t 
98        MOLECULAR  DISSYMMETRY  OP  ORGANIC  PRODUCTS. 
compound  molecules  both  of  these  two  categories  in  which  all 
material  objects  are  distributed.  In  other  words,  it  would  have 
been  very  extraordinary,  if  among  all  chemical  substances,  nat- 
ural or  artificial,  there  had  not  been  individuals  with  a  super- 
posable  image  and  others  with  an  image  non-superposable. 
Things  happen  in  fact  as  might  be  naturally  anticipated  ;  all 
chemical  combinations,  without  exception,  are  equally  distri- 
buted in  two  classes  ;  those  having  an  image  superposable,  and 
those  having  an  image  not  superposable. 
II. 
It  is  easy  to  show  that  this  is  a  legitimate  consequence,  neces- 
sary from  our  first  comparison.  To  place  it  in  clear  light  I  will 
briefly  recall  the  principal  conditions  of  the  decisive  experiment 
which  closed  the  preceding  lecture. 
I  prepare  by  the  aid  of  natural  paratartaric  acid,  the  para- 
tartrate  of  soda  and  ammonia.  It  is  deposited  in  beautiful 
crystals. 
Observed  in  a  polarizing  apparatus,  a  solution  of  any  portion 
of  this  double  salt  offers  no  indication  of  optical  deviation  ;  and 
in  separating  from  the  crystals  the  acid  which  they  contain, 
paratartaric  acid,  identical  with  that  which  served  to  form 
them,  is  reproduced.  So  far  all  is  simple  and  natural,  and  it 
seems  as  if  we  had  to  deal  with  the  crystallization  of  an  ordina- 
ry salt.    There  is  nothing  of  the  kind,  however. 
Take  another  portion  of  the  same  crystals  and  examine  them 
one  by  one.  You  will  find  that  one  half  has  the  form,  a  model 
of  which  I  here  present,  characterized  by  a  non-superposable 
hemihedrity;  that  the  other  half  has  the  inverse  form  identical 
with  the  first  in  all  its  respective  parts,  and  yet  cannot  be  super- 
posed on  it.  Then  let  the  two  kinds  of  crystals  be  isolated  to 
be  dissolved  separately,  and  we  observe  that  one  of  the  two 
solutions  deviates  the  polarized  light  to  the  left  and  the  other  to 
the  right,  and  both  equally. 
If  we  extract  by  the  ordinary  chemical  processes  the  acids 
from  these  two  kinds  of  crystals,  we  perceive  that  one  of  them 
is  identical  with  common  tartaric  acid,  and  that  the  other  is  in 
all  points  similar,  except  that  it  cannot  be  superposed  on  it.  They 
bear  to  each  other  the  relations  of  the  two  salts  from  which  they 
