546  PEOFESSOE  PLtJCKEE  ON  THE  MAGNETIC  ENDUCTION  OF  CETSTALS. 
Fig.  1. 
electro-magnet,  does  not  point  like  an  amorphous  paramagnetic  body.  The  poles  being 
not  too  near  one  another,  the  direction  the  fragment  takes  does  not  depend  on  its 
exterior  form,  but  solely  on  its  interior  crystalline  structure.  On  account  of  this  strong 
extraordinary  magnetic  action,  the  salt  above  mentioned  appears  to  be  pecuharly  fit  for 
exhibiting  the  phenomena  and  the  laws  of  this  action ; the  larger  the  ciystals  you  obtain 
of  this  salt,  the  more  easily  they  may  be  cut  and  worked. 
We  may  regard  a right  prism  with  a rhombic  base  as  the  primitive  form  of  our  salt*. 
There  are  three  crystallographic  axes  perpendicular  to  each  other,  the  axis  of  the 
prism  (a),  the  shorter  diagonal  of  its  base  (k),  and  the  longer  one  (/>.). 
3.  A crystal  of  ferridcyanide  of  potassium,  sus- 
pended between  the  two  poles  in  such  a manner 
that  its  axis  a becomes  vertical  and  therefore  its 
base  X horizontal,  sets  with  energy  » axially 
(fig.  1).  This  axis  %,  remains  similarly  directed 
even  when  we  reduce  the  original  prism  to  a plate, 
by  taking  away  its  obtuse  edges.  Such  a plate 
appears  to  be  repelled  by  the  two  poles  like  a dia- 
magnetic body,  while  a plate  cut  out  of  the  same 
prism  by  grinding  down  its  acute  edges  will  be 
directed,  the  mode  of  suspension  remaining  the 
same,  like  a paramagnetic  body,  as  in  fact  it  is. 
The  same  prism,  suspended  in  such  a manner  that 
its  axis  a may  oscillate  between  the  two  poles  in  the 
horizontal  plane,  points  equatorially,  and  seems  to  be 
repelled  by  the  poles  like  a diamagnetic  body.  We 
can  use  in  this  experiment  any  natural  prism  with  its 
summits,  whose  longest  dimension  is  five  or  six  times 
greater  than  its  thickness ; we  may  use  also  a small 
needle  twenty  or  thirty  times  as  long  as  it  is  thick  f. 
In  all  these  cases  the  oscillating  salt  finds  its  position  of  stable  equilibrium  by  setting 
itself  equatorially.  When  we  reduce  the  oscillating  prism  to  a plate,  by  dimhiishing 
the  dimension  of  its  axis,  such  a plate,  the  axis  a remaining  horizontal,  will  point 
axially  hke  an  amorphous  paramagnetic  body. 
V' 
Fig.  2. 
Ax.- 
* The  primitive  form  of  the  salt  is  disputed.  According  to  the  prevalent  opinion,  we  admitted,  Professor 
Beer  and  myself,  in  a paper  published  some  years  ago,  the  clinorhombic  system,  not  without  some  hesita- 
tion, as  this  opinion  was  neither  supported  by  its  magnetic  nor  its  optic  properties.  I ixfterwards  adopted, 
in  conformity  -with  the  new  measures  recently  made  by  M.  Schabtjs  of  Vienna,  the  rhombic  system.  But 
again,  Noeeembee&’s  admirable  new  arrangement  of  Amici’s  polarizing  microscope  showing  a minute 
diflPerence  between  the  two  systems  of  rings  round  the  optic  axes,  not  seen  in  the  original  apparatus,  the 
question  is  more  doubtful  than  before. 
t Not  knowing  therefore  the  extraordinary  magnetic  action,  one  would  be  inclined  to  range  oui*  crystals 
among  diamagnetic  bodies,  as  reaUy  has  been  done. 
