16       MOLECULAR  DISSYMMETRY  OF    ORGANIC  PRODUCTS. 
These  general  relations  of  properties  and  corresponding 
atomic  arrangements  may  be  summarily  stated  as  follows : — 
1st.  When  the  elementary  atoms  of  organic  products  are  dis- 
symmetrically grouped,  the  crystalline  form  of  the  body 
manifests  this  molecular  dissymmetry  by  non-superposable 
hemihedrity. 
The  cause  of  hemihedrity  is  then  recognized. 
2d.  The  existence  of  this  same  molecular  dissymmetry  is 
translated  beyond  by  the  rotary  optical  property. 
The  cause  of  rotary  polarization  is  also  determined.* 
3d.  When  non-superposable  molecular  dissymmetry  is  found 
realized  in  opposite  directions,  as  happens  in  the  two,  right  and 
left,  tartaric  acids  and  all  their  derivatives,  the  chemical  prop- 
erties of  these  identical  and  inverse  bodies  are  rigorously  the 
same;  whence  it  follows  that  this  mode  of  opposition  and 
of  similitude  does  not  alter  the  ordinary  play  of  chemical 
affinities. 
I  am  mistaken :  upon  this  latter  point  there  is  a  restriction 
to  be  placed,  an  important  restriction,  eminently  instructive. 
Time  would  fail  me  to-day  to  develope  it  at  leisure  and  proper- 
ly.   It  will  find  a  place  in  the  next  lecture. 
Fresnel,  by  one  of  those  views  of  genius  of  which  he  has  had  so  many 
has  in  a  manner  presented  this  cause  of  rotary  polarization. 
He  expresses  himself  thus,  in  one  of  his  memoirs,  in  volume  xxviii.  of 
the  "  Annales  de  Chimie  et  de  Physique,"  year  1825: — "Rock  crystal 
presents  optical  phenomena,  which  cannot  he  reconciled  with  the  com- 
plete parallelism  of  the  molecular  lines,  and  which  would  seem  to  indi- 
cate a  progressive  and  regular  deviation  from  these  lines  in  the  passage, 
from  one  slice  of  the  medium  to  the  next." 
(To  be  continued.) 
