ON  THE  COPPER  TEST  FOR  SUGAR. 
247 
darkness,  and  even  then  all  eyes  are  not  equally  impressionable. 
But,  if  several  persons  unite  in  performing  the  experiment 
together,  there  will  always  be  a  certain  number  who  are  able  to 
see  thephenomena. 
These  facts  of  the  production  of  light  remind  the  author  of 
observations  published  some  time  ago  by  M.  Wullner,  according 
to  which  every  molecular  movement  is  accompanied  by  a  disen- 
gagement of  electricity. — Qhem.  News,  London,  Nov.  23,  1861, 
from  Poggendorff' s  Annalen. 
ON  THE  COPPER  TEST  FOR  SUGAR. 
By  John  Horsley. 
A  few  weeks  since  the  Chemical  News  contained  a  review  of 
"  Braithwaite's  Retrospect,"  in  which  allusion  was  made  to  a 
new  cupreous  preparation,  by  Dr.  Pavy,  for  the  detection  and 
estimation  of  sugar,  which  he  recommended  as  being  superior 
to  either  Barreswill's  or  Fehling's.  In  that  respect  I  differ  with 
him,  having  tried  his  method,  and  object  to  it  on  account  of 
the  excess  of  caustic  potash  it  contains,  having  known  instances 
where  the  copper  was  reduced  per  se  on  standing. 
Like  the  Doctor,  I  have  been  so  dissatisfied  with  the  reduc- 
tion by  copper  solutions  as  to  direct  my  attention  to  other  agents, 
till  I  found  out  that  it  was  not  so  much  the  test  itself  as  the 
mode  of  manipulation  that  required  improvement,  the  copper 
test  being  delicate  enough  when  well  prepared,  and  the  saccha- 
rine fluid  sufficiently  pure.  The  ordinary  method  of  procedure 
is,  I  believe,  pretty  much  as  described  by  Dr.  Pavy, — viz.,  to 
boil  and  decolorise  the  cupreous  liquor  by  the  addition  from 
time  to  time  of  the  saccharine  solution;  it  being  generally  sup- 
posed that  the  powers  of  the  sugar  are  defunct  at  the  moment 
the  blue  color  disappears,  than  which  nothing  is  more  fallacious, 
the  sugar  then  being  in  excess,  as  may  be  proved  by  a  further 
addition  of  the  test  and  re-boiling.  In  fact,  you  may  even  make 
such  an  addition  of  the  test  as  that,  on  boiling  and  filtering,  the 
liquor  shall  pass  through  highly  colored,  and  yet  contain  sugar, 
which  will  not  cease  to  reduce  copper  till  after  two  or  three 
other  boilings  and  filtrations  have  been  made.  The  filtered  liquid 
being  permanently  blue  or  blueish-green,  and  yielding  no  fur- 
