MOLECULAR  DISSYMMETRY  OF  ORGANIC  PRODUCTS.  99 
were  separated.  They  resemble  each  other  as  much  as  the 
right  hand  resembles  the  left,  or  as  two  irregular  symmetric 
tetrahedrons  resemble  each  other,  and  these  analogies  and  these 
differences  are  found  in  all  their  derivatives.  What  can  he  done 
with  one  may  be  repeated  with  the  other  under  the  same  condi- 
tions, and  the  resulting  products  constantly  manifest  the  same 
properties,  with  this  single  difference,  that  in  some  the  deviation 
of  the  plane  of  polarization  is  to  the  right,  and  in  others  to  the 
left,  and  that  the  forms  of  corresponding  species,  although 
identical  in  all  their  details,  cannot  be  superposed. 
All  these  facts,  so  clear,  so  demonstrative,  oblige  us  to  refer 
the  general  exterior  characters  of  these  acids  and  their  combina- 
tions, to  their  individual  chemical  molecules.  Not  to  do  it  would 
be  to  ignore  the  rules  of  the  most  common  logic.  It  is  thus  we 
arrive  at  the  following  conclusions  : 
1st.  The  molecule  of  tartaric  acid,  whatever  it  may  be  other- 
wise, is  dissymmetric,  and  of  a  dissymmetry  with  an  image  non- 
superposable.  2d.  The  molecule  of  the  left  tartaric  acid  is 
formed  precisely  by  the  group  of  inverse  atoms.  And  by  what 
characters  shall  we  recognize  the  existence  of  molecular  dissym- 
metry ?  In  the  one  case  by  non-superposable  hemihedrity ;  in 
the  other,  and  especially  by  the  rotary  optical  property  when 
the  body  is  in  solution. 
These  principles  being  stated,  let  us  examine  all  bodies, 
whether  from  nature  or  the  laboratory,  and  we  shall  find  easily 
that  among  them  a  great  number  possess  both  this  kind  of 
hemihedrity  and  the  molecular  rotary  property,  and  that  all  others 
possess  neither  the  one  nor  the  other  of  these  characters. 
I  was  right,  then,  in  saying  :  The  legitimate  and  forced  conse- 
quence of  our  first  lecture  may  be  thus  expressed  : — 
All  bodies  (I  use  this  expression  chemically)  are  divided  into 
two  great  classes  ;  bodies  with  a  superposable  image,  and  bodies 
with  a  non-superposable  image  ;  bodies  constructed  of  dissym- 
metric atoms,  and  those  formed  of  homohedric  atoms. 
III. 
Here  we  encounter  a  fact  worthy  of  fixing  the  attention, 
even  though  we  should  regard  it  alone  and  isolated  from  the 
whole  of  the  considerations  about  to  follow.    It  is  this: 
