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XXVII.  On  the  Magnetic  Induction  of  Crystals. 
By  Professor  Julius  Pluceee,  of  Bonn,  F.M.B.S.,  H.M.B.I. 
Eeceived  March  26, — Eead  April  23,  1857. 
INTEODUCTION. 
Ijf  repeating  Professor  Faeaday’s  experiments,  by  which  he  proved  that  magnetism  is  a 
universal  agency  of  nature,  to  whose  influence  all  bodies  are  subject,  I observed  in  the 
year  1847  some  strange  anomalies,  which  1 attributed  to  the  peculiar  structure  of  the 
bodies  examined.  Thus  I was  led  to  examine  crystalhne  substances.  Between  the  two 
poles  of  a strong  electro-magnet  I flrst  suspended  a plate  of  tourmaline,  then  a plate  of 
calcareous  spar ; and  I remarked  that  these  plates,  both  taken  from  a polarizing  apparatus, 
were  acted  upon  in  an  extraordinary  way,  not  dependent  on  their  exterior  shape,  but 
solely  on  theii’  crystalline  structure.  Since  these,  my  flrst  observations,  I have  diligently 
investigated  the  magnetism  of  crystals, — this  difficult  subject  of  experimental  and 
mathematical  inquiry, — guided,  as  I was,  by  the  conviction  that  the  study  of  crystals 
would  advance  the  theory  of  magnetism  as  it  did  previously  the  theory  of  light. 
The  experimental  results  relating  to  crystals  of  the  different  systems,  which,  in  common 
with  Professor  Beee,  I have  hitherto  obtained,  are  partly  published  in  Poggendoefe’s 
‘ Annalen.’  My  intention  here  is  not  to  complete  the  series  of  these  results.  In  order 
to  discover  the  true  law  of  nature,  I thought  it  more  important  to  select,  out  of  the 
great  number  of  examined  crystals,  a few  proper  to  be  subjected  to  a closer  examina- 
tion : I chose  red  ferridcyanide  of  potassium,  sulphate  of  zinc,  and  formiate  of  copper. 
I had  proposed  in  the  earliest  period  of  these  researches  an  empirical  law,  intended 
to  connect  all  observations  concerning  extraordinary  magnetic  action  exerted  on  crystals 
not  belonging  to  the  tesseral  system.  This  law,  modified  subsequently  according  to  new 
facts  observed  by  Professor  Faeaday  and  myself,  does  hold  with  regard  to  uniaxal 
crystals  ha\ing  one  principal  ciystallographic  axis.  Indeed  such  a crystal,  like  tour- 
maline and  calcareous  spar,  when  freely  oscillating  between  the  two  poles,  is  directed 
by  them  exactly  in  the  same  way  as  if  the  forces  resulting  from  the  magnetic  action  on 
each  particle  of  the  crystal  acted  upon  a fixed  line  within  the  crystal — its  magnetic 
axis, — this  axis  being  always  forced  either  into  the  axial  or  into  the  equatorial  plane. 
These  conditions  will  be  satisfied  by  conceiving  the  least  particles  of  the  crystal  to  be 
small  magnetic  needles,  becoming  magnetic  by  induction.  Crystals  of  a more  compli- 
cated structure,  like  those  above  mentioned,  could  not  be  brought  under  the  same  law ; 
for  these  therefore  I supposed  generally  two  such  magnetic  axes.  But  here  I became 
convinced  that  the  proposed  law  does  not  hold  when  such  crystals  are  examined  in  all 
MDCCCLVIII.  4 c 
