8 
MOLECULAR  DISSYMMETRY  OF   ORGANIC  PRODUCTS. 
known,  published  a  handsome  work  on  the  crystalline  forms  of 
tartaric  acid,  paratartaric  acid,  and  their  saline  combinations. 
I  took  hold  of  this  memoir.  I  crystallized  tartaric  acid  and  its 
salts,  and  I  studied  the  forms  of  their  crystals.  But,  in  my 
progress,  I  perceived  that  a  very  interesting  fact  had  escaped 
the  learned  physicist.  All  the  tartrates  I  studied  afforded  de- 
cided indications  of  hemihedric  faces. 
This  particularity  of  form  in  the  tartrates  was  not  very  evi- 
dent. This  may  be  readily  conceived,  since  it  had  not  yet  been 
observed.  But  when,  in  one  species,  it  appeared  in  doubtful 
characters,  I  always  succeeded  in  rendering  it  more  manifest, 
by  recommencing  the  crystallization  and  slightly  modifying 
the  conditions.  Sometimes  the  crystals  bore  all  the  faces  re- 
quired by  the  law  of  symmetry,  but  hemihedrity  had  credit  for 
an  unequal  development  of  one  half  the  faces.  This  was  seen 
for  example  in  the  ordinary  tartar  emetic.  It  may  be  said  that 
what  adds  to  the  difficulty  in  recognising  hemihedrity,  are  the 
frequent  irregularities  of  crystals  which  are  never  easily  de- 
veloped. Hence  their  results,  deformities,  arrests  of  develop- 
ment in  this  or  that  direction,  faces  accidentally  suppressed, 
&c.  Except  under  circumstances  almost  exceptional,  the  ascer- 
taining of  hemihedrity,  especially  in  crystals  of  the  laboratory, 
requires  a  very  attentive  study.  Besides  that,  although  hemi- 
hedrity may  be  possible  in  a  form,  although  it  may  be  a  function 
of  the  internal  structure  of  the  body,  it  may  not  be  externally 
manifest,  any  more  than  is  found  upon  every  crystal  of  the 
cubic  species  all  the  forms  compatible  with  the  cube. 
But  be  that  as  it  may,  I  repeat,  I  found  hemihedric  tartrates. 
This  observation  would  have  been  barren,  probably,  without 
the  following  : — 
Let  a,  b,  c,  be  the  parameters  of  the  crystalline  form  of  any, 
tartrate ;  a,  ^,  y,  be  the  angles  of  the  crystallographic  axes. 
These  are  ordinarily  right  angles  or  a  little  oblique.  Besides, 
the  relation  of  the  two  parameters,  such  as  a  and  b,  is  nearly 
the  same  in  the  different  tartrates,  whatever  may  be  their  com- 
position, the  quantity  of  their  water  of  crystallization,  the 
nature  of  their  bases  ;  y,  alone  differs  sensibly.  There  is  a  kind 
of  semi- isomorphism  among  all  the  tartrates.  One  might  say 
that  the  tartaric  group  prevails  and  impresses  a  stamp  of  resem- 
