MOLECULAR   DISSYMMETRY  OF  ORGANIC  PRODUCTS. 
9 
blance  between  these  different  forms,  in  spite  of  the  difference 
of  the  other  constituent  elements. 
It  follows  from  this  that  there  is  something  in  common  in  the 
forms  of  all  the  tartrates,  and  it  is  possible  to  arrange  them 
similarly,  in  taking  for  example,  as  character  of  similar  position, 
the  position  of  the  axes  a  and  p. 
Now,  if  the  disposition  of  the  hemihedric  faces  be  compared 
on  all  the  prisms  of  the  primitive  forms  of  the  tartrates,  ar- 
ranged in  the  same  manner,  this  disposition  is  found  to  be  the 
same. 
Let  us  sum  up  in  two  words  these  results,  which  have  been  the 
point  of  departure  in  all  my  ulterior  researches:  the  tartrates 
are  hemihedric,  and  they  are  so  in  the  same  direction. 
Guided  on  the  one  hand  by  the  fact  of  the  existence  of 
molecular  rotary  polarization,  discovered  by  M.  Biot  in  tartaric 
acid  and  in  all  its  combinations  ';  on  the  other,  by  the  ingenious 
approximation  of  Herschell ;  in  the  third  place,  by  the  learned 
views  of  M.  Delafosse,  with  whom  hemihedrity  has  always  been 
a  law  of  structure  and  not  an  accident  of  crystallization,  I  pre- 
sumed there  might  be  a  correlation  between  the  hemihedrity  of 
the  tartrates  and  their  property  of  deviating  the  plane  of  polar- 
ized light. 
It  is  important  to  seize  here  the  sequence  of  ideas  : 
Haiiy  and  Weiss  ascertained  that  in  quartz  there  exists  hemi- 
hedric faces,  and  that  these  faces  fall  to  the  right  in  certain 
specimens,  and  to  the  left  in  others.  On  his  side,  M.  Biot  found 
that  crystals  of  quartz  are  also  divided  into  two  groups,  under 
the  relation  of  their  optical  properties,  some  deviating  to  the 
right,  and  the  others  deviating  to  the  left,  the  plane  of  polarized 
light  according  to  the  same  laws.  Herschell  comes  in  his  turn, 
places  between  these  two  facts,  until  then  isolated,  a  line  of 
connexion,  and  says  :  The  plagihedrals  of  one  direction  deviate 
in  the  same  direction ;  the  plagihedrals  of  the  other  direction 
deviate  in  the  opposite  direction. 
For  my  part  I  find  that  all  the  tartrates  are  plagihedral,  if  I 
may  so  express  myself,  and  that  they  are  all  in  the  same  di- 
rection. I  should  then  presume  that  here,  as  in  quartz,  there 
was  a  correlation  between  hemihedrity  and  circular  polarization. 
At  all  events,  the  essential  differences  which  I  have  just  noticed 
