PEOFESSOE  PLUCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CEYSTALS. 
Diamagnetic  crystals. 
Positive  (a-=5“). 
Calcareous  spar. 
Antimony. 
Molybdate  of  lead. 
Arseniate  of  lead. 
Sulphate  of  potash. 
Nitrate  of  soda. 
Negative 
Bismuth. 
Arsenic. 
Ice. 
Zircon. 
Honey  stone. 
Cyanide  of  mercury. 
Arseniate  of  ammonia. 
79.  In  all  crystals  belonging  to  the  tesseral  system,  the  ellipsoid  of  magnetic  induc- 
tion is  reduced  to  a sphere,  its  three  axes  being  equal  to  each  other.  There  is  no  extra- 
ordinary action. 
80.  In  different  biaxal  crystals  the  angle  (2(i;)  between  the  two  magnetic  axes  passes 
through  all  degrees  from  0°  to  180°.  It  is  about  90°  in  sulphate  of  zinc.  It  is  small  in 
staurolite,  near  180°  in  Seigxette’s  salt.  In  both  cases  the  two  magnetic  axes  approach 
to  the  longer  diagonal  of  the  base  of  the  primitive  right  prism ; in  the  first  case  the 
difference  between  the  greatest  and  mean,  in  the  second  case  the  difference  between  the 
mean  and  least  axis  of  induction  is  small.  Along  the  longer  diagonal  the  paramagnetic 
induction  of  staurolite  is  greatest,  the  diamagnetic  induction  of  Seignette’s  salt  least. 
The  plane  containing  the  two  axes  has  been  determined  by  observation ; it  passes  in  the 
first  case  through  the  axis  of  the  prism,  in  the  second  case  it  coincides  with  its  base. 
But  if,  in  the  case  of  three  unequal  axes  of  induction,  the  difference  between  any  two 
of  these  axes  becomes  too  small,  the  crystal,  when  examined  between  the  poles,  will 
exhibit  the  appearances  of  a crystal  with  only  one  magnetic  axis.  Crystals  of  this  kind 
are  the  following  ones ; — Sulphate  of  iron,  succinic  acid,  borax,  cyanide  of  nickel  and 
potassium. 
Sulphate  of  iron.,  showdng  very  strongly  the  extraordinary  paramagnetic  induction, 
may  be  regarded  as  a positive  uniaxal  crystal ; its  magnetic  axis  coincides  with  Fresnel’s 
axis  of  greatest  elasticity,  being,  within  the  plane  of  symmetry,  inclined  75°  to  the 
cleavage  plane. 
Succinic  acid,  showing  strongly  the  extraordinary  diamagnetic  induction,  may  be 
regarded  as  a positive  uniaxal  crystal,  whose  magnetic  axis  coincides  likewise  with 
Fresnel’s  axis  of  greatest  elasticity. 
Borax  may  be  regarded  as  a diamagnetic  negative  crystal;  its  magnetic  axis,  coin- 
ciding with  Fresnel’s  axis  of  least  elasticity,  is  perpendicular  to  the  plane  of  symmetry 
(the  cleavage  plane). 
Cyanide  of  7iickel  and  potassium  also  ranges  among  diamagnetic  negative  crystals;  its 
magnetic  axis  coincides  with  Fresnel’s  axis  of  least  elasticity,  and  accordingly  lies  in 
the  plane  of  symmetry. 
81.  A crystal  not  belonging  to  the  tesseral  system  will  nevertheless  show  no  extra- 
ordinary magnetic  action,  if  all  its  three  axes  of  induction  are  nearly  equal  to  one 
MDCCCLVni.  4 H 
