PEOrESSOE  PLiiCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CETSTALS.  547 
4.  When,  always  on  the  supposition  of  the  axis  a oscillating  horizontally,  the  shorter 
diagonal  of  the  rhombic  base  is  vertical,  the  longer  one  points  axially ; when  the  longer 
one  is  vertical,  the  shorter  one  points  axially.  Hence,  when  any  fragment  of  ferrid- 
cyanide  of  potassium  is  brought  between  the  two  poles,  rotating  round  any  one  of  its 
three  crystallographic  axes  a,  a,  X,  this  axis  being  vertical,  one  of  the  two  remaining 
axes  points  axially,  and  consequently  the  other  equatorially.  There  are  not  within  the 
crystal  any  three  other  directions  enjoying  the  same  property.  Oh  this  account  the 
three  crj'stallographic  axes  may  also,  in  the  case  of  our  salt,  be  called  the  three  axes  of 
paramagnetic  induction.  You  may  distinguish  these  three  axes  according  to  the  strength 
of  paramagnetic  induction,  this  induction  being  in  the  present  case  greatest  along  k,  mean 
along  A,  and  least  along  a. 
6.  YTien,  1st,  we  cut  out  of  a crystal  a cylinder  with 
a circular  base,  whose  axis  is  a,  we  may  expect,  that 
such  a cylinder  horizontally  suspended  will,  when  turned 
round  its  horizontal  axis,  retain  in  all  its  positions  the 
equatorial  direction  (fig.  3).  But  the  directing  power 
emanating  from  the  poles  is  not  the  same  in  the  different 
positions  of  the  rotating  cylinder ; it  is  greatest  when  k 
is  horizontal  and  X vertical ; it  is  least  when  X is  horizontal 
and  K vertical.  While  the  cylinder  rotates  from  the 
first  to  the  second  position,  this  power  gradually  diminishes. 
When,  2nd,  we  cut  out  of  the  same  crystal  a cylinder  whose  axis  coincides  with  the 
shorter  diagonal  «,  such  a cylinder,  however  you  may  turn  it  round  its  axis,  supposed 
horizontal,  will  always  point  axially.  The  position  agrees  with  the  paramagnetic  con- 
dition of  its  substance.  But,  contrary  to  this  condition,  it  retains  invariably  the  same 
position,  when,  by  diminishing  its  axis,  the  cylinder  is  transformed  into  a circular  plate 
(fig.  4).  Such  a plate  is  repelled  by  the  poles  with  different  energy,  this  energy  being 
greatest  when  a,  and  least  when  X oscillates  horizontally. 
Att. 
/CC 
Fig.  4.  Fig.  5.  Fig.  6. 
When,  3rd,  we  cut  out  of  the  crystal  a circular  cylinder  with  its  axis  parallel  to  X, 
such  a cylinder,  when  horizontally  suspended  between  the  two  poles,  points  either  equa- 
