548  PEOFESSOE  PLtJCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CETSTADS. 
torially  (fig.  5)  when  a,  or  axially  when  % is  vertical.  When  brought,  by  turning  it  round 
the  horizontal  axis  X,  from  the  first  position  to  the  second,  its  directive  power  at  first 
diminishes,  till  in  a certain  position  it  quite  vanishes;  and  finally,  when  the  cylinder 
has  passed  through  this  intermediate  position,  reappears,  and  increases  till  the  cylinder 
reaches  the  second  position.  The  cylinder,  when  setting  its  axis  equatorially,  is  directed 
contrary  to  the  paramagnetic  condition  of  its  substance ; when  axially,  in  conformity  with 
it.  When,  as  before,  we  reduce  the  cylinder  to  a circular  plate,  the  axis  remaining  the 
same,  the  change  of  direction,  by  turning  the  plate  round  its  horizontal  axis,  takes  place 
in  the  same  way.  But  then  in  the  first  position  the  cylinder  is  directed  like  a common 
paramagnetic  body,  in  the  second  position  (fig.  6)  like  a diamagnetic  body. 
6.  We  shall  now  describe  the  results  obtained  by  operating  with  circular  cylinders  cut 
out  of  crystals  of  ferridcyanide  of  potassium,  in  such  a way  that  their  axis  lies  in  one  of 
the  three  principal  planes,  which,  according  to  the  two  crystallographic  axes  they  con- 
tain, we  shall  denote  by  the  symbols  a»,  aX,  zX.  The  axis  perpendicular  to  the  principal 
plane  in  which  the  axis  of  the  cyhnder  lies,  may  be  marked,  before  experimenting,  on  its 
bases.  In  all  the  following  experiments  we  can  replace  the  circular  cylinder  by  a cir- 
cular plate  having  the  same  axis. 
7.  I.  Let  the  axis  of  the  cylinder  lie  in  the  plane  zX,  forming  with  z any  angle  §. 
a.  Whatever  may  be  the  angle  g>,  the  cyhnder,  when  oschlating  round  its  axis,  this 
axis  being  vertical,  takes  always  such  a position  that  the  plane  contaming  the  axes  z and 
X becomes  axial,  the  axis  a equatorial  (fig.  7). 
Lig.  7.  Fig.  8.  Fig.  9. 
h.  The  cylinder,  when  suspended  in  such  a way  that  its  axis  vibrates  horizontally,  gene- 
rally points  in  an  oblique  direction,  in  respect  to  the  axial  line.  Only  in  the  case  where 
also  the  axis  a is  horizontal  and  therefore  the  plane  zX  vertical,  this  axis  a points  equa- 
torially, and  therefore  the  axis  of  the  cylinder  aodally  (fig.  8).  When,  starting  from  this 
position,  we  turn  the  cylinder  round  its  horizontal  axis,  the  direction  of  this  axis  declines 
from  the  line  joining  the  two  poles;  the  angle  of  declination  increases  till  the  angle  of 
rotation  equals  90°,  and  therefore  the  principal  plane  zX  becomes  horizontal  (fig.  9) ; its 
maximum  is  When  the  rotation  still  continues,  the  axis  of  the  cylinder  retuius  towards 
the  axial  line,  where  it  arrives  again  after  a rotation  of  180°.  From  180°  to  360°  the 
