PEOFESSOE  PLtiCKEE  ON  THE  MAGNETIC  INDUCTION  OF  CEYSTALS.  551 
their  length,  with  the  axis  of  greatest  induction  (z),  the  axis  of  mean  induction  (X),  and  the 
axis  of  least  induction  (a). 
11.  Sulphate  of  Zinc  (ZnS)  is  a diamagnetic  body,  showing  the  extraordinary  mag- 
netic action  not  nearly  so  strong  as  the  red  ferridcyanide  of  potassium,  but  strong  enough 
for  exhibiting  with  nicety  all  the  phenomena  analogous  to  those  described  in  the  case  of  the 
salt  before  examined.  It  was  especially  selected  with  the  intention  to  examine  whether, 
in  regard  to  the  extraordinary  magnetic  action  also,  the  diamagnetic  induction  be 
altogether  the  contrary  of  the  paramagnetic.  This  was  fully  confirmed  by  experiment. 
We  may,  as  in  the  former  case,  denote  the  three  crystallographic  axes  by  a,  z,  and  X, 
a being  the  axis  of  the  primitive  prism,  z the  shorter,  X the  longer  diagonal  of  its  base. 
12.  A natural  prism  oscillating  between  the  two  poles,  which  we  always  suppose 
distant  enough  from  each  other  with  regard  to  the  dimensions  of  the  prism,  will,  when 
suspended  along  its  axis  (a),  set  equatorially  the  shorter  diagonal  of  its  base  («);  when 
suspended  along  its  shorter  diagonal  (z),  as  well  as  along  the  longer  one  (X),  it  sets  equa- 
torially its  axis  (a).  These  directions  remain  unchanged  when  the  dimensions  of  the 
crystal  along  its  crystallographic  axes  are,  in  the  different  modes  of  suspending,  such 
that  the  oscillating  body,  supposed  to  be  an  amorphous  diamagnetic  one,  would  be 
directed  in  the  contrary  way.  Hence,  in  the  case  of  our  salt,  the  three  crystallographic 
axes  may  be  called  the  axes  of  diamagnetic  induction^  a being  the  axis  of  greatest^  z of 
mean,  and  X of  least  induction. 
Fig.  15. 
Att. 
I-'X' 
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Fig.  17. 
'A'tt  - 
/ 
y<x- 
/ 
Ec{^. 
13.  When  we  cut  out  of  the  crystal  a circular  plate  perpendicular  to  the  axis  a,  this 
axis,  when  oscillating  horizontally  between  the  two  poles,  points  equatorially  (fig.  14),  as 
in  the  case  of  an  amorphous  paramagnetic  body.  The  directing  power  emanating  from 
the  poles  is  a maximum  if  a minimum  if  X be  vertical. 
MDCCCLVIII. 
4 D 
