PROCESS  FOR  HELIOGRAPHIC  ENGRAVING. 
549 
coloring  matter.  The  question  now  arises,  whether  these  juices 
act  as  reducing  bodies,  or  whether  they  simply  form  colorless  com- 
binations. The  experiments  to  which  I  have  referred  above  may, 
I  think,  serve  to  answer  this  question  ;  for  if  reduction  takes  place, 
sulphurous  acid  would  not  reproduce  the  color.  I  consider  there- 
fore, that  the  coloring  matter  does  not  experience  any  reduction, 
and  that  it  forms  with  the  elements  of  the  colorless  juices  a  color- 
less combination.  In  infusions  prepared  by  the  action  of  alcohol 
or  water  upon  flowers,  one  portion  of  the  coloring  matter  remains 
free,  whilst  the  other  enters  into  the  combination  just  mentioned. 
It  is  easy  to  separate  the  colored  portion  from  the  colorless,  by 
triturating  the  liquid  with  a  little  artificial  phosphate  of  lime  or 
dry  hydrate  of  alumina  ;  the  colored  part  is  the  first  to  fix  upon 
the  solid  body,  whilst  that  of  which  the  color  is  dissembled  remains 
for  the  most  part  dissolved.  If  the  liquid  be  filtered,  it  passes 
without  color.  It  may  then  be  colored  red  by  acid,  and  green  or 
blue  by  an  alkaline  solution. — Chern.  Gaz.,  Sept.  1,  1S54,  from 
Comptes  Rendus,  July  24,  1854,  p.  194. 
PROCESS  FOR  HELIOGRAPHIC  ENGRAVING. 
M.  Baldus  takes  simply  a  plate  of  copper,  and  spreads  upon  it 
a  sensitive  coating  of  bitumen  of  Judea.  Upon  this  plate,  thus 
covered,  he  lays  a  photograph  on  paper  of  the  object  to  be  en- 
graved. This  photograph  is  positive,  and  must  therefore  impress 
a  negative  on  the  metal  by  the  action  of  the  light.  After  about  a 
quarter  of  an  hour's  exposure  to  the  sun,  the  image  is  produced 
upon  the  resinous  coating,  but  is  not  visible,  and  it  is  made  to  ap- 
pear by  washing  the  plate  with  a  solvent,  which  removes  the  parts 
not  affected  by  the  light,  and  allows  the  picture  to  be  seen  repre- 
sented by  the  resinous  lines  of  the  bitumen.  The  design,  however, 
is  formed  by  a  veil  so  delicate  and  thin,  that  it  would  soon  partly 
disappear  if  the  plate  remained  in  the  liquid.  To  give  it  the  pro- 
per firmness  and  resistance,  it  is  exposed  for  two  clays  to  the  ac- 
tion of  diffused  light  ;  the  picture  being  thus  strengthened  by  its 
exposure  to  daylight,  the  plate  is  plunged  into  a  galvano-plastic 
bath  of  -sulphate  of  copper,  If,  then,  you  attach  the  plate  to  the 
negative  pole  of  the  battery,  youdeposite,  on  the  unprotected  parts 
of  the  metal,  a  coating  of  copper  in  relief;  but  if  you  attach  it  to 
