552  PEOFESSOE  PLtiCKEE  OX  THE  MAGXETIC  IXOrCTIOX  OP  CETSTALS. 
A circular  cylinder,  whose  axis  coincides  with  the  longer  diagonal  (/.),  when  oscilla- 
ting horizontally  between  the  poles,  sets  its  axis  axially  (fig.  15),  as  an  amoi-phous  para- 
magnetic body  of  the  same  shape  would  do.  The  directive  power  is  a maximum  if  z,  a 
minimum  if  a be  vertical. 
A circular  plate,  as  well  as  a circular  cylinder,  whose  axes  coincide  with  the  shorter 
diagonal  (z),  when  suspended  with  their  axes  horizontal,  set  these  axes,  when  rotating 
round  them,  either  axially  or  equatorially  (figs.  16,  17).  There  is  an  intermediate  case 
where  no  sensible  extraordinary  magnetic  action  is  observed. 
14.  A cu'cular  cylinder,  which  also,  by  diminishing  the  length  of  its  axis,  may  be 
reduced  to  a cu’cular  plate,  if  its  axis  do  not  coincide  with  one  of  the  three  crystallogra- 
phic axes  (a),  (z),  (X),  points  generally  obliquely  when  oscillating  with  its  axis  horizontal. 
It  passes,  when  rotating  round  its  horizontal  axis,  either  through  the  axial  or  through 
the  equatorial  position.  The  first  case  always  takes  place  if  its  axis  lie  in  the  plane  za. 
the  second  if  it  lie  in  the  plane  az.  But  if  the  axis  lie  in  the  plane  a/.,  according  to  the 
angles  between  it  and  the  two  axes  a.  A,  the  rotating  cylinder  passes  either  through  the 
axial  or  through  the  equatorial  position. 
15.  The  experiments  just  described  establish  the  existence,  within  a diamagnetic  cry- 
stal also,  of  two  directions,  having  the  property,  that  the  ciystal,  when  suspended  along 
them,  is  not  acted  upon  by  the  magnet  in  an  extraordinary  way.  W e may  call  these 
directions,  as  we  did  in  the  former  case,  the  magnetic  axes  of  the  crystal.  In  order  to 
determine  the  position  of  these  axes,  we  proceeded  in  the  following  way. 
First,  there  was  cut  out  of  a large  crystal 
of  sulphate  of  zinc,  very  easily  procurable, 
a circular  plate  (nearly  0'5  of  an  inch  dia- 
meter and  0‘2  of  an  inch  thick)  perpendi- 
cular to  the  plane  aA  and  inclined  45°  to  the 
base  of  the  prism.  On  the  upper  base  of 
the  plate  was  sketched  the  direction  of  the 
shorter  diagonal  [z).  When  suspended 
horizontally  (its  axis  being  vertical),  the 
sketched  diagonal  [z)  pointed  equatorially, 
just  as  a circular  plate  parallel  to  the  base  of 
the  primitive  prism  would  do  when  horizon- 
tally suspended.  Secondly,  another  similar 
plate  was  cut  out  of  the  crystal,  inclined 
50°  instead  of  45°  to  the  base  of  the  pri- 
mitive form.  The  new  plate  horizontally 
suspended  set  the  shorter  diagonal  [z)  axially.  Hence,  according  to  these  observations, 
the  angle,  with  which  an  indifierent  cu’cular  plate  may  be  obtained,  is  between  45° 
and  50°.  It  would  be  difficult  to  get  by  this  process  \\dth  certainty  closer  hmits. 
including  the  value  of  this  angle.  Therefore,  the  angle  between  the  two  magnetic  axes 
