Dec.  x,  1892.] 
THE  TROPICAL  AGRICULTURIST. 
4x9 
for  hydrogen  to  induce  water  to  part  with  its 
oxygen  which  in  the  nascent  state  is  able  to 
bleach  as  well  as  disinfect,  that  is  to  convert 
colouring  matters  and  many  offensive  gases  and 
vapours  into  colourless  and  inodorous  substances. 
We  next  come  to  “bleaching  powder”  which 
has  acquired  the  erroneous  name— that  has  stuck 
to  it — of  “chloride  of  lime.”  Bleaching  powder 
is  in  reality  a compound  of  calcium  chloride  and 
calcium  hypochlorite  with  the  formula  CaCL, 
CaCl202-  It  is  prepared  on  a large  scale  by 
passing  chlorine  gas  into  chambers  on  the  floors 
of  which  a layer  of  slaked  time  about  2 inches 
thick  is  spread — the  gas  is  all  absorbed  and 
bleaching  powder  formed,  i.  e.,  slaked  lime 
and  chlorine  give  bleaching  powder  and  water. 
In  all  processes  of  bleaching  by  means  of 
“bleaching  powder,”  the  material  to  he  bleached 
has  first  to  he  “ soured  ” or  dipped  in  an  acid 
solution.  This  is  a very  important  operation  as 
I shall  show  you  presently.  In  the  early  days 
of  bleaching  “sour  milk”  was  used  in  carrying 
out  this  operation,  owing  to  the  lactic  acid  it 
contained,  but  it  was  afterwards  found  that  a 
stronger  acid,  namely  sulphuric  acid,  reduced  the 
time  required  for  bleaching  by  one  half.  Now, 
what  is  the  importance  of  this  “souring”?  Well, 
in  bleaching  powders  we  see  that  we  have 
chlorine,  but  the  chlorine  is,  as  it  were,  locked 
up ; and  we  must  somehow  set  it  free.  The 
setting  free  of  chlorine  is  brought  about  by  the 
action  of  the  acid.  When  sulphuric  acid  comes 
in  contact  with  bleaching  powder  the  calcium 
hypochlorite  is  first  acted  upon  and  hypochlorous 
acid  formed ; next  the  calcium  chloride  is  acted 
up  and  hydrochloric  acid  formed  ; and  these  two 
acids  reach  upon  each  _other  to  form  chlorine 
and  water. 
Thus  by  the  use  of  an  acid,  or  by  the 
process  of  “souring”  the  bleaching  powder  is 
decomposed  and  finally  yields  the  bleaching  agent, 
namely  chlorine,  whose  action  in  combination 
with  water  as  “ wet  chlorine  ” we  have  already 
seen ; and  goods  to  be  bleached  by  bleaching 
powder  must  be  dipped  in  a solution  of  it  as 
well  as  in  a dilute  solution  of  acid ; it  does 
not  matter  which  operation  is  carried  out  first, 
the  only  essential  being  that  the  bleaching 
powder  and  the  acid  should  meet  together  in 
the  material  and  liberate  within  its  tissues  the 
chlorine  which  in  conjunction  with  water  does 
the  bleaching  work.  Chlorine  as  gas  or  as  bleach- 
ing powder  is  also  used  for  bleaching  paper 
materials  such  as  old  rags,  paper  pulp  as  well 
as  cotton  waste.  Printed  paper  as  books,  en- 
gravings, maps  &c.  that  have  been  stained  or  dis- 
coloured may  also  be  whitened  by  the  use  of 
dilute  solutions  of  bleaching  powder.  Again, 
chlorine  is  useful  in  the  process  of  bleaching 
sponges,  and  as  dilute  bleaching  powder  solution, 
for  whitening  straw. 
Another  bleaching  agent  that  is  worthy  of  some 
note  is  sulphur  dioxide  or  sulphurous  acid  gas. 
It  is  used  in  the  bleaching  of  silks,  woollens, 
straw,  &c. 
The  gas,  which  is  produced  when  sulphur  is 
burnt  in  air  or  oxygen,  has  the  peculiar  suffo- 
cating odour  of  burning  brimstone,  and  possesses  ' 
the  power  of  extinguishing  flame,  on  which  account 
it  has  been  used  with  success  in  the  putting  out 
of  chimney  fires. 
When  silk  is  required  to  be  very  white,  as 
for  gloves,  stockings,  &c.,  the  goods  are  cautiously 
submitted  for  two  or  three  hours  to  the  action  of 
the  fumes  of  burning  sulphur,  or  they  are 
sometimes  immersed  in  a solution  of  the  gas. 
Woollen  goods  and  fleeces  are  sulphured  in  sealed 
rooms  in  which  sulphur  is  left  to  burn.  In  the 
process  of  removating  printed  paper,  such  as 
engravings,  maps,  books,  Ac.,  the  articles  are  ex- 
posed to  sulphur  fumes  after  being  slightly 
moistened.  In  bleaching  straw  goods  sulphuring 
is  done  in  a “ sulphur  chamber  ” or  stove. 
Sulphuring  is  a method  of  sophistication  employed 
almost  universally  where  fruit  drying  and  preserv- 
ing are  carried  on  wholesale.  The  object  of 
sulphuring  fruit  is  to  brighten  the  colour  of  the 
fruit,  and  to  secure  it  from  the  attacks  of  insects. 
The  operation  has,  however,  been  condemned  as 
being  more  or  less  prejudicial  to  the  health  of 
the  consumers.  Not  long  ago  another  objection- 
able mode  of  sophistication  was  brought  to  light, 
namely  the  colouring  of  “ green  peas  ” by  means 
of  sulphate  of  iron  or  green  vitriol,  and  this 
imposture  was  traced  out  owing  to  symptoms 
of  poisoning  following  the  consumption  of  the 
adulterated  articles.  In  Ceylon  sulphuring  is  known 
to  be  used  in  the  bleaching  of  cardamoms,  and 
it  is  to  be  hoped  it  will  not  go  further  and  extend 
itself  to  other  products  whose  value  is  ruled 
by  outward  appearance  as  well  as  other  qualities 
such  as  cocoa.* 
And  how  does  sulphur  bleach  ? Sulphur  in 
burning  takes  up  two  atoms  of  oxygen  from  the 
air,  forming  sulphurous  acid  gas,  SO  2 which  is 
the  bleaching  agent  under  consideration.  The 
fumes  may  be  used  or  a solution  of  the  gas 
in  water  is  sometimes  conveniently  employed,  for 
one  volume  of  water  dissolves  about  30  volumes 
of  the  gas  at  the  ordinary  temperature.  Now 
sulphur  dioxide,  to  use  the  shorter  name,  SO2 
in  bleaching,  acts  in  a manner  exactly  opposite 
to  that  in  which  chlorine  acts,  inasmuch  as  it 
either  directly  removes  oxygen  from  the  colouring 
matter  present,  or  it  does  so  indirectly  in  the 
presence  of  water  by  itself  removing  the  O 
from  water  and  allowing  the  Hydrogen  to  rob 
the  colouring  matter  of  some  of  its  oxygen.  With 
the  oxygen  it  takes  to  itself  SO2.  unites  to  form 
a higher  oxide  SO3 
Thus  the  bleaching  action  of  SO2  is  a “de- 
oxidising” or  “reducing”  action,  (since  it  takes 
away  oxygen),  while  the  bleaching  action  of 
chlorine  as  well  as  of  ozone  and  peroxide  of 
hydrogen  is  an  oxidising  action. 
Sulphur  dioxide  does  not  entirely  destroy  colour, 
as  chlorine  does  by  oxidation,  but  by  its  reducing 
action  produces  a loss  of  colour  which  in  a 
sense  is  not  permanent,  for  dilute  acid  or 
alkaline  solutions  tend  to  restore  the  lost  colour. 
Flowers  bleached  by  sulphur  dioxide  regain  their 
colour  when  dipped  into  dilute  acid.  Again,  new 
woollen  goods  or  garments,  such  as  flannels, 
blankets,  Ac.,  though  almost  colourless  when  new 
return  to  their  natural  yellow  after  repeated 
washing,  for  the  washing  soda  which  may  be 
used,  or  the  potash  or  soda  present  in  the 
soap  employed,  helps  to  destroy  the  colourless 
compound  formed  in  the  texture  of  the  wool 
during  sulphuring  and  resuscitates  the  original 
colour. 
The  last  bleaching  agent  of  any  importance  is 
chromic  acid  in  the  anhydrous  form,  represented 
chemically  as  CO3  and  also  called  cromium 
trioxide.  It  is  prepared  by  the  action  of  sulphuric 
acid  on  bichromate  of  potash.  Chromic  acid 
is  thus  got  as  red  needle-shaped  chrystals, 
soluble  in  water.  Now  this  chromium  trioxide 
is  easily  reduced  to  the  sesqui-oxide  C2O3  in  the 
* In  preparing  the  “ uncoated  ginger”  of  Jamaica 
for  medicinal  use,  the  fresh  rhizomes  after  being 
scraped,  washed  and  dried,  are  afterwards  bleached  by 
being  subjected  to  the  fumes  of  burning  sulphur,  or 
immersed  in  a chlorinated  bath. 
