MOLECULAR  DISSYMMETRY  OP   ORGANIC  PRODUCTS.  15 
Bear  in  mind  the  definition  of  chemical  species  just  now  men- 
tioned, namely,  the  collection  of  all  individuals  identical  by  the 
nature,  the  proportion  and  arrangement  of  elements.  All  the 
properties  of  bodies  are  the  functions  of  these  three  terms,  and 
the  object  of  all  our  efforts  consists  in  recurring  by  experiment 
on  properties  to  the  knowledge  of  these  three  things. 
In  isomeric  bodies,  the  nature  and  proportion  [of  constituents] 
are  the  same.  The  arrangement  alone  differs.  The  great  in- 
terest of  isomerism  has  been  to  introduce  into  science  this 
principle — that  bodies  may  be  and  are  essentially  different  by 
that  alone,  that  the  arrangement  of  atoms  is  not  the  same  in 
their  chemical  molecules. 
But  no  isomeric  bodies  existed  whose  relations  of  molecular 
arrangements  were  known.    This  gap  is  filled  for  the  first  time 
by  the  discovery  of  the  constitution  of  paratartaric  acid  and 
the  relations  of  constitution  of  the  right  and  left  tartaric  acids. 
We  know,  in  fact,  on  the  one  hand,  that  the  molecular  arrange- 
ments of  the  tartaric  acids  are  dissymmetric,  and  on  the  other, 
that  they  are  rigorously  the  same,  with  the  sole  difference  of 
presenting  dissymmetries  in  opposite  directions.  Are  the  atoms 
of  the  right  acid  grouped  according  to  the  spire  of  a  dextrorse 
helix,  or  placed  at  the  summits  of  an  irregular  tetrahedron,  or 
disposed  according  to  such  or  such  determined  dissymmetric  as- 
semblage?   We  are  unable  to  reply  to  these  questions.  But 
what  cannot  be  doubted  is,  that  there  is  a  grouping  of  atoms 
according  to  an  order  dissymmetric  to  a  non-superposable 
image.    What  is  not  less  certain  is,  that  the  atoms  of  the  left 
acid  precisely  realize  the  inverse  dissymmetric  grouping  of  this 
one.    We  also  know,  that  paratartaric  acid  results  from  the 
juxtaposition  of  these  two  groupings  of  inversely  dissymmetric 
atoms. 
From  this  moment  the  ascertaining  the  resemblances  and 
the  chemical  and  physical  differences  which  correspond  to  these 
arrangements  whose  relations  are  known,  affords  particular  in- 
terest, and  gives  to  the  molecular  mechanism  assured  bases.  It 
permits  us  to  establish  the  connexion  of  physical  and  chemical 
properties  with  the  molecular  arrangement  which  determines 
their  proper  existence,  or  inversely  it  permits  us  to  recur  from 
properties  to  their  first  cause. 
