100       MOLECULAR  DISSYMMETRY  OF  ORGANIC  PRODUCTS. 
All  artificial  products  of  the  laboratory  and  all  mineral 
species  have  a  superposable  image.  On  the  contrary,  most  of 
the  natural  organic  products,  (I  might  say  all,  if  I  had  to  name 
only  those  which  play  an  essential  part  in  the  phenomena  of 
animal  and  vegetable  life,)  all  products  essential  to  life  are 
dissymmetric  and  of  the  dissymmetry  which  causes  their  image 
to  be  non-superposable. 
Before  proceeding  farther,  I  desire  to  set  aside  some  objec- 
tions which  cannot  fail  to  present  themselves  to  your  minds. 
IV. 
Quartz,  you  will  say  at  once  ?  We  have  seen  in  the  last 
lecture  that  quartz  possesses  the  two  characters  of  dissymmetry, 
the  hemihedrity  in  form  observed  by  Hauy,  and  the  rotary 
phenomenon  discovered  by  Arago.  Nevertheless,  all  molecular 
dissymmetry  is  absent  in  quartz.  To  understand  it,  let  us  ad- 
vance a  little  further  in  the  knowledge  of  the  phenomena  we  are 
considering.  We  shall  find,  moreover,  the  explanation  of  the 
analogies  and  differences  already  previously  indicated  between 
quartz  and  natural  organic  products. 
Permit  me  to  describe  roughly,  though  in  the  main  accurately, 
the  structure  of  quartz  and  that  of  natural  organic  products. 
Imagine  a  winding  stair,  the  steps  of  which  shall  be  cubes,  or 
any  other  object  with  a  superposable  image.  Destroy  the  stair, 
and  the  dissymmetry  will  have  disappeared.  The  dissymmetry 
of  the  stair  was  the  result  only  of  the  mode  of  putting  together 
its  elementary  steps.  Such  is  quartz.  The  crystal  of  quartz  is 
the  stair  all  built.  It  is  hemihedric.  For  this  reason  it  acts  on 
polarized  light.  But  let  the  crystal  be  dissolved,  melted,  its 
physical  structure  destroyed  in  any  manner,  and  its  dissymmetry 
is  suppressed,  and  with  it  all  action  on  polarized  light,  as  would 
happen,  for  example,  in  a  solution  of  alum,  a  liquid  formed  of 
molecules  of  a  cubic  structure  distributed  without  order. 
Imagine,  on  the  contrary,  the  same  winding  stair  formed  of 
irregular  tetrahedrons  for  steps.  Destroy  the  stair  ,  and  the 
dissymmetry  will  still  exist,  because  you  have  to  deal  with  an 
assemblage  of  tetrahedrons.  They  may  be  in  any  position,  but 
each  one  of  them  will  have  its  proper  dissymmetry,  neverthe- 
less.   Such  are  organic  bodies  in  which  all  the  molecules  have 
