PEOFESSOE  PLOCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CETSTALS.  553 
is  about  95°,  and  is  bisected  by  the  axis  of  the  primitive  prism ; these  axes  lie  in  the 
plane  containing  the  acfide  edges  of  this  prism  (fig.  18). 
16.  Let  us  conceive  an  ellipsoid  of  amorphous  sulphate  of  zinc  or  another  diamagnetic 
substance,  having  within  the  crystal  of  this  salt  its  three  unequal  axes,  according  to 
then-  length,  directed  along  a,  X.  The  positions  which  the  crystal  assumes  in  all  the 
above  described  modes  of  suspension,  will  be  imitated  by  such  an  ellipsoid  when 
suspended  along  its  corresponding  diameters. 
17.  Formiate  of  copper  (CuFo). — We  join  to  the  two  examined  salts  belonging  to 
the  same  system,  one  of  them  being  paramagnetic,  the  other  diamagnetic,  a third 
salt,  whose  primitive  form  is  an  obhque  prism.  There  is  a plane  of  symmetry  passing 
through  the  axis  of  the  prism  and  the  longer  diagonal  of  its  rhombic  base.  The 
iaclination  of  the  axis  to  the  base  is  78°  55';  the  angles  between  the  lateral  faces  differ 
52'  from  a right  angle  (Heusser):  the  plane  of  the  base  is  one  of  perfect  cleavage. 
This  salt,  easily  crystalhzing  in  large  and  homogeneous  crystals,  is  paramagnetic,  and 
shows  very  distinctly  the  extraordinary  magnetic  action. 
18.  At  first  natural  crystals  were  examined,  whose  exterior  shape  had  been  varied 
by  cleaving  them  parallel  to  the  base.  A horizontal  plate  bounded  by  cleavage  planes 
set  the  symmetrical  plane,  which,  being  perpendicular  to  it,  was  marked  on  its  upper 
base,  exactly  equatorially,  even  then,  when  this  position  did  not  agree  with  the  position 
of  a similar  plate  consisting  of  an  amorphous  paramagnetic  substance.  Our  plate, 
when  suspended  vertically,  set  the  cleavage  plane  nearly  equatorially ; in  this  case  the 
plate  would  rotate  through  nearly  90°,  if  it  were  not  crystalhzed.  When  turned  round 
the  horizontal  line  perpendicular  to  its  bases,  it  passed  through  the  equatorial  position, 
its  declination  from  this  position  remaining  always  very  small. 
19.  Then,  out  of  a large  crystal  was  cut  a chcular  plate  bounded  by  planes  of  sym- 
metry, three  times  as  broad  as  thick.  On  the  surface  of  the  plate,  when  horizontally 
suspended  and  in  equilibrium  between  the  two  poles,  were  marked  the  axial  and  the 
equatorial  line.  Let  us  denote  these  two  fines,  perpendicular  to  each  other,  by  a and  c, 
the  fine  perpendicular  to  the  plate  being  denoted  by  h.  The  angle  within  the  sym- 
metric plane  between  the  normal  to  the  cleavage  plane  and  a was  found  to  be  3°,  taken 
from  the  normal  towards  the  obtuse  angles  of  the  symmetric  plane.  The  approximate 
measure  of  this  angle  was  verified  afterwards  on  different  crystals.  The  same  plate 
oscillating  vertically  pointed  axially  when  c,  equatorially  when  a was  vertical.  In  the 
first  case  only  an  amorphous  paramagnetic  body  of  the  same  shape  would  assume  the 
same  position.  But  here  also  the  position  of  the  crystal  did  not  change,  after  having 
changed  its  dimensions  in  such  a way  that  an  amorphous  body  of  the  same  shape  would 
rotate  through  an  angle  of  90°  round  the  vertical  axis. 
20.  The  last  series  of  experiments  may  be  described  thus:  the  crystal 
When  suspended  along  a sets  axially  h ; 
When  suspended  along  h sets  axially  a ; 
When  suspended  along  c sets  axially  a. 
4 D 2 
