PEOFESSOE  PLtiCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CEYSTALS.  545 
the  two  optic  axes  you  may  obtain  the  direction  of  any  wave  of  light  entering  the 
crystal,  its  plane  of  polarization,  and  its  velocity  of  propagation.  By  means  of  the  two 
magnetic  axes,  you  may  obtain,  using  analytical  expressions  of  nearly  the  same  form, 
the  couple  of  magnetic  forces  acting  upon  a crystal,  when  suspended  between  the  two 
poles  along  any  direction  whatever,  the  position  of  the  crystal,  and  the  law  of  its  oscil- 
lations *. 
I.  On  the  direction  which  biaxial  crystals  assume  when  suspended  between  the  two  poles 
along  different  lines,  having  a determined  position  with  regard  to  the  primitive  form 
of  such  crystals. 
1.  In  all  the  observations  I shall  describe  in  this  section  I made  use  of  a large 
electro-magnet,  excited  by  six  of  Geove’s  elements,  whose  pointed  poles  were  at  a 
distance  from  each  other  of  1‘6  inch.  The  crystals,  oscillating  in  the  horizontal  plane 
which  passes  through  both  poles,  and  equally  distant  from  these  poles,  were  attached 
to  the  double  cocoon  thread  of  the  torsion  balance  by  means  of  a hoop,  without  any 
other  support. 
2.  Bedfexridcyanide  of  potassium  (SKUy-j-FeGy^)  is  paramagnetic.  I observed,  in  the 
year  1847,  that  any  fragment  of  this  salt,  freely  suspended  between  the  two  poles  of  an 
* I think  it  scarcely  necessary  here  to  prove  that  the  theoretical  views  imputed  to  me  by  Professor 
Ttitdail  (Philosophical  Transactions,  vol.  cxlv.  p.  2)  are  not  mine,  and  never  have  been  mine.  I never 
ascribed  the  phenomena,  first  observed  by  myself,  to  a new  force  acting  upon  an  ideal  Line,  like  the  optic 
axis,  quite  independent  of  the  paramagnetic  condition  of  the  mass  of  the  crystal.  Convinced,  as  I was  from 
the  beginning,  that  there  ought  to  be  analogies  between  the  optic  and  magnetic  properties  of  crystals,  I 
never  sought  the  reason  for  it  anywhere  else  but  in  the  influence  of  the  crystalline  structure  on  both  the 
luminiferous  ether  and  the  magnetic  induction.  I no  more  intended  to  imply  a real  repulsion  or  attraction  of 
the  optic  axes,  than  the  celebrated  French  philosopher,  when  he  said  a beam  of  light  in  positive  crystals  was 
attracted,  in  negative  ones  was  repelled,  intended  to  announce  a mysterious  action  emanating  in  fact  from 
these  axes.  Such  expressions  are  intended  to  describe  a newly  observed  fact,  but  not  theoretical  views.  So 
also  the  true  meaning  of  the  German  words,  translated  thus,  “ independent  of  the  paramagnetic  or  diamag- 
netic condition  of  the  mass  of  the  crystal,”  is  only  this,  “whether  the  mass  of  the  crystal  (tourmaline  and 
calcareous  spar)  be  paramagnetic  or  diamagnetic,  the  direction  of  the  axis  is  the  same.”  If,  notwithstanding 
these  remarks,  there  should  remain  any  doubt  whatever,  I can  refer  to  a paper  sent  to  the  Haarlem  Society, 
December  1849,  before  other  philosophers,  except  Professor  Faeadat,  had  published  anything  about  the 
magnetic  induction  of  crystals.  Starting  fi'om  mechanical  principles,  I communicated  in  this  paper  a 
mathematical  explanation  of  what  I had  observed,  for  instance,  in  the  case  of  tourmaline,  conceiving  this 
crystal  to  consist  of  an  infinite  number  of  infinitely  small  needles,  becoming  paramagnetic  by  induction,  and 
being  all  perpendicular  to  its  axis  (see  Poggendoeef’s  Annalen,  Ixxxvi.  p.  1).  The  physical  conditions 
of  the  question,  as  there  stated,  seem  to  be  the  same  as  those  which  Professor  Tyndall  has  also  adopted 
in  his  memoir  (p.  45).  And  though  I have  recently  foimd  reason  to  modify  them,  yet  there  is  no  trace  to 
be  found  of  the  supposition  imputed  to  myself,  nor  even  of  “ the  supposition  that  the  assuming  of  the  axial 
position  proved  a body  to  be  magnetic,  while  the  assuming  of  the  equatorial  position  proved  a body  to  be 
diamagnetic”  (p.  13).  When  there  is  an  analytical  expression,  representing  the  resulting  action  exerted 
on  a body,  now  attracted,  now  repelled,  according  to  distance,  it  is  in  most  cases  mathematically  legitimate 
to  speak  of  two  “ conflicting  forces,”  by  dividing  the  whole  expression  into  two  members,  one  of  which 
represents  an  attractive,  the  other  a repulsive  force. 
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