550  PROFESSOE  PL1JCE3]E  THE  ]MAGXETIC  IXDrCTIOX  OF  CHTSTALS. 
From  r=0  till  r=a;  it  gradually  decreases,  and  finally  altogether  disappears;  from  T=a/ 
till  r=90°  it  increases  again. 
The  angle  &;  may  be  taken  on  either  side  of  the  axis 
Hence  there  are,  within  crystallized  ferridcyanide 
of  potassium,  two  different  directions  (QQ  and  Q'Q', 
fig.  13),  enjoying  each  the  property  that  the  crystal, 
when  suspended  along  it  between  the  two  poles  of  a 
magnet,  is  acted  upon  like  an  amorphous  paramagnetic 
body.  It  merely  points  in  conformity  with  its  exterior 
shape ; when  symmetric  round  its  axis  of  suspension, 
it  is  not  directed  at  all.  These  two  directions,  lying 
in  the  plane  which  contains  the  axis  of  the  primitive 
prism  (a)  and  the  shortest  diagonal  of  its  base  (%.),  i.  e. 
the  axes  of  greatest  and  least  paramagnetic  induction, 
shall  be  called  the  magnetic  axes  of  the  crystal.  The 
angle  between  them  is  about  140°,  it  is  bisected  by 
the  crystallographic  axis  z. 
h.  When  the  same  circular  cylinder  oscillates  be- 
tween the  two  poles,  its  axis  being  horizontal,  this  axis 
points  generally  in  an  oblique  direction.  The  cylinder 
rotating  round  it,  two  positions  will  be  found  where  the  axis  points  either  axially  or 
equatorially : it  depends  upon  the  value  of  the  angle  r which  of  these  two  cases  takes 
place.  In  this  position  the  crystallographic  axis  X lies  in  the  horizontal  plane.  ^^Tien  the 
same  axis  (X)  is  vertical,  the  deviation  of  the  axis  of  the  cyhnder  from  the  equatorial  or 
axial  line  is  a maximum,  the  angle  between  this  axis  and  the  axial  hne  being  r.  Between 
the  two  mentioned  cases,  where  the  axis  of  the  rotating  cylhider,  according  to  the  value 
of  r,  passes  either  through  the  axial  or  equatorial  line,  there  ought  to  be  an  hitermediate 
case,  in  which  the  rotating  cylinder  passes  through  such  a position  that  the  directing 
power  emanating  from  the  poles  becomes  uncertain.  It  takes  place  if  r equals  about  20° 
( = 90  — co);  then  the  magnetic  axis  passes  through  the  vertical  line.  Hence,  when  the 
angle  r varies  from  about  —20°  to  20°,  the  axis  of  the  rotating  cylinder  passes  thi-ough 
the  axial  line;  when  r varies  from  about  20°  to  160°,  the  axis  passes  through  the  equa- 
torial line.  On  this  passage  the  directing  power  emanating  fi’om  the  poles  diminishes 
when  we  approach  to  the  intermediate  case,  where  it  is  zero. 
10.  In  looking  over  the  above-described  results  obtained  by  operatmg  on  prisms, 
cyhnders,  and  plates  cut  out  of  crystallized  ferridcyanide  of  potassium  in  various  dffec- 
tions  with  regard  to  the  primitive  form  of  this  salt,  and  suspended  in  different  ways,  we 
may  easily  remark,  that  all  the  observed  positions  of  the  crystals  between  the  two  poles 
are  exactly  the  same  as  those  of  an  ellipsoid  made  from  uncrystallized  ferridcyanide  or 
any  amorphous  paramagnetic  substance,  and  suspended  along  its  different  diameters.  The 
three  axes  of  this  ellipsoid  are  to  be  supposed  unequal ; they  are  coincident,  according  to 
Fig.  13. 
c 
