PEOFESSOE  PLtlCKEE  ON  THE  MAGNETIC  INDUCTION  OE  CETSTALS.  581 
symmetrical  plane,  and  mark  within  this  plane  the  directions  both  axial  and  equatorial. 
These  two  directions,  representing  the  two  axes  of  induction  within  the  symmetrical 
plane,  are  not  at  all  indicated  by  the  crystalline  form ; they  are  to  be  found  by  experi- 
ment. The  crystal,  when  successively  suspended  along  each  of  the  three  axes  of  para- 
magnetic induction,  sets  one  of  these  axes  twice  axially,  another  one  twice  equatorially. 
In  the  case  of  paramagnetic  induction,  the  first  axis  is  the  longest,  the  second  the  short- 
est. In  the  case  of  diamagnetic  induction,  the  second  axis,  pointing  twice  equatorially, 
is  the  longest,  the  first  the  shortest  one.  In  both  cases,  the  two  magnetic  axes  lie 
in  the  plane  passing  through  these  two  axes  of  induction.  Therefore  the  plane  con- 
taining the  two  magnetic  axes  is  either  coincident  with  the  symmetrical  plane,  or  is 
perpendicular  to  it.  We  may  distinguish  three  different  cases,  whether  the  crystal  be 
paramagnetic  or  diamagnetic : 1.  the  axis  of  greatest,  2.  the  axis  of  least,  3.  the  axis  of 
mean  magnetic  induction  is  perpendicular  to  the  plane  of  symmetry.  In  the  first  two 
cases,  the  plane  containing  the  two  magnetic  axes  is  perpendicular  to  the  symmetrical 
plane,  in  the  third  case  it  falls  within  it. 
First  case  . 
Second  case. 
Third  case  . 
Paramagnetic  crystals. 
JDiopside. 
lEed  ferridcyanide  of  potassium. 
JFormiate  of  copper. 
lAcetate  of  copper. 
Diamagnetic  crystals. 
Hyposulphite  of  soda  (NaS). 
Acetate  of  soda. 
Acetate  of  lead. 
77.  The  crystals  whose  primitive  form  can  be  referred  to  a right  prism  with  a 
rhombic  base  have  three  planes  of  symmetry,  intersecting  each  other  along  the  three 
axes  of  magnetic  induction.  We  can,  as  we  did  in  the  former  case,  determine,  by 
suspending  the  crystal  successively  along  each  of  these  axes,  which  of  them  is  the 
longest,  mean,  and  shortest.  The  two  magnetic  axes,  always  to  be  sought  within  the 
plane  of  the  longest  and  the  shortest  axis  of  induction,  lie  in  any  one  of  the  three 
planes  of  symmetry.  We  may  distinguish  six  different  cases,  as  well  of  paramagnetic 
as  diamagnetic  induction. 
We  shall  denote,  as  we  did  in  the  first  section,  the  axis  of  the  right  prism,  the  shorter 
and  the  longer  diagonal  of  its  base,  by  a,  z,,  X,  the  longest  axis  of  the  ellipsoid  of  induc- 
tion (art.  47.)  being  always  a^,  the  mean  and  the  shortest  (f.  The  six  cases  above 
mentioned  may  then  be  enumerated  thus : — 
b'^  & 
falling  within 
I.  K a X paramagnetic.  Sulphate  of  nickel. 
, Sulphate  of  nickel  and  zinc. 
diamagnetic,  Seignette’s  salt. 
2i.  X a X 
