MOLECULAR  DISSYMMETRY  OF  ORGANIC  PRODUCTS.  7 
of  bestowing  epithets  appropriate  to  each  variety  of  a  species, 
applied  the  term  plagihedral  to  the  variety  of  quartz  having  the 
face  x.  Crystals  in  which  the  face  x  inclined  to  the  right,  are 
designated  under  the  name  right  plagihedrals,  the  crystal  being 
adjusted  in  a  suitable  manner  ;  and  those  crystals  in  which  the 
face  x  inclined  in  an  opposite  direction  are  termed  left  plagihe- 
drals. 
Nothing  is  more  variable  than  this  character.  Here  it  ex- 
ists ;  there  it  is  absent.  Upon  the  same  crystal  there  are  angles 
which  bear  the  face  x  ;  others  which  should  have  it,  have  it  not. 
Sometimes  we  find  plagihedral  faces  to  the  right  and  to  the  left. 
Nevertheless,  all  persons  versed  in  a  knowledge  of  crystals,  ad- 
mit that  there  is  in  quartz  a  true  hemihedrity  in  two  opposite 
directions. 
Here  is  placed  a  very  ingenious  approximation  due  to  Sir 
John  Herschell,  communicated  to  the  Royal  Society  of  London, 
in  1820. 
M.  Biot,  as  I  previously  said,  made  the  remarkable  observa- 
tion, that  among  the  specimens  of  quartz,  some  deviated  in  the 
direction  of  the  plane  of  polarized  light,,  and  others  in  an 
opposite  direction,  to  the  right  and  to  the  left.  This  stated, 
John  Herschell  associated  the  crystallographic  observation  of 
Haiiy  with  the  physical  remark  due  to  M.  Biot.  Experiment 
confirmed  the  idea  of  a  relation,  in  fact,  between  the  right  and 
left  plagihedrals  and  the  right  and  left  optical  deviations. 
Specimens  of  quartz  which  bear  in  one  direction  the  face  x, 
deviate  the  plane  of  polarized  light  in  the  same  direction. 
Such  is  an  exposition  of  the  principal  facts  which  have  pre- 
ceded the  researches,  an  abridged  history  of  which  I  have  to 
relate. 
III. 
When  I  began  to  devote  myself  to  particular  pursuits  I  sought 
to  fortify  myself  in  the  study  of  crystals,  anticipating  that  I 
would  derive  assistance  from  it  in  my  chemical  researches.  The 
most  simple  means,  it  seemed  to  me,  was  to  take  a  somewhat 
extensive  work  upon  crystalline  forms,  to  repeat  all  the  measure- 
ments and  to  compare  my  determinations  with  those  of  the 
'  author.    In  1841,  M.  de  la  Provostaye,  whose  exactness  is  well 
