554  PEOFESSOE  PLtJCKEE  THE  ]MAaXETIC  IXOrCTIOX  OF  CETSTALS. 
Hence  there  is  proved  here  also  the  existence  of  three  axes  of  paramagnetic  induciion, 
enjoying  the  property,  that  the  crystal,  when  vertically  suspended  along  any  one  of 
them,  sets  one  of  the  remaining  two  axes  axially,  the  other  equatorially.  In  the  case 
of  our  salt,  the  normal  to  the  plane  of  symmetry  is  the  axis  of  '^mean  induction  (5). 
The  axes  of  greatest  and  least  induction,  {a)  and  (c),  both  lie  in  the  plane  of  symmetry, 
the  first  forming  an  angle  of  3°  vfith  the  normal  to  the  cleavage  plane. 
We  may  again  conceive  an  ellipsoid  consisting  of  an  amorphous  paramagnetic  substance, 
having  three  unequal  axes  coincident,  according  to  their  length,  with  a,  h,  c.  Then,  in  all 
the  difierent  modes  of  suspension,  the  crystal  will  be  dii’ected  like  such  an  ellipsoid. 
21.  The  two  magnetic  axes  are  to  be 
sought  for  within  the  plane  of  symmetry 
containing  the  greatest  and  the  least  axis  of 
induction  («),  (c).  Therefore  a circular  plate 
was  prepared,  with  a diameter  of  nearly  0’3 
of  an  inch,  bounded  by  planes  perpendicular 
to  the  symmetric  plane  ABCD  (fig.  19),  and 
inclined  to  the  cleavage  plane  (the  base  DC  of 
the  primitive  oblique  prism)  22^°.  Horizon- 
tally oscillating,  this  plate  set  the  plane  of 
symmetry,  marked  by  a line  on  its  upper  base, 
equatorially,  as  a plate  bounded  by  cleavage 
planes  does.  After  some  trials  we  prepared 
a similar  plate,  II.,  substituting  only  an  angle 
of  28°  for  the  above  angle  of  22^°.  Such  a 
plate,  horizontally  oscillating,  was  not  sen- 
sibly directed  by  the  poles.  Hence  the  two  magnetic  axes,  QQ  and  Q'Q',  of  formiate 
of  copper,  lie  in  the  symmetric  plane,  including  an  angle  of  about  50°,  bisected  by  the 
above  determined  axis  of  greatest  induction  («). 
II.  On  finite  ellipsoids  influenced  hy  an  infinitely  distant  pole. 
22.  We  have  shown  that  a natural  prism  of  ferridcyanide  of  potassium,  for  instance, 
oscillating  between  the  two  poles  of  a magnet,  is  kept  by  them  in  a fixed  position, 
dependent  solely  upon  the  direction  of  its  crystallographic  axes  mth  regai'd  to  the  axis 
of  suspension.  It  will  invariably  retain  the  same  position,  whatever  part  we  may  take 
away  from  it,  whatever  the  shape  of  the  remaining  fragment  may  become.  From  this 
fact  we  may  conclude  that,  in  the  case  of  this  salt,  the  pai’ticles  of  the  infiuenced  crystal 
do  not  act  sensibly  on  each  other,  and  hence  deduce  that  the  dii’ection  of  the  whole 
mass  of  the  crystal  is  plainly  defined  by  the  action  upon  a single  one  of  its  molecular 
particles.  According  to  the  above-described  experiments,  we  may  infer  too,  that  such  a 
particle  is  acted  upon  like  a certain  amorphous  ellipsoid  consisting  of  the  substance  of 
the  crystal.  Again,  the  action  of  a magnetic  pole  upon  a single  molecule  is  throughout 
Fig.  19. 
