580  PEOFESSOE  PLtiCEIEE  OX  THE  MAGXETIC  IXDrCTIOX  OF  CETSTALS. 
IV.  Position  of  the  three  axes  of  magnetic  induction  with  regard  to  the  primitive  form, 
in  crystals  belonging  to  different  systems. 
74.  A great  number  of  crystals,  the  finest  of  which  were  supphed  by  the  extreme 
hberality  of  Professor  Bottger  of  Frankfort,  have  been  examined  by  myself  in  connexion 
with  Professor  Beer,  in  order  to  establish  them  optic  as  well  as  magnetic  properties, 
with  regard  to  them  crystalline  form.  The  results  are  pubhshed  in  two  papers  in 
PoGGENDORFP’s  ‘ Annalen.’  A third  paper,  ready  to  be  sent  to  press  nearly  two  years 
ago,  did  not  appear,  since,  according  to  the  newly-adopted  theor}',  I found  it  necessary 
to  present  the  results  obtained  by  experiment  in  a different  way.  From  these  papers 
are  taken  nearly  all  the  observations  referred  to  in  this  section. 
75.  In  crystals  having  no  plane  of  symmetry,  there  is  not  the  least  indication  of  the 
position  of  the  three  unequal  axes  of  magnetic  induction.  In  no  case  has  the  position 
of  these  axes  been  determined. 
Sulphate  of  copper  is  a crystal  of  this  kind,  strongly  exhibiting  the  extraordinary 
action.  I provided  a sphere,  turned  out  of  a large  crystal,  in  order  to  determine  the 
position  of  the  three  axes  of  induction,  and  hence  the  two  magnetic  axes.  According 
to  art.  39,  I intended  to  trace  on  its  surface  the  two  curves  indicating  the  diameters 
which  point  axially  and  equatorially,  when  the  sphere,  suspended  between  the  two  poles, 
rotates  successively  round  any  two  horizontal  diameters.  The  practicability  of  this  pro- 
ceeding had  been  previously  proved  in  the  case  of  crystals  belonging  to  a difierent 
system ; but  here  we  did  not  succeed,  on  account  of  the  want  of  homogeneity  of  the 
sphere  of  sulphate  of  copper,  which  in  its  different  parts  contained  different  quantities 
of  sulphate  of  iron 
Cyanite  furnishes  another  instance  of  extraordinary  paramagnetic  induction.  Strongly 
paramagnetic  prisms  of  this  mineral  show  the  extraordinary  action,  when  suspended 
horizontally,  in  a sensible  way,  even  under  the  magnetic  induction  of  the  earth.  Eotathig 
round  their  horizontal  axis,  they  point  towards  different  azimuths. 
Bichromate  of  potash  shows  very  distinctly  the  extraordinary  paramagnetic,  racemic 
acid  the  extraordinary  diamagnetic  induction. 
76.  The  crystals  having  a single  plane  of  symmetry  mscg  be  referred  to  an  obhque 
prism  with  a rhombic  base,  the  plane  of  symmetry  passing  through  one  of  the  diagonals 
of  the  base.  Two  axes  of  paramagnetic  induction  always  lie  hi  the  plane  of  sym- 
metry, the  remaining  third  one  being  perpendicular  to  it.  When  any  crystal  of  this 
system  is  to  be  examined,  you  may  suspend  it  first  along  the  line  perpendicular  to  the 
* All  specimens  of  copper,  except  of  English,  which  I had  the  opportunity'  to  examine,  were  found 
to  he  strongly  paramagnetic,  even  such  specimens  as  were  obtained  hy  galvano-plastic  deposition ; so  were 
all  salts  of  its  oxides.  I was  first  inclined  to  attribute  the  paramagnetic  condition  in  both  eases  to  un- 
mixed iron ; but  on  closer  examination,  I convinced  myself  that  oxide  of  copper  is  paramagnetic  per  se,  and 
so  are  aU  its  salts  ; in  like  manner  as  copper,  dioxide  of  copper  and  its  salts  (sulphite  of  copper  and  ammonia, 
GuS-t-Am  S)  are  diamagnetic. 
