 
Sulphate  of  zinc. 
4.  k X a paramagnetic,  Staurolite. 
, Ferridcyanide  of  lead  (SPbGy  + FeGv^). 
, Sulphate  of  zinc  and  iron. 
, Sulphate  of  magnesia  and  iron. 
diamagnetic.  Anhydrite. 
, Hyposulphate  of  soda  (NaS). 
5.  a X « 
6.  X k 06  paramagnetic,  Ferridcyanide  of  potassium. 
diamagnetic.  Sulphur. 
, Citric  acid. 
In  the  cases  1 and  2,  the  two  magnetic  axes  lie  in  the  base  of  the  right  prism ; in  the 
cases  3 and  4,  in  the  plane  passing  through  its  axis  and  the  longer  diagonal  of  its  base ; in 
the  cases  5 and  6,  in  the  plane  passing  through  its  axis  and  the  shorter  diagonal  of  its  base. 
In  all  cases,  either  the  acute  or  the  obtuse  angle  between  the  two  magnetic  axes  is 
bisected  by  the  axis  of  greatest  magnetic  induction.  We  shall  count  the  angle  2a  from 
0°  to  180°,  this  angle  being  always  bisected  by  the  axis  of  greatest  induction.  [If  this 
angle  be  counted  up  to  90°  only,  we  may,  analogously  as  in  optics,  according  as  the 
acute  angle  between  the  magnetic  axes  is  bisected  by  the  axis  of  greatest  or  by  the  axis 
of  least  induction,  call  the  crystals  magnetically  positive  or  negative.] 
78.  In  crystals  having  one  straight  line  of  symmetry — primitive  form:  rhombohedron. 
prism  with  a hexagonal  base,  prism  with  a square  base — the  ellipsoid  of  induction 
becomes  a spheroid  whose  axis  of  revolution  coincides  with  the  line  of  s^Tnmetry.  This 
line  is  likewise  the  magnetic  axis.  We  meet  in  each  of  the  two  cases  of  magnetic  induc- 
tion with  two  classes  of  crystals.  Here  the  crystals  of  the  fii’st  class  shall  be  called 
positive,  of  the  second  class,  negative.  In  positive  crystals  the  line  of  symmetry  coincides 
with  the  axis  of  greatest  paramagnetic  or  diamagnetic  induction,  in  negative  crystals 
with  the  axis  of  least  induction.  A uniaxal  crystal,  when  suspended  along  any  ver- 
tical axis,  sets,  if  paramagnetic  and  positive  or  diamagnetic  and  negative,  its  magnetic 
axis  in  the  axial;  if  paramagnetic  and  negative  or  diamagnetic  and  positive,  in  the 
equatorial  plane. 
Paramagnetic  ct'ystals. 
Positive  (a^=¥).  Negative  (¥=¥). 
Carbonate  of  iron. 
Scapohte. 
Green  uranite. 
Sulphate  of  copper  and  calcium. 
Sulphate  of  magnesia  (containing  iron). 
Tourmaline. 
Beryl. 
Dioptase. 
Vesuvian. 
Sulphate  of  nickel. 
Chloride  of  ammonium  and  copper 
(AmCl-j-CuCl+2aq). 
